# BIND 9 Administrator Reference Manual

The Internet Domain Name System (DNS) consists of the syntax to specify the names of entities in the Internet in a hierarchical manner, the rules used for delegating authority over names, and the system implementation that actually maps names to Internet addresses. DNS data is maintained in a group of distributed hierarchical databases.

The Berkeley Internet Name Domain (BIND) implements a domain name server for a number of operating systems. This document provides basic information about the installation and care of the Internet Systems Consortium (ISC) BIND version 9 software package for system administrators.

In this document, Chapter 1 introduces the basic DNS and BIND concepts. Chapter 2 describes resource requirements for running BIND in various environments. Information in Chapter 3 is task-oriented in its presentation and is organized functionally, to aid in the process of installing the BIND 9 software. The task-oriented section is followed by Chapter 4, which contains more advanced concepts that the system administrator may need for implementing certain options. The contents of Chapter 5 are organized as in a reference manual to aid in the ongoing maintenance of the software. Chapter 6 addresses security considerations, and Chapter 7 contains troubleshooting help. The main body of the document is followed by several appendices which contain useful reference information, such as a bibliography and historic information related to BIND and the Domain Name System.

In this document, we use the following general typographic conventions:

To describe: | We use the style:
a pathname, filename, URL, hostname, mailing list name, or new term or concept | `Fixed width`
literal user input | `Fixed Width Bold`
program output | `Fixed Width`

The following conventions are used in descriptions of the BIND configuration file:

To describe: | We use the style:
keywords | `Fixed Width`
variables | `Fixed Width`
Optional input | Text is enclosed in square brackets

The purpose of this document is to explain the installation and upkeep of the BIND (Berkeley Internet Name Domain) software package, and we begin by reviewing the fundamentals of the Domain Name System (DNS) as they relate to BIND.

The Domain Name System (DNS) is a hierarchical, distributed database. It stores information for mapping Internet host names to IP addresses and vice versa, mail routing information, and other data used by Internet applications.

Clients look up information in the DNS by calling a resolver library, which sends queries to one or more name servers and interprets the responses. The BIND 9 software distribution contains a name server, `named`, and a set of associated tools.

The data stored in the DNS is identified by domain names that are organized as a tree according to organizational or administrative boundaries. Each node of the tree, called a domain, is given a label. The domain name of the node is the concatenation of all the labels on the path from the node to the root node. This is represented in written form as a string of labels listed from right to left and separated by dots. A label need only be unique within its parent domain.

For example, a domain name for a host at the company Example, Inc. could be `ourhost.example.com`, where `com` is the top level domain to which `ourhost.example.com` belongs, `example` is a subdomain of `com`, and `ourhost` is the name of the host.

For administrative purposes, the name space is partitioned into areas called zones, each starting at a node and extending down to the leaf nodes or to nodes where other zones start. The data for each zone is stored in a name server, which answers queries about the zone using the DNS protocol.

The data associated with each domain name is stored in the form of resource records (RRs). Some of the supported resource record types are described in types_of_resource_records_and_when_to_use_them.

For more detailed information about the design of the DNS and the DNS protocol, please refer to the standards documents listed in rfcs.

To properly operate a name server, it is important to understand the difference between a zone and a domain.

As stated previously, a zone is a point of delegation in the DNS tree. A zone consists of those contiguous parts of the domain tree for which a name server has complete information and over which it has authority. It contains all domain names from a certain point downward in the domain tree except those which are delegated to other zones. A delegation point is marked by one or more NS records in the parent zone, which should be matched by equivalent NS records at the root of the delegated zone.

For instance, consider the `example.com` domain which includes names such as `host.aaa.example.com` and `host.bbb.example.com` even though the `example.com` zone includes only delegations for the `aaa.example.com` and `bbb.example.com` zones. A zone can map exactly to a single domain, but could also include only part of a domain, the rest of which could be delegated to other name servers. Every name in the DNS tree is a domain, even if it is terminal, that is, has no subdomains. Every subdomain is a domain and every domain except the root is also a subdomain. The terminology is not intuitive and we suggest that you read RFCs 1033, 1034 and 1035 to gain a complete understanding of this difficult and subtle topic.

Though BIND is called a "domain name server", it deals primarily in terms of zones. The master and slave declarations in the `named.conf` file specify zones, not domains. When you ask some other site if it is willing to be a slave server for your domain, you are actually asking for slave service for some collection of zones.

Each zone is served by at least one authoritative name server, which contains the complete data for the zone. To make the DNS tolerant of server and network failures, most zones have two or more authoritative servers, on different networks.

Responses from authoritative servers have the "authoritative answer" (AA) bit set in the response packets. This makes them easy to identify when debugging DNS configurations using tools like `dig` (diagnostic_tools).

The authoritative server where the master copy of the zone data is maintained is called the primary master server, or simply the primary. Typically it loads the zone contents from some local file edited by humans or perhaps generated mechanically from some other local file which is edited by humans. This file is called the zone file or master file.

In some cases, however, the master file may not be edited by humans at all, but may instead be the result of dynamic update operations.

The other authoritative servers, the slave servers (also known as secondary servers) load the zone contents from another server using a replication process known as a zone transfer. Typically the data are transferred directly from the primary master, but it is also possible to transfer it from another slave. In other words, a slave server may itself act as a master to a subordinate slave server.

Periodically, the slave server must send a refresh query to determine whether the zone contents have been updated. This is done by sending a query for the zone's SOA record and checking whether the SERIAL field has been updated; if so, a new transfer request is initiated. The timing of these refresh queries is controlled by the SOA REFRESH and RETRY fields, but can be overrridden with the `max-refresh-time`, `min-refresh-time`, `max-retry-time`, and `min-retry-time` options.

If the zone data cannot be updated within the time specified by the SOA EXPIRE option (up to a hard-coded maximum of 24 weeks) then the slave zone expires and will no longer respond to queries.

Usually all of the zone's authoritative servers are listed in NS records in the parent zone. These NS records constitute a delegation of the zone from the parent. The authoritative servers are also listed in the zone file itself, at the top level or apex of the zone. You can list servers in the zone's top-level NS records that are not in the parent's NS delegation, but you cannot list servers in the parent's delegation that are not present at the zone's top level.

A stealth server is a server that is authoritative for a zone but is not listed in that zone's NS records. Stealth servers can be used for keeping a local copy of a zone to speed up access to the zone's records or to make sure that the zone is available even if all the "official" servers for the zone are inaccessible.

A configuration where the primary master server itself is a stealth server is often referred to as a "hidden primary" configuration. One use for this configuration is when the primary master is behind a firewall and therefore unable to communicate directly with the outside world.

The resolver libraries provided by most operating systems are stub resolvers, meaning that they are not capable of performing the full DNS resolution process by themselves by talking directly to the authoritative servers. Instead, they rely on a local name server to perform the resolution on their behalf. Such a server is called a recursive name server; it performs recursive lookups for local clients.

To improve performance, recursive servers cache the results of the lookups they perform. Since the processes of recursion and caching are intimately connected, the terms recursive server and caching server are often used synonymously.

The length of time for which a record may be retained in the cache of a caching name server is controlled by the Time To Live (TTL) field associated with each resource record.

Even a caching name server does not necessarily perform the complete recursive lookup itself. Instead, it can forward some or all of the queries that it cannot satisfy from its cache to another caching name server, commonly referred to as a forwarder.

There may be one or more forwarders, and they are queried in turn until the list is exhausted or an answer is found. Forwarders are typically used when you do not wish all the servers at a given site to interact directly with the rest of the Internet servers. A typical scenario would involve a number of internal DNS servers and an Internet firewall. Servers unable to pass packets through the firewall would forward to the server that can do it, and that server would query the Internet DNS servers on the internal server's behalf.

The BIND name server can simultaneously act as a master for some zones, a slave for other zones, and as a caching (recursive) server for a set of local clients.

However, since the functions of authoritative name service and caching/recursive name service are logically separate, it is often advantageous to run them on separate server machines. A server that only provides authoritative name service (an authoritative-only server) can run with recursion disabled, improving reliability and security. A server that is not authoritative for any zones and only provides recursive service to local clients (a caching-only server) does not need to be reachable from the Internet at large and can be placed inside a firewall.

DNS hardware requirements have traditionally been quite modest. For many installations, servers that have been pensioned off from active duty have performed admirably as DNS servers.

The DNSSEC features of BIND 9 may prove to be quite CPU intensive however, so organizations that make heavy use of these features may wish to consider larger systems for these applications. BIND 9 is fully multithreaded, allowing full utilization of multiprocessor systems for installations that need it.

CPU requirements for BIND 9 range from i486-class machines for serving of static zones without caching, to enterprise-class machines if you intend to process many dynamic updates and DNSSEC signed zones, serving many thousands of queries per second.

The memory of the server has to be large enough to fit the cache and zones loaded off disk. The `max-cache-size` option can be used to limit the amount of memory used by the cache, at the expense of reducing cache hit rates and causing more DNS traffic. It is still good practice to have enough memory to load all zone and cache data into memory — unfortunately, the best way to determine this for a given installation is to watch the name server in operation. After a few weeks the server process should reach a relatively stable size where entries are expiring from the cache as fast as they are being inserted.

For name server intensive environments, there are two alternative configurations that may be used. The first is where clients and any second-level internal name servers query a main name server, which has enough memory to build a large cache. This approach minimizes the bandwidth used by external name lookups. The second alternative is to set up second-level internal name servers to make queries independently. In this configuration, none of the individual machines needs to have as much memory or CPU power as in the first alternative, but this has the disadvantage of making many more external queries, as none of the name servers share their cached data.

ISC BIND 9 compiles and runs on a large number of Unix-like operating systems and on Microsoft Windows Server 2003 and 2008, and Windows XP and Vista. For an up-to-date list of supported systems, see the README file in the top level directory of the BIND 9 source distribution.

In this chapter we provide some suggested configurations along with guidelines for their use. We suggest reasonable values for certain option settings.

The following sample configuration is appropriate for a caching-only name server for use by clients internal to a corporation. All queries from outside clients are refused using the `allow-query` option. Alternatively, the same effect could be achieved using suitable firewall rules.

    // Two corporate subnets we wish to allow queries from.
    acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
    options {
         // Working directory
         directory "/etc/namedb";

         allow-query { corpnets; };
    };
    // Provide a reverse mapping for the loopback
    // address 127.0.0.1
    zone "0.0.127.in-addr.arpa" {
         type master;
         file "localhost.rev";
         notify no;
    };

This sample configuration is for an authoritative-only server that is the master server for "`example.com`" and a slave for the subdomain "`eng.example.com`".

    options {
         // Working directory
         directory "/etc/namedb";
         // Do not allow access to cache
         allow-query-cache { none; };
         // This is the default
         allow-query { any; };
         // Do not provide recursive service
         recursion no;
    };

    // Provide a reverse mapping for the loopback
    // address 127.0.0.1
    zone "0.0.127.in-addr.arpa" {
         type master;
         file "localhost.rev";
         notify no;
    };
    // We are the master server for example.com
    zone "example.com" {
         type master;
         file "example.com.db";
         // IP addresses of slave servers allowed to
         // transfer example.com
         allow-transfer {
    	  192.168.4.14;
    	  192.168.5.53;
         };
    };
    // We are a slave server for eng.example.com
    zone "eng.example.com" {
         type slave;
         file "eng.example.com.bk";
         // IP address of eng.example.com master server
         masters { 192.168.4.12; };
    };

A primitive form of load balancing can be achieved in the DNS by using multiple records (such as multiple A records) for one name.

For example, if you have three WWW servers with network addresses of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the following means that clients will connect to each machine one third of the time:

Name | TTL | CLASS | TYPE | Resource Record (RR) Data
`www` | `600` | `IN` | `A` | `10.0.0.1`
| `600` | `IN` | `A` | `10.0.0.2`
| `600` | `IN` | `A` | `10.0.0.3`

When a resolver queries for these records, BIND will rotate them and respond to the query with the records in a different order. In the example above, clients will randomly receive records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients will use the first record returned and discard the rest.

For more detail on ordering responses, check the `rrset-order` sub-statement in the `options` statement, see rrset_ordering_title.

This section describes several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the name server daemon.

The `dig`, `host`, and `nslookup` programs are all command line tools for manually querying name servers. They differ in style and output format.

`dig`

- `dig` is the most versatile and complete of these lookup tools. It has two modes: simple interactive mode for a single query, and batch mode which executes a query for each in a list of several query lines. All query options are accessible from the command line.

      		  dig
      		  @server
      		  domain
      		  query-type
      		  query-class
      		  +query-option
      		  -dig-option
      		  %comment

  The usual simple use of `dig` will take the form

  `dig @server domain query-type query-class`

  For more information and a list of available commands and options, see the `dig` man page.

`host`

- The `host` utility emphasizes simplicity and ease of use. By default, it converts between host names and Internet addresses, but its functionality can be extended with the use of options.

      		  host
      		  -aCdlnrsTwv
      		  -c class
      		  -N ndots
      		  -t type
      		  -W timeout
      		  -R retries
      		  -m flag
      		  -4
      		  -6
      		  hostname
      		  server

  For more information and a list of available commands and options, see the `host` man page.

`nslookup`

- `nslookup` has two modes: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain.

      		  nslookup
      		  -option

      		    host-to-find
      		    - server

  Interactive mode is entered when no arguments are given (the default name server will be used) or when the first argument is a hyphen (`-') and the second argument is the host name or Internet address of a name server.

  Non-interactive mode is used when the name or Internet address of the host to be looked up is given as the first argument. The optional second argument specifies the host name or address of a name server.

  Due to its arcane user interface and frequently inconsistent behavior, we do not recommend the use of `nslookup`. Use `dig` instead.

Administrative tools play an integral part in the management of a server.

`named-checkconf`

- The `named-checkconf` program checks the syntax of a `named.conf` file.

      		  named-checkconf
      		  -jvz
      		  -t directory
      		  filename

`named-checkzone`

- The `named-checkzone` program checks a master file for syntax and consistency.

      		  named-checkzone
      		  -djqvD
      		  -c class
      		  -o output
      		  -t directory
      		  -w directory
      		  -k (ignore|warn|fail)
      		  -n (ignore|warn|fail)
      		  -W (ignore|warn)
      		  zone
      		  filename

`named-compilezone`

- Similar to `named-checkzone,` but it always dumps the zone content to a specified file (typically in a different format).

`rndc`

- The remote name daemon control (`rndc`) program allows the system administrator to control the operation of a name server. If you run `rndc` without any options, it will display a usage message as follows:

      		  rndc
      		  -c config
      		  -s server
      		  -p port
      		  -y key
      		  command
      		  command

  See man.rndc for details of the available `rndc` commands.

  `rndc` requires a configuration file, since all communication with the server is authenticated with digital signatures that rely on a shared secret, and there is no way to provide that secret other than with a configuration file. The default location for the `rndc` configuration file is `/etc/rndc.conf`, but an alternate location can be specified with the `-c` option. If the configuration file is not found, `rndc` will also look in `/etc/rndc.key` (or whatever `sysconfdir` was defined when the BIND build was configured). The `rndc.key` file is generated by running `rndc-confgen -a` as described in controls_statement_definition_and_usage.

  The format of the configuration file is similar to that of `named.conf`, but limited to only four statements, the `options`, `key`, `server` and `include` statements. These statements are what associate the secret keys to the servers with which they are meant to be shared. The order of statements is not significant.

  The `options` statement has three clauses: `default-server`, `default-key`, and `default-port`. `default-server` takes a host name or address argument and represents the server that will be contacted if no `-s` option is provided on the command line. `default-key` takes the name of a key as its argument, as defined by a `key` statement. `default-port` specifies the port to which `rndc` should connect if no port is given on the command line or in a `server` statement.

  The `key` statement defines a key to be used by `rndc` when authenticating with `named`. Its syntax is identical to the `key` statement in `named.conf`. The keyword `key` is followed by a key name, which must be a valid domain name, though it need not actually be hierarchical; thus, a string like "`rndc_key`" is a valid name. The `key` statement has two clauses: `algorithm` and `secret`. While the configuration parser will accept any string as the argument to algorithm, currently only the strings "`hmac-md5`", "`hmac-sha1`", "`hmac-sha224`", "`hmac-sha256`", "`hmac-sha384`" and "`hmac-sha512`" have any meaning. The secret is a Base64 encoded string as specified in RFC 3548.

  The `server` statement associates a key defined using the `key` statement with a server. The keyword `server` is followed by a host name or address. The `server` statement has two clauses: `key` and `port`. The `key` clause specifies the name of the key to be used when communicating with this server, and the `port` clause can be used to specify the port `rndc` should connect to on the server.

  A sample minimal configuration file is as follows:

      key rndc_key {
           algorithm "hmac-sha256";
           secret
             "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
      };
      options {
           default-server 127.0.0.1;
           default-key    rndc_key;
      };

  This file, if installed as `/etc/rndc.conf`, would allow the command:

  `$` `rndc reload`

  to connect to 127.0.0.1 port 953 and cause the name server to reload, if a name server on the local machine were running with following controls statements:

      controls {
      	inet 127.0.0.1
      	    allow { localhost; } keys { rndc_key; };
      };

  and it had an identical key statement for `rndc_key`.

  Running the `rndc-confgen` program will conveniently create a `rndc.conf` file for you, and also display the corresponding `controls` statement that you need to add to `named.conf`. Alternatively, you can run `rndc-confgen -a` to set up a `rndc.key` file and not modify `named.conf` at all.

Certain UNIX signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the `kill` command.

`SIGHUP` | Causes the server to read `named.conf` and reload the database.
`SIGTERM` | Causes the server to clean up and exit.
`SIGINT` | Causes the server to clean up and exit.

DNS NOTIFY is a mechanism that allows master servers to notify their slave servers of changes to a zone's data. In response to a `NOTIFY` from a master server, the slave will check to see that its version of the zone is the current version and, if not, initiate a zone transfer.

For more information about DNS `NOTIFY`, see the description of the `notify` option in boolean_options and the description of the zone option `also-notify` in zone_transfers. The `NOTIFY` protocol is specified in RFC 1996.

As a slave zone can also be a master to other slaves, `named`, by default, sends `NOTIFY` messages for every zone it loads. Specifying `notify master-only;` will cause `named` to only send `NOTIFY` for master zones that it loads.

Dynamic Update is a method for adding, replacing or deleting records in a master server by sending it a special form of DNS messages. The format and meaning of these messages is specified in RFC 2136.

Dynamic update is enabled by including an `allow-update` or an `update-policy` clause in the `zone` statement.

If the zone's `update-policy` is set to `local`, updates to the zone will be permitted for the key `local-ddns`, which will be generated by `named` at startup. See dynamic_update_policies for more details.

Dynamic updates using Kerberos signed requests can be made using the TKEY/GSS protocol by setting either the `tkey-gssapi-keytab` option, or alternatively by setting both the `tkey-gssapi-credential` and `tkey-domain` options. Once enabled, Kerberos signed requests will be matched against the update policies for the zone, using the Kerberos principal as the signer for the request.

Updating of secure zones (zones using DNSSEC) follows RFC 3007: RRSIG, NSEC and NSEC3 records affected by updates are automatically regenerated by the server using an online zone key. Update authorization is based on transaction signatures and an explicit server policy.

All changes made to a zone using dynamic update are stored in the zone's journal file. This file is automatically created by the server when the first dynamic update takes place. The name of the journal file is formed by appending the extension `.jnl` to the name of the corresponding zone file unless specifically overridden. The journal file is in a binary format and should not be edited manually.

The server will also occasionally write ("dump") the complete contents of the updated zone to its zone file. This is not done immediately after each dynamic update, because that would be too slow when a large zone is updated frequently. Instead, the dump is delayed by up to 15 minutes, allowing additional updates to take place. During the dump process, transient files will be created with the extensions `.jnw` and `.jbk`; under ordinary circumstances, these will be removed when the dump is complete, and can be safely ignored.

When a server is restarted after a shutdown or crash, it will replay the journal file to incorporate into the zone any updates that took place after the last zone dump.

Changes that result from incoming incremental zone transfers are also journaled in a similar way.

The zone files of dynamic zones cannot normally be edited by hand because they are not guaranteed to contain the most recent dynamic changes — those are only in the journal file. The only way to ensure that the zone file of a dynamic zone is up to date is to run `rndc stop`.

If you have to make changes to a dynamic zone manually, the following procedure will work: Disable dynamic updates to the zone using `rndc freeze `zone``. This will update the zone's master file with the changes stored in its `.jnl` file. Edit the zone file. Run `rndc thaw `zone`` to reload the changed zone and re-enable dynamic updates.

`rndc sync `zone`` will update the zone file with changes from the journal file without stopping dynamic updates; this may be useful for viewing the current zone state. To remove the `.jnl` file after updating the zone file, use `rndc sync -clean`.

The incremental zone transfer (IXFR) protocol is a way for slave servers to transfer only changed data, instead of having to transfer the entire zone. The IXFR protocol is specified in RFC 1995. See proposed_standards.

When acting as a master, BIND 9 supports IXFR for those zones where the necessary change history information is available. These include master zones maintained by dynamic update and slave zones whose data was obtained by IXFR. For manually maintained master zones, and for slave zones obtained by performing a full zone transfer (AXFR), IXFR is supported only if the option `ixfr-from-differences` is set to `yes`.

When acting as a slave, BIND 9 will attempt to use IXFR unless it is explicitly disabled. For more information about disabling IXFR, see the description of the `request-ixfr` clause of the `server` statement.

Setting up different views, or visibility, of the DNS space to internal and external resolvers is usually referred to as a Split DNS setup. There are several reasons an organization would want to set up its DNS this way.

One common reason for setting up a DNS system this way is to hide "internal" DNS information from "external" clients on the Internet. There is some debate as to whether or not this is actually useful. Internal DNS information leaks out in many ways (via email headers, for example) and most savvy "attackers" can find the information they need using other means. However, since listing addresses of internal servers that external clients cannot possibly reach can result in connection delays and other annoyances, an organization may choose to use a Split DNS to present a consistent view of itself to the outside world.

Another common reason for setting up a Split DNS system is to allow internal networks that are behind filters or in RFC 1918 space (reserved IP space, as documented in RFC 1918) to resolve DNS on the Internet. Split DNS can also be used to allow mail from outside back in to the internal network.

Let's say a company named Example, Inc. (`example.com`) has several corporate sites that have an internal network with reserved Internet Protocol (IP) space and an external demilitarized zone (DMZ), or "outside" section of a network, that is available to the public.

Example, Inc. wants its internal clients to be able to resolve external hostnames and to exchange mail with people on the outside. The company also wants its internal resolvers to have access to certain internal-only zones that are not available at all outside of the internal network.

In order to accomplish this, the company will set up two sets of name servers. One set will be on the inside network (in the reserved IP space) and the other set will be on bastion hosts, which are "proxy" hosts that can talk to both sides of its network, in the DMZ.

The internal servers will be configured to forward all queries, except queries for `site1.internal`, `site2.internal`, `site1.example.com`, and `site2.example.com`, to the servers in the DMZ. These internal servers will have complete sets of information for `site1.example.com`, `site2.example.com`, `site1.internal`, and `site2.internal`.

To protect the `site1.internal` and `site2.internal` domains, the internal name servers must be configured to disallow all queries to these domains from any external hosts, including the bastion hosts.

The external servers, which are on the bastion hosts, will be configured to serve the "public" version of the `site1` and `site2.example.com` zones. This could include things such as the host records for public servers (`www.example.com` and `ftp.example.com`), and mail exchange (MX) records (`a.mx.example.com` and `b.mx.example.com`).

In addition, the public `site1` and `site2.example.com` zones should have special MX records that contain wildcard (`*') records pointing to the bastion hosts. This is needed because external mail servers do not have any other way of looking up how to deliver mail to those internal hosts. With the wildcard records, the mail will be delivered to the bastion host, which can then forward it on to internal hosts.

Here's an example of a wildcard MX record:

    *   IN MX 10 external1.example.com.

Now that they accept mail on behalf of anything in the internal network, the bastion hosts will need to know how to deliver mail to internal hosts. In order for this to work properly, the resolvers on the bastion hosts will need to be configured to point to the internal name servers for DNS resolution.

Queries for internal hostnames will be answered by the internal servers, and queries for external hostnames will be forwarded back out to the DNS servers on the bastion hosts.

In order for all this to work properly, internal clients will need to be configured to query only the internal name servers for DNS queries. This could also be enforced via selective filtering on the network.

If everything has been set properly, Example, Inc.'s internal clients will now be able to:

- Look up any hostnames in the `site1` and `site2.example.com` zones.

- Look up any hostnames in the `site1.internal` and `site2.internal` domains.

- Look up any hostnames on the Internet.

- Exchange mail with both internal and external people.

Hosts on the Internet will be able to:

- Look up any hostnames in the `site1` and `site2.example.com` zones.

- Exchange mail with anyone in the `site1` and `site2.example.com` zones.

Here is an example configuration for the setup we just described above. Note that this is only configuration information; for information on how to configure your zone files, see sample_configuration.

Internal DNS server config:

    acl internals { 172.16.72.0/24; 192.168.1.0/24; };

    acl externals { bastion-ips-go-here; };

    options {
        ...
        ...
        forward only;
        // forward to external servers
        forwarders {
    	bastion-ips-go-here;
        };
        // sample allow-transfer (no one)
        allow-transfer { none; };
        // restrict query access
        allow-query { internals; externals; };
        // restrict recursion
        allow-recursion { internals; };
        ...
        ...
    };

    // sample master zone
    zone "site1.example.com" {
      type master;
      file "m/site1.example.com";
      // do normal iterative resolution (do not forward)
      forwarders { };
      allow-query { internals; externals; };
      allow-transfer { internals; };
    };

    // sample slave zone
    zone "site2.example.com" {
      type slave;
      file "s/site2.example.com";
      masters { 172.16.72.3; };
      forwarders { };
      allow-query { internals; externals; };
      allow-transfer { internals; };
    };

    zone "site1.internal" {
      type master;
      file "m/site1.internal";
      forwarders { };
      allow-query { internals; };
      allow-transfer { internals; }
    };

    zone "site2.internal" {
      type slave;
      file "s/site2.internal";
      masters { 172.16.72.3; };
      forwarders { };
      allow-query { internals };
      allow-transfer { internals; }
    };

External (bastion host) DNS server config:

    acl internals { 172.16.72.0/24; 192.168.1.0/24; };

    acl externals { bastion-ips-go-here; };

    options {
      ...
      ...
      // sample allow-transfer (no one)
      allow-transfer { none; };
      // default query access
      allow-query { any; };
      // restrict cache access
      allow-query-cache { internals; externals; };
      // restrict recursion
      allow-recursion { internals; externals; };
      ...
      ...
    };

    // sample slave zone
    zone "site1.example.com" {
      type master;
      file "m/site1.foo.com";
      allow-transfer { internals; externals; };
    };

    zone "site2.example.com" {
      type slave;
      file "s/site2.foo.com";
      masters { another_bastion_host_maybe; };
      allow-transfer { internals; externals; }
    };

In the `resolv.conf` (or equivalent) on the bastion host(s):

    search ...
    nameserver 172.16.72.2
    nameserver 172.16.72.3
    nameserver 172.16.72.4

TSIG (Transaction SIGnatures) is a mechanism for authenticating DNS messages, originally specified in RFC 2845. It allows DNS messages to be cryptographically signed using a shared secret. TSIG can be used in any DNS transaction, as a way to restrict access to certain server functions (e.g., recursive queries) to authorized clients when IP-based access control is insufficient or needs to be overridden, or as a way to ensure message authenticity when it is critical to the integrity of the server, such as with dynamic UPDATE messages or zone transfers from a master to a slave server.

This is a guide to setting up TSIG in BIND. It describes the configuration syntax and the process of creating TSIG keys.

`named` supports TSIG for server-to-server communication, and some of the tools included with BIND support it for sending messages to `named`:

- man.nsupdate supports TSIG via the `-k`, `-l` and `-y` command line options, or via the `key` command when running interactively.
- man.dig supports TSIG via the `-k` and `-y` command line options.

TSIG keys can be generated using the `tsig-keygen` command; the output of the command is a `key` directive suitable for inclusion in `named.conf`. The key name, algorithm and size can be specified by command line parameters; the defaults are "tsig-key", HMAC-SHA256, and 256 bits, respectively.

Any string which is a valid DNS name can be used as a key name. For example, a key to be shared between servers called host1 and host2 could be called "host1-host2.", and this key could be generated using:

      $ tsig-keygen host1-host2. > host1-host2.key

This key may then be copied to both hosts. The key name and secret must be identical on both hosts. (Note: copying a shared secret from one server to another is beyond the scope of the DNS. A secure transport mechanism should be used: secure FTP, SSL, ssh, telephone, encrypted email, etc.)

`tsig-keygen` can also be run as `ddns-confgen`, in which case its output includes additional configuration text for setting up dynamic DNS in `named`. See man.ddns-confgen for details.

For a key shared between servers called host1 and host2, the following could be added to each server's `named.conf` file:

    key "host1-host2." {
    	algorithm hmac-sha256;
    	secret "DAopyf1mhCbFVZw7pgmNPBoLUq8wEUT7UuPoLENP2HY=";
    };

(This is the same key generated above using `tsig-keygen`.)

Since this text contains a secret, it is recommended that either `named.conf` not be world-readable, or that the `key` directive be stored in a file which is not world-readable, and which is included in `named.conf` via the `include` directive.

Once a key has been added to `named.conf` and the server has been restarted or reconfigured, the server can recognize the key. If the server receives a message signed by the key, it will be able to verify the signature. If the signature is valid, the response will be signed using the same key.

TSIG keys that are known to a server can be listed using the command `rndc tsig-list`.

A server sending a request to another server must be told whether to use a key, and if so, which key to use.

For example, a key may be specified for each server in the `masters` statement in the definition of a slave zone; in this case, all SOA QUERY messages, NOTIFY messages, and zone transfer requests (AXFR or IXFR) will be signed using the specified key. Keys may also be specified in the `also-notify` statement of a master or slave zone, causing NOTIFY messages to be signed using the specified key.

Keys can also be specified in a `server` directive. Adding the following on host1, if the IP address of host2 is 10.1.2.3, would cause all requests from host1 to host2, including normal DNS queries, to be signed using the `host1-host2.` key:

    server 10.1.2.3 {
    	keys { host1-host2. ;};
    };

Multiple keys may be present in the `keys` statement, but only the first one is used. As this directive does not contain secrets, it can be used in a world-readable file.

Requests sent by host2 to host1 would not be signed, unless a similar `server` directive were in host2's configuration file.

Whenever any server sends a TSIG-signed DNS request, it will expect the response to be signed with the same key. If a response is not signed, or if the signature is not valid, the response will be rejected.

TSIG keys may be specified in ACL definitions and ACL directives such as `allow-query`, `allow-transfer` and `allow-update`. The above key would be denoted in an ACL element as `key host1-host2.`

An example of an `allow-update` directive using a TSIG key:

    allow-update { !{ !localnets; any; }; key host1-host2. ;};

This allows dynamic updates to succeed only if the UPDATE request comes from an address in `localnets`, and if it is signed using the `host1-host2.` key.

See dynamic_update_policies for a discussion of the more flexible `update-policy` statement.

Processing of TSIG-signed messages can result in several errors:

- If a TSIG-aware server receives a message signed by an unknown key, the response will be unsigned, with the TSIG extended error code set to BADKEY.
- If a TSIG-aware server receives a message from a known key but with an invalid signature, the response will be unsigned, with the TSIG extended error code set to BADSIG.
- If a TSIG-aware server receives a message with a time outside of the allowed range, the response will be signed, with the TSIG extended error code set to BADTIME, and the time values will be adjusted so that the response can be successfully verified.

In all of the above cases, the server will return a response code of NOTAUTH (not authenticated).

TKEY (Transaction KEY) is a mechanism for automatically negotiating a shared secret between two hosts, originally specified in RFC 2930.

There are several TKEY "modes" that specify how a key is to be generated or assigned. BIND 9 implements only one of these modes: Diffie-Hellman key exchange. Both hosts are required to have a KEY record with algorithm DH (though this record is not required to be present in a zone).

The TKEY process is initiated by a client or server by sending a query of type TKEY to a TKEY-aware server. The query must include an appropriate KEY record in the additional section, and must be signed using either TSIG or SIG(0) with a previously established key. The server's response, if successful, will contain a TKEY record in its answer section. After this transaction, both participants will have enough information to calculate a shared secret using Diffie-Hellman key exchange. The shared secret can then be used by to sign subsequent transactions between the two servers.

TSIG keys known by the server, including TKEY-negotiated keys, can be listed using `rndc tsig-list`.

TKEY-negotiated keys can be deleted from a server using `rndc tsig-delete`. This can also be done via the TKEY protocol itself, by sending an authenticated TKEY query specifying the "key deletion" mode.

BIND partially supports DNSSEC SIG(0) transaction signatures as specified in RFC 2535 and RFC 2931. SIG(0) uses public/private keys to authenticate messages. Access control is performed in the same manner as TSIG keys; privileges can be granted or denied in ACL directives based on the key name.

When a SIG(0) signed message is received, it will only be verified if the key is known and trusted by the server. The server will not attempt to recursively fetch or validate the key.

SIG(0) signing of multiple-message TCP streams is not supported.

The only tool shipped with BIND 9 that generates SIG(0) signed messages is `nsupdate`.

Cryptographic authentication of DNS information is possible through the DNS Security (DNSSEC-bis) extensions, defined in RFC 4033, RFC 4034, and RFC 4035. This section describes the creation and use of DNSSEC signed zones.

In order to set up a DNSSEC secure zone, there are a series of steps which must be followed. BIND 9 ships with several tools that are used in this process, which are explained in more detail below. In all cases, the `-h` option prints a full list of parameters. Note that the DNSSEC tools require the keyset files to be in the working directory or the directory specified by the `-d` option, and that the tools shipped with BIND 9.2.x and earlier are not compatible with the current ones.

There must also be communication with the administrators of the parent and/or child zone to transmit keys. A zone's security status must be indicated by the parent zone for a DNSSEC capable resolver to trust its data. This is done through the presence or absence of a `DS` record at the delegation point.

For other servers to trust data in this zone, they must either be statically configured with this zone's zone key or the zone key of another zone above this one in the DNS tree.

The `dnssec-keygen` program is used to generate keys.

A secure zone must contain one or more zone keys. The zone keys will sign all other records in the zone, as well as the zone keys of any secure delegated zones. Zone keys must have the same name as the zone, a name type of `ZONE`, and must be usable for authentication. It is recommended that zone keys use a cryptographic algorithm designated as "mandatory to implement" by the IETF; currently the are two algorithms: RSASHA256 and ECDSAP256SHA256. ECDSAP256SHA256 is recommended for current and future deployments.

