Secure TCP/IP Connections with SSL

PostgreSQL 9.4.1 Documentation
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# 17.9. Secure TCP/IP Connections with SSL

PostgreSQL has native support for using SSL connections to encrypt client/server communications for increased security. This requires that OpenSSL is installed on both client and server systems and that support in PostgreSQL is enabled at build time (see Chapter 15).

With SSL support compiled in, the PostgreSQL server can be started with SSL enabled by setting the parameter ssl to `on` in `postgresql.conf`. The server will listen for both normal and SSL connections on the same TCP port, and will negotiate with any connecting client on whether to use SSL. By default, this is at the client's option; see Section 19.1 about how to set up the server to require use of SSL for some or all connections.

PostgreSQL reads the system-wide