The following command will generate a ECDSAP256SHA256 key for the `child.example` zone:

`dnssec-keygen -a ECDSAP256SHA256 -n ZONE child.example.`

Two output files will be produced: `Kchild.example.+013+12345.key` and `Kchild.example.+013+12345.private` (where 12345 is an example of a key tag). The key filenames contain the key name (`child.example.`), algorithm (5 is RSASHA1, 8 is RSASHA256, 13 is ECDSAP256SHA256, 15 is ED25519 etc.), and the key tag (12345 in this case). The private key (in the `.private` file) is used to generate signatures, and the public key (in the `.key` file) is used for signature verification.

To generate another key with the same properties (but with a different key tag), repeat the above command.

The `dnssec-keyfromlabel` program is used to get a key pair from a crypto hardware and build the key files. Its usage is similar to `dnssec-keygen`.

The public keys should be inserted into the zone file by including the `.key` files using `$INCLUDE` statements.

The `dnssec-signzone` program is used to sign a zone.

Any `keyset` files corresponding to secure sub-zones should be present. The zone signer will generate `NSEC`, `NSEC3` and `RRSIG` records for the zone, as well as `DS` for the child zones if `'-g'` is specified. If `'-g'` is not specified, then DS RRsets for the secure child zones need to be added manually.

The following command signs the zone, assuming it is in a file called `zone.child.example`. By default, all zone keys which have an available private key are used to generate signatures.

`dnssec-signzone -o child.example zone.child.example`

One output file is produced: `zone.child.example.signed`. This file should be referenced by `named.conf` as the input file for the zone.

`dnssec-signzone` will also produce a keyset and dsset files and optionally a dlvset file. These are used to provide the parent zone administrators with the `DNSKEYs` (or their corresponding `DS` records) that are the secure entry point to the zone.

To enable `named` to respond appropriately to DNS requests from DNSSEC-aware clients, `dnssec-enable` must be set to `yes`. This is the default setting.

To enable `named` to validate answers received from other servers, the `dnssec-validation` option must be set to either `yes` or `auto`.

When `dnssec-validation` is set to `auto`, a trust anchor for the DNS root zone will automatically be used. This trust anchor is provided as part of BIND and is kept up to date using RFC 5011 key management.

When `dnssec-validation` is set to `yes`, DNSSEC validation will only occur if at least one trust anchor has been explicitly configured in `named.conf` using a `trusted-keys` or `managed-keys` statement.

When `dnssec-validation` is set to `no`, DNSSEC validation will not occur.

The default is `auto` unless BIND is built with `configure --disable-auto-validation`, in which case the default is `yes`.

If `dnssec-enable` is set to `no`, then the default for `dnssec-validation` is also changed to `no`. If `dnssec-validation` is set to `yes`, the server will perform DNSSEC validation internally, but will not return signatures when queried - but it will not be turned on automatically.

`trusted-keys` are copies of DNSKEY RRs for zones that are used to form the first link in the cryptographic chain of trust. All keys listed in `trusted-keys` (and corresponding zones) are deemed to exist and only the listed keys will be used to validated the DNSKEY RRset that they are from.

`managed-keys` are trusted keys which are automatically kept up to date via RFC 5011 trust anchor maintenance.

`trusted-keys` and `managed-keys` are described in more detail later in this document.

Unlike BIND 8, BIND 9 does not verify signatures on load, so zone keys for authoritative zones do not need to be specified in the configuration file.

After DNSSEC gets established, a typical DNSSEC configuration will look something like the following. It has one or more public keys for the root. This allows answers from outside the organization to be validated. It will also have several keys for parts of the namespace the organization controls. These are here to ensure that `named` is immune to compromises in the DNSSEC components of the security of parent zones.

    managed-keys {
    	/* Root Key */
    	"." initial-key 257 3 3 "BNY4wrWM1nCfJ+CXd0rVXyYmobt7sEEfK3clRbGaTwS
    				 JxrGkxJWoZu6I7PzJu/E9gx4UC1zGAHlXKdE4zYIpRh
    				 aBKnvcC2U9mZhkdUpd1Vso/HAdjNe8LmMlnzY3zy2Xy
    				 4klWOADTPzSv9eamj8V18PHGjBLaVtYvk/ln5ZApjYg
    				 hf+6fElrmLkdaz MQ2OCnACR817DF4BBa7UR/beDHyp
    				 5iWTXWSi6XmoJLbG9Scqc7l70KDqlvXR3M/lUUVRbke
    				 g1IPJSidmK3ZyCllh4XSKbje/45SKucHgnwU5jefMtq
    				 66gKodQj+MiA21AfUVe7u99WzTLzY3qlxDhxYQQ20FQ
    				 97S+LKUTpQcq27R7AT3/V5hRQxScINqwcz4jYqZD2fQ
    				 dgxbcDTClU0CRBdiieyLMNzXG3";
    };

    trusted-keys {
    	/* Key for our organization's forward zone */
    	example.com. 257 3 5 "AwEAAaxPMcR2x0HbQV4WeZB6oEDX+r0QM6
    			      5KbhTjrW1ZaARmPhEZZe3Y9ifgEuq7vZ/z
    			      GZUdEGNWy+JZzus0lUptwgjGwhUS1558Hb
    			      4JKUbbOTcM8pwXlj0EiX3oDFVmjHO444gL
    			      kBOUKUf/mC7HvfwYH/Be22GnClrinKJp1O
    			      g4ywzO9WglMk7jbfW33gUKvirTHr25GL7S
    			      TQUzBb5Usxt8lgnyTUHs1t3JwCY5hKZ6Cq
    			      FxmAVZP20igTixin/1LcrgX/KMEGd/biuv
    			      F4qJCyduieHukuY3H4XMAcR+xia2nIUPvm
    			      /oyWR8BW/hWdzOvnSCThlHf3xiYleDbt/o
    			      1OTQ09A0=";

    	/* Key for our reverse zone. */
    	2.0.192.IN-ADDRPA.NET. 257 3 5 "AQOnS4xn/IgOUpBPJ3bogzwc
    				       xOdNax071L18QqZnQQQAVVr+i
    				       LhGTnNGp3HoWQLUIzKrJVZ3zg
    				       gy3WwNT6kZo6c0tszYqbtvchm
    				       gQC8CzKojM/W16i6MG/eafGU3
    				       siaOdS0yOI6BgPsw+YZdzlYMa
    				       IJGf4M4dyoKIhzdZyQ2bYQrjy
    				       Q4LB0lC7aOnsMyYKHHYeRvPxj
    				       IQXmdqgOJGq+vsevG06zW+1xg
    				       YJh9rCIfnm1GX/KMgxLPG2vXT
    				       D/RnLX+D3T3UL7HJYHJhAZD5L
    				       59VvjSPsZJHeDCUyWYrvPZesZ
    				       DIRvhDD52SKvbheeTJUm6Ehkz
    				       ytNN2SN96QRk8j/iI8ib";
    };

    options {
    	...
    	dnssec-enable yes;
    	dnssec-validation yes;
    };

None of the keys listed in this example are valid. In particular, the root key is not valid.

When DNSSEC validation is enabled and properly configured, the resolver will reject any answers from signed, secure zones which fail to validate, and will return SERVFAIL to the client.

Responses may fail to validate for any of several reasons, including missing, expired, or invalid signatures, a key which does not match the DS RRset in the parent zone, or an insecure response from a zone which, according to its parent, should have been secure.

When the validator receives a response from an unsigned zone that has a signed parent, it must confirm with the parent that the zone was intentionally left unsigned. It does this by verifying, via signed and validated NSEC/NSEC3 records, that the parent zone contains no DS records for the child.

If the validator can prove that the zone is insecure, then the response is accepted. However, if it cannot, then it must assume an insecure response to be a forgery; it rejects the response and logs an error.

The logged error reads "insecurity proof failed" and "got insecure response; parent indicates it should be secure".

BIND 9 fully supports all currently defined forms of IPv6 name to address and address to name lookups. It will also use IPv6 addresses to make queries when running on an IPv6 capable system.

For forward lookups, BIND 9 supports only AAAA records. RFC 3363 deprecated the use of A6 records, and client-side support for A6 records was accordingly removed from BIND 9. However, authoritative BIND 9 name servers still load zone files containing A6 records correctly, answer queries for A6 records, and accept zone transfer for a zone containing A6 records.

For IPv6 reverse lookups, BIND 9 supports the traditional "nibble" format used in the ip6.arpa domain, as well as the older, deprecated ip6.int domain. Older versions of BIND 9 supported the "binary label" (also known as "bitstring") format, but support of binary labels has been completely removed per RFC 3363. Many applications in BIND 9 do not understand the binary label format at all any more, and will return an error if given. In particular, an authoritative BIND 9 name server will not load a zone file containing binary labels.

For an overview of the format and structure of IPv6 addresses, see ipv6addresses.

The IPv6 AAAA record is a parallel to the IPv4 A record, and, unlike the deprecated A6 record, specifies the entire IPv6 address in a single record. For example,

    $ORIGIN example.com.
    host            3600    IN      AAAA    2001:db8::1

Use of IPv4-in-IPv6 mapped addresses is not recommended. If a host has an IPv4 address, use an A record, not a AAAA, with `::ffff:192.168.42.1` as the address.

When looking up an address in nibble format, the address components are simply reversed, just as in IPv4, and `ip6.arpa.` is appended to the resulting name. For example, the following would provide reverse name lookup for a host with address `2001:db8::1`.

    $ORIGIN 0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.
    1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0  14400   IN    PTR    (
    				    host.example.com. )

BIND 9 configuration is broadly similar to BIND 8; however, there are a few new areas of configuration, such as views. BIND 8 configuration files should work with few alterations in BIND 9, although more complex configurations should be reviewed to check if they can be more efficiently implemented using the new features found in BIND 9.

BIND 4 configuration files can be converted to the new format using the shell script `contrib/named-bootconf/named-bootconf.sh`.

Following is a list of elements used throughout the BIND configuration file documentation:

`acl_name` | The name of an `address_match_list` as defined by the `acl` statement.
`address_match_list` | A list of one or more `ip_addr`, `ip_prefix`, `key_id`, or `acl_name` elements, see address_match_lists.
`masters_list` | A named list of one or more `ip_addr` with optional `key_id` and/or `ip_port`. A `masters_list` may include other `masters_lists`.
`domain_name` | A quoted string which will be used as a DNS name, for example "`my.test.domain`".
`namelist` | A list of one or more `domain_name` elements.
`dotted_decimal` | One to four integers valued 0 through 255 separated by dots (`.'), such as `123`, `45.67` or `89.123.45.67`.
`ip4_addr` | An IPv4 address with exactly four elements in `dotted_decimal` notation.
`ip6_addr` | An IPv6 address, such as `2001:db8::1234`. IPv6 scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (`%') as delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, in order to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, currently only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address `fe80::1` on the link attached to the interface `ne0` can be specified as `fe80::1%ne0`. Note that on most systems link-local addresses always have the ambiguity, and need to be disambiguated.
`ip_addr` | An `ip4_addr` or `ip6_addr`.
`ip_dscp` | A `number` between 0 and 63, used to select a differentiated services code point (DSCP) value for use with outgoing traffic on operating systems that support DSCP.
`ip_port` | An IP port `number`. The `number` is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (`*') character can be used as a placeholder to select a random high-numbered port.
`ip_prefix` | An IP network specified as an `ip_addr`, followed by a slash (`/') and then the number of bits in the netmask. Trailing zeros in a `ip_addr` may omitted. For example, `127/8` is the network `127.0.0.0` with netmask `255.0.0.0` and `1.2.3.0/28` is network `1.2.3.0` with netmask `255.255.255.240`. When specifying a prefix involving a IPv6 scoped address the scope may be omitted. In that case the prefix will match packets from any scope.
`key_id` | A `domain_name` representing the name of a shared key, to be used for transaction security.
`key_list` | A list of one or more `key_id`s, separated by semicolons and ending with a semicolon.
`number` | A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used.
`fixedpoint` | A non-negative real number that can be specified to the nearest one hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the context in which it is used.
`path_name` | A quoted string which will be used as a pathname, such as `zones/master/my.test.domain`.
`port_list` | A list of an `ip_port` or a port range. A port range is specified in the form of `range` followed by two `ip_port`s, `port_low` and `port_high`, which represents port numbers from `port_low` through `port_high`, inclusive. `port_low` must not be larger than `port_high`. For example, `range 1024 65535` represents ports from 1024 through 65535. In either case an asterisk (`*') character is not allowed as a valid `ip_port`.
`size_spec` | A 64-bit unsigned integer, or the keywords `unlimited` or `default`. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as `max-journal-size`) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as possible", depending on the context. See the explanations of particular parameters that use `size_spec` for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: `K` or `k` for kilobytes, `M` or `m` for megabytes, and `G` or `g` for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively. `unlimited` generally means "as big as possible", and is usually the best way to safely set a very large number. `default` uses the limit that was in force when the server was started.
`size_or_percent` | `size_spec` or integer value followed by '%' to represent percents. The behavior is exactly the same as `size_spec`, but `size_or_percent` allows also to specify a positive integer value followed by '%' sign to represent percents.
`yes_or_no` | Either `yes` or `no`. The words `true` and `false` are also accepted, as are the numbers `1` and `0`.
`dialup_option` | One of `yes`, `no`, `notify`, `notify-passive`, `refresh` or `passive`. When used in a zone, `notify-passive`, `refresh`, and `passive` are restricted to slave and stub zones.

    address_match_list = address_match_list_element ; ...

    address_match_list_element = [ ! ] ( ip_address | ip_prefix |
         key key_id | acl_name | { address_match_list } )

Address match lists are primarily used to determine access control for various server operations. They are also used in the `listen-on` and `sortlist` statements. The elements which constitute an address match list can be any of the following:

- an IP address (IPv4 or IPv6)

- an IP prefix (in `/' notation)

- a key ID, as defined by the `key` statement

- the name of an address match list defined with the `acl` statement

- a nested address match list enclosed in braces

Elements can be negated with a leading exclamation mark (`!'), and the match list names "any", "none", "localhost", and "localnets" are predefined. More information on those names can be found in the description of the acl statement.

The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate access without regard to a host or network address. Nonetheless, the term "address match list" is still used throughout the documentation.

When a given IP address or prefix is compared to an address match list, the comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

The interpretation of a match depends on whether the list is being used for access control, defining `listen-on` ports, or in a `sortlist`, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses `allow-notify`, `allow-recursion`, `allow-recursion-on`, `allow-query`, `allow-query-on`, `allow-query-cache`, `allow-query-cache-on`, `allow-transfer`, `allow-update`, `allow-update-forwarding`, `blackhole`, and `keep-response-order` all use address match lists. Similarly, the `listen-on` option will cause the server to refuse queries on any of the machine's addresses which do not match the list.

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference will be given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in `1.2.3/24; ! 1.2.3.13;` the 1.2.3.13 element is completely useless because the algorithm will match any lookup for 1.2.3.13 to the 1.2.3/24 element. Using `! 1.2.3.13; 1.2.3/24` fixes that problem by having 1.2.3.13 blocked by the negation, but all other 1.2.3.* hosts fall through.

The BIND 9 comment syntax allows for comments to appear anywhere that whitespace may appear in a BIND configuration file. To appeal to programmers of all kinds, they can be written in the C, C++, or shell/perl style.

    /* This is a BIND comment as in C */

    // This is a BIND comment as in C++

    # This is a BIND comment as in common UNIX shells
    # and perl

Comments may appear anywhere that whitespace may appear in a BIND configuration file.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.

C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:

    /* This is the start of a comment.
       This is still part of the comment.
    /* This is an incorrect attempt at nesting a comment. */
       This is no longer in any comment. */

C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:

    // This is the start of a comment.  The next line
    // is a new comment, even though it is logically
    // part of the previous comment.

Shell-style (or perl-style, if you prefer) comments start with the character `#` (number sign) and continue to the end of the physical line, as in C++ comments. For example:

    # This is the start of a comment.  The next line
    # is a new comment, even though it is logically
    # part of the previous comment.

You cannot use the semicolon (`;') character to start a comment such as you would in a zone file. The semicolon indicates the end of a configuration statement.

A BIND 9 configuration consists of statements and comments. Statements end with a semicolon. Statements and comments are the only elements that can appear without enclosing braces. Many statements contain a block of sub-statements, which are also terminated with a semicolon.

The following statements are supported:

`acl` | defines a named IP address matching list, for access control and other uses.
`controls` | declares control channels to be used by the `rndc` utility.
`include` | includes a file.
`key` | specifies key information for use in authentication and authorization using TSIG.
`logging` | specifies what the server logs, and where the log messages are sent.
`masters` | defines a named masters list for inclusion in stub and slave zones' `masters` or `also-notify` lists.
`options` | controls global server configuration options and sets defaults for other statements.
`server` | sets certain configuration options on a per-server basis.
`statistics-channels` | declares communication channels to get access to `named` statistics.
`trusted-keys` | defines trusted DNSSEC keys.
`managed-keys` | lists DNSSEC keys to be kept up to date using RFC 5011 trust anchor maintenance.
`view` | defines a view.
`zone` | defines a zone.

The `logging` and `options` statements may only occur once per configuration.

The `acl` statement assigns a symbolic name to an address match list. It gets its name from a primary use of address match lists: Access Control Lists (ACLs).

The following ACLs are built-in:

`any` | Matches all hosts.
`none` | Matches no hosts.
`localhost` | Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the `localhost` ACL element is updated to reflect the changes.
`localnets` | Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the `localnets` ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses. In such a case, `localnets` only matches the local IPv6 addresses, just like `localhost`.

The `controls` statement declares control channels to be used by system administrators to control the operation of the name server. These control channels are used by the `rndc` utility to send commands to and retrieve non-DNS results from a name server.

An `inet` control channel is a TCP socket listening at the specified `ip_port` on the specified `ip_addr`, which can be an IPv4 or IPv6 address. An `ip_addr` of `*` (asterisk) is interpreted as the IPv4 wildcard address; connections will be accepted on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address, use an `ip_addr` of `::`. If you will only use `rndc` on the local host, using the loopback address (`127.0.0.1` or `::1`) is recommended for maximum security.

If no port is specified, port 953 is used. The asterisk "`*`" cannot be used for `ip_port`.

The ability to issue commands over the control channel is restricted by the `allow` and `keys` clauses. Connections to the control channel are permitted based on the `address_match_list`. This is for simple IP address based filtering only; any `key_id` elements of the `address_match_list` are ignored.

A `unix` control channel is a UNIX domain socket listening at the specified path in the file system. Access to the socket is specified by the `perm`, `owner` and `group` clauses. Note on some platforms (SunOS and Solaris) the permissions (`perm`) are applied to the parent directory as the permissions on the socket itself are ignored.

The primary authorization mechanism of the command channel is the `key_list`, which contains a list of `key_id`s. Each `key_id` in the `key_list` is authorized to execute commands over the control channel. See rndc in admin_tools) for information about configuring keys in `rndc`.

If the `read-only` clause is enabled, the control channel is limited to the following set of read-only commands: `nta -dump`, `null`, `status`, `showzone`, `testgen`, and `zonestatus`. By default, `read-only` is not enabled and the control channel allows read-write access.

If no `controls` statement is present, `named` will set up a default control channel listening on the loopback address 127.0.0.1 and its IPv6 counterpart ::1. In this case, and also when the `controls` statement is present but does not have a `keys` clause, `named` will attempt to load the command channel key from the file `rndc.key` in `/etc` (or whatever `sysconfdir` was specified as when BIND was built). To create a `rndc.key` file, run `rndc-confgen -a`.

The `rndc.key` feature was created to ease the transition of systems from BIND 8, which did not have digital signatures on its command channel messages and thus did not have a `keys` clause. It makes it possible to use an existing BIND 8 configuration file in BIND 9 unchanged, and still have `rndc` work the same way `ndc` worked in BIND 8, simply by executing the command `rndc-confgen -a` after BIND 9 is installed.

Since the `rndc.key` feature is only intended to allow the backward-compatible usage of BIND 8 configuration files, this feature does not have a high degree of configurability. You cannot easily change the key name or the size of the secret, so you should make a `rndc.conf` with your own key if you wish to change those things. The `rndc.key` file also has its permissions set such that only the owner of the file (the user that `named` is running as) can access it. If you desire greater flexibility in allowing other users to access `rndc` commands, then you need to create a `rndc.conf` file and make it group readable by a group that contains the users who should have access.

To disable the command channel, use an empty `controls` statement: `controls { };`.

    include filename;

The `include` statement inserts the specified file at the point where the `include` statement is encountered. The `include` statement facilitates the administration of configuration files by permitting the reading or writing of some things but not others. For example, the statement could include private keys that are readable only by the name server.

The `key` statement defines a shared secret key for use with TSIG (see tsig) or the command channel (see controls_statement_definition_and_usage).

The `key` statement can occur at the top level of the configuration file or inside a `view` statement. Keys defined in top-level `key` statements can be used in all views. Keys intended for use in a `controls` statement (see controls_statement_definition_and_usage) must be defined at the top level.

The `key_id`, also known as the key name, is a domain name uniquely identifying the key. It can be used in a `server` statement to cause requests sent to that server to be signed with this key, or in address match lists to verify that incoming requests have been signed with a key matching this name, algorithm, and secret.

The `algorithm_id` is a string that specifies a security/authentication algorithm. The `named` server supports `hmac-md5`, `hmac-sha1`, `hmac-sha224`, `hmac-sha256`, `hmac-sha384` and `hmac-sha512` TSIG authentication. Truncated hashes are supported by appending the minimum number of required bits preceded by a dash, e.g. `hmac-sha1-80`. The `secret_string` is the secret to be used by the algorithm, and is treated as a Base64 encoded string.

The `logging` statement configures a wide variety of logging options for the name server. Its `channel` phrase associates output methods, format options and severity levels with a name that can then be used with the `category` phrase to select how various classes of messages are logged.

Only one `logging` statement is used to define as many channels and categories as are wanted. If there is no `logging` statement, the logging configuration will be:

    logging {
         category default { default_syslog; default_debug; };
         category unmatched { null; };
    };

If `named` is started with the `-L` option, it logs to the specified file at startup, instead of using syslog. In this case the logging configuration will be:

    logging {
         category default { default_logfile; default_debug; };
         category unmatched { null; };
    };

The logging configuration is only established when the entire configuration file has been parsed. When the server is starting up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the `-g` option was specified.

All log output goes to one or more channels; you can make as many of them as you want.

Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, to a particular syslog facility, to the standard error stream, or are discarded. It can optionally also limit the message severity level that will be accepted by the channel (the default is `info`), and whether to include a `named`-generated time stamp, the category name and/or severity level (the default is not to include any).

The `null` destination clause causes all messages sent to the channel to be discarded; in that case, other options for the channel are meaningless.

The `file` destination clause directs the channel to a disk file. It can include additional arguments to specify how large the file is allowed to become before it is rolled to a backup file (`size`), how many backup versions of the file will be saved each time this happens (`versions`), and the format to use for naming backup versions (`suffix`).

The `size` option is used to limit log file growth. If the file ever exceeds the specified size, then `named` will stop writing to the file unless it has a `versions` option associated with it. If backup versions are kept, the files are rolled as described below. If there is no `versions` option, no more data will be written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

File rolling only occurs when the file exceeds the size specified with the `size` option. No backup versions are kept by default; any existing log file is simply appended. The `versions` option specifies how many backup versions of the file should be kept. If set to `unlimited`, there is no limit.

The `suffix` option can be set to either `increment` or `timestamp`. If set to `timestamp`, then when a log file is rolled, it is saved with the current timestamp as a file suffix. If set to `increment`, then backup files are saved with incrementing numbers as suffixes; older files are renamed when rolling. For example, if `versions` is set to 3 and `suffix` to `increment`, then when `filename.log` reaches the size specified by `size`, `filename.log.1` is renamed to `filename.log.2`, `filename.log.0` is renamed to `filename.log.1`, and `filename.log` is renamed to `filename.log.0`, whereupon a new `filename.log` is opened.

Example usage of the `size`, `versions`, and `suffix` options:

    channel an_example_channel {
        file "example.log" versions 3 size 20m suffix increment;
        print-time yes;
        print-category yes;
    };

The `syslog` destination clause directs the channel to the system log. Its argument is a syslog facility as described in the `syslog` man page. Known facilities are `kern`, `user`, `mail`, `daemon`, `auth`, `syslog`, `lpr`, `news`, `uucp`, `cron`, `authpriv`, `ftp`, `local0`, `local1`, `local2`, `local3`, `local4`, `local5`, `local6` and `local7`, however not all facilities are supported on all operating systems. How `syslog` will handle messages sent to this facility is described in the `syslog.conf` man page. If you have a system which uses a very old version of `syslog` that only uses two arguments to the `openlog()` function, then this clause is silently ignored.

On Windows machines syslog messages are directed to the EventViewer.

The `severity` clause works like `syslog`'s "priorities", except that they can also be used if you are writing straight to a file rather than using `syslog`. Messages which are not at least of the severity level given will not be selected for the channel; messages of higher severity levels will be accepted.

If you are using `syslog`, then the `syslog.conf` priorities will also determine what eventually passes through. For example, defining a channel facility and severity as `daemon` and `debug` but only logging `daemon.warning` via `syslog.conf` will cause messages of severity `info` and `notice` to be dropped. If the situation were reversed, with `named` writing messages of only `warning` or higher, then `syslogd` would print all messages it received from the channel.

The `stderr` destination clause directs the channel to the server's standard error stream. This is intended for use when the server is running as a foreground process, for example when debugging a configuration.

The server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, then debugging mode will be active. The global debug level is set either by starting the `named` server with the `-d` flag followed by a positive integer, or by running `rndc trace`. The global debug level can be set to zero, and debugging mode turned off, by running `rndc notrace`. All debugging messages in the server have a debug level, and higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

    channel specific_debug_level {
        file "foo";
        severity debug 3;
    };

will get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with `dynamic` severity use the server's global debug level to determine what messages to print.

`print-time` can be set to `yes`, `no`, or a time format specifier, which may be one of `local`, `iso8601` or `iso8601-utc`. If set to `no`, then the date and time will not be logged. If set to `yes` or `local`, the date and time are logged in a human readable format, using the local time zone. If set to `iso8601` the local time is logged in ISO8601 format. If set to `iso8601-utc`, then the date and time are logged in ISO8601 format, with time zone set to UTC. The default is `no`.

`print-time` may be specified for a `syslog` channel, but it is usually pointless since `syslog` also logs the date and time.

If `print-category` is requested, then the category of the message will be logged as well. Finally, if `print-severity` is on, then the severity level of the message will be logged. The `print-` options may be used in any combination, and will always be printed in the following order: time, category, severity. Here is an example where all three `print-` options are on:

`28-Feb-2000 15:05:32.863 general: notice: running`

If `buffered` has been turned on the output to files will not be flushed after each log entry. By default all log messages are flushed.

There are four predefined channels that are used for `named`'s default logging as follows. If `named` is started with the `-L` then a fifth channel `default_logfile` is added. How they are used is described in the_category_phrase.

    channel default_syslog {
        // send to syslog's daemon facility
        syslog daemon;
        // only send priority info and higher
        severity info;
    };

    channel default_debug {
        // write to named.run in the working directory
        // Note: stderr is used instead of "named.run" if
        // the server is started with the '-g' option.
        file "named.run";
        // log at the server's current debug level
        severity dynamic;
    };

    channel default_stderr {
        // writes to stderr
        stderr;
        // only send priority info and higher
        severity info;
    };

    channel null {
       // toss anything sent to this channel
       null;
    };

    channel default_logfile {
        // this channel is only present if named is
        // started with the -L option, whose argument
        // provides the file name
        file "...";
        // log at the server's current debug level
        severity dynamic;
    };

The `default_debug` channel has the special property that it only produces output when the server's debug level is nonzero. It normally writes to a file called `named.run` in the server's working directory.

For security reasons, when the `-u` command line option is used, the `named.run` file is created only after `named` has changed to the new UID, and any debug output generated while `named` is starting up and still running as root is discarded. If you need to capture this output, you must run the server with the `-L` option to specify a default logfile, or the `-g` option to log to standard error which you can redirect to a file.

Once a channel is defined, it cannot be redefined. Thus you cannot alter the built-in channels directly, but you can modify the default logging by pointing categories at channels you have defined.

There are many categories, so you can send the logs you want to see wherever you want, without seeing logs you don't want. If you don't specify a list of channels for a category, then log messages in that category will be sent to the `default` category instead. If you don't specify a default category, the following "default default" is used:

    category default { default_syslog; default_debug; };

If you start `named` with the `-L` option then the default category is:

    category default { default_logfile; default_debug; };

As an example, let's say you want to log security events to a file, but you also want keep the default logging behavior. You'd specify the following:

    channel my_security_channel {
        file "my_security_file";
        severity info;
    };
    category security {
        my_security_channel;
        default_syslog;
        default_debug;
    };

To discard all messages in a category, specify the `null` channel:

    category xfer-out { null; };
    category notify { null; };

Following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future BIND releases.

The `query-errors` category is specifically intended for debugging purposes: To identify why and how specific queries result in responses which indicate an error. Messages of this category are therefore only logged with `debug` levels.

At the debug levels of 1 or higher, each response with the rcode of SERVFAIL is logged as follows:

`client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880`

This means an error resulting in SERVFAIL was detected at line 3880 of source file `query.c`. Log messages of this level will particularly help identify the cause of SERVFAIL for an authoritative server.

At the debug levels of 2 or higher, detailed context information of recursive resolutions that resulted in SERVFAIL is logged. The log message will look like as follows:

    fetch completed at resolver.c:2970 for www.example.com/A
    in 30.000183: timed out/success [domain:example.com,
    referral:2,restart:7,qrysent:8,timeout:5,lame:0,neterr:0,
    badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 30.000183 seconds and the final result that led to the SERVFAIL was determined at line 2970 of source file `resolver.c`.

The following part shows the detected final result and the latest result of DNSSEC validation. The latter is always success when no validation attempt is made. In this example, this query resulted in SERVFAIL probably because all name servers are down or unreachable, leading to a timeout in 30 seconds. DNSSEC validation was probably not attempted.

The last part enclosed in square brackets shows statistics information collected for this particular resolution attempt. The `domain` field shows the deepest zone that the resolver reached; it is the zone where the error was finally detected. The meaning of the other fields is summarized in the following table.

`referral` | The number of referrals the resolver received throughout the resolution process. In the above example this is 2, which are most likely com and example.com.
`restart` | The number of cycles that the resolver tried remote servers at the `domain` zone. In each cycle the resolver sends one query (possibly resending it, depending on the response) to each known name server of the `domain` zone.
`qrysent` | The number of queries the resolver sent at the `domain` zone.
`timeout` | The number of timeouts since the resolver received the last response.
`lame` | The number of lame servers the resolver detected at the `domain` zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND9's address database (ADB), where lame servers are cached.
`neterr` | The number of erroneous results that the resolver encountered in sending queries at the `domain` zone. One common case is the remote server is unreachable and the resolver receives an ICMP unreachable error message.
`badresp` | The number of unexpected responses (other than `lame`) to queries sent by the resolver at the `domain` zone.
`adberr` | Failures in finding remote server addresses of the `domain` zone in the ADB. One common case of this is that the remote server's name does not have any address records.
`findfail` | Failures of resolving remote server addresses. This is a total number of failures throughout the resolution process.
`valfail` | Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the `domain` zone), but should only happen in `domain`.

At the debug levels of 3 or higher, the same messages as those at the debug 1 level are logged for other errors than SERVFAIL. Note that negative responses such as NXDOMAIN are not regarded as errors here.

At the debug levels of 4 or higher, the same messages as those at the debug 2 level are logged for other errors than SERVFAIL. Unlike the above case of level 3, messages are logged for negative responses. This is because any unexpected results can be difficult to debug in the recursion case.

`masters` lists allow for a common set of masters to be easily used by multiple stub and slave zones in their `masters` or `also-notify` lists.

This is the grammar of the `options` statement in the `named.conf` file:

The `options` statement sets up global options to be used by BIND. This statement may appear only once in a configuration file. If there is no `options` statement, an options block with each option set to its default will be used.

`attach-cache`

- Allows multiple views to share a single cache database. Each view has its own cache database by default, but if multiple views have the same operational policy for name resolution and caching, those views can share a single cache to save memory and possibly improve resolution efficiency by using this option.

  The `attach-cache` option may also be specified in `view` statements, in which case it overrides the global `attach-cache` option.

  The `cache_name` specifies the cache to be shared. When the `named` server configures views which are supposed to share a cache, it creates a cache with the specified name for the first view of these sharing views. The rest of the views will simply refer to the already created cache.

  One common configuration to share a cache would be to allow all views to share a single cache. This can be done by specifying the `attach-cache` as a global option with an arbitrary name.

  Another possible operation is to allow a subset of all views to share a cache while the others to retain their own caches. For example, if there are three views A, B, and C, and only A and B should share a cache, specify the `attach-cache` option as a view A (or B)'s option, referring to the other view name:

        view "A" {
          // this view has its own cache
          ...
        };
        view "B" {
          // this view refers to A's cache
          attach-cache "A";
        };
        view "C" {
          // this view has its own cache
          ...
        };

  Views that share a cache must have the same policy on configurable parameters that may affect caching. The current implementation requires the following configurable options be consistent among these views: `check-names`, `cleaning-interval`, `dnssec-accept-expired`, `dnssec-validation`, `max-cache-ttl`, `max-ncache-ttl`, `max-stale-ttl`, `max-cache-size`, and `min-cache-ttl`, `min-ncache-ttl`, `zero-no-soa-ttl`.

  Note that there may be other parameters that may cause confusion if they are inconsistent for different views that share a single cache. For example, if these views define different sets of forwarders that can return different answers for the same question, sharing the answer does not make sense or could even be harmful. It is administrator's responsibility to ensure configuration differences in different views do not cause disruption with a shared cache.

`directory`

- The working directory of the server. Any non-absolute pathnames in the configuration file will be taken as relative to this directory. The default location for most server output files (e.g. `named.run`) is this directory. If a directory is not specified, the working directory defaults to ``.`', the directory from which the server was started. The directory specified should be an absolute path, and must be writable by the effective user ID of the `named` process.

`dnstap`

- `dnstap` is a fast, flexible method for capturing and logging DNS traffic. Developed by Robert Edmonds at Farsight Security, Inc., and supported by multiple DNS implementations, `dnstap` uses `libfstrm` (a lightweight high-speed framing library, see https://github.com/farsightsec/fstrm) to send event payloads which are encoded using Protocol Buffers (`libprotobuf-c`, a mechanism for serializing structured data developed by Google, Inc.; see https://developers.google.com/protocol-buffers).

  To enable `dnstap` at compile time, the `fstrm` and `protobuf-c` libraries must be available, and BIND must be configured with `--enable-dnstap`.

  The `dnstap` option is a bracketed list of message types to be logged. These may be set differently for each view. Supported types are `client`, `auth`, `resolver`, `forwarder`, and `update`. Specifying type `all` will cause all `dnstap` messages to be logged, regardless of type.

  Each type may take an additional argument to indicate whether to log `query` messages or `response` messages; if not specified, both queries and responses are logged.

  Example: To log all authoritative queries and responses, recursive client responses, and upstream queries sent by the resolver, use:

      dnstap {
        auth;
        client response;
        resolver query;
      };

  Logged `dnstap` messages can be parsed using the `dnstap-read` utility (see man.dnstap-read for details).

  For more information on `dnstap`, see http://dnstap.info.

  The fstrm library has a number of tunables that are exposed in `named.conf`, and can be modified if necessary to improve performance or prevent loss of data. These are:

  - `fstrm-set-buffer-hint`: The threshold number of bytes to accumulate in the output buffer before forcing a buffer flush. The minimum is 1024, the maximum is 65536, and the default is 8192.

  - `fstrm-set-flush-timeout`: The number of seconds to allow unflushed data to remain in the output buffer. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 1 second.

  - `fstrm-set-output-notify-threshold`: The number of outstanding queue entries to allow on an input queue before waking the I/O thread. The minimum is 1 and the default is 32.

  - `fstrm-set-output-queue-model`: Controls the queuing semantics to use for queue objects. The default is `mpsc` (multiple producer, single consumer); the other option is `spsc` (single producer, single consumer).

  - `fstrm-set-input-queue-size`: The number of queue entries to allocate for each input queue. This value must be a power of 2. The minimum is 2, the maximum is 16384, and the default is 512.

  - `fstrm-set-output-queue-size`: The number of queue entries to allocate for each output queue. The minimum is 2, the maximum is system-dependent and based on `IOV_MAX`, and the default is 64.

  - `fstrm-set-reopen-interval`: The number of seconds to wait between attempts to reopen a closed output stream. The minimum is 1 second, the maximum is 600 seconds (10 minutes), and the default is 5 seconds. For convenience, TTL-style time unit suffixes may be used to specify the value.

  Note that all of the above minimum, maximum, and default values are set by the `libfstrm` library, and may be subject to change in future versions of the library. See the `libfstrm` documentation for more information.

`dnstap-output`

- Configures the path to which the `dnstap` frame stream will be sent if `dnstap` is enabled at compile time and active.

  The first argument is either `file` or `unix`, indicating whether the destination is a file or a UNIX domain socket. The second argument is the path of the file or socket. (Note: when using a socket, `dnstap` messages will only be sent if another process such as `fstrm_capture` (provided with `libfstrm`) is listening on the socket.)

  If the first argument is `file`, then up to three additional options can be added: `size` indicates the size to which a `dnstap` log file can grow before being rolled to a new file; `versions` specifies the number of rolled log files to retain; and `suffix` indicates whether to retain rolled log files with an incrementing counter as the suffix (`increment`) or with the current timestamp (`timestamp`). These are similar to the `size`, `versions`, and `suffix` options in a `logging` channel. The default is to allow `dnstap` log files to grow to any size without rolling.

  `dnstap-output` can only be set globally in `options`. Currently, it can only be set once while `named` is running; once set, it cannot be changed by `rndc reload` or `rndc reconfig`.

`dnstap-identity`

- Specifies an `identity` string to send in `dnstap` messages. If set to `hostname`, which is the default, the server's hostname will be sent. If set to `none`, no identity string will be sent.

`dnstap-version`

- Specifies a `version` string to send in `dnstap` messages. The default is the version number of the BIND release. If set to `none`, no version string will be sent.

`geoip-directory`

- When `named` is compiled using the MaxMind GeoIP2 geolocation API, or the legacy GeoIP API, this specifies the directory containing GeoIP database files. By default, the option is set based on the prefix used to build the `libmaxminddb` module: for example, if the library is installed in `/usr/local/lib`, then the default `geoip-directory` will be `/usr/local/share/GeoIP`. On Windows, the default is the `named` working directory. See acl for details about `geoip` ACLs.

`key-directory`

- When performing dynamic update of secure zones, the directory where the public and private DNSSEC key files should be found, if different than the current working directory. (Note that this option has no effect on the paths for files containing non-DNSSEC keys such as `bind.keys`, `rndc.key` or `session.key`.)

`lmdb-mapsize`

- When `named` is built with liblmdb, this option sets a maximum size for the memory map of the new-zone database (NZD) in LMDB database format. This database is used to store configuration information for zones added using `rndc addzone`. Note that this is not the NZD database file size, but the largest size that the database may grow to.

  Because the database file is memory mapped, its size is limited by the address space of the named process. The default of 32 megabytes was chosen to be usable with 32-bit `named` builds. The largest permitted value is 1 terabyte. Given typical zone configurations without elaborate ACLs, a 32 MB NZD file ought to be able to hold configurations of about 100,000 zones.

`managed-keys-directory`

- Specifies the directory in which to store the files that track managed DNSSEC keys. By default, this is the working directory. The directory must be writable by the effective user ID of the `named` process.

  If `named` is not configured to use views, then managed keys for the server will be tracked in a single file called `managed-keys.bind`. Otherwise, managed keys will be tracked in separate files, one file per view; each file name will be the view name (or, if it contains characters that are incompatible with use as a file name, the SHA256 hash of the view name), followed by the extension `.mkeys`.

  (Note: in previous releases, file names for views always used the SHA256 hash of the view name. To ensure compatibility after upgrade, if a file using the old name format is found to exist, it will be used instead of the new format.)

`new-zones-directory`

- Specifies the directory in which to store the configuration parameters for zones added via `rndc addzone`. By default, this is the working directory. If set to a relative path, it will be relative to the working directory. The directory must be writable by the effective user ID of the `named` process.

`qname-minimization`

- This option controls QNAME minimization behaviour in the BIND resolver. When set to `strict`, BIND will follow the QNAME minimization algorithm to the letter, as specified in RFC 7816. Setting this option to `relaxed` will cause BIND to fall back to normal (non-minimized) query mode when it receives either NXDOMAIN or other unexpected responses (e.g. SERVFAIL, improper zone cut, REFUSED) to a minimized query. `disabled` disables QNAME minimization completely. The current default is `relaxed`, but it might be changed to `strict` in a future release.

`tkey-gssapi-keytab`

- The KRB5 keytab file to use for GSS-TSIG updates. If this option is set and tkey-gssapi-credential is not set, then updates will be allowed with any key matching a principal in the specified keytab.

`tkey-gssapi-credential`

- The security credential with which the server should authenticate keys requested by the GSS-TSIG protocol. Currently only Kerberos 5 authentication is available and the credential is a Kerberos principal which the server can acquire through the default system key file, normally `/etc/krb5.keytab`. The location keytab file can be overridden using the tkey-gssapi-keytab option. Normally this principal is of the form "`DNS/``server.domain`". To use GSS-TSIG, `tkey-domain` must also be set if a specific keytab is not set with tkey-gssapi-keytab.

`tkey-domain`

- The domain appended to the names of all shared keys generated with `TKEY`. When a client requests a `TKEY` exchange, it may or may not specify the desired name for the key. If present, the name of the shared key will be `client specified part` + `tkey-domain`. Otherwise, the name of the shared key will be `random hex digits` + `tkey-domain`. In most cases, the `domainname` should be the server's domain name, or an otherwise non-existent subdomain like "_tkey.`domainname`". If you are using GSS-TSIG, this variable must be defined, unless you specify a specific keytab using tkey-gssapi-keytab.

`tkey-dhkey`

- The Diffie-Hellman key used by the server to generate shared keys with clients using the Diffie-Hellman mode of `TKEY`. The server must be able to load the public and private keys from files in the working directory. In most cases, the `key_name` should be the server's host name.

`cache-file`

- This is for testing only. Do not use.

`dump-file`

- The pathname of the file the server dumps the database to when instructed to do so with `rndc dumpdb`. If not specified, the default is `named_dump.db`.

`memstatistics-file`

- The pathname of the file the server writes memory usage statistics to on exit. If not specified, the default is `named.memstats`.

`lock-file`

- The pathname of a file on which `named` will attempt to acquire a file lock when starting up for the first time; if unsuccessful, the server will will terminate, under the assumption that another server is already running. If not specified, the default is `none`.

  Specifying `lock-file none` disables the use of a lock file. `lock-file` is ignored if `named` was run using the `-X` option, which overrides it. Changes to `lock-file` are ignored if `named` is being reloaded or reconfigured; it is only effective when the server is first started up.

`pid-file`

- The pathname of the file the server writes its process ID in. If not specified, the default is `/var/run/named/named.pid`. The PID file is used by programs that want to send signals to the running name server. Specifying `pid-file none` disables the use of a PID file — no file will be written and any existing one will be removed. Note that `none` is a keyword, not a filename, and therefore is not enclosed in double quotes.

`recursing-file`

- The pathname of the file the server dumps the queries that are currently recursing when instructed to do so with `rndc recursing`. If not specified, the default is `named.recursing`.

`statistics-file`

- The pathname of the file the server appends statistics to when instructed to do so using `rndc stats`. If not specified, the default is `named.stats` in the server's current directory. The format of the file is described in statsfile.

`bindkeys-file`

- The pathname of a file to override the built-in trusted keys provided by `named`. See the discussion of `dnssec-validation` for details. If not specified, the default is `/etc/bind.keys`.

`secroots-file`

- The pathname of the file the server dumps security roots to when instructed to do so with `rndc secroots`. If not specified, the default is `named.secroots`.

`session-keyfile`

- The pathname of the file into which to write a TSIG session key generated by `named` for use by `nsupdate -l`. If not specified, the default is `/var/run/named/session.key`. (See dynamic_update_policies, and in particular the discussion of the `update-policy` statement's `local` option for more information about this feature.)

`session-keyname`

- The key name to use for the TSIG session key. If not specified, the default is "local-ddns".

`session-keyalg`

- The algorithm to use for the TSIG session key. Valid values are hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, hmac-sha512 and hmac-md5. If not specified, the default is hmac-sha256.

`port`

- The UDP/TCP port number the server uses for receiving and sending DNS protocol traffic. The default is 53. This option is mainly intended for server testing; a server using a port other than 53 will not be able to communicate with the global DNS.

`dscp`

- The global Differentiated Services Code Point (DSCP) value to classify outgoing DNS traffic on operating systems that support DSCP. Valid values are 0 through 63. It is not configured by default.

`random-device`

- Specifies a source of entropy to be used by the server. This is a device or file from which to read entropy. If it is a file, operations requiring entropy will fail when the file has been exhausted.

  Entropy is needed for cryptographic operations such as TKEY transactions, dynamic update of signed zones, and generation of TSIG session keys. It is also used for seeding and stirring the pseudo-random number generator, which is used for less critical functions requiring randomness such as generation of DNS message transaction ID's.

  If `random-device` is not specified, or if it is set to `none`, entropy will be read from the random number generation function supplied by the cryptographic library with which BIND was linked (i.e. OpenSSL or a PKCS#11 provider).

  The `random-device` option takes effect during the initial configuration load at server startup time and is ignored on subsequent reloads.

`preferred-glue`

- If specified, the listed type (A or AAAA) will be emitted before other glue in the additional section of a query response. The default is to prefer A records when responding to queries that arrived via IPv4 and AAAA when responding to queries that arrived via IPv6.

`root-delegation-only`

- Turn on enforcement of delegation-only in TLDs (top level domains) and root zones with an optional exclude list.

  DS queries are expected to be made to and be answered by delegation only zones. Such queries and responses are treated as an exception to delegation-only processing and are not converted to NXDOMAIN responses provided a CNAME is not discovered at the query name.

  If a delegation only zone server also serves a child zone it is not always possible to determine whether an answer comes from the delegation only zone or the child zone. SOA NS and DNSKEY records are apex only records and a matching response that contains these records or DS is treated as coming from a child zone. RRSIG records are also examined to see if they are signed by a child zone or not. The authority section is also examined to see if there is evidence that the answer is from the child zone. Answers that are determined to be from a child zone are not converted to NXDOMAIN responses. Despite all these checks there is still a possibility of false negatives when a child zone is being served.

  Similarly false positives can arise from empty nodes (no records at the name) in the delegation only zone when the query type is not ANY.

  Note some TLDs are not delegation only (e.g. "DE", "LV", "US" and "MUSEUM"). This list is not exhaustive.

      options {
      	root-delegation-only exclude { "de"; "lv"; "us"; "museum"; };
      };

`disable-algorithms`

- Disable the specified DNSSEC algorithms at and below the specified name. Multiple `disable-algorithms` statements are allowed. Only the best match `disable-algorithms` clause will be used to determine which algorithms are used.

  If all supported algorithms are disabled, the zones covered by the `disable-algorithms` will be treated as insecure.

  Configured trust anchors in `trusted-keys` or `managed-keys` that match a disabled algorithm will be ignored and treated as if they were not configured at all.

`disable-ds-digests`

- Disable the specified DS/DLV digest types at and below the specified name. Multiple `disable-ds-digests` statements are allowed. Only the best match `disable-ds-digests` clause will be used to determine which digest types are used.

  If all supported digest types are disabled, the zones covered by the `disable-ds-digests` will be treated as insecure.

`dnssec-lookaside`

- When set, `dnssec-lookaside` provides the validator with an alternate method to validate DNSKEY records at the top of a zone. When a DNSKEY is at or below a domain specified by the deepest `dnssec-lookaside`, and the normal DNSSEC validation has left the key untrusted, the trust-anchor will be appended to the key name and a DLV record will be looked up to see if it can validate the key. If the DLV record validates a DNSKEY (similarly to the way a DS record does) the DNSKEY RRset is deemed to be trusted.

  If `dnssec-lookaside` is set to `no`, then dnssec-lookaside is not used.

  NOTE: The ISC-provided DLV service at `dlv.isc.org`, has been shut down. The `dnssec-lookaside auto;` configuration option, which set `named` up to use ISC DLV with minimal configuration, has accordingly been removed.

`dnssec-must-be-secure`

- Specify hierarchies which must be or may not be secure (signed and validated). If `yes`, then `named` will only accept answers if they are secure. If `no`, then normal DNSSEC validation applies allowing for insecure answers to be accepted. The specified domain must be under a `trusted-keys` or `managed-keys` statement, or `dnssec-validation auto` must be active.

`dns64`

- This directive instructs `named` to return mapped IPv4 addresses to AAAA queries when there are no AAAA records. It is intended to be used in conjunction with a NAT64. Each `dns64` defines one DNS64 prefix. Multiple DNS64 prefixes can be defined.

  Compatible IPv6 prefixes have lengths of 32, 40, 48, 56, 64 and 96 as per RFC 6052. Bits 64..71 inclusive must be zero with the most significate bit of the prefix in position 0.

  Additionally a reverse IP6.ARPA zone will be created for the prefix to provide a mapping from the IP6.ARPA names to the corresponding IN-ADDR.ARPA names using synthesized CNAMEs. `dns64-server` and `dns64-contact` can be used to specify the name of the server and contact for the zones. These are settable at the view / options level. These are not settable on a per-prefix basis.

  Each `dns64` supports an optional `clients` ACL that determines which clients are affected by this directive. If not defined, it defaults to `any;`.

  Each `dns64` supports an optional `mapped` ACL that selects which IPv4 addresses are to be mapped in the corresponding A RRset. If not defined it defaults to `any;`.

  Normally, DNS64 won't apply to a domain name that owns one or more AAAA records; these records will simply be returned. The optional `exclude` ACL allows specification of a list of IPv6 addresses that will be ignored if they appear in a domain name's AAAA records, and DNS64 will be applied to any A records the domain name owns. If not defined, `exclude` defaults to ::ffff:0.0.0.0/96.

  A optional `suffix` can also be defined to set the bits trailing the mapped IPv4 address bits. By default these bits are set to `::`. The bits matching the prefix and mapped IPv4 address must be zero.

  If `recursive-only` is set to `yes` the DNS64 synthesis will only happen for recursive queries. The default is `no`.

  If `break-dnssec` is set to `yes` the DNS64 synthesis will happen even if the result, if validated, would cause a DNSSEC validation failure. If this option is set to `no` (the default), the DO is set on the incoming query, and there are RRSIGs on the applicable records, then synthesis will not happen.

      	acl rfc1918 { 10/8; 192.168/16; 172.16/12; };

      	dns64 64:FF9B::/96 {
      		clients { any; };
      		mapped { !rfc1918; any; };
      		exclude { 64:FF9B::/96; ::ffff:0000:0000/96; };
      		suffix ::;
      	};

`dnssec-loadkeys-interval`

- When a zone is configured with `auto-dnssec maintain;` its key repository must be checked periodically to see if any new keys have been added or any existing keys' timing metadata has been updated (see man.dnssec-keygen and man.dnssec-settime). The `dnssec-loadkeys-interval` option sets the frequency of automatic repository checks, in minutes. The default is `60` (1 hour), the minimum is `1` (1 minute), and the maximum is `1440` (24 hours); any higher value is silently reduced.

`dnssec-update-mode`

- If this option is set to its default value of `maintain` in a zone of type `master` which is DNSSEC-signed and configured to allow dynamic updates (see dynamic_update_policies), and if `named` has access to the private signing key(s) for the zone, then `named` will automatically sign all new or changed records and maintain signatures for the zone by regenerating RRSIG records whenever they approach their expiration date.

  If the option is changed to `no-resign`, then `named` will sign all new or changed records, but scheduled maintenance of signatures is disabled.

  With either of these settings, `named` will reject updates to a DNSSEC-signed zone when the signing keys are inactive or unavailable to `named`. (A planned third option, `external`, will disable all automatic signing and allow DNSSEC data to be submitted into a zone via dynamic update; this is not yet implemented.)

`nta-lifetime`

- Species the default lifetime, in seconds, that will be used for negative trust anchors added via `rndc nta`.

  A negative trust anchor selectively disables DNSSEC validation for zones that are known to be failing because of misconfiguration rather than an attack. When data to be validated is at or below an active NTA (and above any other configured trust anchors), `named` will abort the DNSSEC validation process and treat the data as insecure rather than bogus. This continues until the NTA's lifetime is elapsed. NTAs persist across `named` restarts.

  For convenience, TTL-style time unit suffixes can be used to specify the NTA lifetime in seconds, minutes or hours. `nta-lifetime` defaults to one hour. It cannot exceed one week.

`nta-recheck`

- Species how often to check whether negative trust anchors added via `rndc nta` are still necessary.

  A negative trust anchor is normally used when a domain has stopped validating due to operator error; it temporarily disables DNSSEC validation for that domain. In the interest of ensuring that DNSSEC validation is turned back on as soon as possible, `named` will periodically send a query to the domain, ignoring negative trust anchors, to find out whether it can now be validated. If so, the negative trust anchor is allowed to expire early.

  Validity checks can be disabled for an individual NTA by using `rndc nta -f`, or for all NTAs by setting `nta-recheck` to zero.

  For convenience, TTL-style time unit suffixes can be used to specify the NTA recheck interval in seconds, minutes or hours. The default is five minutes. It cannot be longer than `nta-lifetime` (which cannot be longer than a week).

`max-zone-ttl`

- Specifies a maximum permissible TTL value in seconds. For convenience, TTL-style time unit suffixes may be used to specify the maximum value. When loading a zone file using a `masterfile-format` of `text` or `raw`, any record encountered with a TTL higher than `max-zone-ttl` will cause the zone to be rejected.

  This is useful in DNSSEC-signed zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. The `max-zone-ttl` option guarantees that the largest TTL in the zone will be no higher than the set value.

  (NOTE: Because `map`-format files load directly into memory, this option cannot be used with them.)

  The default value is `unlimited`. A `max-zone-ttl` of zero is treated as `unlimited`.

`stale-answer-ttl`

- Specifies the TTL to be returned on stale answers. The default is 1 second. The minimum allowed is also 1 second; a value of 0 will be updated silently to 1 second.

  For stale answers to be returned, they must be enabled, either in the configuration file using `stale-answer-enable` or via `rndc serve-stale on`.

`serial-update-method`

- Zones configured for dynamic DNS may use this option to set the update method that will be used for the zone serial number in the SOA record.

  With the default setting of `serial-update-method increment;`, the SOA serial number will be incremented by one each time the zone is updated.

  When set to `serial-update-method unixtime;`, the SOA serial number will be set to the number of seconds since the UNIX epoch, unless the serial number is already greater than or equal to that value, in which case it is simply incremented by one.

  When set to `serial-update-method date;`, the new SOA serial number will be the current date in the form "YYYYMMDD", followed by two zeroes, unless the existing serial number is already greater than or equal to that value, in which case it is incremented by one.

`zone-statistics`

- If `full`, the server will collect statistical data on all zones (unless specifically turned off on a per-zone basis by specifying `zone-statistics terse` or `zone-statistics none` in the `zone` statement). These include, for example, DNSSEC signing operations and the number of authoritative answers per query type. The default is `terse`, providing minimal statistics on zones (including name and current serial number, but not query type counters).

  These statistics may be accessed via the `statistics-channel` or using `rndc stats`, which will dump them to the file listed in the `statistics-file`. See also statsfile.

  For backward compatibility with earlier versions of BIND 9, the `zone-statistics` option can also accept `yes` or `no`; `yes` has the same meaning as `full`. As of BIND 9.10, `no` has the same meaning as `none`; previously, it was the same as `terse`.

`automatic-interface-scan`

- If `yes` and supported by the operating system, automatically rescan network interfaces when the interface addresses are added or removed. The default is `yes`. This configuration option does not affect time based `interface-interval` option, and it is recommended to set the time based `interface-interval` to 0 when the operator confirms that automatic interface scanning is supported by the operating system.

  The `automatic-interface-scan` implementation uses routing sockets for the network interface discovery, and therefore the operating system has to support the routing sockets for this feature to work.

`allow-new-zones`

- If `yes`, then zones can be added at runtime via `rndc addzone`. The default is `no`.

  Newly added zones' configuration parameters are stored so that they can persist after the server is restarted. The configuration information is saved in a file called ``viewname`.nzf` (or, if `named` is compiled with liblmdb, in an LMDB database file called ``viewname`.nzd`). `viewname` is the name of the view, unless the view name contains characters that are incompatible with use as a file name, in which case a cryptographic hash of the view name is used instead.

  Zones added at runtime will have their configuration stored either in a new-zone file (NZF) or a new-zone database (NZD) depending on whether `named` was linked with liblmdb at compile time. See man.rndc for further details about `rndc addzone`.

`auth-nxdomain`

- If `yes`, then the `AA` bit is always set on NXDOMAIN responses, even if the server is not actually authoritative. The default is `no`. If you are using very old DNS software, you may need to set it to `yes`.

`deallocate-on-exit`

- This option was used in BIND 8 to enable checking for memory leaks on exit. BIND 9 ignores the option and always performs the checks.

`memstatistics`

- Write memory statistics to the file specified by `memstatistics-file` at exit. The default is `no` unless '-m record' is specified on the command line in which case it is `yes`.

`dialup`

- If `yes`, then the server treats all zones as if they are doing zone transfers across a dial-on-demand dialup link, which can be brought up by traffic originating from this server. This has different effects according to zone type and concentrates the zone maintenance so that it all happens in a short interval, once every `heartbeat-interval` and hopefully during the one call. It also suppresses some of the normal zone maintenance traffic. The default is `no`.

  The `dialup` option may also be specified in the `view` and `zone` statements, in which case it overrides the global `dialup` option.

  If the zone is a master zone, then the server will send out a NOTIFY request to all the slaves (default). This should trigger the zone serial number check in the slave (providing it supports NOTIFY) allowing the slave to verify the zone while the connection is active. The set of servers to which NOTIFY is sent can be controlled by `notify` and `also-notify`.

  If the zone is a slave or stub zone, then the server will suppress the regular "zone up to date" (refresh) queries and only perform them when the `heartbeat-interval` expires in addition to sending NOTIFY requests.

  Finer control can be achieved by using `notify` which only sends NOTIFY messages, `notify-passive` which sends NOTIFY messages and suppresses the normal refresh queries, `refresh` which suppresses normal refresh processing and sends refresh queries when the `heartbeat-interval` expires, and `passive` which just disables normal refresh processing.

  dialup mode | normal refresh | heart-beat refresh | heart-beat notify
  `no` (default) | yes | no | no
  `yes` | no | yes | yes
  `notify` | yes | no | yes
  `refresh` | no | yes | no
  `passive` | no | no | no
  `notify-passive` | no | no | yes

  Note that normal NOTIFY processing is not affected by `dialup`.

`flush-zones-on-shutdown`

- When the nameserver exits due receiving SIGTERM, flush or do not flush any pending zone writes. The default is `flush-zones-on-shutdown` `no`.

`geoip-use-ecs`

- This option was part of an experimental implementation of the EDNS CLIENT-SUBNET for authoritative servers, but is now obsolete.

`root-key-sentinel`

- Respond to root key sentinel probes as described in draft-ietf-dnsop-kskroll-sentinel-08. The default is `yes`.

`message-compression`

- If `yes`, DNS name compression is used in responses to regular queries (not including AXFR or IXFR, which always uses compression). Setting this option to `no` reduces CPU usage on servers and may improve throughput. However, it increases response size, which may cause more queries to be processed using TCP; a server with compression disabled is out of compliance with RFC 1123 Section 6.1.3.2. The default is `yes`.

`minimal-responses`

- This option controls the addition of records to the authority and additional sections of responses. Such records may be included in responses to be helpful to clients; for example, NS or MX records may have associated address records included in the additional section, obviating the need for a separate address lookup. However, adding these records to responses is not mandatory and requires additional database lookups, causing extra latency when marshalling responses. `minimal-responses` takes one of four values:

  - `no`: the server will be as complete as possible when generating responses.
  - `yes`: the server will only add records to the authority and additional sections when such records are required by the DNS protocol (for example, when returning delegations or negative responses). This provides the best server performance but may result in more client queries.
  - `no-auth`: the server will omit records from the authority section except when they are required, but it may still add records to the additional section.
  - `no-auth-recursive`: the same as `no-auth` when recursion is requested in the query (RD=1), or the same as `no` if recursion is not requested.

  `no-auth` and `no-auth-recursive` are useful when answering stub clients, which usually ignore the authority section. `no-auth-recursive` is meant for use in mixed-mode servers that handle both authoritative and recursive queries.

  The default is `no-auth-recursive`.

`glue-cache`

- When set to `yes`, a cache is used to improve query performance when adding address-type (A and AAAA) glue records to the additional section of DNS response messages that delegate to a child zone.

  The glue cache uses memory proportional to the number of delegations in the zone. The default setting is `yes`, which improves performance at the cost of increased memory usage for the zone. If you don't want this, set it to `no`.

`minimal-any`

- If set to `yes`, then when generating a positive response to a query of type ANY over UDP, the server will reply with only one of the RRsets for the query name, and its covering RRSIGs if any, instead of replying with all known RRsets for the name. Similarly, a query for type RRSIG will be answered with the RRSIG records covering only one type. This can reduce the impact of some kinds of attack traffic, without harming legitimate clients. (Note, however, that the RRset returned is the first one found in the database; it is not necessarily the smallest available RRset.) Additionally, `minimal-responses` is turned on for these queries, so no unnecessary records will be added to the authority or additional sections. The default is `no`.

`notify`

- If `yes` (the default), DNS NOTIFY messages are sent when a zone the server is authoritative for changes, see notify. The messages are sent to the servers listed in the zone's NS records (except the master server identified in the SOA MNAME field), and to any servers listed in the `also-notify` option.

  If `master-only`, notifies are only sent for master zones. If `explicit`, notifies are sent only to servers explicitly listed using `also-notify`. If `no`, no notifies are sent.

  The `notify` option may also be specified in the `zone` statement, in which case it overrides the `options notify` statement. It would only be necessary to turn off this option if it caused slaves to crash.

`notify-to-soa`

- If `yes` do not check the nameservers in the NS RRset against the SOA MNAME. Normally a NOTIFY message is not sent to the SOA MNAME (SOA ORIGIN) as it is supposed to contain the name of the ultimate master. Sometimes, however, a slave is listed as the SOA MNAME in hidden master configurations and in that case you would want the ultimate master to still send NOTIFY messages to all the nameservers listed in the NS RRset.

`recursion`

- If `yes`, and a DNS query requests recursion, then the server will attempt to do all the work required to answer the query. If recursion is off and the server does not already know the answer, it will return a referral response. The default is `yes`. Note that setting `recursion no` does not prevent clients from getting data from the server's cache; it only prevents new data from being cached as an effect of client queries. Caching may still occur as an effect the server's internal operation, such as NOTIFY address lookups.

`request-nsid`

- If `yes`, then an empty EDNS(0) NSID (Name Server Identifier) option is sent with all queries to authoritative name servers during iterative resolution. If the authoritative server returns an NSID option in its response, then its contents are logged in the `nsid` category at level `info`. The default is `no`.

`request-sit`

- This experimental option is obsolete.

`require-server-cookie`

- Require a valid server cookie before sending a full response to a UDP request from a cookie aware client. BADCOOKIE is sent if there is a bad or no existent server cookie. The default is `no`.

  Set this to `yes` to test that DNS COOKIE clients correctly handle BADCOOKIE or if you are getting a lot of forged DNS requests with DNS COOKIES present. Setting this to `yes` will result in reduced amplification effect in a reflection attack, as the BADCOOKIE response will be smaller than a full response, while also requiring a legitimate client to follow up with a second query with the new, valid, cookie.

`answer-cookie`

- When set to the default value of `yes`, COOKIE EDNS options will be sent when applicable in replies to client queries. If set to `no`, COOKIE EDNS options will not be sent in replies. This can only be set at the global options level, not per-view.

  `answer-cookie no` is intended as a temporary measure, for use when `named` shares an IP address with other servers that do not yet support DNS COOKIE. A mismatch between servers on the same address is not expected to cause operational problems, but the option to disable COOKIE responses so that all servers have the same behavior is provided out of an abundance of caution. DNS COOKIE is an important security mechanism, and should not be disabled unless absolutely necessary.

`send-cookie`

- If `yes`, then a COOKIE EDNS option is sent along with the query. If the resolver has previously talked to the server, the COOKIE returned in the previous transaction is sent. This is used by the server to determine whether the resolver has talked to it before. A resolver sending the correct COOKIE is assumed not to be an off-path attacker sending a spoofed-source query; the query is therefore unlikely to be part of a reflection/amplification attack, so resolvers sending a correct COOKIE option are not subject to response rate limiting (RRL). Resolvers which do not send a correct COOKIE option may be limited to receiving smaller responses via the `nocookie-udp-size` option. The default is `yes`.

`stale-answer-enable`

- Enable the returning of "stale" cached answers when the nameservers for a zone are not answering. The default is not to return stale answers.

  Stale answers can also be enabled or disabled at runtime via `rndc serve-stale on` or `rndc serve-stale off`; these override the configured setting. `rndc serve-stale reset` restores the setting to the one specified in `named.conf`. Note that if stale answers have been disabled by `rndc`, then they cannot be re-enabled by reloading or reconfiguring `named`; they must be re-enabled with `rndc serve-stale on`, or the server must be restarted.

  Information about stale answers is logged under the `serve-stale` log category.

`nocookie-udp-size`

- Sets the maximum size of UDP responses that will be sent to queries without a valid server COOKIE. A value below 128 will be silently raised to 128. The default value is 4096, but the `max-udp-size` option may further limit the response size.

`sit-secret`

- This experimental option is obsolete.

`cookie-algorithm`

- Set the algorithm to be used when generating the server cookie. One of "aes", "sha1" or "sha256". The default is "aes" if supported by the cryptographic library or otherwise "sha256".

`cookie-secret`

- If set, this is a shared secret used for generating and verifying EDNS COOKIE options within an anycast cluster. If not set, the system will generate a random secret at startup. The shared secret is encoded as a hex string and needs to be 128 bits for AES128, 160 bits for SHA1 and 256 bits for SHA256.

  If there are multiple secrets specified, the first one listed in `named.conf` is used to generate new server cookies. The others will only be used to verify returned cookies.

`response-padding`

- The EDNS Padding option is intended to improve confidentiality when DNS queries are sent over an encrypted channel by reducing the variability in packet sizes. If a query:

  1. contains an EDNS Padding option,
  2. includes a valid server cookie or uses TCP,
  3. is not signed using TSIG or SIG(0), and
  4. is from a client whose address matches the specified ACL,
  then the response is padded with an EDNS Padding option to a multiple of `block-size` bytes. If these conditions are not met, the response is not padded.

  If `block-size` is 0 or the ACL is `none;`, then this feature is disabled and no padding will occur; this is the default. If `block-size` is greater than 512, a warning is logged and the value is truncated to 512. Block sizes are ordinarily expected to be powers of two (for instance, 128), but this is not mandatory.

`trust-anchor-telemetry`

- Causes `named` to send specially-formed queries once per day to domains for which trust anchors have been configured via `trusted-keys`, `managed-keys`, or `dnssec-validation auto`.

  The query name used for these queries has the form "_ta-xxxx(-xxxx)(...)".<domain>, where each "xxxx" is a group of four hexadecimal digits representing the key ID of a trusted DNSSEC key. The key IDs for each domain are sorted smallest to largest prior to encoding. The query type is NULL.

  By monitoring these queries, zone operators will be able to see which resolvers have been updated to trust a new key; this may help them decide when it is safe to remove an old one.

  The default is `yes`.

`use-ixfr`

- This option is obsolete. If you need to disable IXFR to a particular server or servers, see the information on the `provide-ixfr` option in server_statement_definition_and_usage. See also incremental_zone_transfers.

`provide-ixfr`

- See the description of `provide-ixfr` in server_statement_definition_and_usage.

`request-ixfr`

- See the description of `request-ixfr` in server_statement_definition_and_usage.

`request-expire`

- See the description of `request-expire` in server_statement_definition_and_usage.

`match-mapped-addresses`

- If `yes`, then an IPv4-mapped IPv6 address will match any address match list entries that match the corresponding IPv4 address.

  This option was introduced to work around a kernel quirk in some operating systems that causes IPv4 TCP connections, such as zone transfers, to be accepted on an IPv6 socket using mapped addresses. This caused address match lists designed for IPv4 to fail to match. However, `named` now solves this problem internally. The use of this option is discouraged.

`ixfr-from-differences`

- When `yes` and the server loads a new version of a master zone from its zone file or receives a new version of a slave file via zone transfer, it will compare the new version to the previous one and calculate a set of differences. The differences are then logged in the zone's journal file such that the changes can be transmitted to downstream slaves as an incremental zone transfer.

  By allowing incremental zone transfers to be used for non-dynamic zones, this option saves bandwidth at the expense of increased CPU and memory consumption at the master. In particular, if the new version of a zone is completely different from the previous one, the set of differences will be of a size comparable to the combined size of the old and new zone version, and the server will need to temporarily allocate memory to hold this complete difference set.

  `ixfr-from-differences` also accepts `master` (or `primary`) and `slave` (or `secondary`) at the view and options levels, which causes `ixfr-from-differences` to be enabled for all primary or secondary zones, respectively. It is off for all zones by default.

  Note: if inline signing is enabled for a zone, the user-provided `ixfr-from-differences` setting is ignored for that zone.

`multi-master`

- This should be set when you have multiple masters for a zone and the addresses refer to different machines. If `yes`, `named` will not log when the serial number on the master is less than what `named` currently has. The default is `no`.

`auto-dnssec`

- Zones configured for dynamic DNS may use this option to allow varying levels of automatic DNSSEC key management. There are three possible settings:

  `auto-dnssec allow;` permits keys to be updated and the zone fully re-signed whenever the user issues the command `rndc sign `zonename``.

  `auto-dnssec maintain;` includes the above, but also automatically adjusts the zone's DNSSEC keys on schedule, according to the keys' timing metadata (see man.dnssec-keygen and man.dnssec-settime). The command `rndc sign `zonename`` causes `named` to load keys from the key repository and sign the zone with all keys that are active. `rndc loadkeys `zonename`` causes `named` to load keys from the key repository and schedule key maintenance events to occur in the future, but it does not sign the full zone immediately. Note: once keys have been loaded for a zone the first time, the repository will be searched for changes periodically, regardless of whether `rndc loadkeys` is used. The recheck interval is defined by `dnssec-loadkeys-interval`.)

  The default setting is `auto-dnssec off`.

`dnssec-enable`

- This indicates whether DNSSEC-related resource records are to be returned by `named`. If set to `no`, `named` will not return DNSSEC-related resource records unless specifically queried for. The default is `yes`.

`dnssec-validation`

- This enables DNSSEC validation in `named`. Note that `dnssec-enable` also needs to be set to `yes` for signatures to be returned to the client along with validated answers.

  If set to `auto`, DNSSEC validation is enabled, and a default trust anchor for the DNS root zone is used.

  If set to `yes`, DNSSEC validation is enabled, but a trust anchor must be manually configured using a `trusted-keys` or `managed-keys` statement; if there is no configured trust anchor, validation will not take place.

  If set to `no`, DNSSEC validation is disabled.

  The default is `auto`, unless BIND is built with `configure --disable-auto-validation`, in which case the default is `yes`. If `dnssec-enable` is set to `no`, then the default for `dnssec-validation` is also `no`. Validation can still be turned on if desired - this results in a server that performs DNSSEC validation but does not return signatures when queried - but it will not be turned on automatically.

  The default root trust anchor is stored in the file `bind.keys`. `named` will load that key at startup if `dnssec-validation` is set to `auto`. A copy of the file is installed along with BIND 9, and is current as of the release date. If the root key expires, a new copy of `bind.keys` can be downloaded from https://www.isc.org/bind-keys.

  (To prevent problems if `bind.keys` is not found, the current trust anchor is also compiled in to `named`. Relying on this is not recommended, however, as it requires `named` to be recompiled with a new key when the root key expires.)

  `named` loads only the root key from `bind.keys`. The file cannot be used to store keys for other zones. The root key in `bind.keys` is ignored if `dnssec-validation auto` is not in use.

  Whenever the resolver sends out queries to an EDNS-compliant server, it always sets the DO bit indicating it can support DNSSEC responses even if `dnssec-validation` is off.

`validate-except`

- Specifies a list of domain names at and beneath which DNSSEC validation should not be performed, regardless of the presence of a trust anchor at or above those names. This may be used, for example, when configuring a top-level domain intended only for local use, so that the lack of a secure delegation for that domain in the root zone will not cause validation failures. (This is similar to setting a negative trust anchor, except that it is a permanent configuration, whereas negative trust anchors expire and are removed after a set period of time.)

`dnssec-accept-expired`

- Accept expired signatures when verifying DNSSEC signatures. The default is `no`. Setting this option to `yes` leaves `named` vulnerable to replay attacks.

`querylog`

- Specify whether query logging should be started when `named` starts. If `querylog` is not specified, then the query logging is determined by the presence of the logging category `queries`.

`check-names`

- This option is used to restrict the character set and syntax of certain domain names in master files and/or DNS responses received from the network. The default varies according to usage area. For `master` zones the default is `fail`. For `slave` zones the default is `warn`. For answers received from the network (`response`) the default is `ignore`.

  The rules for legal hostnames and mail domains are derived from RFC 952 and RFC 821 as modified by RFC 1123.

  `check-names` applies to the owner names of A, AAAA and MX records. It also applies to the domain names in the RDATA of NS, SOA, MX, and SRV records. It also applies to the RDATA of PTR records where the owner name indicated that it is a reverse lookup of a hostname (the owner name ends in IN-ADDR.ARPA, IP6.ARPA, or IP6.INT).

`check-dup-records`

- Check master zones for records that are treated as different by DNSSEC but are semantically equal in plain DNS. The default is to `warn`. Other possible values are `fail` and `ignore`.

`check-mx`

- Check whether the MX record appears to refer to a IP address. The default is to `warn`. Other possible values are `fail` and `ignore`.

`check-wildcard`

- This option is used to check for non-terminal wildcards. The use of non-terminal wildcards is almost always as a result of a failure to understand the wildcard matching algorithm (RFC 1034). This option affects master zones. The default (`yes`) is to check for non-terminal wildcards and issue a warning.

`check-integrity`

- Perform post load zone integrity checks on master zones. This checks that MX and SRV records refer to address (A or AAAA) records and that glue address records exist for delegated zones. For MX and SRV records only in-zone hostnames are checked (for out-of-zone hostnames use `named-checkzone`). For NS records only names below top of zone are checked (for out-of-zone names and glue consistency checks use `named-checkzone`). The default is `yes`.

  The use of the SPF record for publishing Sender Policy Framework is deprecated as the migration from using TXT records to SPF records was abandoned. Enabling this option also checks that a TXT Sender Policy Framework record exists (starts with "v=spf1") if there is an SPF record. Warnings are emitted if the TXT record does not exist and can be suppressed with `check-spf`.

`check-mx-cname`

- If `check-integrity` is set then fail, warn or ignore MX records that refer to CNAMES. The default is to `warn`.

`check-srv-cname`

- If `check-integrity` is set then fail, warn or ignore SRV records that refer to CNAMES. The default is to `warn`.

`check-sibling`

- When performing integrity checks, also check that sibling glue exists. The default is `yes`.

`check-spf`

- If `check-integrity` is set then check that there is a TXT Sender Policy Framework record present (starts with "v=spf1") if there is an SPF record present. The default is `warn`.

`zero-no-soa-ttl`

- When returning authoritative negative responses to SOA queries set the TTL of the SOA record returned in the authority section to zero. The default is `yes`.

`zero-no-soa-ttl-cache`

- When caching a negative response to a SOA query set the TTL to zero. The default is `no`.

`update-check-ksk`

- When set to the default value of `yes`, check the KSK bit in each key to determine how the key should be used when generating RRSIGs for a secure zone.

  Ordinarily, zone-signing keys (that is, keys without the KSK bit set) are used to sign the entire zone, while key-signing keys (keys with the KSK bit set) are only used to sign the DNSKEY RRset at the zone apex. However, if this option is set to `no`, then the KSK bit is ignored; KSKs are treated as if they were ZSKs and are used to sign the entire zone. This is similar to the `dnssec-signzone -z` command line option.

  When this option is set to `yes`, there must be at least two active keys for every algorithm represented in the DNSKEY RRset: at least one KSK and one ZSK per algorithm. If there is any algorithm for which this requirement is not met, this option will be ignored for that algorithm.

`dnssec-dnskey-kskonly`

- When this option and `update-check-ksk` are both set to `yes`, only key-signing keys (that is, keys with the KSK bit set) will be used to sign the DNSKEY, CDNSKEY, and CDS RRsets at the zone apex. Zone-signing keys (keys without the KSK bit set) will be used to sign the remainder of the zone, but not the DNSKEY RRset. This is similar to the `dnssec-signzone -x` command line option.

  The default is `no`. If `update-check-ksk` is set to `no`, this option is ignored.

`try-tcp-refresh`

- Try to refresh the zone using TCP if UDP queries fail. The default is `yes`.

`dnssec-secure-to-insecure`

- Allow a dynamic zone to transition from secure to insecure (i.e., signed to unsigned) by deleting all of the DNSKEY records. The default is `no`. If set to `yes`, and if the DNSKEY RRset at the zone apex is deleted, all RRSIG and NSEC records will be removed from the zone as well.

  If the zone uses NSEC3, then it is also necessary to delete the NSEC3PARAM RRset from the zone apex; this will cause the removal of all corresponding NSEC3 records. (It is expected that this requirement will be eliminated in a future release.)

  Note that if a zone has been configured with `auto-dnssec maintain` and the private keys remain accessible in the key repository, then the zone will be automatically signed again the next time `named` is started.

`synth-from-dnssec`

- Synthesize answers from cached NSEC, NSEC3 and other RRsets that have been proved to be correct using DNSSEC. The default is `no`, but it will become `yes` again in the future releases.

  Note:

  - DNSSEC validation must be enabled for this option to be effective.

    This initial implementation only covers synthesis of answers from NSEC records. Synthesis from NSEC3 is planned for the future. This will also be controlled by `synth-from-dnssec`.

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external name servers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

`forward`

- This option is only meaningful if the forwarders list is not empty. A value of `first`, the default, causes the server to query the forwarders first — and if that doesn't answer the question, the server will then look for the answer itself. If `only` is specified, the server will only query the forwarders.

`forwarders`

- Specifies the IP addresses to be used for forwarding. The default is the empty list (no forwarding).

Forwarding can also be configured on a per-domain basis, allowing for the global forwarding options to be overridden in a variety of ways. You can set particular domains to use different forwarders, or have a different `forward only/first` behavior, or not forward at all, see zone_statement_grammar.

Dual-stack servers are used as servers of last resort to work around problems in reachability due the lack of support for either IPv4 or IPv6 on the host machine.

`dual-stack-servers`

- Specifies host names or addresses of machines with access to both IPv4 and IPv6 transports. If a hostname is used, the server must be able to resolve the name using only the transport it has. If the machine is dual stacked, then the `dual-stack-servers` have no effect unless access to a transport has been disabled on the command line (e.g. `named -4`).

Access to the server can be restricted based on the IP address of the requesting system. See address_match_lists for details on how to specify IP address lists.

`allow-notify`

- This ACL specifies which hosts may send NOTIFY messages to inform this server of changes to zones for which it is acting as a secondary server. This is only applicable for secondary zones (i.e., type `secondary` or `slave`).

  If this option is set in `view` or `options`, it is globally applied to all secondary zones. If set in the `zone` statement, the global value is overridden.

  If not specified, the default is to process NOTIFY messages only from the configured `masters` for the zone. `allow-notify` can be used to expand the list of permitted hosts, not to reduce it.

`allow-query`

- Specifies which hosts are allowed to ask ordinary DNS questions. `allow-query` may also be specified in the `zone` statement, in which case it overrides the `options allow-query` statement. If not specified, the default is to allow queries from all hosts.

  `allow-query-cache` is now used to specify access to the cache.

`allow-query-on`

- Specifies which local addresses can accept ordinary DNS questions. This makes it possible, for instance, to allow queries on internal-facing interfaces but disallow them on external-facing ones, without necessarily knowing the internal network's addresses.

  Note that `allow-query-on` is only checked for queries that are permitted by `allow-query`. A query must be allowed by both ACLs, or it will be refused.

  `allow-query-on` may also be specified in the `zone` statement, in which case it overrides the `options allow-query-on` statement.

  If not specified, the default is to allow queries on all addresses.

  `allow-query-cache` is used to specify access to the cache.

`allow-query-cache`

- Specifies which hosts are allowed to get answers from the cache. If `allow-query-cache` is not set then `allow-recursion` is used if set, otherwise `allow-query` is used if set unless `recursion no;` is set in which case `none;` is used, otherwise the default (`localnets;` `localhost;`) is used.

`allow-query-cache-on`

- Specifies which local addresses can send answers from the cache. If `allow-query-cache-on` is not set, then `allow-recursion-on` is used if set. Otherwise, the default is to allow cache responses to be sent from any address. Note: Both `allow-query-cache` and `allow-query-cache-on` must be satisfied before a cache response can be sent; a client that is blocked by one cannot be allowed by the other.

`allow-recursion`

- Specifies which hosts are allowed to make recursive queries through this server. If `allow-recursion` is not set then `allow-query-cache` is used if set, otherwise `allow-query` is used if set, otherwise the default (`localnets;` `localhost;`) is used.

`allow-recursion-on`

- Specifies which local addresses can accept recursive queries. If `allow-recursion-on` is not set, then `allow-query-cache-on` is used if set; otherwise, the default is to allow recursive queries on all addresses: Any client permitted to send recursive queries can send them to any address on which `named` is listening. Note: Both `allow-recursion` and `allow-recursion-on` must be satisfied before recursion is allowed; a client that is blocked by one cannot be allowed by the other.

`allow-update`

- When set in the `zone` statement for a master zone, specifies which hosts are allowed to submit Dynamic DNS updates to that zone. The default is to deny updates from all hosts.

  Note that allowing updates based on the requestor's IP address is insecure; see dynamic_update_security for details.

  In general this option should only be set at the `zone` level. While a default value can be set at the `options` or `view` level and inherited by zones, this could lead to some zones unintentionally allowing updates.

`allow-update-forwarding`

- When set in the `zone` statement for a slave zone, specifies which hosts are allowed to submit Dynamic DNS updates and have them be forwarded to the master. The default is `{ none; }`, which means that no update forwarding will be performed.

  To enable update forwarding, specify `allow-update-forwarding { any; };`. in the `zone` statement. Specifying values other than `{ none; }` or `{ any; }` is usually counterproductive; the responsibility for update access control should rest with the master server, not the slave.

  Note that enabling the update forwarding feature on a slave server may expose master servers to attacks if they rely on insecure IP-address-based access control; see dynamic_update_security for more details.

  In general this option should only be set at the `zone` level. While a default value can be set at the `options` or `view` level and inherited by zones, this can lead to some zones unintentionally forwarding updates.

`allow-v6-synthesis`

- This option was introduced for the smooth transition from AAAA to A6 and from "nibble labels" to binary labels. However, since both A6 and binary labels were then deprecated, this option was also deprecated. It is now ignored with some warning messages.

`allow-transfer`

- Specifies which hosts are allowed to receive zone transfers from the server. `allow-transfer` may also be specified in the `zone` statement, in which case it overrides the `allow-transfer` statement set in `options` or `view`. If not specified, the default is to allow transfers to all hosts.

`blackhole`

- Specifies a list of addresses that the server will not accept queries from or use to resolve a query. Queries from these addresses will not be responded to. The default is `none`.

`keep-response-order`

- Specifies a list of addresses to which the server will send responses to TCP queries in the same order in which they were received. This disables the processing of TCP queries in parallel. The default is `none`.

`no-case-compress`

- Specifies a list of addresses which require responses to use case-insensitive compression. This ACL can be used when `named` needs to work with clients that do not comply with the requirement in RFC 1034 to use case-insensitive name comparisons when checking for matching domain names.

  If left undefined, the ACL defaults to `none`: case-insensitive compression will be used for all clients. If the ACL is defined and matches a client, then case will be ignored when compressing domain names in DNS responses sent to that client.

  This can result in slightly smaller responses: if a response contains the names "example.com" and "example.COM", case-insensitive compression would treat the second one as a duplicate. It also ensures that the case of the query name exactly matches the case of the owner names of returned records, rather than matching the case of the records entered in the zone file. This allows responses to exactly match the query, which is required by some clients due to incorrect use of case-sensitive comparisons.

  Case-insensitive compression is always used in AXFR and IXFR responses, regardless of whether the client matches this ACL.

  There are circumstances in which `named` will not preserve the case of owner names of records: if a zone file defines records of different types with the same name, but the capitalization of the name is different (e.g., "www.example.com/A" and "WWW.EXAMPLE.COM/AAAA"), then all responses for that name will use the first version of the name that was used in the zone file. This limitation may be addressed in a future release. However, domain names specified in the rdata of resource records (i.e., records of type NS, MX, CNAME, etc) will always have their case preserved unless the client matches this ACL.

`resolver-query-timeout`

- The amount of time in milliseconds that the resolver will spend attempting to resolve a recursive query before failing. The default and minimum is `10000` and the maximum is `30000`. Setting it to `0` will result in the default being used.

  This value was originally specified in seconds. Values less than or equal to 300 will be be treated as seconds and converted to milliseconds before applying the above limits.

The interfaces and ports that the server will answer queries from may be specified using the `listen-on` option. `listen-on` takes an optional port and an `address_match_list` of IPv4 addresses. (IPv6 addresses are ignored, with a logged warning.) The server will listen on all interfaces allowed by the address match list. If a port is not specified, port 53 will be used.

Multiple `listen-on` statements are allowed. For example,

    listen-on { 5.6.7.8; };
    listen-on port 1234 { !1.2.3.4; 1.2/16; };

will enable the name server on port 53 for the IP address 5.6.7.8, and on port 1234 of an address on the machine in net 1.2 that is not 1.2.3.4.

If no `listen-on` is specified, the server will listen on port 53 on all IPv4 interfaces.

The `listen-on-v6` option is used to specify the interfaces and the ports on which the server will listen for incoming queries sent using IPv6. If not specified, the server will listen on port 53 on all IPv6 interfaces.

When

    { any; }

is specified as the `address_match_list` for the `listen-on-v6` option, the server does not bind a separate socket to each IPv6 interface address as it does for IPv4 if the operating system has enough API support for IPv6 (specifically if it conforms to RFC 3493 and RFC 3542). Instead, it listens on the IPv6 wildcard address. If the system only has incomplete API support for IPv6, however, the behavior is the same as that for IPv4.

A list of particular IPv6 addresses can also be specified, in which case the server listens on a separate socket for each specified address, regardless of whether the desired API is supported by the system. IPv4 addresses specified in `listen-on-v6` will be ignored, with a logged warning.

Multiple `listen-on-v6` options can be used. For example,

    listen-on-v6 { any; };
    listen-on-v6 port 1234 { !2001:db8::/32; any; };

will enable the name server on port 53 for any IPv6 addresses (with a single wildcard socket), and on port 1234 of IPv6 addresses that is not in the prefix 2001:db8::/32 (with separate sockets for each matched address.)

To make the server not listen on any IPv6 address, use

    listen-on-v6 { none; };

If the server doesn't know the answer to a question, it will query other name servers. `query-source` specifies the address and port used for such queries. For queries sent over IPv6, there is a separate `query-source-v6` option. If `address` is `*` (asterisk) or is omitted, a wildcard IP address (`INADDR_ANY`) will be used.

If `port` is `*` or is omitted, a random port number from a pre-configured range is picked up and will be used for each query. The port range(s) is that specified in the `use-v4-udp-ports` (for IPv4) and `use-v6-udp-ports` (for IPv6) options, excluding the ranges specified in the `avoid-v4-udp-ports` and `avoid-v6-udp-ports` options, respectively.

The defaults of the `query-source` and `query-source-v6` options are:

    query-source address * port *;
    query-source-v6 address * port *;

If `use-v4-udp-ports` or `use-v6-udp-ports` is unspecified, `named` will check if the operating system provides a programming interface to retrieve the system's default range for ephemeral ports. If such an interface is available, `named` will use the corresponding system default range; otherwise, it will use its own defaults:

    use-v4-udp-ports { range 1024 65535; };
    use-v6-udp-ports { range 1024 65535; };

Note: make sure the ranges be sufficiently large for security. A desirable size depends on various parameters, but we generally recommend it contain at least 16384 ports (14 bits of entropy). Note also that the system's default range when used may be too small for this purpose, and that the range may even be changed while `named` is running; the new range will automatically be applied when `named` is reloaded. It is encouraged to configure `use-v4-udp-ports` and `use-v6-udp-ports` explicitly so that the ranges are sufficiently large and are reasonably independent from the ranges used by other applications.

Note: the operational configuration where `named` runs may prohibit the use of some ports. For example, UNIX systems will not allow `named` running without a root privilege to use ports less than 1024. If such ports are included in the specified (or detected) set of query ports, the corresponding query attempts will fail, resulting in resolution failures or delay. It is therefore important to configure the set of ports that can be safely used in the expected operational environment.

The defaults of the `avoid-v4-udp-ports` and `avoid-v6-udp-ports` options are:

    avoid-v4-udp-ports {};
    avoid-v6-udp-ports {};

Note: BIND 9.5.0 introduced the `use-queryport-pool` option to support a pool of such random ports, but this option is now obsolete because reusing the same ports in the pool may not be sufficiently secure. For the same reason, it is generally strongly discouraged to specify a particular port for the `query-source` or `query-source-v6` options; it implicitly disables the use of randomized port numbers.

`use-queryport-pool`

- This option is obsolete.

`queryport-pool-ports`

- This option is obsolete.

`queryport-pool-updateinterval`

- This option is obsolete.

The address specified in the `query-source` option is used for both UDP and TCP queries, but the port applies only to UDP queries. TCP queries always use a random unprivileged port.

Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

See also `transfer-source` and `notify-source`.

BIND has mechanisms in place to facilitate zone transfers and set limits on the amount of load that transfers place on the system. The following options apply to zone transfers.

`also-notify`

- Defines a global list of IP addresses of name servers that are also sent NOTIFY messages whenever a fresh copy of the zone is loaded, in addition to the servers listed in the zone's NS records. This helps to ensure that copies of the zones will quickly converge on stealth servers. Optionally, a port may be specified with each `also-notify` address to send the notify messages to a port other than the default of 53. An optional TSIG key can also be specified with each address to cause the notify messages to be signed; this can be useful when sending notifies to multiple views. In place of explicit addresses, one or more named `masters` lists can be used.

  If an `also-notify` list is given in a `zone` statement, it will override the `options also-notify` statement. When a `zone notify` statement is set to `no`, the IP addresses in the global `also-notify` list will not be sent NOTIFY messages for that zone. The default is the empty list (no global notification list).

`max-transfer-time-in`

- Inbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

`max-transfer-idle-in`

- Inbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

`max-transfer-time-out`

- Outbound zone transfers running longer than this many minutes will be terminated. The default is 120 minutes (2 hours). The maximum value is 28 days (40320 minutes).

`max-transfer-idle-out`

- Outbound zone transfers making no progress in this many minutes will be terminated. The default is 60 minutes (1 hour). The maximum value is 28 days (40320 minutes).

`notify-rate`

- The rate at which NOTIFY requests will be sent during normal zone maintenance operations. (NOTIFY requests due to initial zone loading are subject to a separate rate limit; see below.) The default is 20 per second. The lowest possible rate is one per second; when set to zero, it will be silently raised to one.

`startup-notify-rate`

- The rate at which NOTIFY requests will be sent when the name server is first starting up, or when zones have been newly added to the nameserver. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it will be silently raised to one.

`serial-query-rate`

- Slave servers will periodically query master servers to find out if zone serial numbers have changed. Each such query uses a minute amount of the slave server's network bandwidth. To limit the amount of bandwidth used, BIND 9 limits the rate at which queries are sent. The value of the `serial-query-rate` option, an integer, is the maximum number of queries sent per second. The default is 20 per second. The lowest possible rate is one per second; when set to zero, it will be silently raised to one.

`transfer-format`

- Zone transfers can be sent using two different formats, `one-answer` and `many-answers`. The `transfer-format` option is used on the master server to determine which format it sends. `one-answer` uses one DNS message per resource record transferred. `many-answers` packs as many resource records as possible into a message. `many-answers` is more efficient, but is only supported by relatively new slave servers, such as BIND 9, BIND 8.x and BIND 4.9.5 onwards. The `many-answers` format is also supported by recent Microsoft Windows nameservers. The default is `many-answers`. `transfer-format` may be overridden on a per-server basis by using the `server` statement.

`transfer-message-size`

- This is an upper bound on the uncompressed size of DNS messages used in zone transfers over TCP. If a message grows larger than this size, additional messages will be used to complete the zone transfer. (Note, however, that this is a hint, not a hard limit; if a message contains a single resource record whose RDATA does not fit within the size limit, a larger message will be permitted so the record can be transferred.)

  Valid values are between 512 and 65535 octets, and any values outside that range will be adjusted to the nearest value within it. The default is `20480`, which was selected to improve message compression: most DNS messages of this size will compress to less than 16536 bytes. Larger messages cannot be compressed as effectively, because 16536 is the largest permissible compression offset pointer in a DNS message.

  This option is mainly intended for server testing; there is rarely any benefit in setting a value other than the default.

`transfers-in`

- The maximum number of inbound zone transfers that can be running concurrently. The default value is `10`. Increasing `transfers-in` may speed up the convergence of slave zones, but it also may increase the load on the local system.

`transfers-out`

- The maximum number of outbound zone transfers that can be running concurrently. Zone transfer requests in excess of the limit will be refused. The default value is `10`.

`transfers-per-ns`

- The maximum number of inbound zone transfers that can be concurrently transferring from a given remote name server. The default value is `2`. Increasing `transfers-per-ns` may speed up the convergence of slave zones, but it also may increase the load on the remote name server. `transfers-per-ns` may be overridden on a per-server basis by using the `transfers` phrase of the `server` statement.

`transfer-source`

- `transfer-source` determines which local address will be bound to IPv4 TCP connections used to fetch zones transferred inbound by the server. It also determines the source IPv4 address, and optionally the UDP port, used for the refresh queries and forwarded dynamic updates. If not set, it defaults to a system controlled value which will usually be the address of the interface "closest to" the remote end. This address must appear in the remote end's `allow-transfer` option for the zone being transferred, if one is specified. This statement sets the `transfer-source` for all zones, but can be overridden on a per-view or per-zone basis by including a `transfer-source` statement within the `view` or `zone` block in the configuration file.

  Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

`transfer-source-v6`

- The same as `transfer-source`, except zone transfers are performed using IPv6.

`alt-transfer-source`

- An alternate transfer source if the one listed in `transfer-source` fails and `use-alt-transfer-source` is set.

  If you do not wish the alternate transfer source to be used, you should set `use-alt-transfer-source` appropriately and you should not depend upon getting an answer back to the first refresh query.

`alt-transfer-source-v6`

- An alternate transfer source if the one listed in `transfer-source-v6` fails and `use-alt-transfer-source` is set.

`use-alt-transfer-source`

- Use the alternate transfer sources or not. If views are specified this defaults to `no`, otherwise it defaults to `yes`.

`notify-source`

- `notify-source` determines which local source address, and optionally UDP port, will be used to send NOTIFY messages. This address must appear in the slave server's `masters` zone clause or in an `allow-notify` clause. This statement sets the `notify-source` for all zones, but can be overridden on a per-zone or per-view basis by including a `notify-source` statement within the `zone` or `view` block in the configuration file.

  Solaris 2.5.1 and earlier does not support setting the source address for TCP sockets.

`notify-source-v6`

- Like `notify-source`, but applies to notify messages sent to IPv6 addresses.

`use-v4-udp-ports`, `avoid-v4-udp-ports`, `use-v6-udp-ports`, and `avoid-v6-udp-ports` specify a list of IPv4 and IPv6 UDP ports that will be used or not used as source ports for UDP messages. See query_address about how the available ports are determined. For example, with the following configuration

    use-v6-udp-ports { range 32768 65535; };
    avoid-v6-udp-ports { 40000; range 50000 60000; };

UDP ports of IPv6 messages sent from `named` will be in one of the following ranges: 32768 to 39999, 40001 to 49999, and 60001 to 65535.

`avoid-v4-udp-ports` and `avoid-v6-udp-ports` can be used to prevent `named` from choosing as its random source port a port that is blocked by your firewall or a port that is used by other applications; if a query went out with a source port blocked by a firewall, the answer would not get by the firewall and the name server would have to query again. Note: the desired range can also be represented only with `use-v4-udp-ports` and `use-v6-udp-ports`, and the `avoid-` options are redundant in that sense; they are provided for backward compatibility and to possibly simplify the port specification.

The server's usage of many system resources can be limited. Scaled values are allowed when specifying resource limits. For example, `1G` can be used instead of `1073741824` to specify a limit of one gigabyte. `unlimited` requests unlimited use, or the maximum available amount. `default` uses the limit that was in force when the server was started. See the description of `size_spec` in configuration_file_elements.

The following options set operating system resource limits for the name server process. Some operating systems don't support some or any of the limits. On such systems, a warning will be issued if the unsupported limit is used.

`coresize`

- The maximum size of a core dump. The default is `default`.

`datasize`

- The maximum amount of data memory the server may use. The default is `default`. This is a hard limit on server memory usage. If the server attempts to allocate memory in excess of this limit, the allocation will fail, which may in turn leave the server unable to perform DNS service. Therefore, this option is rarely useful as a way of limiting the amount of memory used by the server, but it can be used to raise an operating system data size limit that is too small by default. If you wish to limit the amount of memory used by the server, use the `max-cache-size` and `recursive-clients` options instead.

`files`

- The maximum number of files the server may have open concurrently. The default is `unlimited`.

`stacksize`

- The maximum amount of stack memory the server may use. The default is `default`.

The following options set limits on the server's resource consumption that are enforced internally by the server rather than the operating system.

`max-journal-size`

- Sets a maximum size for each journal file (see journal), expressed in bytes or, if followed by an optional unit suffix ('k', 'm', or 'g'), in kilobytes, megabytes, or gigabytes. When the journal file approaches the specified size, some of the oldest transactions in the journal will be automatically removed. The largest permitted value is 2 gigabytes. Very small values are rounded up to 4096 bytes. You can specify `unlimited`, which also means 2 gigabytes. If you set the limit to `default` or leave it unset, the journal is allowed to grow up to twice as large as the zone. (There is little benefit in storing larger journals.)

  This option may also be set on a per-zone basis.

`max-records`

- The maximum number of records permitted in a zone. The default is zero which means unlimited.

`recursive-clients`

- The maximum number ("hard quota") of simultaneous recursive lookups the server will perform on behalf of clients. The default is `1000`. Because each recursing client uses a fair bit of memory (on the order of 20 kilobytes), the value of the `recursive-clients` option may have to be decreased on hosts with limited memory.

  `recursive-clients` defines a "hard quota" limit for pending recursive clients: when more clients than this are pending, new incoming requests will not be accepted, and for each incoming request a previous pending request will also be dropped.

  A "soft quota" is also set. When this lower quota is exceeded, incoming requests are accepted, but for each one, a pending request will be dropped. If `recursive-clients` is greater than 1000, the soft quota is set to `recursive-clients` minus 100; otherwise it is set to 90% of `recursive-clients`.

`tcp-clients`

- The maximum number of simultaneous client TCP connections that the server will accept. The default is `150`.

`clients-per-query`

`max-clients-per-query`

- These set the initial value (minimum) and maximum number of recursive simultaneous clients for any given query (<qname,qtype,qclass>) that the server will accept before dropping additional clients. `named` will attempt to self tune this value and changes will be logged. The default values are 10 and 100.

  This value should reflect how many queries come in for a given name in the time it takes to resolve that name. If the number of queries exceed this value, `named` will assume that it is dealing with a non-responsive zone and will drop additional queries. If it gets a response after dropping queries, it will raise the estimate. The estimate will then be lowered in 20 minutes if it has remained unchanged.

  If `clients-per-query` is set to zero, then there is no limit on the number of clients per query and no queries will be dropped.

  If `max-clients-per-query` is set to zero, then there is no upper bound other than imposed by `recursive-clients`.

`fetches-per-zone`

- The maximum number of simultaneous iterative queries to any one domain that the server will permit before blocking new queries for data in or beneath that zone. This value should reflect how many fetches would normally be sent to any one zone in the time it would take to resolve them. It should be smaller than `recursive-clients`.

  When many clients simultaneously query for the same name and type, the clients will all be attached to the same fetch, up to the `max-clients-per-query` limit, and only one iterative query will be sent. However, when clients are simultaneously querying for different names or types, multiple queries will be sent and `max-clients-per-query` is not effective as a limit.

  Optionally, this value may be followed by the keyword `drop` or `fail`, indicating whether queries which exceed the fetch quota for a zone will be dropped with no response, or answered with SERVFAIL. The default is `drop`.

  If `fetches-per-zone` is set to zero, then there is no limit on the number of fetches per query and no queries will be dropped. The default is zero.

  The current list of active fetches can be dumped by running `rndc recursing`. The list includes the number of active fetches for each domain and the number of queries that have been passed or dropped as a result of the `fetches-per-zone` limit. (Note: these counters are not cumulative over time; whenever the number of active fetches for a domain drops to zero, the counter for that domain is deleted, and the next time a fetch is sent to that domain, it is recreated with the counters set to zero.)

`fetches-per-server`

- The maximum number of simultaneous iterative queries that the server will allow to be sent to a single upstream name server before blocking additional queries. This value should reflect how many fetches would normally be sent to any one server in the time it would take to resolve them. It should be smaller than `recursive-clients`.

  Optionally, this value may be followed by the keyword `drop` or `fail`, indicating whether queries will be dropped with no response, or answered with SERVFAIL, when all of the servers authoritative for a zone are found to have exceeded the per-server quota. The default is `fail`.

  If `fetches-per-server` is set to zero, then there is no limit on the number of fetches per query and no queries will be dropped. The default is zero.

  The `fetches-per-server` quota is dynamically adjusted in response to detected congestion. As queries are sent to a server and are either answered or time out, an exponentially weighted moving average is calculated of the ratio of timeouts to responses. If the current average timeout ratio rises above a "high" threshold, then `fetches-per-server` is reduced for that server. If the timeout ratio drops below a "low" threshold, then `fetches-per-server` is increased. The `fetch-quota-params` options can be used to adjust the parameters for this calculation.

`fetch-quota-params`

- Sets the parameters to use for dynamic resizing of the `fetches-per-server` quota in response to detected congestion.

  The first argument is an integer value indicating how frequently to recalculate the moving average of the ratio of timeouts to responses for each server. The default is 100, meaning we recalculate the average ratio after every 100 queries have either been answered or timed out.

  The remaining three arguments represent the "low" threshold (defaulting to a timeout ratio of 0.1), the "high" threshold (defaulting to a timeout ratio of 0.3), and the discount rate for the moving average (defaulting to 0.7). A higher discount rate causes recent events to weigh more heavily when calculating the moving average; a lower discount rate causes past events to weigh more heavily, smoothing out short-term blips in the timeout ratio. These arguments are all fixed-point numbers with precision of 1/100: at most two places after the decimal point are significant.

`reserved-sockets`

- The number of file descriptors reserved for TCP, stdio, etc. This needs to be big enough to cover the number of interfaces `named` listens on plus `tcp-clients`, as well as to provide room for outgoing TCP queries and incoming zone transfers. The default is `512`. The minimum value is `128` and the maximum value is `128` less than maxsockets (-S). This option may be removed in the future.

  This option has little effect on Windows.

`max-cache-size`

- The maximum amount of memory to use for the server's cache, in bytes or % of total physical memory. When the amount of data in the cache reaches this limit, the server will cause records to expire prematurely based on an LRU based strategy so that the limit is not exceeded. The keyword `unlimited`, or the value 0, will place no limit on cache size; records will be purged from the cache only when their TTLs expire. Any positive values less than 2MB will be ignored and reset to 2MB. In a server with multiple views, the limit applies separately to the cache of each view. The default is `90%`. On systems where detection of amount of physical memory is not supported values represented as % fall back to unlimited. Note that the detection of physical memory is done only once at startup, so `named` will not adjust the cache size if the amount of physical memory is changed during runtime.

`tcp-listen-queue`

- The listen queue depth. The default and minimum is 10. If the kernel supports the accept filter "dataready" this also controls how many TCP connections that will be queued in kernel space waiting for some data before being passed to accept. Nonzero values less than 10 will be silently raised. A value of 0 may also be used; on most platforms this sets the listen queue length to a system-defined default value.

`tcp-initial-timeout`

- The amount of time (in units of 100 milliseconds) the server waits on a new TCP connection for the first message from the client. The default is 300 (30 seconds), the minimum is 25 (2.5 seconds), and the maximum is 1200 (two minutes). Values above the maximum or below the minimum will be adjusted with a logged warning. (Note: This value must be greater than the expected round trip delay time; otherwise no client will ever have enough time to submit a message.) This value can be updated at runtime by using `rndc tcp-timeouts`.

`tcp-idle-timeout`

- The amount of time (in units of 100 milliseconds) the server waits on an idle TCP connection before closing it when the client is not using the EDNS TCP keepalive option. The default is 300 (30 seconds), the maximum is 1200 (two minutes), and the minimum is 1 (one tenth of a second). Values above the maximum or below the minimum will be adjusted with a logged warning. See `tcp-keepalive-timeout` for clients using the EDNS TCP keepalive option. This value can be updated at runtime by using `rndc tcp-timeouts`.

`tcp-keepalive-timeout`

- The amount of time (in units of 100 milliseconds) the server waits on an idle TCP connection before closing it when the client is using the EDNS TCP keepalive option. The default is 300 (30 seconds), the maximum is 65535 (about 1.8 hours), and the minimum is 1 (one tenth of a second). Values above the maximum or below the minimum will be adjusted with a logged warning. This value may be greater than `tcp-idle-timeout`, because clients using the EDNS TCP keepalive option are expected to use TCP connections for more than one message. This value can be updated at runtime by using `rndc tcp-timeouts`.

`tcp-advertised-timeout`

- The timeout value (in units of 100 milliseconds) the server will send in respones containing the EDNS TCP keepalive option. This informs a client of the amount of time it may keep the session open. The default is 300 (30 seconds), the maximum is 65535 (about 1.8 hours), and the minimum is 0, which signals that the clients must close TCP connections immediately. Ordinarily this should be set to the same value as `tcp-keepalive-timeout`. This value can be updated at runtime by using `rndc tcp-timeouts`.

`cleaning-interval`

- This interval is effectively obsolete. Previously, the server would remove expired resource records from the cache every `cleaning-interval` minutes. BIND 9 now manages cache memory in a more sophisticated manner and does not rely on the periodic cleaning any more. Specifying this option therefore has no effect on the server's behavior.

`heartbeat-interval`

- The server will perform zone maintenance tasks for all zones marked as `dialup` whenever this interval expires. The default is 60 minutes. Reasonable values are up to 1 day (1440 minutes). The maximum value is 28 days (40320 minutes). If set to 0, no zone maintenance for these zones will occur.

`interface-interval`

- The server will scan the network interface list every `interface-interval` minutes. The default is 60 minutes. The maximum value is 28 days (40320 minutes). If set to 0, interface scanning will only occur when the configuration file is loaded, or when `automatic-interface-scan` is enabled and supported by the operating system. After the scan, the server will begin listening for queries on any newly discovered interfaces (provided they are allowed by the `listen-on` configuration), and will stop listening on interfaces that have gone away. For convenience, TTL-style time unit suffixes may be used to specify the value.

The response to a DNS query may consist of multiple resource records (RRs) forming a resource record set (RRset). The name server will normally return the RRs within the RRset in an indeterminate order (but see the `rrset-order` statement in rrset_ordering). The client resolver code should rearrange the RRs as appropriate, that is, using any addresses on the local net in preference to other addresses. However, not all resolvers can do this or are correctly configured. When a client is using a local server, the sorting can be performed in the server, based on the client's address. This only requires configuring the name servers, not all the clients.

The `sortlist` statement (see below) takes an `address_match_list` and interprets it in a special way. Each top level statement in the `sortlist` must itself be an explicit `address_match_list` with one or two elements. The first element (which may be an IP address, an IP prefix, an ACL name or a nested `address_match_list`) of each top level list is checked against the source address of the query until a match is found. When the addresses in the first element overlap, the first rule to match gets selected.

Once the source address of the query has been matched, if the top level statement contains only one element, the actual primitive element that matched the source address is used to select the address in the response to move to the beginning of the response. If the statement is a list of two elements, then the second element is interpreted as a topology preference list. Each top level element is assigned a distance and the address in the response with the minimum distance is moved to the beginning of the response.

In the following example, any queries received from any of the addresses of the host itself will get responses preferring addresses on any of the locally connected networks. Next most preferred are addresses on the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24 network with no preference shown between these two networks. Queries received from a host on the 192.168.1/24 network will prefer other addresses on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries received from a host on the 192.168.4/24 or the 192.168.5/24 network will only prefer other addresses on their directly connected networks.

    sortlist {
        // IF the local host
        // THEN first fit on the following nets
        { localhost;
    	{ localnets;
    	    192.168.1/24;
    	    { 192.168.2/24; 192.168.3/24; }; }; };
        // IF on class C 192.168.1 THEN use .1, or .2 or .3
        { 192.168.1/24;
    	{ 192.168.1/24;
    	    { 192.168.2/24; 192.168.3/24; }; }; };
        // IF on class C 192.168.2 THEN use .2, or .1 or .3
        { 192.168.2/24;
    	{ 192.168.2/24;
    	    { 192.168.1/24; 192.168.3/24; }; }; };
        // IF on class C 192.168.3 THEN use .3, or .1 or .2
        { 192.168.3/24;
    	{ 192.168.3/24;
    	    { 192.168.1/24; 192.168.2/24; }; }; };
        // IF .4 or .5 THEN prefer that net
        { { 192.168.4/24; 192.168.5/24; };
        };
    };

The following example will give reasonable behavior for the local host and hosts on directly connected networks. It is similar to the behavior of the address sort in BIND 4.9.x. Responses sent to queries from the local host will favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network will prefer addresses on that same network. Responses to other queries will not be sorted.

    sortlist {
    	   { localhost; localnets; };
    	   { localnets; };
    };

When multiple records are returned in an answer it may be useful to configure the order of the records placed into the response. The `rrset-order` statement permits configuration of the ordering of the records in a multiple-record response. See also the `sortlist` statement, the_sortlist_statement.

An `order_spec` is defined as follows:

class `class_name` type `type_name` name `"domain_name"` order `ordering`

If no class is specified, the default is `ANY`. If no type is specified, the default is `ANY`. If no name is specified, the default is "`*`" (asterisk).

The legal values for `ordering` are:

`fixed` | Records are returned in the order they are defined in the zone file. This option is only available if BIND is configured with "--enable-fixed-rrset" at compile time.
`random` | Records are returned in some random order.
`cyclic` | Records are returned in a cyclic round-robin order, rotating by one record per query. If BIND is configured with "--enable-fixed-rrset" at compile time, then the initial ordering of the RRset will match the one specified in the zone file; otherwise the initial ordering is indeterminate.
`none` | Records are returned in whatever order they were retrieved from the database. This order is indeterminate, but will be consistent as long as the database is not modified. When no ordering is specified, this is the default.

For example:

    rrset-order {
       class IN type A name "host.example.com" order random;
       order cyclic;
    };

will cause any responses for type A records in class IN that have "`host.example.com`" as a suffix, to always be returned in random order. All other records are returned in cyclic order.

If multiple `rrset-order` statements appear, they are not combined — the last one applies.

By default, records are returned in `random` order.

In this release of BIND 9, the `rrset-order` statement does not support "fixed" ordering by default. Fixed ordering can be enabled at compile time by specifying "--enable-fixed-rrset" on the "configure" command line.

`lame-ttl`

- Sets the number of seconds to cache a lame server indication. 0 disables caching. (This is NOT recommended.) The default is `600` (10 minutes) and the maximum value is `1800` (30 minutes).

`servfail-ttl`

- Sets the number of seconds to cache a SERVFAIL response due to DNSSEC validation failure or other general server failure. If set to `0`, SERVFAIL caching is disabled. The SERVFAIL cache is not consulted if a query has the CD (Checking Disabled) bit set; this allows a query that failed due to DNSSEC validation to be retried without waiting for the SERVFAIL TTL to expire.

  The maximum value is `30` seconds; any higher value will be silently reduced. The default is `1` second.

`min-ncache-ttl`

- To reduce network traffic and increase performance, the server stores negative answers. `min-ncache-ttl` is used to set a minimum retention time for these answers in the server in seconds. For convenience, TTL-style time unit suffixes may be used to specify the value. The default `min-ncache-ttl` is `0` seconds. `min-ncache-ttl` cannot exceed 90 seconds and will be truncated to 90 seconds if set to a greater value.

`min-cache-ttl`

- Sets the minimum time for which the server will cache ordinary (positive) answers in seconds. For convenience, TTL-style time unit suffixes may be used to specify the value. The default `min-cache-ttl` is `0` seconds. `min-cache-ttl` cannot exceed 90 seconds and will be truncated to 90 seconds if set to a greater value.

`max-ncache-ttl`

- To reduce network traffic and increase performance, the server stores negative answers. `max-ncache-ttl` is used to set a maximum retention time for these answers in the server in seconds. For convenience, TTL-style time unit suffixes may be used to specify the value. The default `max-ncache-ttl` is `10800` seconds (3 hours). `max-ncache-ttl` cannot exceed 7 days and will be silently truncated to 7 days if set to a greater value.

`max-cache-ttl`

- Sets the maximum time for which the server will cache ordinary (positive) answers in seconds. For convenience, TTL-style time unit suffixes may be used to specify the value. The default is 604800 (one week). A value of zero may cause all queries to return SERVFAIL, because of lost caches of intermediate RRsets (such as NS and glue AAAA/A records) in the resolution process.

`max-stale-ttl`

- If stale answers are enabled, `max-stale-ttl` sets the maximum time for which the server will retain records past their normal expiry to return them as stale records when the servers for those records are not reachable. The default is 1 week. The minimum allowed is 1 second; a value of 0 will be updated silently to 1 second.

  For stale answers to be returned, they must be enabled, either in the configuration file using `stale-answer-enable` or via `rndc serve-stale on`.

`resolver-nonbackoff-tries`

- Specifies how many retries occur before exponential backoff kicks in. The default is `3`.

`resolver-retry-interval`

- The base retry interval in milliseconds. The default is `800`.

`sig-validity-interval`

- Specifies the number of days into the future when DNSSEC signatures automatically generated as a result of dynamic updates (dynamic_update) will expire. There is an optional second field which specifies how long before expiry that the signatures will be regenerated. If not specified, the signatures will be regenerated at 1/4 of base interval. The second field is specified in days if the base interval is greater than 7 days otherwise it is specified in hours. The default base interval is `30` days giving a re-signing interval of 7 1/2 days. The maximum values are 10 years (3660 days).

  The signature inception time is unconditionally set to one hour before the current time to allow for a limited amount of clock skew.

  The `sig-validity-interval` can be overridden for DNSKEY records by setting `dnskey-sig-validity`.

  The `sig-validity-interval` should be, at least, several multiples of the SOA expire interval to allow for reasonable interaction between the various timer and expiry dates.

`dnskey-sig-validity`

- Specifies the number of days into the future when DNSSEC signatures that are automatically generated for DNSKEY RRsets as a result of dynamic updates (dynamic_update) will expire. If set to a non-zero value, this overrides the value set by `sig-validity-interval`. The default is zero, meaning `sig-validity-interval` is used. The maximum value is 3660 days (10 years), and higher values will be rejected.

`sig-signing-nodes`

- Specify the maximum number of nodes to be examined in each quantum when signing a zone with a new DNSKEY. The default is `100`.

`sig-signing-signatures`

- Specify a threshold number of signatures that will terminate processing a quantum when signing a zone with a new DNSKEY. The default is `10`.

`sig-signing-type`

- Specify a private RDATA type to be used when generating signing state records. The default is `65534`.

  It is expected that this parameter may be removed in a future version once there is a standard type.

  Signing state records are used to internally by `named` to track the current state of a zone-signing process, i.e., whether it is still active or has been completed. The records can be inspected using the command `rndc signing -list `zone``. Once `named` has finished signing a zone with a particular key, the signing state record associated with that key can be removed from the zone by running `rndc signing -clear `keyid/algorithm` `zone``. To clear all of the completed signing state records for a zone, use `rndc signing -clear all `zone``.

`min-refresh-time`

`max-refresh-time`

`min-retry-time`

`max-retry-time`

- These options control the server's behavior on refreshing a zone (querying for SOA changes) or retrying failed transfers. Usually the SOA values for the zone are used, up to a hard-coded maximum expiry of 24 weeks. However, these values are set by the master, giving slave server administrators little control over their contents.

  These options allow the administrator to set a minimum and maximum refresh and retry time in seconds per-zone, per-view, or globally. These options are valid for slave and stub zones, and clamp the SOA refresh and retry times to the specified values.

  The following defaults apply. `min-refresh-time` 300 seconds, `max-refresh-time` 2419200 seconds (4 weeks), `min-retry-time` 500 seconds, and `max-retry-time` 1209600 seconds (2 weeks).

`edns-udp-size`

- Sets the maximum advertised EDNS UDP buffer size in bytes, to control the size of packets received from authoritative servers in response to recursive queries. Valid values are 512 to 4096 (values outside this range will be silently adjusted to the nearest value within it). The default value is 4096.

  The usual reason for setting `edns-udp-size` to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP DNS packets that are greater than 512 bytes.

  When `named` first queries a remote server, it will advertise a UDP buffer size of 512, as this has the greatest chance of success on the first try.

  If the initial query is successful with EDNS advertising a buffer size of 512, then `named` will advertise progressively larger buffer sizes on successive queries, until responses begin timing out or `edns-udp-size` is reached.

  The default buffer sizes used by `named` are 512, 1232, 1432, and 4096, but never exceeding `edns-udp-size`. (The values 1232 and 1432 are chosen to allow for an IPv4/IPv6 encapsulated UDP message to be sent without fragmentation at the minimum MTU sizes for Ethernet and IPv6 networks.)

`max-udp-size`

- Sets the maximum EDNS UDP message size `named` will send in bytes. Valid values are 512 to 4096 (values outside this range will be silently adjusted to the nearest value within it). The default value is 4096.

  This value applies to responses sent by a server; to set the advertised buffer size in queries, see `edns-udp-size`.

  The usual reason for setting `max-udp-size` to a non-default value is to get UDP answers to pass through broken firewalls that block fragmented packets and/or block UDP packets that are greater than 512 bytes. This is independent of the advertised receive buffer (`edns-udp-size`).

  Setting this to a low value will encourage additional TCP traffic to the nameserver.

`masterfile-format`

- Specifies the file format of zone files (see zonefile_format). The default value is `text`, which is the standard textual representation, except for slave zones, in which the default value is `raw`. Files in other formats than `text` are typically expected to be generated by the `named-compilezone` tool, or dumped by `named`.

  Note that when a zone file in a different format than `text` is loaded, `named` may omit some of the checks which would be performed for a file in the `text` format. In particular, `check-names` checks do not apply for the `raw` format. This means a zone file in the `raw` format must be generated with the same check level as that specified in the `named` configuration file. Also, `map` format files are loaded directly into memory via memory mapping, with only minimal checking.

  This statement sets the `masterfile-format` for all zones, but can be overridden on a per-zone or per-view basis by including a `masterfile-format` statement within the `zone` or `view` block in the configuration file.

`masterfile-style`

- Specifies the formatting of zone files during dump when the `masterfile-format` is `text`. (This option is ignored with any other `masterfile-format`.)

  When set to `relative`, records are printed in a multi-line format with owner names expressed relative to a shared origin. When set to `full`, records are printed in a single-line format with absolute owner names. The `full` format is most suitable when a zone file needs to be processed automatically by a script. The `relative` format is more human-readable, and is thus suitable when a zone is to be edited by hand. The default is `relative`.

`max-recursion-depth`

- Sets the maximum number of levels of recursion that are permitted at any one time while servicing a recursive query. Resolving a name may require looking up a name server address, which in turn requires resolving another name, etc; if the number of indirections exceeds this value, the recursive query is terminated and returns SERVFAIL. The default is 7.

`max-recursion-queries`

- Sets the maximum number of iterative queries that may be sent while servicing a recursive query. If more queries are sent, the recursive query is terminated and returns SERVFAIL. Queries to look up top level domains such as "com" and "net" and the DNS root zone are exempt from this limitation. The default is 75.

`notify-delay`

- The delay, in seconds, between sending sets of notify messages for a zone. The default is five (5) seconds.

  The overall rate that NOTIFY messages are sent for all zones is controlled by `serial-query-rate`.

`max-rsa-exponent-size`

- The maximum RSA exponent size, in bits, that will be accepted when validating. Valid values are 35 to 4096 bits. The default zero (0) is also accepted and is equivalent to 4096.

`prefetch`

- When a query is received for cached data which is to expire shortly, `named` can refresh the data from the authoritative server immediately, ensuring that the cache always has an answer available.

  The `prefetch` specifies the "trigger" TTL value at which prefetch of the current query will take place: when a cache record with a lower TTL value is encountered during query processing, it will be refreshed. Valid trigger TTL values are 1 to 10 seconds. Values larger than 10 seconds will be silently reduced to 10. Setting a trigger TTL to zero (0) causes prefetch to be disabled. The default trigger TTL is `2`.

  An optional second argument specifies the "eligibility" TTL: the smallest original TTL value that will be accepted for a record to be eligible for prefetching. The eligibility TTL must be at least six seconds longer than the trigger TTL; if it isn't, `named` will silently adjust it upward. The default eligibility TTL is `9`.

`v6-bias`

- When determining the next nameserver to try preference IPv6 nameservers by this many milliseconds. The default is `50` milliseconds.

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain `bind` in the `CHAOS` class. These zones are part of a built-in view (see view_statement_grammar) of class `CHAOS` which is separate from the default view of class `IN`. Most global configuration options (`allow-query`, etc) will apply to this view, but some are locally overridden: `notify`, `recursion` and `allow-new-zones` are always set to `no`, and `rate-limit` is set to allow three responses per second.

If you need to disable these zones, use the options below, or hide the built-in `CHAOS` view by defining an explicit view of class `CHAOS` that matches all clients.

`version`

- The version the server should report via a query of the name `version.bind` with type `TXT`, class `CHAOS`. The default is the real version number of this server. Specifying `version none` disables processing of the queries.

`hostname`

- The hostname the server should report via a query of the name `hostname.bind` with type `TXT`, class `CHAOS`. This defaults to the hostname of the machine hosting the name server as found by the gethostname() function. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying `hostname none;` disables processing of the queries.

`server-id`

- The ID the server should report when receiving a Name Server Identifier (NSID) query, or a query of the name `ID.SERVER` with type `TXT`, class `CHAOS`. The primary purpose of such queries is to identify which of a group of anycast servers is actually answering your queries. Specifying `server-id none;` disables processing of the queries. Specifying `server-id hostname;` will cause `named` to use the hostname as found by the gethostname() function. The default `server-id` is `none`.

The `named` server has some built-in empty zones (SOA and NS records only). These are for zones that should normally be answered locally and which queries should not be sent to the Internet's root servers. The official servers which cover these namespaces return NXDOMAIN responses to these queries. In particular, these cover the reverse namespaces for addresses from RFC 1918, RFC 4193, RFC 5737 and RFC 6598. They also include the reverse namespace for IPv6 local address (locally assigned), IPv6 link local addresses, the IPv6 loopback address and the IPv6 unknown address.

The server will attempt to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and will not create an empty zone in that case.

The current list of empty zones is:

- 10.IN-ADDR.ARPA
- 16.172.IN-ADDR.ARPA
- 17.172.IN-ADDR.ARPA
- 18.172.IN-ADDR.ARPA
- 19.172.IN-ADDR.ARPA
- 20.172.IN-ADDR.ARPA
- 21.172.IN-ADDR.ARPA
- 22.172.IN-ADDR.ARPA
- 23.172.IN-ADDR.ARPA
- 24.172.IN-ADDR.ARPA
- 25.172.IN-ADDR.ARPA
- 26.172.IN-ADDR.ARPA
- 27.172.IN-ADDR.ARPA
- 28.172.IN-ADDR.ARPA
- 29.172.IN-ADDR.ARPA
- 30.172.IN-ADDR.ARPA
- 31.172.IN-ADDR.ARPA
- 168.192.IN-ADDR.ARPA
- 64.100.IN-ADDR.ARPA
- 65.100.IN-ADDR.ARPA
- 66.100.IN-ADDR.ARPA
- 67.100.IN-ADDR.ARPA
- 68.100.IN-ADDR.ARPA
- 69.100.IN-ADDR.ARPA
- 70.100.IN-ADDR.ARPA
- 71.100.IN-ADDR.ARPA
- 72.100.IN-ADDR.ARPA
- 73.100.IN-ADDR.ARPA
- 74.100.IN-ADDR.ARPA
- 75.100.IN-ADDR.ARPA
- 76.100.IN-ADDR.ARPA
- 77.100.IN-ADDR.ARPA
- 78.100.IN-ADDR.ARPA
- 79.100.IN-ADDR.ARPA
- 80.100.IN-ADDR.ARPA
- 81.100.IN-ADDR.ARPA
- 82.100.IN-ADDR.ARPA
- 83.100.IN-ADDR.ARPA
- 84.100.IN-ADDR.ARPA
- 85.100.IN-ADDR.ARPA
- 86.100.IN-ADDR.ARPA
- 87.100.IN-ADDR.ARPA
- 88.100.IN-ADDR.ARPA
- 89.100.IN-ADDR.ARPA
- 90.100.IN-ADDR.ARPA
- 91.100.IN-ADDR.ARPA
- 92.100.IN-ADDR.ARPA
- 93.100.IN-ADDR.ARPA
- 94.100.IN-ADDR.ARPA
- 95.100.IN-ADDR.ARPA
- 96.100.IN-ADDR.ARPA
- 97.100.IN-ADDR.ARPA
- 98.100.IN-ADDR.ARPA
- 99.100.IN-ADDR.ARPA
- 100.100.IN-ADDR.ARPA
- 101.100.IN-ADDR.ARPA
- 102.100.IN-ADDR.ARPA
- 103.100.IN-ADDR.ARPA
- 104.100.IN-ADDR.ARPA
- 105.100.IN-ADDR.ARPA
- 106.100.IN-ADDR.ARPA
- 107.100.IN-ADDR.ARPA
- 108.100.IN-ADDR.ARPA
- 109.100.IN-ADDR.ARPA
- 110.100.IN-ADDR.ARPA
- 111.100.IN-ADDR.ARPA
- 112.100.IN-ADDR.ARPA
- 113.100.IN-ADDR.ARPA
- 114.100.IN-ADDR.ARPA
- 115.100.IN-ADDR.ARPA
- 116.100.IN-ADDR.ARPA
- 117.100.IN-ADDR.ARPA
- 118.100.IN-ADDR.ARPA
- 119.100.IN-ADDR.ARPA
- 120.100.IN-ADDR.ARPA
- 121.100.IN-ADDR.ARPA
- 122.100.IN-ADDR.ARPA
- 123.100.IN-ADDR.ARPA
- 124.100.IN-ADDR.ARPA
- 125.100.IN-ADDR.ARPA
- 126.100.IN-ADDR.ARPA
- 127.100.IN-ADDR.ARPA
- 0.IN-ADDR.ARPA
- 127.IN-ADDR.ARPA
- 254.169.IN-ADDR.ARPA
- 2.0.192.IN-ADDR.ARPA
- 100.51.198.IN-ADDR.ARPA
- 113.0.203.IN-ADDR.ARPA
- 255.255.255.255.IN-ADDR.ARPA
- 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- 8.B.D.0.1.0.0.2.IP6.ARPA
- D.F.IP6.ARPA
- 8.E.F.IP6.ARPA
- 9.E.F.IP6.ARPA
- A.E.F.IP6.ARPA
- B.E.F.IP6.ARPA
- EMPTY.AS112.ARPA
- HOME.ARPA

Empty zones are settable at the view level and only apply to views of class IN. Disabled empty zones are only inherited from options if there are no disabled empty zones specified at the view level. To override the options list of disabled zones, you can disable the root zone at the view level, for example:

    	    disable-empty-zone ".";

If you are using the address ranges covered here, you should already have reverse zones covering the addresses you use. In practice this appears to not be the case with many queries being made to the infrastructure servers for names in these spaces. So many in fact that sacrificial servers were needed to be deployed to channel the query load away from the infrastructure servers.

The real parent servers for these zones should disable all empty zone under the parent zone they serve. For the real root servers, this is all built-in empty zones. This will enable them to return referrals to deeper in the tree.

`empty-server`

- Specify what server name will appear in the returned SOA record for empty zones. If none is specified, then the zone's name will be used.

`empty-contact`

- Specify what contact name will appear in the returned SOA record for empty zones. If none is specified, then "." will be used.

`empty-zones-enable`

- Enable or disable all empty zones. By default, they are enabled.

`disable-empty-zone`

- Disable individual empty zones. By default, none are disabled. This option can be specified multiple times.

BIND 9 provides the ability to filter out DNS responses from external DNS servers containing certain types of data in the answer section. Specifically, it can reject address (A or AAAA) records if the corresponding IPv4 or IPv6 addresses match the given `address_match_list` of the `deny-answer-addresses` option. It can also reject CNAME or DNAME records if the "alias" name (i.e., the CNAME alias or the substituted query name due to DNAME) matches the given `namelist` of the `deny-answer-aliases` option, where "match" means the alias name is a subdomain of one of the `name_list` elements. If the optional `namelist` is specified with `except-from`, records whose query name matches the list will be accepted regardless of the filter setting. Likewise, if the alias name is a subdomain of the corresponding zone, the `deny-answer-aliases` filter will not apply; for example, even if "example.com" is specified for `deny-answer-aliases`,

    www.example.com. CNAME xxx.example.com.

returned by an "example.com" server will be accepted.

In the `address_match_list` of the `deny-answer-addresses` option, only `ip_addr` and `ip_prefix` are meaningful; any `key_id` will be silently ignored.

If a response message is rejected due to the filtering, the entire message is discarded without being cached, and a SERVFAIL error will be returned to the client.

This filtering is intended to prevent "DNS rebinding attacks," in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within your own network or an alias name within your own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of your local network that couldn't be externally accessed otherwise. See the paper available at http://portal.acm.org/citation.cfm?id=1315245.1315298 for more details about the attacks.

For example, if you own a domain named "example.net" and your internal network uses an IPv4 prefix 192.0.2.0/24, you might specify the following rules:

    deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; };
    deny-answer-aliases { "example.net"; };

If an external attacker lets a web browser in your local network look up an IPv4 address of "attacker.example.com", the attacker's DNS server would return a response like this:

    attacker.example.com. A 192.0.2.1

in the answer section. Since the rdata of this record (the IPv4 address) matches the specified prefix 192.0.2.0/24, this response will be ignored.

On the other hand, if the browser looks up a legitimate internal web server "www.example.net" and the following response is returned to the BIND 9 server

    www.example.net. A 192.0.2.2

it will be accepted since the owner name "www.example.net" matches the `except-from` element, "example.net".

Note that this is not really an attack on the DNS per se. In fact, there is nothing wrong for an "external" name to be mapped to your "internal" IP address or domain name from the DNS point of view. It might actually be provided for a legitimate purpose, such as for debugging. As long as the mapping is provided by the correct owner, it is not possible or does not make sense to detect whether the intent of the mapping is legitimate or not within the DNS. The "rebinding" attack must primarily be protected at the application that uses the DNS. For a large site, however, it may be difficult to protect all possible applications at once. This filtering feature is provided only to help such an operational environment; it is generally discouraged to turn it on unless you are very sure you have no other choice and the attack is a real threat for your applications.

Care should be particularly taken if you want to use this option for addresses within 127.0.0.0/8. These addresses are obviously "internal", but many applications conventionally rely on a DNS mapping from some name to such an address. Filtering out DNS records containing this address spuriously can break such applications.

BIND 9 includes a limited mechanism to modify DNS responses for requests analogous to email anti-spam DNS blacklists. Responses can be changed to deny the existence of domains (NXDOMAIN), deny the existence of IP addresses for domains (NODATA), or contain other IP addresses or data.

Response policy zones are named in the `response-policy` option for the view or among the global options if there is no response-policy option for the view. Response policy zones are ordinary DNS zones containing RRsets that can be queried normally if allowed. It is usually best to restrict those queries with something like `allow-query { localhost; };`. Note that zones using `masterfile-format map` cannot be used as policy zones.

A `response-policy` option can support multiple policy zones. To maximize performance, a radix tree is used to quickly identify response policy zones containing triggers that match the current query. This imposes an upper limit of 64 on the number of policy zones in a single `response-policy` option; more than that is a configuration error.

Rules encoded in response policy zones are processed after Access Control Lists (ACLs). All queries from clients which are not permitted access to the resolver will be answered with a status code of REFUSED, regardless of configured RPZ rules.

Five policy triggers can be encoded in RPZ records.

`RPZ-CLIENT-IP`

- IP records are triggered by the IP address of the DNS client. Client IP address triggers are encoded in records that have owner names that are subdomains of `rpz-client-ip` relativized to the policy zone origin name and encode an address or address block. IPv4 addresses are represented as `prefixlength.B4.B3.B2.B1.rpz-client-ip`. The IPv4 prefix length must be between 1 and 32. All four bytes, B4, B3, B2, and B1, must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA.

  IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, `prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip`. Each of W8,...,W1 is a one to four digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IP6.ARPA. (Note that this representation of IPv6 address is different from IP6.ARPA where each hex digit occupies a label.) All 8 words must be present except when one set of consecutive zero words is replaced with `.zz.` analogous to double colons (::) in standard IPv6 text encodings. The IPv6 prefix length must be between 1 and 128.

`QNAME`

- QNAME policy records are triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME policy record is the query name relativized to the policy zone.

`RPZ-IP`

- IP triggers are IP addresses in an A or AAAA record in the ANSWER section of a response. They are encoded like client-IP triggers except as subdomains of `rpz-ip`.

`RPZ-NSDNAME`

- NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for query name, or a parent of a CNAME. They are encoded as subdomains of `rpz-nsdname` relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records. The `nsdname-enable` phrase turns NSDNAME triggers off or on for a single policy zone or all zones.

`RPZ-NSIP`

- NSIP triggers match the IP addresses of authoritative servers. They are enncoded like IP triggers, except as subdomains of `rpz-nsip`. NSDNAME and NSIP triggers are checked only for names with at least `min-ns-dots` dots. The default value of `min-ns-dots` is 1, to exclude top level domains. The `nsip-enable` phrase turns NSIP triggers off or on for a single policy zone or all zones.

  If a name server's IP address is not yet known, `named` will recursively look up the IP address before applying an RPZ-NSIP rule. This can cause a processing delay. To speed up processing at the cost of precision, the `nsip-wait-recurse` option can be used: when set to `no`, RPZ-NSIP rules will only be applied when a name servers's IP address has already been looked up and cached. If a server's IP address is not in the cache, then the RPZ-NSIP rule will be ignored, but the address will be looked up in the background, and the rule will be applied to subsequent queries. The default is `yes`, meaning RPZ-NSIP rules should always be applied even if an address needs to be looked up first.

The query response is checked against all response policy zones, so two or more policy records can be triggered by a response. Because DNS responses are rewritten according to at most one policy record, a single record encoding an action (other than `DISABLED` actions) must be chosen. Triggers or the records that encode them are chosen for the rewriting in the following order:

1. Choose the triggered record in the zone that appears first in the `response-policy` option.
2. Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a single zone.
3. Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.
4. Among IP or NSIP triggers, prefer the trigger with the longest prefix.
5. Among triggers with the same prefix length, prefer the IP or NSIP trigger that matches the smallest IP address.

When the processing of a response is restarted to resolve DNAME or CNAME records and a policy record set has not been triggered, all response policy zones are again consulted for the DNAME or CNAME names and addresses.

RPZ record sets are any types of DNS record except DNAME or DNSSEC that encode actions or responses to individual queries. Any of the policies can be used with any of the triggers. For example, while the `TCP-only` policy is commonly used with `client-IP` triggers, it can be used with any type of trigger to force the use of TCP for responses with owner names in a zone.

`PASSTHRU`

- The whitelist policy is specified by a CNAME whose target is `rpz-passthru`. It causes the response to not be rewritten and is most often used to "poke holes" in policies for CIDR blocks.

`DROP`

- The blacklist policy is specified by a CNAME whose target is `rpz-drop`. It causes the response to be discarded. Nothing is sent to the DNS client.

`TCP-Only`

- The "slip" policy is specified by a CNAME whose target is `rpz-tcp-only`. It changes UDP responses to short, truncated DNS responses that require the DNS client to try again with TCP. It is used to mitigate distributed DNS reflection attacks.

`NXDOMAIN`

- The domain undefined response is encoded by a CNAME whose target is the root domain (.)

`NODATA`

- The empty set of resource records is specified by CNAME whose target is the wildcard top-level domain (*.). It rewrites the response to NODATA or ANCOUNT=0.

`Local Data`

- A set of ordinary DNS records can be used to answer queries. Queries for record types not the set are answered with NODATA.

  A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if were an ordinary CNAME after the asterisk (*) has been replaced with the query name. The purpose for this special form is query logging in the walled garden's authority DNS server.

All of the actions specified in all of the individual records in a policy zone can be overridden with a `policy` clause in the `response-policy` option. An organization using a policy zone provided by another organization might use this mechanism to redirect domains to its own walled garden.

`GIVEN`

- The placeholder policy says "do not override but perform the action specified in the zone."

`DISABLED`

- The testing override policy causes policy zone records to do nothing but log what they would have done if the policy zone were not disabled. The response to the DNS query will be written (or not) according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they will often not be logged if a higher precedence trigger is found first.

`PASSTHRU`

,

`DROP`

,

`TCP-Only`

,

`NXDOMAIN`

, and

`NODATA`

- override with the corresponding per-record policy.

`CNAME domain`

- causes all RPZ policy records to act as if they were "cname domain" records.

By default, the actions encoded in a response policy zone are applied only to queries that ask for recursion (RD=1). That default can be changed for a single policy zone or all response policy zones in a view with a `recursive-only no` clause. This feature is useful for serving the same zone files both inside and outside an RFC 1918 cloud and using RPZ to delete answers that would otherwise contain RFC 1918 values on the externally visible name server or view.

Also by default, RPZ actions are applied only to DNS requests that either do not request DNSSEC metadata (DO=0) or when no DNSSEC records are available for request name in the original zone (not the response policy zone). This default can be changed for all response policy zones in a view with a `break-dnssec yes` clause. In that case, RPZ actions are applied regardless of DNSSEC. The name of the clause option reflects the fact that results rewritten by RPZ actions cannot verify.

No DNS records are needed for a QNAME or Client-IP trigger. The name or IP address itself is sufficient, so in principle the query name need not be recursively resolved. However, not resolving the requested name can leak the fact that response policy rewriting is in use and that the name is listed in a policy zone to operators of servers for listed names. To prevent that information leak, by default any recursion needed for a request is done before any policy triggers are considered. Because listed domains often have slow authoritative servers, this behavior can cost significant time. The `qname-wait-recurse yes` option overrides the default and enables that behavior when recursion cannot change a non-error response. The option does not affect QNAME or client-IP triggers in policy zones listed after other zones containing IP, NSIP and NSDNAME triggers, because those may depend on the A, AAAA, and NS records that would be found during recursive resolution. It also does not affect DNSSEC requests (DO=1) unless `break-dnssec yes` is in use, because the response would depend on whether or not RRSIG records were found during resolution. Using this option can cause error responses such as SERVFAIL to appear to be rewritten, since no recursion is being done to discover problems at the authoritative server.

The `dnsrps-enable yes` option turns on the DNS Rsponse Policy Service (DNSRPS) interface, if it has been compiled in to `named` using `configure --enable-dnsrps`.

The `dnsrps-options` block provides additional RPZ configuration settings, which are passed through to the DNSRPS provider library. Multiple DNSRPS settings in an `dnsrps-options` string should be separated with semi-colons. The DNSRPS provider, librpz, is passed a configuration string consisting of the `dnsrps-options` text, concatenated with settings derived from the `response-policy` statement.

Note: The `dnsrps-options` text should only include configuration settings that are specific to the DNSRPS provider. For example, the DNSRPS provider from Farsight Security takes options such as `dnsrpzd-conf`, `dnsrpzd-sock`, and `dnzrpzd-args` (for details of these options, see the `librpz` documentation). Other RPZ configuration settings could be included in `dnsrps-options` as well, but if `named` were switched back to traditional RPZ by setting `dnsrps-enable` to "no", those options would be ignored.

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in policy zone. It is then limited to a maximum value. The `max-policy-ttl` clause changes the maximum seconds from its default of 5. For convenience, TTL-style time unit suffixes may be used to specify the value.

For example, you might use this option statement

        response-policy { zone "badlist"; };

and this zone statement

        zone "badlist" {type master; file "master/badlist"; allow-query {none;}; };

with this zone file

    $TTL 1H
    @                       SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h)
    			NS  LOCALHOST.

    ; QNAME policy records.  There are no periods (.) after the owner names.
    nxdomain.domain.com     CNAME   .               ; NXDOMAIN policy
    *.nxdomain.domain.com   CNAME   .               ; NXDOMAIN policy
    nodata.domain.com       CNAME   *.              ; NODATA policy
    *.nodata.domain.com     CNAME   *.              ; NODATA policy
    bad.domain.com          A       10.0.0.1        ; redirect to a walled garden
    			AAAA    2001:2::1
    bzone.domain.com        CNAME   garden.example.com.

    ; do not rewrite (PASSTHRU) OK.DOMAIN.COM
    ok.domain.com           CNAME   rpz-passthru.

    ; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com
    *.bzone.domain.com      CNAME   *.garden.example.com.

    ; IP policy records that rewrite all responses containing A records in 127/8
    ;       except 127.0.0.1
    8.0.0.0.127.rpz-ip      CNAME   .
    32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

    ; NSDNAME and NSIP policy records
    ns.domain.com.rpz-nsdname   CNAME   .
    48.zz.2.2001.rpz-nsip       CNAME   .

    ; blacklist and whitelist some DNS clients
    112.zz.2001.rpz-client-ip    CNAME   rpz-drop.
    8.0.0.0.127.rpz-client-ip    CNAME   rpz-drop.

    ; force some DNS clients and responses in the example.com zone to TCP
    16.0.0.1.10.rpz-client-ip   CNAME   rpz-tcp-only.
    example.com                 CNAME   rpz-tcp-only.
    *.example.com               CNAME   rpz-tcp-only.

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers, QNAME, IP, NSIP, and NSDNAME, requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A BIND9 server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the `RPZRewrites` statistics.

The `log` clause can be used to optionally turn off rewrite logging for a particular response policy zone. By default, all rewrites are logged.

The `add-soa` option controls whether the RPZ's SOA record is added to the additional section for traceback of changes from this zone or not. This can be set at the individual policy zone level or at the response-policy level. The default is `yes`.

Updates to RPZ zones are processed asynchronously; if there is more than one update pending they are bundled together. If an update to a RPZ zone (for example, via IXFR) happens less than `min-update-interval` seconds after the most recent update, then the changes will not be carried out until this interval has elapsed. The default is `60` seconds. For convenience, TTL-style time unit suffixes may be used to specify the value.

Excessive almost identical UDP responses can be controlled by configuring a `rate-limit` clause in an `options` or `view` statement. This mechanism keeps authoritative BIND 9 from being used in amplifying reflection denial of service (DoS) attacks. Short truncated (TC=1) responses can be sent to provide rate-limited responses to legitimate clients within a range of forged, attacked IP addresses. Legitimate clients react to dropped or truncated response by retrying with UDP or with TCP respectively.

This mechanism is intended for authoritative DNS servers. It can be used on recursive servers but can slow applications such as SMTP servers (mail receivers) and HTTP clients (web browsers) that repeatedly request the same domains. When possible, closing "open" recursive servers is better.

Response rate limiting uses a "credit" or "token bucket" scheme. Each combination of identical response and client has a conceptual account that earns a specified number of credits every second. A prospective response debits its account by one. Responses are dropped or truncated while the account is negative. Responses are tracked within a rolling window of time which defaults to 15 seconds, but can be configured with the `window` option to any value from 1 to 3600 seconds (1 hour). The account cannot become more positive than the per-second limit or more negative than `window` times the per-second limit. When the specified number of credits for a class of responses is set to 0, those responses are not rate limited.

The notions of "identical response" and "DNS client" for rate limiting are not simplistic. All responses to an address block are counted as if to a single client. The prefix lengths of addresses blocks are specified with `ipv4-prefix-length` (default 24) and `ipv6-prefix-length` (default 56).

All non-empty responses for a valid domain name (qname) and record type (qtype) are identical and have a limit specified with `responses-per-second` (default 0 or no limit). All empty (NODATA) responses for a valid domain, regardless of query type, are identical. Responses in the NODATA class are limited by `nodata-per-second` (default `responses-per-second`). Requests for any and all undefined subdomains of a given valid domain result in NXDOMAIN errors, and are identical regardless of query type. They are limited by `nxdomains-per-second` (default `responses-per-second`). This controls some attacks using random names, but can be relaxed or turned off (set to 0) on servers that expect many legitimate NXDOMAIN responses, such as from anti-spam blacklists. Referrals or delegations to the server of a given domain are identical and are limited by `referrals-per-second` (default `responses-per-second`).

Responses generated from local wildcards are counted and limited as if they were for the parent domain name. This controls flooding using random.wild.example.com.

All requests that result in DNS errors other than NXDOMAIN, such as SERVFAIL and FORMERR, are identical regardless of requested name (qname) or record type (qtype). This controls attacks using invalid requests or distant, broken authoritative servers. By default the limit on errors is the same as the `responses-per-second` value, but it can be set separately with `errors-per-second`.

Many attacks using DNS involve UDP requests with forged source addresses. Rate limiting prevents the use of BIND 9 to flood a network with responses to requests with forged source addresses, but could let a third party block responses to legitimate requests. There is a mechanism that can answer some legitimate requests from a client whose address is being forged in a flood. Setting `slip` to 2 (its default) causes every other UDP request to be answered with a small truncated (TC=1) response. The small size and reduced frequency, and so lack of amplification, of "slipped" responses make them unattractive for reflection DoS attacks. `slip` must be between 0 and 10. A value of 0 does not "slip": no truncated responses are sent due to rate limiting, all responses are dropped. A value of 1 causes every response to slip; values between 2 and 10 cause every n'th response to slip. Some error responses including REFUSED and SERVFAIL cannot be replaced with truncated responses and are instead leaked at the `slip` rate.

(NOTE: Dropped responses from an authoritative server may reduce the difficulty of a third party successfully forging a response to a recursive resolver. The best security against forged responses is for authoritative operators to sign their zones using DNSSEC and for resolver operators to validate the responses. When this is not an option, operators who are more concerned with response integrity than with flood mitigation may consider setting `slip` to 1, causing all rate-limited responses to be truncated rather than dropped. This reduces the effectiveness of rate-limiting against reflection attacks.)

When the approximate query per second rate exceeds the `qps-scale` value, then the `responses-per-second`, `errors-per-second`, `nxdomains-per-second` and `all-per-second` values are reduced by the ratio of the current rate to the `qps-scale` value. This feature can tighten defenses during attacks. For example, with `qps-scale 250; responses-per-second 20;` and a total query rate of 1000 queries/second for all queries from all DNS clients including via TCP, then the effective responses/second limit changes to (250/1000)*20 or 5. Responses sent via TCP are not limited but are counted to compute the query per second rate.

Communities of DNS clients can be given their own parameters or no rate limiting by putting `rate-limit` statements in `view` statements instead of the global `option` statement. A `rate-limit` statement in a view replaces, rather than supplementing, a `rate-limit` statement among the main options. DNS clients within a view can be exempted from rate limits with the `exempt-clients` clause.

UDP responses of all kinds can be limited with the `all-per-second` phrase. This rate limiting is unlike the rate limiting provided by `responses-per-second`, `errors-per-second`, and `nxdomains-per-second` on a DNS server which are often invisible to the victim of a DNS reflection attack. Unless the forged requests of the attack are the same as the legitimate requests of the victim, the victim's requests are not affected. Responses affected by an `all-per-second` limit are always dropped; the `slip` value has no effect. An `all-per-second` limit should be at least 4 times as large as the other limits, because single DNS clients often send bursts of legitimate requests. For example, the receipt of a single mail message can prompt requests from an SMTP server for NS, PTR, A, and AAAA records as the incoming SMTP/TCP/IP connection is considered. The SMTP server can need additional NS, A, AAAA, MX, TXT, and SPF records as it considers the STMP `Mail From` command. Web browsers often repeatedly resolve the same names that are repeated in HTML <IMG> tags in a page. `all-per-second` is similar to the rate limiting offered by firewalls but often inferior. Attacks that justify ignoring the contents of DNS responses are likely to be attacks on the DNS server itself. They usually should be discarded before the DNS server spends resources make TCP connections or parsing DNS requests, but that rate limiting must be done before the DNS server sees the requests.

The maximum size of the table used to track requests and rate limit responses is set with `max-table-size`. Each entry in the table is between 40 and 80 bytes. The table needs approximately as many entries as the number of requests received per second. The default is 20,000. To reduce the cold start of growing the table, `min-table-size` (default 500) can set the minimum table size. Enable `rate-limit` category logging to monitor expansions of the table and inform choices for the initial and maximum table size.

Use `log-only yes` to test rate limiting parameters without actually dropping any requests.

Responses dropped by rate limits are included in the `RateDropped` and `QryDropped` statistics. Responses that truncated by rate limits are included in `RateSlipped` and `RespTruncated`.

Named supports NXDOMAIN redirection via two methods:

- Redirect zone zone_statement_grammar
- Redirect namespace

With both methods when named gets a NXDOMAIN response it examines a separate namespace to see if the NXDOMAIN response should be replaced with an alternative response.

With a redirect zone (`zone "." { type redirect; };`), the data used to replace the NXDOMAIN is held in a single zone which is not part of the normal namespace. All the redirect information is contained in the zone; there are no delegations.

With a redirect namespace (`option { nxdomain-redirect <suffix> };`) the data used to replace the NXDOMAIN is part of the normal namespace and is looked up by appending the specified suffix to the original query name. This roughly doubles the cache required to process NXDOMAIN responses as you have the original NXDOMAIN response and the replacement data or a NXDOMAIN indicating that there is no replacement.

If both a redirect zone and a redirect namespace are configured, the redirect zone is tried first.

The `server` statement defines characteristics to be associated with a remote name server. If a prefix length is specified, then a range of servers is covered. Only the most specific server clause applies regardless of the order in `named.conf`.

The `server` statement can occur at the top level of the configuration file or inside a `view` statement. If a `view` statement contains one or more `server` statements, only those apply to the view and any top-level ones are ignored. If a view contains no `server` statements, any top-level `server` statements are used as defaults.

If you discover that a remote server is giving out bad data, marking it as bogus will prevent further queries to it. The default value of `bogus` is `no`.

The `provide-ixfr` clause determines whether the local server, acting as master, will respond with an incremental zone transfer when the given remote server, a slave, requests it. If set to `yes`, incremental transfer will be provided whenever possible. If set to `no`, all transfers to the remote server will be non-incremental. If not set, the value of the `provide-ixfr` option in the view or global options block is used as a default.

The `request-ixfr` clause determines whether the local server, acting as a slave, will request incremental zone transfers from the given remote server, a master. If not set, the value of the `request-ixfr` option in the view or global options block is used as a default. It may also be set in the zone block and, if set there, it will override the global or view setting for that zone.

IXFR requests to servers that do not support IXFR will automatically fall back to AXFR. Therefore, there is no need to manually list which servers support IXFR and which ones do not; the global default of `yes` should always work. The purpose of the `provide-ixfr` and `request-ixfr` clauses is to make it possible to disable the use of IXFR even when both master and slave claim to support it, for example if one of the servers is buggy and crashes or corrupts data when IXFR is used.

The `request-expire` clause determines whether the local server, when acting as a slave, will request the EDNS EXPIRE value. The EDNS EXPIRE value indicates the remaining time before the zone data will expire and need to be be refreshed. This is used when a secondary server transfers a zone from another secondary server; when transferring from the primary, the expiration timer is set from the EXPIRE field of the SOA record instead. The default is `yes`.

The `edns` clause determines whether the local server will attempt to use EDNS when communicating with the remote server. The default is `yes`.

The `edns-udp-size` option sets the EDNS UDP size that is advertised by `named` when querying the remote server. Valid values are 512 to 4096 bytes (values outside this range will be silently adjusted to the nearest value within it). This option is useful when you wish to advertise a different value to this server than the value you advertise globally, for example, when there is a firewall at the remote site that is blocking large replies. (Note: Currently, this sets a single UDP size for all packets sent to the server; `named` will not deviate from this value. This differs from the behavior of `edns-udp-size` in `options` or `view` statements, where it specifies a maximum value. The `server` statement behavior may be brought into conformance with the `options/view` behavior in future releases.)

The `edns-version` option sets the maximum EDNS VERSION that will be sent to the server(s) by the resolver. The actual EDNS version sent is still subject to normal EDNS version negotiation rules (see RFC 6891), the maximum EDNS version supported by the server, and any other heuristics that indicate that a lower version should be sent. This option is intended to be used when a remote server reacts badly to a given EDNS version or higher; it should be set to the highest version the remote server is known to support. Valid values are 0 to 255; higher values will be silently adjusted. This option will not be needed until higher EDNS versions than 0 are in use.

The `max-udp-size` option sets the maximum EDNS UDP message size `named` will send. Valid values are 512 to 4096 bytes (values outside this range will be silently adjusted). This option is useful when you know that there is a firewall that is blocking large replies from `named`.

The `padding` option adds EDNS Padding options to outgoing messages, increasing the packet size to a multiple of the specified block size. Valid block sizes range from 0 (the default, which disables the use of EDNS Padding) to 512 bytes. Larger values will be reduced to 512, with a logged warning. Note: This option is not currently compatible with no TSIG or SIG(0), as the EDNS OPT record containing the padding would have to be added to the packet after it had already been signed.

The `tcp-only` option sets the transport protocol to TCP. The default is to use the UDP transport and to fallback on TCP only when a truncated response is received.

The `tcp-keepalive` option adds EDNS TCP keepalive to messages sent over TCP. Note currently idle timeouts in responses are ignored.

The server supports two zone transfer methods. The first, `one-answer`, uses one DNS message per resource record transferred. `many-answers` packs as many resource records as possible into a message. `many-answers` is more efficient, but is only known to be understood by BIND 9, BIND 8.x, and patched versions of BIND 4.9.5. You can specify which method to use for a server with the `transfer-format` option. If `transfer-format` is not specified, the `transfer-format` specified by the `options` statement will be used.

`transfers` is used to limit the number of concurrent inbound zone transfers from the specified server. If no `transfers` clause is specified, the limit is set according to the `transfers-per-ns` option.

The `keys` clause identifies a `key_id` defined by the `key` statement, to be used for transaction security (TSIG, tsig) when talking to the remote server. When a request is sent to the remote server, a request signature will be generated using the key specified here and appended to the message. A request originating from the remote server is not required to be signed by this key.

Only a single key per server is currently supported.

The `transfer-source` and `transfer-source-v6` clauses specify the IPv4 and IPv6 source address to be used for zone transfer with the remote server, respectively. For an IPv4 remote server, only `transfer-source` can be specified. Similarly, for an IPv6 remote server, only `transfer-source-v6` can be specified. For more details, see the description of `transfer-source` and `transfer-source-v6` in zone_transfers.

The `notify-source` and `notify-source-v6` clauses specify the IPv4 and IPv6 source address to be used for notify messages sent to remote servers, respectively. For an IPv4 remote server, only `notify-source` can be specified. Similarly, for an IPv6 remote server, only `notify-source-v6` can be specified.

The `query-source` and `query-source-v6` clauses specify the IPv4 and IPv6 source address to be used for queries sent to remote servers, respectively. For an IPv4 remote server, only `query-source` can be specified. Similarly, for an IPv6 remote server, only `query-source-v6` can be specified.

The `request-nsid` clause determines whether the local server will add a NSID EDNS option to requests sent to the server. This overrides `request-nsid` set at the view or option level.

The `send-cookie` clause determines whether the local server will add a COOKIE EDNS option to requests sent to the server. This overrides `send-cookie` set at the view or option level. The `named` server may determine that COOKIE is not supported by the remote server and not add a COOKIE EDNS option to requests.

The `statistics-channels` statement declares communication channels to be used by system administrators to get access to statistics information of the name server.

This statement intends to be flexible to support multiple communication protocols in the future, but currently only HTTP access is supported. It requires that BIND 9 be compiled with libxml2 and/or json-c (also known as libjson0); the `statistics-channels` statement is still accepted even if it is built without the library, but any HTTP access will fail with an error.

An `inet` control channel is a TCP socket listening at the specified `ip_port` on the specified `ip_addr`, which can be an IPv4 or IPv6 address. An `ip_addr` of `*` (asterisk) is interpreted as the IPv4 wildcard address; connections will be accepted on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address, use an `ip_addr` of `::`.

If no port is specified, port 80 is used for HTTP channels. The asterisk "`*`" cannot be used for `ip_port`.

The attempt of opening a statistics channel is restricted by the optional `allow` clause. Connections to the statistics channel are permitted based on the `address_match_list`. If no `allow` clause is present, `named` accepts connection attempts from any address; since the statistics may contain sensitive internal information, it is highly recommended to restrict the source of connection requests appropriately.

If no `statistics-channels` statement is present, `named` will not open any communication channels.

The statistics are available in various formats and views depending on the URI used to access them. For example, if the statistics channel is configured to listen on 127.0.0.1 port 8888, then the statistics are accessible in XML format at http://127.0.0.1:8888/ or http://127.0.0.1:8888/xml. A CSS file is included which can format the XML statistics into tables when viewed with a stylesheet-capable browser, and into charts and graphs using the Google Charts API when using a javascript-capable browser.

Broken-out subsets of the statistics can be viewed at http://127.0.0.1:8888/xml/v3/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/xml/v3/server (server and resolver statistics), http://127.0.0.1:8888/xml/v3/zones (zone statistics), http://127.0.0.1:8888/xml/v3/net (network status and socket statistics), http://127.0.0.1:8888/xml/v3/mem (memory manager statistics), http://127.0.0.1:8888/xml/v3/tasks (task manager statistics), and http://127.0.0.1:8888/xml/v3/traffic (traffic sizes).

The full set of statistics can also be read in JSON format at http://127.0.0.1:8888/json, with the broken-out subsets at http://127.0.0.1:8888/json/v1/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/json/v1/server (server and resolver statistics), http://127.0.0.1:8888/json/v1/zones (zone statistics), http://127.0.0.1:8888/json/v1/net (network status and socket statistics), http://127.0.0.1:8888/json/v1/mem (memory manager statistics), http://127.0.0.1:8888/json/v1/tasks (task manager statistics), and http://127.0.0.1:8888/json/v1/traffic (traffic sizes).

The `trusted-keys` statement defines DNSSEC security roots. DNSSEC is described in DNSSEC. A security root is defined when the public key for a non-authoritative zone is known, but cannot be securely obtained through DNS, either because it is the DNS root zone or because its parent zone is unsigned. Once a key has been configured as a trusted key, it is treated as if it had been validated and proven secure. The resolver attempts DNSSEC validation on all DNS data in subdomains of a security root.

All keys (and corresponding zones) listed in `trusted-keys` are deemed to exist regardless of what parent zones say. Similarly for all keys listed in `trusted-keys` only those keys are used to validate the DNSKEY RRset. The parent's DS RRset will not be used.

The `trusted-keys` statement can contain multiple key entries, each consisting of the key's domain name, flags, protocol, algorithm, and the Base64 representation of the key data. Spaces, tabs, newlines and carriage returns are ignored in the key data, so the configuration may be split up into multiple lines.

`trusted-keys` may be set at the top level of `named.conf` or within a view. If it is set in both places, they are additive: keys defined at the top level are inherited by all views, but keys defined in a view are only used within that view.

Validation below specified names can be temporarily disabled by using `rndc nta`.

The `managed-keys` statement, like `trusted-keys`, defines DNSSEC security roots. The difference is that `managed-keys` can be kept up to date automatically, without intervention from the resolver operator.

Suppose, for example, that a zone's key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the old key in a `trusted-keys` statement would be unable to validate this zone any longer; it would reply with a SERVFAIL response code. This would continue until the resolver operator had updated the `trusted-keys` statement with the new key.

If, however, the zone were listed in a `managed-keys` statement instead, then the zone owner could add a "stand-by" key to the zone in advance. `named` would store the stand-by key, and when the original key was revoked, `named` would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked, and cease using that key to validate answers, minimizing the damage that the compromised key could do.

A `managed-keys` statement contains a list of the keys to be managed, along with information about how the keys are to be initialized for the first time. The only initialization method currently supported is `initial-key`. This means the `managed-keys` statement must contain a copy of the initializing key. (Future releases may allow keys to be initialized by other methods, eliminating this requirement.)

Consequently, a `managed-keys` statement appears similar to a `trusted-keys`, differing in the presence of the second field, containing the keyword `initial-key`. The difference is, whereas the keys listed in a `trusted-keys` continue to be trusted until they are removed from `named.conf`, an initializing key listed in a `managed-keys` statement is only trusted once: for as long as it takes to load the managed key database and start the RFC 5011 key maintenance process.

The first time `named` runs with a managed key configured in `named.conf`, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the key specified in the `managed-keys` statement. If the DNSKEY RRset is validly signed, then it is used as the basis for a new managed keys database.

From that point on, whenever `named` runs, it sees the `managed-keys` statement, checks to make sure RFC 5011 key maintenance has already been initialized for the specified domain, and if so, it simply moves on. The key specified in the `managed-keys` statement is not used to validate answers; it has been superseded by the key or keys stored in the managed keys database.

The next time `named` runs after a name has been removed from the `managed-keys` statement, the corresponding zone will be removed from the managed keys database, and RFC 5011 key maintenance will no longer be used for that domain.

In the current implementation, the managed keys database is stored as a master-format zone file.

On servers which do not use views, this file is named `managed-keys.bind`. When views are in use, there will be a separate managed keys database for each view; the filename will be the view name (or, if a view name contains characters which would make it illegal as a filename, a hash of the view name), followed by the suffix `.mkeys`.

When the key database is changed, the zone is updated. As with any other dynamic zone, changes will be written into a journal file, e.g., `managed-keys.bind.jnl` or `internal.mkeys.jnl`. Changes are committed to the master file as soon as possible afterward; this will usually occur within 30 seconds. So, whenever `named` is using automatic key maintenance, the zone file and journal file can be expected to exist in the working directory. (For this reason among others, the working directory should be always be writable by `named`.)

If the `dnssec-validation` option is set to `auto`, `named` will automatically initialize a managed key for the root zone. The key that is used to initialize the key maintenance process is stored in `bind.keys`; the location of this file can be overridden with the `bindkeys-file` option. As a fallback in the event no `bind.keys` can be found, the initializing key is also compiled directly into `named`.

    view view_name [ class ] {
        match-clients { address_match_list } ;
        match-destinations { address_match_list } ;
        match-recursive-only yes_or_no ;
      [ view_option ; ... ]
      [ zone_statement ; ... ]
    } ;

The `view` statement is a powerful feature of BIND 9 that lets a name server answer a DNS query differently depending on who is asking. It is particularly useful for implementing split DNS setups without having to run multiple servers.

Each `view` statement defines a view of the DNS namespace that will be seen by a subset of clients. A client matches a view if its source IP address matches the `address_match_list` of the view's `match-clients` clause and its destination IP address matches the `address_match_list` of the view's `match-destinations` clause. If not specified, both `match-clients` and `match-destinations` default to matching all addresses. In addition to checking IP addresses `match-clients` and `match-destinations` can also take `keys` which provide an mechanism for the client to select the view. A view can also be specified as `match-recursive-only`, which means that only recursive requests from matching clients will match that view. The order of the `view` statements is significant — a client request will be resolved in the context of the first `view` that it matches.

Zones defined within a `view` statement will only be accessible to clients that match the `view`. By defining a zone of the same name in multiple views, different zone data can be given to different clients, for example, "internal" and "external" clients in a split DNS setup.

Many of the options given in the `options` statement can also be used within a `view` statement, and then apply only when resolving queries with that view. When no view-specific value is given, the value in the `options` statement is used as a default. Also, zone options can have default values specified in the `view` statement; these view-specific defaults take precedence over those in the `options` statement.

Views are class specific. If no class is given, class IN is assumed. Note that all non-IN views must contain a hint zone, since only the IN class has compiled-in default hints.

If there are no `view` statements in the config file, a default view that matches any client is automatically created in class IN. Any `zone` statements specified on the top level of the configuration file are considered to be part of this default view, and the `options` statement will apply to the default view. If any explicit `view` statements are present, all `zone` statements must occur inside `view` statements.

Here is an example of a typical split DNS setup implemented using `view` statements:

    view "internal" {
          // This should match our internal networks.
          match-clients { 10.0.0.0/8; };

          // Provide recursive service to internal
          // clients only.
          recursion yes;

          // Provide a complete view of the example.com
          // zone including addresses of internal hosts.
          zone "example.com" {
    	    type master;
    	    file "example-internal.db";
          };
    };

    view "external" {
          // Match all clients not matched by the
          // previous view.
          match-clients { any; };

          // Refuse recursive service to external clients.
          recursion no;

          // Provide a restricted view of the example.com
          // zone containing only publicly accessible hosts.
          zone "example.com" {
    	   type master;
    	   file "example-external.db";
          };
    };

The `type` keyword is required for the `zone` configuration unless it is an `in-view` configuration. Its acceptable values include: `master` (or `primary`), `slave` (or `secondary`), `mirror`, `delegation-only`, `forward`, `hint`, `redirect`, `static-stub`, and `stub`.

`master` | The server has a master copy of the data for the zone and will be able to provide authoritative answers for it. Type `primary` is a synonym for `master`.
`slave` | A slave zone is a replica of a master zone. Type `secondary` is a synonym for `slave`. The `masters` list specifies one or more IP addresses of master servers that the slave contacts to update its copy of the zone. Masters list elements can also be names of other masters lists. By default, transfers are made from port 53 on the servers; this can be changed for all servers by specifying a port number before the list of IP addresses, or on a per-server basis after the IP address. Authentication to the master can also be done with per-server TSIG keys. If a file is specified, then the replica will be written to this file whenever the zone is changed, and reloaded from this file on a server restart. Use of a file is recommended, since it often speeds server startup and eliminates a needless waste of bandwidth. Note that for large numbers (in the tens or hundreds of thousands) of zones per server, it is best to use a two-level naming scheme for zone filenames. For example, a slave server for the zone `example.com` might place the zone contents into a file called `ex/example.com` where `ex/` is just the first two letters of the zone name. (Most operating systems behave very slowly if you put 100000 files into a single directory.)
`stub` | A stub zone is similar to a slave zone, except that it replicates only the NS records of a master zone instead of the entire zone. Stub zones are not a standard part of the DNS; they are a feature specific to the BIND implementation. Stub zones can be used to eliminate the need for glue NS record in a parent zone at the expense of maintaining a stub zone entry and a set of name server addresses in `named.conf`. This usage is not recommended for new configurations, and BIND 9 supports it only in a limited way. In BIND 4/8, zone transfers of a parent zone included the NS records from stub children of that zone. This meant that, in some cases, users could get away with configuring child stubs only in the master server for the parent zone. BIND 9 never mixes together zone data from different zones in this way. Therefore, if a BIND 9 master serving a parent zone has child stub zones configured, all the slave servers for the parent zone also need to have the same child stub zones configured. Stub zones can also be used as a way of forcing the resolution of a given domain to use a particular set of authoritative servers. For example, the caching name servers on a private network using RFC1918 addressing may be configured with stub zones for `10.in-addr.arpa` to use a set of internal name servers as the authoritative servers for that domain.
`mirror` | Note: using this zone type with any zone other than the root zone should be considered experimental and may cause performance issues, especially for zones which are large and/or frequently updated. A mirror zone acts like a zone of type `secondary` whose data is subject to DNSSEC validation before being used in answers. Validation is performed during the zone transfer process (for both AXFR and IXFR), and again when the zone file is loaded from disk when `named` is restarted. If validation of a new version of a mirror zone fails, a retransfer is scheduled and the most recent correctly validated version of that zone is used until it expires; if a newer version of that zone is later correctly validated, it replaces the previously used version. If no usable zone data is available for a mirror zone (either because it was never loaded from disk and has not yet been transferred from a primary server or because its most recent correctly validated version expired), traditional DNS recursion will be used to look up the answers instead. While any zone may be configured with this type, it is intended to be used to set up a fast local copy of the root zone, similar to the one described in RFC 7706. Note, however, that mirror zones are not supposed to augment the example configuration provided by RFC 7706 but rather to replace it altogether. A default list of primary servers for the IANA root zone is built into `named` and thus its mirroring can be enabled using the following configuration:
    zone "." {
    	type mirror;
    };
In order to set up mirroring of any other zone, an explicit list of primary servers needs to be provided using the `masters` option (see masters_grammar for details). To make mirror zone contents persist between `named` restarts, use the file_option_term option. Mirror zone validation always happens for the entire zone contents, i.e. no "incremental validation" takes place, even for IXFRs. This is required to ensure that each version of the zone used by the resolver is fully self-consistent with respect to DNSSEC. Other, more efficient zone verification methods may be added in the future. For validation to succeed, a key-signing key (KSK) for the zone must be configured as a trust anchor in `named.conf`: that is, a key for the zone must either be specified in `managed-keys` or `trusted-keys`. In the case of the root zone, you may also rely on the built-in root trust anchor, which is enabled when dnssec_validation_term is set to the default value `auto`. Answers coming from a mirror zone look almost exactly like answers from a zone of type `secondary`, with the notable exceptions that the AA bit ("authoritative answer") is not set, and the AD bit ("authenticated data") is. Since mirror zones are intended to be used by recursive resolvers, adding one to a view with recursion disabled is considered to be a configuration error. When configuring NOTIFY for a mirror zone, only `notify no;` and `notify explicit;` can be used. Using any other `notify` setting at the zone level is a configuration error. Using any other `notify` setting at the `options` or `view` level will cause that setting to be overridden with `notify explicit;` for the mirror zone in question. Since the global default for the `notify` option is `yes`, mirror zones are by default configured with `notify explicit;`. Outgoing transfers of mirror zones are disabled by default but may be enabled using allow_transfer_term.
`static-stub` | A static-stub zone is similar to a stub zone with the following exceptions: the zone data is statically configured, rather than transferred from a master server; when recursion is necessary for a query that matches a static-stub zone, the locally configured data (nameserver names and glue addresses) is always used even if different authoritative information is cached. Zone data is configured via the `server-addresses` and `server-names` zone options. The zone data is maintained in the form of NS and (if necessary) glue A or AAAA RRs internally, which can be seen by dumping zone databases by `rndc dumpdb -all`. The configured RRs are considered local configuration parameters rather than public data. Non recursive queries (i.e., those with the RD bit off) to a static-stub zone are therefore prohibited and will be responded with REFUSED. Since the data is statically configured, no zone maintenance action takes place for a static-stub zone. For example, there is no periodic refresh attempt, and an incoming notify message will be rejected with an rcode of NOTAUTH. Each static-stub zone is configured with internally generated NS and (if necessary) glue A or AAAA RRs
`forward` | A "forward zone" is a way to configure forwarding on a per-domain basis. A `zone` statement of type `forward` can contain a `forward` and/or `forwarders` statement, which will apply to queries within the domain given by the zone name. If no `forwarders` statement is present or an empty list for `forwarders` is given, then no forwarding will be done for the domain, canceling the effects of any forwarders in the `options` statement. Thus if you want to use this type of zone to change the behavior of the global `forward` option (that is, "forward first" to, then "forward only", or vice versa, but want to use the same servers as set globally) you need to re-specify the global forwarders.
`hint` | The initial set of root name servers is specified using a "hint zone". When the server starts up, it uses the root hints to find a root name server and get the most recent list of root name servers. If no hint zone is specified for class IN, the server uses a compiled-in default set of root servers hints. Classes other than IN have no built-in defaults hints.
`redirect` | Redirect zones are used to provide answers to queries when normal resolution would result in NXDOMAIN being returned. Only one redirect zone is supported per view. `allow-query` can be used to restrict which clients see these answers. If the client has requested DNSSEC records (DO=1) and the NXDOMAIN response is signed then no substitution will occur. To redirect all NXDOMAIN responses to 100.100.100.2 and 2001:ffff:ffff::100.100.100.2, one would configure a type redirect zone named ".", with the zone file containing wildcard records that point to the desired addresses: `"*. IN A 100.100.100.2"` and `"*. IN AAAA 2001:ffff:ffff::100.100.100.2"`. To redirect all Spanish names (under .ES) one would use similar entries but with the names "*.ES." instead of "*.". To redirect all commercial Spanish names (under COM.ES) one would use wildcard entries called "*.COM.ES.". Note that the redirect zone supports all possible types; it is not limited to A and AAAA records. If a redirect zone is configured with a `masters` option, then it is transfered in as if it were a slave zone. Otherwise, it is loaded from a file as if it were a master zone. Because redirect zones are not referenced directly by name, they are not kept in the zone lookup table with normal master and slave zones. To reload a redirect zone, use `rndc reload -redirect`, and to retransfer a redirect zone configured as slave, use `rndc retransfer -redirect`. When using `rndc reload` without specifying a zone name, redirect zones will be reloaded along with other zones.
`delegation-only` | This is used to enforce the delegation-only status of infrastructure zones (e.g. COM, NET, ORG). Any answer that is received without an explicit or implicit delegation in the authority section will be treated as NXDOMAIN. This does not apply to the zone apex. This should not be applied to leaf zones. `delegation-only` has no effect on answers received from forwarders. See caveats in root_delegation_only.

The zone's name may optionally be followed by a class. If a class is not specified, class `IN` (for `Internet`), is assumed. This is correct for the vast majority of cases.

The `hesiod` class is named for an information service from MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on. The keyword `HS` is a synonym for hesiod.

Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the `CHAOS` class.

`allow-notify`

- See the description of `allow-notify` in access_control.

`allow-query`

- See the description of `allow-query` in access_control.

`allow-query-on`

- See the description of `allow-query-on` in access_control.

`allow-transfer`

- See the description of `allow-transfer` in access_control.

`allow-update`

- See the description of `allow-update` in access_control.

`update-policy`

- Specifies a "Simple Secure Update" policy. See dynamic_update_policies.

`allow-update-forwarding`

- See the description of `allow-update-forwarding` in access_control.

`also-notify`

- Only meaningful if `notify` is active for this zone. The set of machines that will receive a `DNS NOTIFY` message for this zone is made up of all the listed name servers (other than the primary master) for the zone plus any IP addresses specified with `also-notify`. A port may be specified with each `also-notify` address to send the notify messages to a port other than the default of 53. A TSIG key may also be specified to cause the `NOTIFY` to be signed by the given key. `also-notify` is not meaningful for stub zones. The default is the empty list.

`check-names`

- This option is used to restrict the character set and syntax of certain domain names in master files and/or DNS responses received from the network. The default varies according to zone type. For `master` zones the default is `fail`. For `slave` zones the default is `warn`. It is not implemented for `hint` zones.

`check-mx`

- See the description of `check-mx` in boolean_options.

`check-spf`

- See the description of `check-spf` in boolean_options.

`check-wildcard`

- See the description of `check-wildcard` in boolean_options.

`check-integrity`

- See the description of `check-integrity` in boolean_options.

`check-sibling`

- See the description of `check-sibling` in boolean_options.

`zero-no-soa-ttl`

- See the description of `zero-no-soa-ttl` in boolean_options.

`update-check-ksk`

- See the description of `update-check-ksk` in boolean_options.

`dnssec-loadkeys-interval`

- See the description of `dnssec-loadkeys-interval` in options.

`dnssec-update-mode`

- See the description of `dnssec-update-mode` in options.

`dnssec-dnskey-kskonly`

- See the description of `dnssec-dnskey-kskonly` in boolean_options.

`try-tcp-refresh`

- See the description of `try-tcp-refresh` in boolean_options.

`database`

- Specify the type of database to be used for storing the zone data. The string following the `database` keyword is interpreted as a list of whitespace-delimited words. The first word identifies the database type, and any subsequent words are passed as arguments to the database to be interpreted in a way specific to the database type.

  The default is `"rbt"`, BIND 9's native in-memory red-black-tree database. This database does not take arguments.

  Other values are possible if additional database drivers have been linked into the server. Some sample drivers are included with the distribution but none are linked in by default.

`dialup`

- See the description of `dialup` in boolean_options.

`delegation-only`

- The flag only applies to forward, hint and stub zones. If set to `yes`, then the zone will also be treated as if it is also a delegation-only type zone.

  See caveats in root_delegation_only.

`file`

- Set the zone's filename. In `master`, `hint`, and `redirect` zones which do not have `masters` defined, zone data is loaded from this file. In `slave`, `mirror`, `stub`, and `redirect` zones which do have `masters` defined, zone data is retrieved from another server and saved in this file. This option is not applicable to other zone types.

`forward`

- Only meaningful if the zone has a forwarders list. The `only` value causes the lookup to fail after trying the forwarders and getting no answer, while `first` would allow a normal lookup to be tried.

`forwarders`

- Used to override the list of global forwarders. If it is not specified in a zone of type `forward`, no forwarding is done for the zone and the global options are not used.

`journal`

- Allow the default journal's filename to be overridden. The default is the zone's filename with "`.jnl`" appended. This is applicable to `master` and `slave` zones.

`max-journal-size`

- See the description of `max-journal-size` in server_resource_limits.

`max-records`

- See the description of `max-records` in server_resource_limits.

`max-transfer-time-in`

- See the description of `max-transfer-time-in` in zone_transfers.

`max-transfer-idle-in`

- See the description of `max-transfer-idle-in` in zone_transfers.

`max-transfer-time-out`

- See the description of `max-transfer-time-out` in zone_transfers.

`max-transfer-idle-out`

- See the description of `max-transfer-idle-out` in zone_transfers.

`notify`

- See the description of `notify` in boolean_options.

`notify-delay`

- See the description of `notify-delay` in tuning.

`notify-to-soa`

- See the description of `notify-to-soa` in boolean_options.

`zone-statistics`

- See the description of `zone-statistics` in options.

`server-addresses`

- Only meaningful for static-stub zones. This is a list of IP addresses to which queries should be sent in recursive resolution for the zone. A non empty list for this option will internally configure the apex NS RR with associated glue A or AAAA RRs.

  For example, if "example.com" is configured as a static-stub zone with 192.0.2.1 and 2001:db8::1234 in a `server-addresses` option, the following RRs will be internally configured.

      example.com. NS example.com.
      example.com. A 192.0.2.1
      example.com. AAAA 2001:db8::1234

  These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server will initiate recursive resolution and send queries to 192.0.2.1 and/or 2001:db8::1234.

`server-names`

- Only meaningful for static-stub zones. This is a list of domain names of nameservers that act as authoritative servers of the static-stub zone. These names will be resolved to IP addresses when `named` needs to send queries to these servers. To make this supplemental resolution successful, these names must not be a subdomain of the origin name of static-stub zone. That is, when "example.net" is the origin of a static-stub zone, "ns.example" and "master.example.com" can be specified in the `server-names` option, but "ns.example.net" cannot, and will be rejected by the configuration parser.

  A non empty list for this option will internally configure the apex NS RR with the specified names. For example, if "example.com" is configured as a static-stub zone with "ns1.example.net" and "ns2.example.net" in a `server-names` option, the following RRs will be internally configured.

      example.com. NS ns1.example.net.
      example.com. NS ns2.example.net.

  These records are internally used to resolve names under the static-stub zone. For instance, if the server receives a query for "www.example.com" with the RD bit on, the server initiate recursive resolution, resolve "ns1.example.net" and/or "ns2.example.net" to IP addresses, and then send queries to (one or more of) these addresses.

`sig-validity-interval`

- See the description of `sig-validity-interval` in tuning.

`sig-signing-nodes`

- See the description of `sig-signing-nodes` in tuning.

`sig-signing-signatures`

- See the description of `sig-signing-signatures` in tuning.

`sig-signing-type`

- See the description of `sig-signing-type` in tuning.

`transfer-source`

- See the description of `transfer-source` in zone_transfers.

`transfer-source-v6`

- See the description of `transfer-source-v6` in zone_transfers.

`alt-transfer-source`

- See the description of `alt-transfer-source` in zone_transfers.

`alt-transfer-source-v6`

- See the description of `alt-transfer-source-v6` in zone_transfers.

`use-alt-transfer-source`

- See the description of `use-alt-transfer-source` in zone_transfers.

`notify-source`

- See the description of `notify-source` in zone_transfers.

`notify-source-v6`

- See the description of `notify-source-v6` in zone_transfers.

`min-refresh-time`

`max-refresh-time`

`min-retry-time`

`max-retry-time`

- See the description in tuning.

`ixfr-from-differences`

- See the description of `ixfr-from-differences` in boolean_options. (Note that the `ixfr-from-differences` `master` and `slave` choices are not available at the zone level.)

`key-directory`

- See the description of `key-directory` in options.

`auto-dnssec`

- See the description of `auto-dnssec` in options.

`serial-update-method`

- See the description of `serial-update-method` in options.

`inline-signing`

- If `yes`, this enables "bump in the wire" signing of a zone, where a unsigned zone is transferred in or loaded from disk and a signed version of the zone is served, with possibly, a different serial number. This behavior is disabled by default.

`multi-master`

- See the description of `multi-master` in boolean_options.

`masterfile-format`

- See the description of `masterfile-format` in tuning.

`max-zone-ttl`

- See the description of `max-zone-ttl` in options.

`dnssec-secure-to-insecure`

- See the description of `dnssec-secure-to-insecure` in boolean_options.

BIND 9 supports two alternative methods of granting clients the right to perform dynamic updates to a zone, configured by the `allow-update` and `update-policy` option, respectively.

The `allow-update` clause is a simple access control list. Any client that matches the ACL is granted permission to update any record in the zone.

The `update-policy` clause allows more fine-grained control over what updates are allowed. It specifies a set of rules, in which each rule either grants or denies permission for one or more names in the zone to be updated by one or more identities. Identity is determined by the key that signed the update request using either TSIG or SIG(0). In most cases, `update-policy` rules only apply to key-based identities. There is no way to specify update permissions based on client source address.

`update-policy` rules are only meaningful for zones of type `master`, and are not allowed in any other zone type. It is a configuration error to specify both `allow-update` and `update-policy` at the same time.

A pre-defined `update-policy` rule can be switched on with the command `update-policy local;`. Using this in a zone causes `named` to generate a TSIG session key when starting up and store it in a file; this key can then be used by local clients to update the zone while `named` is running. By default, the session key is stored in the file `/var/run/named/session.key`, the key name is "local-ddns", and the key algorithm is HMAC-SHA256. These values are configurable with the `session-keyfile`, `session-keyname` and `session-keyalg` options, respectively. A client running on the local system, if run with appropriate permissions, may read the session key from the key file and use it to sign update requests. The zone's update policy will be set to allow that key to change any record within the zone. Assuming the key name is "local-ddns", this policy is equivalent to:

    update-policy { grant local-ddns zonesub any; };

...with the additional restriction that only clients connecting from the local system will be permitted to send updates.

Note that only one session key is generated by `named`; all zones configured to use `update-policy local` will accept the same key.

The command `nsupdate -l` implements this feature, sending requests to localhost and signing them using the key retrieved from the session key file.

Other rule definitions look like this:

    ( grant | deny ) identity ruletype  name   types

Each rule grants or denies privileges. Rules are checked in the order in which they are specified in the `update-policy` statement. Once a message has successfully matched a rule, the operation is immediately granted or denied, and no further rules are examined. There are 13 types of rules; the rule type is specified by the `ruletype` field, and the interpretation of other fields varies depending on the rule type.

In general, a rule is matched when the key that signed an update request matches the `identity` field, the name of the record to be updated matches the `name` field (in the manner specified by the `ruletype` field), and the type of the record to be updated matches the `types` field. Details for each rule type are described below.

The `identity` field must be set to a fully-qualified domain name. In most cases, this represensts the name of the TSIG or SIG(0) key that must be used to sign the update request. If the specified name is a wildcard, it is subject to DNS wildcard expansion, and the rule may apply to multiple identities. When a TKEY exchange has been used to create a shared secret, the identity of the key used to authenticate the TKEY exchange will be used as the identity of the shared secret. Some rule types use identities matching the client's Kerberos principal (e.g, `"host/machine@REALM"`) or Windows realm (`machine$@REALM`).

The `name` field also specifies a fully-qualified domain name. This often represents the name of the record to be updated. Interpretation of this field is dependent on rule type.

If no `types` are explicitly specified, then a rule matches all types except RRSIG, NS, SOA, NSEC and NSEC3. Types may be specified by name, including "ANY" (ANY matches all types except NSEC and NSEC3, which can never be updated). Note that when an attempt is made to delete all records associated with a name, the rules are checked for each existing record type.

The `ruletype` field has 16 values: `name`, `subdomain`, `wildcard`, `self`, `selfsub`, `selfwild`, `krb5-self`, `ms-self`, `krb5-selfsub`, `ms-selfsub`, `krb5-subdomain`, `ms-subdomain`, `tcp-self`, `6to4-self`, `zonesub`, and `external`.

`name` | Exact-match semantics. This rule matches when the name being updated is identical to the contents of the `name` field.
`subdomain` | This rule matches when the name being updated is a subdomain of, or identical to, the contents of the `name` field.
`zonesub` | This rule is similar to subdomain, except that it matches when the name being updated is a subdomain of the zone in which the `update-policy` statement appears. This obviates the need to type the zone name twice, and enables the use of a standard `update-policy` statement in multiple zones without modification. When this rule is used, the `name` field is omitted.
`wildcard` | The `name` field is subject to DNS wildcard expansion, and this rule matches when the name being updated is a valid expansion of the wildcard.
`self` | This rule matches when the name of the record being updated matches the contents of the `identity` field. The `name` field is ignored. To avoid confusion, it is recommended that this field be set to the same value as the `identity` field or to "." The `self` rule type is most useful when allowing one key per name to update, where the key has the same name as the record to be updated. In this case, the `identity` field can be specified as `*` (an asterisk).
`selfsub` | This rule is similar to `self` except that subdomains of `self` can also be updated.
`selfwild` | This rule is similar to `self` except that only subdomains of `self` can be updated.
`ms-self` | When a client sends an UPDATE using a Windows machine principal (for example, 'machine$@REALM'), this rule allows records with the absolute name of 'machine.REALM' to be updated. The realm to be matched is specified in the `identity` field. The `name` field has no effect on this rule; it should be set to "." as a placeholder. For example, `grant EXAMPLE.COM ms-self . A AAAA` allows any machine with a valid principal in the realm `EXAMPLE.COM` to update its own address records.
`ms-selfsub` | This is similar to `ms-self` except it also allows updates to any subdomain of the name specified in the Windows machine principal, not just to the name itself.
`ms-subdomain` | When a client sends an UPDATE using a Windows machine principal (for example, 'machine$@REALM'), this rule allows any machine in the specified realm to update any record in the zone or in a specified subdomain of the zone. The realm to be matched is specified in the `identity` field. The `name` field specifies the subdomain that may be updated. If set to "." (or any other name at or above the zone apex), any name in the zone can be updated. For example, if `update-policy` for the zone "example.com" includes `grant EXAMPLE.COM ms-subdomain hosts.example.com. A AAAA`, any machine with a valid principal in the realm `EXAMPLE.COM` will be able to update address records at or below "hosts.example.com".
`krb5-self` | When a client sends an UPDATE using a Kerberos machine principal (for example, 'host/machine@REALM'), this rule allows records with the absolute name of 'machine' to be updated provided it has been authenticated by REALM. This is similar but not identical to `ms-self` due to the 'machine' part of the Kerberos principal being an absolute name instead of a unqualified name. The realm to be matched is specified in the `identity` field. The `name` field has no effect on this rule; it should be set to "." as a placeholder. For example, `grant EXAMPLE.COM krb5-self . A AAAA` allows any machine with a valid principal in the realm `EXAMPLE.COM` to update its own address records.
`krb5-selfsub` | This is similar to `krb5-self` except it also allows updates to any subdomain of the name specified in the 'machine' part of the Kerberos principal, not just to the name itself.
`krb5-subdomain` | This rule is identical to `ms-subdomain`, except that it works with Kerberos machine principals (i.e., 'host/machine@REALM') rather than Windows machine principals.
`tcp-self` | This rule allows updates that have been sent via TCP and for which the standard mapping from the client's IP address into the `in-addr.arpa` and `ip6.arpa` namespaces match the name to be updated. The `identity` field must match that name. The `name` field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. It is theoretically possible to spoof these TCP sessions.
`6to4-self` | This allows the name matching a 6to4 IPv6 prefix, as specified in RFC 3056, to be updated by any TCP connection from either the 6to4 network or from the corresponding IPv4 address. This is intended to allow NS or DNAME RRsets to be added to the `ip6.arpa` reverse tree. The `identity` field must match the 6to4 prefix in `ip6.arpa`. The `name` field should be set to ".". Note that, since identity is based on the client's IP address, it is not necessary for update request messages to be signed. In addition, if specified for an `ip6.arpa` name outside of the `2.0.0.2.ip6.arpa` namespace, the corresponding /48 reverse name can be updated. For example, TCP/IPv6 connections from 2001:DB8:ED0C::/48 can update records at `C.0.D.E.8.B.D.0.1.0.0.2.ip6.arpa`. It is theoretically possible to spoof these TCP sessions.
`external` | This rule allows `named` to defer the decision of whether to allow a given update to an external daemon. The method of communicating with the daemon is specified in the `identity` field, the format of which is "`local:``path`", where `path` is the location of a UNIX-domain socket. (Currently, "local" is the only supported mechanism.) Requests to the external daemon are sent over the UNIX-domain socket as datagrams with the following format:
       Protocol version number (4 bytes, network byte order, currently 1)
       Request length (4 bytes, network byte order)
       Signer (null-terminated string)
       Name (null-terminated string)
       TCP source address (null-terminated string)
       Rdata type (null-terminated string)
       Key (null-terminated string)
       TKEY token length (4 bytes, network byte order)
       TKEY token (remainder of packet)
The daemon replies with a four-byte value in network byte order, containing either 0 or 1; 0 indicates that the specified update is not permitted, and 1 indicates that it is.

When multiple views are in use, a zone may be referenced by more than one of them. Often, the views will contain different zones with the same name, allowing different clients to receive different answers for the same queries. At times, however, it is desirable for multiple views to contain identical zones. The `in-view` zone option provides an efficient way to do this: it allows a view to reference a zone that was defined in a previously configured view. Example:

    view internal {
        match-clients { 10/8; };

        zone example.com {
    	type master;
    	file "example-external.db";
        };
    };

    view external {
        match-clients { any; };

        zone example.com {
    	in-view internal;
        };
    };

An `in-view` option cannot refer to a view that is configured later in the configuration file.

A `zone` statement which uses the `in-view` option may not use any other options with the exception of `forward` and `forwarders`. (These options control the behavior of the containing view, rather than changing the zone object itself.)

Zone level acls (e.g. allow-query, allow-transfer) and other configuration details of the zone are all set in the view the referenced zone is defined in. Care need to be taken to ensure that acls are wide enough for all views referencing the zone.

An `in-view` zone cannot be used as a response policy zone.

An `in-view` zone is not intended to reference a `forward` zone.

This section, largely borrowed from RFC 1034, describes the concept of a Resource Record (RR) and explains when each is used. Since the publication of RFC 1034, several new RRs have been identified and implemented in the DNS. These are also included.

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate RRs. The order of RRs in a set is not significant and need not be preserved by name servers, resolvers, or other parts of the DNS. However, sorting of multiple RRs is permitted for optimization purposes, for example, to specify that a particular nearby server be tried first. See the_sortlist_statement and rrset_ordering.

The components of a Resource Record are:

owner name | The domain name where the RR is found.
type | An encoded 16-bit value that specifies the type of the resource record.
TTL | The time-to-live of the RR. This field is a 32-bit integer in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded.
class | An encoded 16-bit value that identifies a protocol family or instance of a protocol.
RDATA | The resource data. The format of the data is type (and sometimes class) specific.

The following are types of valid RRs:

A | A host address. In the IN class, this is a 32-bit IP address. Described in RFC 1035.
AAAA | IPv6 address. Described in RFC 1886.
A6 | IPv6 address. This can be a partial address (a suffix) and an indirection to the name where the rest of the address (the prefix) can be found. Experimental. Described in RFC 2874.
AFSDB | Location of AFS database servers. Experimental. Described in RFC 1183.
AMTRELAY | Automatic Multicast Tunneling Relay discovery record. Work in progress draft-ietf-mboned-driad-amt-discovery.
APL | Address prefix list. Experimental. Described in RFC 3123.
ATMA | ATM Address.
AVC | Application Visibility and Control record.
CAA | Identifies which Certificate Authorities can issue certificates for this domain and what rules they need to follow when doing so. Defined in RFC 6844.
CDNSKEY | Identifies which DNSKEY records should be published as DS records in the parent zone.
CDS | Contains the set of DS records that should be published by the parent zone.
CERT | Holds a digital certificate. Described in RFC 2538.
CNAME | Identifies the canonical name of an alias. Described in RFC 1035.
CSYNC | Child-to-Parent Synchronization in DNS as described in RFC 7477.
DHCID | Is used for identifying which DHCP client is associated with this name. Described in RFC 4701.
DLV | A DNS Look-aside Validation record which contains the records that are used as trust anchors for zones in a DLV namespace. Described in RFC 4431.
DNAME | Replaces the domain name specified with another name to be looked up, effectively aliasing an entire subtree of the domain name space rather than a single record as in the case of the CNAME RR. Described in RFC 2672.
DNSKEY | Stores a public key associated with a signed DNS zone. Described in RFC 4034.
DOA | Implements the Digital Object Architecture over DNS. Experimental.
DS | Stores the hash of a public key associated with a signed DNS zone. Described in RFC 4034.
EID | End Point Identifier.
EUI48 | A 48-bit EUI address. Described in RFC 7043.
EUI64 | A 64-bit EUI address. Described in RFC 7043.
GID | Reserved.
GPOS | Specifies the global position. Superseded by LOC.
HINFO | Identifies the CPU and OS used by a host. Described in RFC 1035.
HIP | Host Identity Protocol Address. Described in RFC 5205.
IPSECKEY | Provides a method for storing IPsec keying material in DNS. Described in RFC 4025.
ISDN | Representation of ISDN addresses. Experimental. Described in RFC 1183.
KEY | Stores a public key associated with a DNS name. Used in original DNSSEC; replaced by DNSKEY in DNSSECbis, but still used with SIG(0). Described in RFCs 2535 and 2931.
KX | Identifies a key exchanger for this DNS name. Described in RFC 2230.
L32 | Holds 32-bit Locator values for Identifier-Locator Network Protocol. Described in RFC 6742.
L64 | Holds 64-bit Locator values for Identifier-Locator Network Protocol. Described in RFC 6742.
LOC | For storing GPS info. Described in RFC 1876. Experimental.
LP | Identifier-Locator Network Protocol. Described in RFC 6742.
MB | Mail Box. Historical.
MD | Mail Destination. Historical.
MF | Mail Forwarder. Historical.
MG | Mail Group. Historical.
MINFO | Mail Information.
MR | Mail Rename. Historical.
MX | Identifies a mail exchange for the domain with a 16-bit preference value (lower is better) followed by the host name of the mail exchange. Described in RFC 974, RFC 1035.
NAPTR | Name authority pointer. Described in RFC 2915.
NID | Holds values for Node Identifiers in Identifier-Locator Network Protocol. Described in RFC 6742.
NINFO | Contains zone status information.
NIMLOC | Nimrod Locator.
NSAP | A network service access point. Described in RFC 1706.
NSAP-PTR | Historical.
NS | The authoritative name server for the domain. Described in RFC 1035.
NSEC | Used in DNSSECbis to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and indicate what RR types are present for an existing name. Described in RFC 4034.
NSEC3 | Used in DNSSECbis to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and indicate what RR types are present for an existing name. NSEC3 differs from NSEC in that it prevents zone enumeration but is more computationally expensive on both the server and the client than NSEC. Described in RFC 5155.
NSEC3PARAM | Used in DNSSECbis to tell the authoritative server which NSEC3 chains are available to use. Described in RFC 5155.
NULL | This is an opaque container.
NXT | Used in DNSSEC to securely indicate that RRs with an owner name in a certain name interval do not exist in a zone and indicate what RR types are present for an existing name. Used in original DNSSEC; replaced by NSEC in DNSSECbis. Described in RFC 2535.
OPENPGPKEY | Used to hold an OPENPGPKEY.
PTR | A pointer to another part of the domain name space. Described in RFC 1035.
PX | Provides mappings between RFC 822 and X.400 addresses. Described in RFC 2163.
RKEY | Resource key.
RP | Information on persons responsible for the domain. Experimental. Described in RFC 1183.
RRSIG | Contains DNSSECbis signature data. Described in RFC 4034.
RT | Route-through binding for hosts that do not have their own direct wide area network addresses. Experimental. Described in RFC 1183.
SIG | Contains DNSSEC signature data. Used in original DNSSEC; replaced by RRSIG in DNSSECbis, but still used for SIG(0). Described in RFCs 2535 and 2931.
SINK | The kitchen sink record.
SMIMEA | The S/MIME Security Certificate Association.
SOA | Identifies the start of a zone of authority. Described in RFC 1035.
SPF | Contains the Sender Policy Framework information for a given email domain. Described in RFC 4408.
SRV | Information about well known network services (replaces WKS). Described in RFC 2782.
SSHFP | Provides a way to securely publish a secure shell key's fingerprint. Described in RFC 4255.
TA | Trust Anchor. Experimental.
TALINK | Trust Anchor Link. Experimental.
TLSA | Transport Layer Security Certificate Association. Described in RFC 6698.
TXT | Text records. Described in RFC 1035.
UID | Reserved.
UINFO | Reserved.
UNSPEC | Reserved. Historical.
URI | Holds a URI. Described in RFC 7553.
WKS | Information about which well known network services, such as SMTP, that a domain supports. Historical.
X25 | Representation of X.25 network addresses. Experimental. Described in RFC 1183.
ZONEMD | Zone Message Digest. Work in progress draft-wessels-dns-zone-digest.

The following classes of resource records are currently valid in the DNS:

IN | The Internet.
CH | Chaosnet, a LAN protocol created at MIT in the mid-1970s. Rarely used for its historical purpose, but reused for BIND's built-in server information zones, e.g., `version.bind`.
HS | Hesiod, an information service developed by MIT's Project Athena. It is used to share information about various systems databases, such as users, groups, printers and so on.

The owner name is often implicit, rather than forming an integral part of the RR. For example, many name servers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL) which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The meaning of the TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; it is also timed out, but by the refreshing policies for the zone. The TTL is assigned by the administrator for the zone where the data originates. While short TTLs can be used to minimize caching, and a zero TTL prohibits caching, the realities of Internet performance suggest that these times should be on the order of days for the typical host. If a change can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency during the change, and then increased back to its former value following the change.

The data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as "pointers" to other data in the DNS.

RRs are represented in binary form in the packets of the DNS protocol, and are usually represented in highly encoded form when stored in a name server or resolver. In the examples provided in RFC 1034, a style similar to that used in master files was employed in order to show the contents of RRs. In this format, most RRs are shown on a single line, although continuation lines are possible using parentheses.

The start of the line gives the owner of the RR. If a line begins with a blank, then the owner is assumed to be the same as that of the previous RR. Blank lines are often included for readability.

Following the owner, we list the TTL, type, and class of the RR. Class and type use the mnemonics defined above, and TTL is an integer before the type field. In order to avoid ambiguity in parsing, type and class mnemonics are disjoint, TTLs are integers, and the type mnemonic is always last. The IN class and TTL values are often omitted from examples in the interests of clarity.

The resource data or RDATA section of the RR are given using knowledge of the typical representation for the data.

For example, we might show the RRs carried in a message as:

`ISI.EDU.` | `MX` | `10 VENERA.ISI.EDU.`
| `MX` | `10 VAXA.ISI.EDU`
`VENERA.ISI.EDU` | `A` | `128.9.0.32`
| `A` | `10.1.0.52`
`VAXA.ISI.EDU` | `A` | `10.2.0.27`
| `A` | `128.9.0.33`

The MX RRs have an RDATA section which consists of a 16-bit number followed by a domain name. The address RRs use a standard IP address format to contain a 32-bit internet address.

The above example shows six RRs, with two RRs at each of three domain names.

Similarly we might see:

`XX.LCS.MIT.EDU.` | `IN A` | `10.0.0.44`
| `CH A` | `MIT.EDU. 2420`

This example shows two addresses for `XX.LCS.MIT.EDU`, each of a different class.

As described above, domain servers store information as a series of resource records, each of which contains a particular piece of information about a given domain name (which is usually, but not always, a host). The simplest way to think of a RR is as a typed pair of data, a domain name matched with a relevant datum, and stored with some additional type information to help systems determine when the RR is relevant.

MX records are used to control delivery of email. The data specified in the record is a priority and a domain name. The priority controls the order in which email delivery is attempted, with the lowest number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority are responding, the mail transport agent will fall back to the next largest priority. Priority numbers do not have any absolute meaning — they are relevant only respective to other MX records for that domain name. The domain name given is the machine to which the mail will be delivered. It must have an associated address record (A or AAAA) — CNAME is not sufficient.

For a given domain, if there is both a CNAME record and an MX record, the MX record is in error, and will be ignored. Instead, the mail will be delivered to the server specified in the MX record pointed to by the CNAME. For example:

`example.com.` | `IN` | `MX` | `10` | `mail.example.com.`
| `IN` | `MX` | `10` | `mail2.example.com.`
| `IN` | `MX` | `20` | `mail.backup.org.`
`mail.example.com.` | `IN` | `A` | `10.0.0.1` |
`mail2.example.com.` | `IN` | `A` | `10.0.0.2` |

Mail delivery will be attempted to `mail.example.com` and `mail2.example.com` (in any order), and if neither of those succeed, delivery to `mail.backup.org` will be attempted.

The time-to-live of the RR field is a 32-bit integer represented in units of seconds, and is primarily used by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be discarded. The following three types of TTL are currently used in a zone file.

SOA | The last field in the SOA is the negative caching TTL. This controls how long other servers will cache no-such-domain (NXDOMAIN) responses from you. The maximum time for negative caching is 3 hours (3h).
$TTL | The $TTL directive at the top of the zone file (before the SOA) gives a default TTL for every RR without a specific TTL set.
RR TTLs | Each RR can have a TTL as the second field in the RR, which will control how long other servers can cache it.

All of these TTLs default to units of seconds, though units can be explicitly specified, for example, `1h30m`.

Reverse name resolution (that is, translation from IP address to name) is achieved by means of the in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most significant order, read left to right. This is the opposite order to the way IP addresses are usually written. Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the machine or, optionally, multiple PTR records if the machine has more than one name. For example, in the example.com domain:

`$ORIGIN` | `2.1.10.in-addr.arpa`
`3` | `IN PTR foo.example.com.`

The `$ORIGIN` lines in the examples are for providing context to the examples only — they do not necessarily appear in the actual usage. They are only used here to indicate that the example is relative to the listed origin.

The Master File Format was initially defined in RFC 1035 and has subsequently been extended. While the Master File Format itself is class independent all records in a Master File must be of the same class.

Master File Directives include `$ORIGIN`, `$INCLUDE`, and `$TTL.`

When used in the label (or name) field, the asperand or at-sign (@) symbol represents the current origin. At the start of the zone file, it is the <`zone_name`> (followed by trailing dot).

Syntax: `$ORIGIN` `domain-name` `comment`

`$ORIGIN` sets the domain name that will be appended to any unqualified records. When a zone is first read in there is an implicit `$ORIGIN` <`zone_name`>`.` (followed by trailing dot). The current `$ORIGIN` is appended to the domain specified in the `$ORIGIN` argument if it is not absolute.

    $ORIGIN example.com.
    WWW     CNAME   MAIN-SERVER

is equivalent to

    WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM.

Syntax: `$INCLUDE` `filename` `origin` `comment`

Read and process the file `filename` as if it were included into the file at this point. If `origin` is specified the file is processed with `$ORIGIN` set to that value, otherwise the current `$ORIGIN` is used.

The origin and the current domain name revert to the values they had prior to the `$INCLUDE` once the file has been read.

RFC 1035 specifies that the current origin should be restored after an `$INCLUDE`, but it is silent on whether the current domain name should also be restored. BIND 9 restores both of them. This could be construed as a deviation from RFC 1035, a feature, or both.

Syntax: `$TTL` `default-ttl` `comment`

Set the default Time To Live (TTL) for subsequent records with undefined TTLs. Valid TTLs are of the range 0-2147483647 seconds.

`$TTL` is defined in RFC 2308.

Syntax: `$GENERATE` `range` `lhs` `ttl` `class` `type` `rhs` `comment`

`$GENERATE` is used to create a series of resource records that only differ from each other by an iterator. `$GENERATE` can be used to easily generate the sets of records required to support sub /24 reverse delegations described in RFC 2317: Classless IN-ADDR.ARPA delegation.

    $ORIGIN 0.0.192.IN-ADDR.ARPA.
    $GENERATE 1-2 @ NS SERVER$.EXAMPLE.
    $GENERATE 1-127 $ CNAME $.0

is equivalent to

    0.0.0.192.IN-ADDR.ARPA. NS SERVER1.EXAMPLE.
    0.0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE.
    1.0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA.
    2.0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA.
    ...
    127.0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA.

Generate a set of A and MX records. Note the MX's right hand side is a quoted string. The quotes will be stripped when the right hand side is processed.

    $ORIGIN EXAMPLE.
    $GENERATE 1-127 HOST-$ A 1.2.3.$
    $GENERATE 1-127 HOST-$ MX "0 ."

is equivalent to

    HOST-1.EXAMPLE.   A  1.2.3.1
    HOST-1.EXAMPLE.   MX 0 .
    HOST-2.EXAMPLE.   A  1.2.3.2
    HOST-2.EXAMPLE.   MX 0 .
    HOST-3.EXAMPLE.   A  1.2.3.3
    HOST-3.EXAMPLE.   MX 0 .
    ...
    HOST-127.EXAMPLE. A  1.2.3.127
    HOST-127.EXAMPLE. MX 0 .

`range` | This can be one of two forms: start-stop or start-stop/step. If the first form is used, then step is set to 1. start, stop and step must be positive integers between 0 and (2^31)-1. start must not be larger than stop.
`lhs` | This describes the owner name of the resource records to be created. Any single `$` (dollar sign) symbols within the `lhs` string are replaced by the iterator value. To get a $ in the output, you need to escape the `$` using a backslash `\`, e.g. `\$`. The `$` may optionally be followed by modifiers which change the offset from the iterator, field width and base. Modifiers are introduced by a `{` (left brace) immediately following the `$` as `${offset[,width[,base]]}`. For example, `${-20,3,d}` subtracts 20 from the current value, prints the result as a decimal in a zero-padded field of width 3. Available output forms are decimal (`d`), octal (`o`), hexadecimal (`x` or `X` for uppercase) and nibble (`n` or `N`\ for uppercase). The default modifier is `${0,0,d}`. If the `lhs` is not absolute, the current `$ORIGIN` is appended to the name. In nibble mode the value will be treated as if it was a reversed hexadecimal string with each hexadecimal digit as a separate label. The width field includes the label separator. For compatibility with earlier versions, `$$` is still recognized as indicating a literal $ in the output.
`ttl` | Specifies the time-to-live of the generated records. If not specified this will be inherited using the normal TTL inheritance rules. `class` and `ttl` can be entered in either order.
`class` | Specifies the class of the generated records. This must match the zone class if it is specified. `class` and `ttl` can be entered in either order.
`type` | Any valid type.
`rhs` | `rhs`, optionally, quoted string.

The `$GENERATE` directive is a BIND extension and not part of the standard zone file format.

In addition to the standard textual format, BIND 9 supports the ability to read or dump to zone files in other formats.

The `raw` format is a binary representation of zone data in a manner similar to that used in zone transfers. Since it does not require parsing text, load time is significantly reduced.

An even faster alternative is the `map` format, which is an image of a BIND 9 in-memory zone database; it is capable of being loaded directly into memory via the `mmap()` function; the zone can begin serving queries almost immediately.

For a primary server, a zone file in `raw` or `map` format is expected to be generated from a textual zone file by the `named-compilezone` command. For a secondary server or for a dynamic zone, it is automatically generated (if this format is specified by the `masterfile-format` option) when `named` dumps the zone contents after zone transfer or when applying prior updates.

If a zone file in a binary format needs manual modification, it first must be converted to a textual form by the `named-compilezone` command. All necessary modification should go to the text file, which should then be converted to the binary form by the `named-compilezone` command again.

Note that `map` format is extremely architecture-specific. A `map` file cannot be used on a system with different pointer size, endianness or data alignment than the system on which it was generated, and should in general be used only inside a single system. While `raw` format uses network byte order and avoids architecture-dependent data alignment so that it is as portable as possible, it is also primarily expected to be used inside the same single system. To export a zone file in either `raw` or `map` format, or make a portable backup of such a file, conversion to `text` format is recommended.

BIND 9 maintains lots of statistics information and provides several interfaces for users to get access to the statistics. The available statistics include all statistics counters that were available in BIND 8 and are meaningful in BIND 9, and other information that is considered useful.

The statistics information is categorized into the following sections.

Incoming Requests | The number of incoming DNS requests for each OPCODE.
Incoming Queries | The number of incoming queries for each RR type.
Outgoing Queries | The number of outgoing queries for each RR type sent from the internal resolver. Maintained per view.
Name Server Statistics | Statistics counters about incoming request processing.
Zone Maintenance Statistics | Statistics counters regarding zone maintenance operations such as zone transfers.
Resolver Statistics | Statistics counters about name resolution performed in the internal resolver. Maintained per view.
Cache DB RRsets | Statistics counters related to cache contents; maintained per view. The "NXDOMAIN" counter is the number of names that have been cached as nonexistent. Counters named for RR types indicate the number of active RRsets for each type in the cache database. If an RR type name is preceded by an exclamation mark (!), it represents the number of records in the cache which indicate that the type does not exist for a particular name (this is also known as "NXRRSET"). If an RR type name is preceded by a hash mark (#), it represents the number of RRsets for this type that are present in the cache but whose TTLs have expired; these RRsets may only be used if stale answers are enabled. If an RR type name is preceded by a tilde (~), it represents the number of RRsets for this type that are present in the cache database but are marked for garbage collection; these RRsets cannot be used.
Socket I/O Statistics | Statistics counters about network related events.

A subset of Name Server Statistics is collected and shown per zone for which the server has the authority when `zone-statistics` is set to `full` (or `yes` for backward compatibility. See the description of `zone-statistics` in options for further details.

These statistics counters are shown with their zone and view names. The view name is omitted when the server is not configured with explicit views.

There are currently two user interfaces to get access to the statistics. One is in the plain text format dumped to the file specified by the `statistics-file` configuration option. The other is remotely accessible via a statistics channel when the `statistics-channels` statement is specified in the configuration file (see statschannels.)

The text format statistics dump begins with a line, like:

`+++ Statistics Dump +++ (973798949)`

The number in parentheses is a standard Unix-style timestamp, measured as seconds since January 1, 1970. Following that line is a set of statistics information, which is categorized as described above. Each section begins with a line, like:

`++ Name Server Statistics ++`

Each section consists of lines, each containing the statistics counter value followed by its textual description. See below for available counters. For brevity, counters that have a value of 0 are not shown in the statistics file.

The statistics dump ends with the line where the number is identical to the number in the beginning line; for example:

`--- Statistics Dump --- (973798949)`

The following tables summarize statistics counters that BIND 9 provides. For each row of the tables, the leftmost column is the abbreviated symbol name of that counter. These symbols are shown in the statistics information accessed via an HTTP statistics channel. The rightmost column gives the description of the counter, which is also shown in the statistics file (but, in this document, possibly with slight modification for better readability). Additional notes may also be provided in this column. When a middle column exists between these two columns, it gives the corresponding counter name of the BIND 8 statistics, if applicable.

Note: BIND statistics counters are signed 64-bit values on all platforms except one: 32-bit Windows, where they are signed 32-bit values. Given that 32-bit values have a vastly smaller range than 64-bit values, BIND statistics counters in 32-bit Windows builds overflow significantly more quickly than on all other platforms.

Symbol | BIND8 Symbol | Description
`Requestv4` | `RQ` | IPv4 requests received. Note: this also counts non query requests.
`Requestv6` | `RQ` | IPv6 requests received. Note: this also counts non query requests.
`ReqEdns0` | `` | Requests with EDNS(0) received.
`ReqBadEDNSVer` | `` | Requests with unsupported EDNS version received.
`ReqTSIG` | `` | Requests with TSIG received.
`ReqSIG0` | `` | Requests with SIG(0) received.
`ReqBadSIG` | `` | Requests with invalid (TSIG or SIG(0)) signature.
`ReqTCP` | `RTCP` | TCP requests received.
`AuthQryRej` | `RUQ` | Authoritative (non recursive) queries rejected.
`RecQryRej` | `RURQ` | Recursive queries rejected.
`XfrRej` | `RUXFR` | Zone transfer requests rejected.
`UpdateRej` | `RUUpd` | Dynamic update requests rejected.
`Response` | `SAns` | Responses sent.
`RespTruncated` | `` | Truncated responses sent.
`RespEDNS0` | `` | Responses with EDNS(0) sent.
`RespTSIG` | `` | Responses with TSIG sent.
`RespSIG0` | `` | Responses with SIG(0) sent.
`QrySuccess` | `` | Queries resulted in a successful answer. This means the query which returns a NOERROR response with at least one answer RR. This corresponds to the `success` counter of previous versions of BIND 9.
`QryAuthAns` | `` | Queries resulted in authoritative answer.
`QryNoauthAns` | `SNaAns` | Queries resulted in non authoritative answer.
`QryReferral` | `` | Queries resulted in referral answer. This corresponds to the `referral` counter of previous versions of BIND 9.
`QryNxrrset` | `` | Queries resulted in NOERROR responses with no data. This corresponds to the `nxrrset` counter of previous versions of BIND 9.
`QrySERVFAIL` | `SFail` | Queries resulted in SERVFAIL.
`QryFORMERR` | `SFErr` | Queries resulted in FORMERR.
`QryNXDOMAIN` | `SNXD` | Queries resulted in NXDOMAIN. This corresponds to the `nxdomain` counter of previous versions of BIND 9.
`QryRecursion` | `RFwdQ` | Queries which caused the server to perform recursion in order to find the final answer. This corresponds to the `recursion` counter of previous versions of BIND 9.
`QryDuplicate` | `RDupQ` | Queries which the server attempted to recurse but discovered an existing query with the same IP address, port, query ID, name, type and class already being processed. This corresponds to the `duplicate` counter of previous versions of BIND 9.
`QryDropped` | `` | Recursive queries for which the server discovered an excessive number of existing recursive queries for the same name, type and class and were subsequently dropped. This is the number of dropped queries due to the reason explained with the `clients-per-query` and `max-clients-per-query` options (see the description about cpq_term.) This corresponds to the `dropped` counter of previous versions of BIND 9.
`QryFailure` | `` | Other query failures. This corresponds to the `failure` counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with the previous versions. Normally a more fine-grained counters such as `AuthQryRej` and `RecQryRej` that would also fall into this counter are provided, and so this counter would not be of much interest in practice.
`QryNXRedir` | `` | Queries resulted in NXDOMAIN that were redirected.
`QryNXRedirRLookup` | `` | Queries resulted in NXDOMAIN that were redirected and resulted in a successful remote lookup.
`XfrReqDone` | `` | Requested zone transfers completed.
`UpdateReqFwd` | `` | Update requests forwarded.
`UpdateRespFwd` | `` | Update responses forwarded.
`UpdateFwdFail` | `` | Dynamic update forward failed.
`UpdateDone` | `` | Dynamic updates completed.
`UpdateFail` | `` | Dynamic updates failed.
`UpdateBadPrereq` | `` | Dynamic updates rejected due to prerequisite failure.
`RateDropped` | `` | Responses dropped by rate limits.
`RateSlipped` | `` | Responses truncated by rate limits.
`RPZRewrites` | `` | Response policy zone rewrites.

Symbol | Description
`NotifyOutv4` | IPv4 notifies sent.
`NotifyOutv6` | IPv6 notifies sent.
`NotifyInv4` | IPv4 notifies received.
`NotifyInv6` | IPv6 notifies received.
`NotifyRej` | Incoming notifies rejected.
`SOAOutv4` | IPv4 SOA queries sent.
`SOAOutv6` | IPv6 SOA queries sent.
`AXFRReqv4` | IPv4 AXFR requested.
`AXFRReqv6` | IPv6 AXFR requested.
`IXFRReqv4` | IPv4 IXFR requested.
`IXFRReqv6` | IPv6 IXFR requested.
`XfrSuccess` | Zone transfer requests succeeded.
`XfrFail` | Zone transfer requests failed.

Symbol | BIND8 Symbol | Description
`Queryv4` | `SFwdQ` | IPv4 queries sent.
`Queryv6` | `SFwdQ` | IPv6 queries sent.
`Responsev4` | `RR` | IPv4 responses received.
`Responsev6` | `RR` | IPv6 responses received.
`NXDOMAIN` | `RNXD` | NXDOMAIN received.
`SERVFAIL` | `RFail` | SERVFAIL received.
`FORMERR` | `RFErr` | FORMERR received.
`OtherError` | `RErr` | Other errors received.
`EDNS0Fail` | `` | EDNS(0) query failures.
`Mismatch` | `RDupR` | Mismatch responses received. The DNS ID, response's source address, and/or the response's source port does not match what was expected. (The port must be 53 or as defined by the `port` option.) This may be an indication of a cache poisoning attempt.
`Truncated` | `` | Truncated responses received.
`Lame` | `RLame` | Lame delegations received.
`Retry` | `SDupQ` | Query retries performed.
`QueryAbort` | `` | Queries aborted due to quota control.
`QuerySockFail` | `` | Failures in opening query sockets. One common reason for such failures is a failure of opening a new socket due to a limitation on file descriptors.
`QueryTimeout` | `` | Query timeouts.
`GlueFetchv4` | `SSysQ` | IPv4 NS address fetches invoked.
`GlueFetchv6` | `SSysQ` | IPv6 NS address fetches invoked.
`GlueFetchv4Fail` | `` | IPv4 NS address fetch failed.
`GlueFetchv6Fail` | `` | IPv6 NS address fetch failed.
`ValAttempt` | `` | DNSSEC validation attempted.
`ValOk` | `` | DNSSEC validation succeeded.
`ValNegOk` | `` | DNSSEC validation on negative information succeeded.
`ValFail` | `` | DNSSEC validation failed.
`QryRTTnn` | `` | Frequency table on round trip times (RTTs) of queries. Each `nn` specifies the corresponding frequency. In the sequence of `nn_1`, `nn_2`, ..., `nn_m`, the value of `nn_i` is the number of queries whose RTTs are between `nn_(i-1)` (inclusive) and `nn_i` (exclusive) milliseconds. For the sake of convenience we define `nn_0` to be 0. The last entry should be represented as `nn_m+`, which means the number of queries whose RTTs are equal to or over `nn_m` milliseconds.

Socket I/O statistics counters are defined per socket types, which are `UDP4` (UDP/IPv4), `UDP6` (UDP/IPv6), `TCP4` (TCP/IPv4), `TCP6` (TCP/IPv6), `Unix` (Unix Domain), and `FDwatch` (sockets opened outside the socket module). In the following table `<TYPE>` represents a socket type. Not all counters are available for all socket types; exceptions are noted in the description field.

Symbol | Description
`<TYPE>Open` | Sockets opened successfully. This counter is not applicable to the `FDwatch` type.
`<TYPE>OpenFail` | Failures of opening sockets. This counter is not applicable to the `FDwatch` type.
`<TYPE>Close` | Sockets closed.
`<TYPE>BindFail` | Failures of binding sockets.
`<TYPE>ConnFail` | Failures of connecting sockets.
`<TYPE>Conn` | Connections established successfully.
`<TYPE>AcceptFail` | Failures of accepting incoming connection requests. This counter is not applicable to the `UDP` and `FDwatch` types.
`<TYPE>Accept` | Incoming connections successfully accepted. This counter is not applicable to the `UDP` and `FDwatch` types.
`<TYPE>SendErr` | Errors in socket send operations. This counter corresponds to `SErr` counter of `BIND` 8.
`<TYPE>RecvErr` | Errors in socket receive operations. This includes errors of send operations on a connected UDP socket notified by an ICMP error message.

Most statistics counters that were available in `BIND` 8 are also supported in `BIND` 9 as shown in the above tables. Here are notes about other counters that do not appear in these tables.

`RFwdR,SFwdR`

- These counters are not supported because `BIND` 9 does not adopt the notion of forwarding as `BIND` 8 did.

`RAXFR`

- This counter is accessible in the Incoming Queries section.

`RIQ`

- This counter is accessible in the Incoming Requests section.

`ROpts`

- This counter is not supported because `BIND` 9 does not care about IP options in the first place.

Access Control Lists (ACLs) are address match lists that you can set up and nickname for future use in `allow-notify`, `allow-query`, `allow-query-on`, `allow-recursion`, `blackhole`, `allow-transfer`, `match-clients`, etc.

Using ACLs allows you to have finer control over who can access your name server, without cluttering up your config files with huge lists of IP addresses.

It is a good idea to use ACLs, and to control access to your server. Limiting access to your server by outside parties can help prevent spoofing and denial of service (DoS) attacks against your server.

ACLs match clients on the basis of up to three characteristics: 1) The client's IP address; 2) the TSIG or SIG(0) key that was used to sign the request, if any; and 3) an address prefix encoded in an EDNS Client Subnet option, if any.

Here is an example of ACLs based on client addresses:

    // Set up an ACL named "bogusnets" that will block
    // RFC1918 space and some reserved space, which is
    // commonly used in spoofing attacks.
    acl bogusnets {
    	0.0.0.0/8;  192.0.2.0/24; 224.0.0.0/3;
    	10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
    };

    // Set up an ACL called our-nets. Replace this with the
    // real IP numbers.
    acl our-nets { x.x.x.x/24; x.x.x.x/21; };
    options {
      ...
      ...
      allow-query { our-nets; };
      allow-recursion { our-nets; };
      ...
      blackhole { bogusnets; };
      ...
    };

    zone "example.com" {
      type master;
      file "m/example.com";
      allow-query { any; };
    };

This allows authoritative queries for "example.com" from any address, but recursive queries only from the networks specified in "our-nets", and no queries at all from the networks specified in "bogusnets".

In addition to network addresses and prefixes, which are matched against the source address of the DNS request, ACLs may include `key` elements, which specify the name of a TSIG or SIG(0) key.

When BIND 9 is built with GeoIP support, ACLs can also be used for geographic access restrictions. This is done by specifying an ACL element of the form: `geoip db `database` `field` `value``

The `field` indicates which field to search for a match. Available fields are "country", "region", "city", "continent", "postal" (postal code), "metro" (metro code), "area" (area code), "tz" (timezone), "isp", "asnum", and "domain".

`value` is the value to search for within the database. A string may be quoted if it contains spaces or other special characters. An "asnum" search for autonomous system number can be specified using the string "ASNNNN" or the integer NNNN. When "country" search is specified with a string is two characters long, then it must be a standard ISO-3166-1 two-letter country code; otherwise it is interpreted as the full name of the country. Similarly, if this is a "region" search and the string is two characters long, then it treated as a standard two-letter state or province abbreviation; otherwise it treated as the full name of the state or province.

The `database` field indicates which GeoIP database to search for a match. In most cases this is unnecessary, because most search fields can only be found in a single database. However, searches for "continent" or "country" can be answered from either the "city" or "country" databases, so for these search types, specifying a `database` will force the query to be answered from that database and no other. If `database` is not specified, then these queries will be answered from the "city", database if it is installed, or the "country" database if it is installed, in that order. Valid database names are "country", "city", "asnum", "isp", and "domain". (If using the legacy GeoIP API, "netspeed" and "org" databases are also available.)

Some example GeoIP ACLs:

    geoip country US;
    geoip country JP;
    geoip db country country Canada;
    geoip region WA;
    geoip city "San Francisco";
    geoip region Oklahoma;
    geoip postal 95062;
    geoip tz "America/Los_Angeles";
    geoip org "Internet Systems Consortium";

ACLs use a "first-match" logic rather than "best-match": if an address prefix matches an ACL element, then that ACL is considered to have matched even if a later element would have matched more specifically. For example, the ACL `{ 10/8; !10.0.0.1; }` would actually match a query from 10.0.0.1, because the first element indicated that the query should be accepted, and the second element is ignored.

When using "nested" ACLs (that is, ACLs included or referenced within other ACLs), a negative match of a nested ACL will the containing ACL to continue looking for matches. This enables complex ACLs to be constructed, in which multiple client characteristics can be checked at the same time. For example, to construct an ACL which allows queries only when it originates from a particular network and only when it is signed with a particular key, use:

    allow-query { !{ !10/8; any; }; key example; };

Within the nested ACL, any address that is not in the 10/8 network prefix will be rejected, and this will terminate processing of the ACL. Any address that is in the 10/8 network prefix will be accepted, but this causes a negative match of the nested ACL, so the containing ACL continues processing. The query will then be accepted if it is signed by the key "example", and rejected otherwise. The ACL, then, will only matches when both conditions are true.

On UNIX servers, it is possible to run BIND in a chrooted environment (using the `chroot()` function) by specifying the `-t` option for `named`. This can help improve system security by placing BIND in a "sandbox", which will limit the damage done if a server is compromised.

Another useful feature in the UNIX version of BIND is the ability to run the daemon as an unprivileged user ( `-u` `user` ). We suggest running as an unprivileged user when using the `chroot` feature.

Here is an example command line to load BIND in a `chroot` sandbox, `/var/named`, and to run `named` `setuid` to user 202:

`/usr/local/sbin/named -u 202 -t /var/named`

In order for a `chroot` environment to work properly in a particular directory (for example, `/var/named`), you will need to set up an environment that includes everything BIND needs to run. From BIND's point of view, `/var/named` is the root of the filesystem. You will need to adjust the values of options like `directory` and `pid-file` to account for this.

Unlike with earlier versions of BIND, you typically will not need to compile `named` statically nor install shared libraries under the new root. However, depending on your operating system, you may need to set up things like `/dev/zero`, `/dev/random`, `/dev/log`, and `/etc/localtime`.

Prior to running the `named` daemon, use the `touch` utility (to change file access and modification times) or the `chown` utility (to set the user id and/or group id) on files to which you want BIND to write.

If the `named` daemon is running as an unprivileged user, it will not be able to bind to new restricted ports if the server is reloaded.

Access to the dynamic update facility should be strictly limited. In earlier versions of BIND, the only way to do this was based on the IP address of the host requesting the update, by listing an IP address or network prefix in the `allow-update` zone option. This method is insecure since the source address of the update UDP packet is easily forged. Also note that if the IP addresses allowed by the `allow-update` option include the address of a slave server which performs forwarding of dynamic updates, the master can be trivially attacked by sending the update to the slave, which will forward it to the master with its own source IP address causing the master to approve it without question.

For these reasons, we strongly recommend that updates be cryptographically authenticated by means of transaction signatures (TSIG). That is, the `allow-update` option should list only TSIG key names, not IP addresses or network prefixes. Alternatively, the new `update-policy` option can be used.

Some sites choose to keep all dynamically-updated DNS data in a subdomain and delegate that subdomain to a separate zone. This way, the top-level zone containing critical data such as the IP addresses of public web and mail servers need not allow dynamic update at all.

The best solution to solving installation and configuration issues is to take preventative measures by setting up logging files beforehand. The log files provide a source of hints and information that can be used to figure out what went wrong and how to fix the problem.

EDNS (Extended DNS) is a standard that was first specified in 1999. It is required for DNSSEC validation, DNS COOKIE options, and other features. There are broken and outdated DNS servers and firewalls still in use which misbehave when queried with EDNS; for example, they may drop EDNS queries rather than replying with FORMERR. BIND and other recursive name servers have traditionally employed workarounds in this situation, retrying queries in different ways and eventually falling back to plain DNS queries without EDNS.

Such workarounds cause unnecessary resolution delays, increase code complexity, and prevent deployment of new DNS features. As of February 2019, all major DNS software vendors have agreed to remove these workarounds; see https://dnsflagday.net for further details. This change was implemented in BIND as of release 9.14.0.

As a result, some domains may be non-resolvable without manual intervention. In these cases, resolution can be restored by adding `server` clauses for the offending servers, specifying `edns no` or `send-cookie no`, depending on the specific noncompliance.

To determine which `server` clause to use, run the following commands to send queries to the authoritative servers for the broken domain:

    	    dig soa <zone> @<server> +dnssec
    	    dig soa <zone> @<server> +dnssec +nocookie
    	    dig soa <zone> @<server> +noedns

If the first command fails but the second succeeds, the server most likely needs `send-cookie no`. If the first two fail but the third succeeds, then the server needs EDNS to be fully disabled with `edns no`.

Please contact the administrators of noncompliant domains and encourage them to upgrade their broken DNS servers.

Zone serial numbers are just numbers — they aren't date related. A lot of people set them to a number that represents a date, usually of the form YYYYMMDDRR. Occasionally they will make a mistake and set them to a "date in the future" then try to correct them by setting them to the "current date". This causes problems because serial numbers are used to indicate that a zone has been updated. If the serial number on the slave server is lower than the serial number on the master, the slave server will attempt to update its copy of the zone.

Setting the serial number to a lower number on the master server than the slave server means that the slave will not perform updates to its copy of the zone.

The solution to this is to add 2147483647 (2^31-1) to the number, reload the zone and make sure all slaves have updated to the new zone serial number, then reset the number to what you want it to be, and reload the zone again.

The Internet Systems Consortium (ISC) offers a wide range of support and service agreements for BIND and DHCP servers. Four levels of premium support are available and each level includes support for all ISC programs, significant discounts on products and training, and a recognized priority on bug fixes and non-funded feature requests. In addition, ISC offers a standard support agreement package which includes services ranging from bug fix announcements to remote support. It also includes training in BIND and DHCP.

To discuss arrangements for support, contact info@isc.org or visit the ISC web page at http://www.isc.org/services/support/ to read more.

Although the "official" beginning of the Domain Name System occurred in 1984 with the publication of RFC 920, the core of the new system was described in 1983 in RFCs 882 and 883. From 1984 to 1987, the ARPAnet (the precursor to today's Internet) became a testbed of experimentation for developing the new naming/addressing scheme in a rapidly expanding, operational network environment. New RFCs were written and published in 1987 that modified the original documents to incorporate improvements based on the working model. RFC 1034, "Domain Names-Concepts and Facilities", and RFC 1035, "Domain Names-Implementation and Specification" were published and became the standards upon which all DNS implementations are built.

The first working domain name server, called "Jeeves", was written in 1983-84 by Paul Mockapetris for operation on DEC Tops-20 machines located at the University of Southern California's Information Sciences Institute (USC-ISI) and SRI International's Network Information Center (SRI-NIC). A DNS server for Unix machines, the Berkeley Internet Name Domain (BIND) package, was written soon after by a group of graduate students at the University of California at Berkeley under a grant from the US Defense Advanced Research Projects Administration (DARPA).

Versions of BIND through 4.8.3 were maintained by the Computer Systems Research Group (CSRG) at UC Berkeley. Douglas Terry, Mark Painter, David Riggle and Songnian Zhou made up the initial BIND project team. After that, additional work on the software package was done by Ralph Campbell. Kevin Dunlap, a Digital Equipment Corporation employee on loan to the CSRG, worked on BIND for 2 years, from 1985 to 1987. Many other people also contributed to BIND development during that time: Doug Kingston, Craig Partridge, Smoot Carl-Mitchell, Mike Muuss, Jim Bloom and Mike Schwartz. BIND maintenance was subsequently handled by Mike Karels and Øivind Kure.

BIND versions 4.9 and 4.9.1 were released by Digital Equipment Corporation (now Compaq Computer Corporation). Paul Vixie, then a DEC employee, became BIND's primary caretaker. He was assisted by Phil Almquist, Robert Elz, Alan Barrett, Paul Albitz, Bryan Beecher, Andrew Partan, Andy Cherenson, Tom Limoncelli, Berthold Paffrath, Fuat Baran, Anant Kumar, Art Harkin, Win Treese, Don Lewis, Christophe Wolfhugel, and others.

In 1994, BIND version 4.9.2 was sponsored by Vixie Enterprises. Paul Vixie became BIND's principal architect/programmer.

BIND versions from 4.9.3 onward have been developed and maintained by the Internet Systems Consortium and its predecessor, the Internet Software Consortium, with support being provided by ISC's sponsors.

As co-architects/programmers, Bob Halley and Paul Vixie released the first production-ready version of BIND version 8 in May 1997.

BIND version 9 was released in September 2000 and is a major rewrite of nearly all aspects of the underlying BIND architecture.

BIND versions 4 and 8 are officially deprecated. No additional development is done on BIND version 4 or BIND version 8.

BIND development work is made possible today by the sponsorship of several corporations, and by the tireless work efforts of numerous individuals.

IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces which were introduced in the DNS to facilitate scalable Internet routing. There are three types of addresses: Unicast, an identifier for a single interface; Anycast, an identifier for a set of interfaces; and Multicast, an identifier for a set of interfaces. Here we describe the global Unicast address scheme. For more information, see RFC 3587, "Global Unicast Address Format."

IPv6 unicast addresses consist of a global routing prefix, a subnet identifier, and an interface identifier.

The global routing prefix is provided by the upstream provider or ISP, and (roughly) corresponds to the IPv4 network section of the address range. The subnet identifier is for local subnetting, much the same as subnetting an IPv4 /16 network into /24 subnets. The interface identifier is the address of an individual interface on a given network; in IPv6, addresses belong to interfaces rather than to machines.

The subnetting capability of IPv6 is much more flexible than that of IPv4: subnetting can be carried out on bit boundaries, in much the same way as Classless InterDomain Routing (CIDR), and the DNS PTR representation ("nibble" format) makes setting up reverse zones easier.

The Interface Identifier must be unique on the local link, and is usually generated automatically by the IPv6 implementation, although it is usually possible to override the default setting if necessary. A typical IPv6 address might look like: `2001:db8:201:9:a00:20ff:fe81:2b32`

IPv6 address specifications often contain long strings of zeros, so the architects have included a shorthand for specifying them. The double colon (`::') indicates the longest possible string of zeros that can fit, and can be used only once in an address.

Specification documents for the Internet protocol suite, including the DNS, are published as part of the Request for Comments (RFCs) series of technical notes. The standards themselves are defined by the Internet Engineering Task Force (IETF) and the Internet Engineering Steering Group (IESG). RFCs can be obtained online via FTP at:

ftp://www.isi.edu/in-notes/RFC`xxxx`.txt

(where `xxxx` is the number of the RFC). RFCs are also available via the Web at:

http://www.ietf.org/rfc/.

RFC974 Partridge C. Mail Routing and the Domain System

January 1986

RFC1034 Mockapetris P.V. Domain Names — Concepts and Facilities

November 1987

RFC1035 Mockapetris P. V. Domain Names — Implementation and Specification

November 1987

RFC2181 Elz R., R. Bush Clarifications to the DNS Specification

July 1997

RFC2308 Andrews M. Negative Caching of DNS Queries

March 1998

RFC1995 Ohta M. Incremental Zone Transfer in DNS

August 1996

RFC1996 Vixie P. A Mechanism for Prompt Notification of Zone Changes

August 1996

RFC2136
Dynamic Updates in the Domain Name System

April 1997

RFC2671
Extension Mechanisms for DNS (EDNS0)

August 1997

RFC2672
Non-Terminal DNS Name Redirection

August 1999

RFC2845
Secret Key Transaction Authentication for DNS (TSIG)

May 2000

RFC2930
Secret Key Establishment for DNS (TKEY RR)

September 2000

RFC2931
DNS Request and Transaction Signatures (SIG(0)s)

September 2000

RFC3007
Secure Domain Name System (DNS) Dynamic Update

November 2000

RFC3645
Generic Security Service Algorithm for Secret Key Transaction Authentication for DNS (GSS-TSIG)

October 2003

RFC3225
Indicating Resolver Support of DNSSEC

December 2001

RFC3833
Threat Analysis of the Domain Name System (DNS)

August 2004

RFC4033
DNS Security Introduction and Requirements

March 2005

RFC4034
Resource Records for the DNS Security Extensions

March 2005

RFC4035
Protocol Modifications for the DNS Security Extensions

March 2005

RFC1535 Gavron E. A Security Problem and Proposed Correction With Widely Deployed DNS Software

October 1993

RFC1536
Common DNS Implementation Errors and Suggested Fixes

October 1993

RFC1982
Serial Number Arithmetic

August 1996

RFC4074
Common Misbehaviour Against DNS Queries for IPv6 Addresses

May 2005

RFC1183
New DNS RR Definitions

October 1990

RFC1706
DNS NSAP Resource Records

October 1994

RFC2168
Resolution of Uniform Resource Identifiers using the Domain Name System

June 1997

RFC1876
A Means for Expressing Location Information in the Domain Name System

January 1996

RFC2052
A DNS RR for Specifying the Location of Services

October 1996

RFC2163 Allocchio A. Using the Internet DNS to Distribute MIXER Conformant Global Address Mapping

January 1998

RFC2230 Atkinson R. Key Exchange Delegation Record for the DNS

October 1997

RFC2536 Eastlake D.3rd DSA KEYs and SIGs in the Domain Name System (DNS)

March 1999

RFC2537 Eastlake D.3rd RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)

March 1999

RFC2538
Storing Certificates in the Domain Name System (DNS)

March 1999

RFC2539
Storage of Diffie-Hellman Keys in the Domain Name System (DNS)

March 1999

RFC2540
Detached Domain Name System (DNS) Information

March 1999

RFC2782 Gulbrandsen A. Vixie P. Esibov L. A DNS RR for specifying the location of services (DNS SRV)

February 2000

RFC2915 Mealling M. Daniel R. The Naming Authority Pointer (NAPTR) DNS Resource Record

September 2000

RFC3110 Eastlake D.3rd RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)

May 2001

RFC3123 Koch P. A DNS RR Type for Lists of Address Prefixes (APL RR)

June 2001

RFC3596
DNS Extensions to support IP version 6

October 2003

RFC3597 Gustafsson A. Handling of Unknown DNS Resource Record (RR) Types

September 2003

RFC1101 Mockapetris P. V. DNS Encoding of Network Names and Other Types

April 1989

RFC1123 Braden R. Requirements for Internet Hosts - Application and Support

October 1989

RFC1591 Postel J. Domain Name System Structure and Delegation

March 1994

RFC2317
Classless IN-ADDR.ARPA Delegation

March 1998

RFC2826
IAB Technical Comment on the Unique DNS Root

May 2000

RFC2929
Domain Name System (DNS) IANA Considerations

September 2000

RFC1033 Lottor M. Domain administrators operations guide

November 1987

RFC1537 Beertema P. Common DNS Data File Configuration Errors

October 1993

RFC1912 Barr D. Common DNS Operational and Configuration Errors

February 1996

RFC2010
Operational Criteria for Root Name Servers

October 1996

RFC2219
Use of DNS Aliases for Network Services

October 1997

RFC2825
A Tangled Web: Issues of I18N, Domain Names, and the Other Internet protocols

May 2000

RFC3490
Internationalizing Domain Names in Applications (IDNA)

March 2003

RFC3491
Nameprep: A Stringprep Profile for Internationalized Domain Names

March 2003

RFC3492
Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)

March 2003

Note: the following list of RFCs, although DNS-related, are not concerned with implementing software.

RFC1464 Rosenbaum R. Using the Domain Name System To Store Arbitrary String Attributes

May 1993

RFC1713 Romao A. Tools for DNS Debugging

November 1994

RFC1794 Brisco T. DNS Support for Load Balancing

April 1995

RFC2240 Vaughan O. A Legal Basis for Domain Name Allocation

November 1997

RFC2345
Domain Names and Company Name Retrieval

May 1998

RFC2352 Vaughan O. A Convention For Using Legal Names as Domain Names

May 1998

RFC3071
Reflections on the DNS, RFC 1591, and Categories of Domains

February 2001

RFC3258
Distributing Authoritative Name Servers via Shared Unicast Addresses

April 2002

RFC3901
DNS IPv6 Transport Operational Guidelines

September 2004

RFC1712
DNS Encoding of Geographical Location

November 1994

RFC2673
Binary Labels in the Domain Name System

August 1999

RFC2874
DNS Extensions to Support IPv6 Address Aggregation and Renumbering

July 2000

Most of these have been consolidated into RFC4033, RFC4034 and RFC4035 which collectively describe DNSSECbis.

RFC2065
Domain Name System Security Extensions

January 1997

RFC2137 Eastlake 3rd D. Secure Domain Name System Dynamic Update

April 1997

RFC2535
Domain Name System Security Extensions

March 1999

RFC3008
Domain Name System Security (DNSSEC) Signing Authority

November 2000

RFC3090
DNS Security Extension Clarification on Zone Status

March 2001

RFC3445
Limiting the Scope of the KEY Resource Record (RR)

December 2002

RFC3655
Redefinition of DNS Authenticated Data (AD) bit

November 2003

RFC3658
Delegation Signer (DS) Resource Record (RR)

December 2003

RFC3755
Legacy Resolver Compatibility for Delegation Signer (DS)

May 2004

RFC3757
Domain Name System KEY (DNSKEY) Resource Record (RR) Secure Entry Point (SEP) Flag

April 2004

RFC3845
DNS Security (DNSSEC) NextSECure (NSEC) RDATA Format

August 2004

Internet Drafts (IDs) are rough-draft working documents of the Internet Engineering Task Force. They are, in essence, RFCs in the preliminary stages of development. Implementors are cautioned not to regard IDs as archival, and they should not be quoted or cited in any formal documents unless accompanied by the disclaimer that they are "works in progress." IDs have a lifespan of six months after which they are deleted unless updated by their authors.

DNS and BIND

1998 Sebastopol, CA: O'Reilly and Associates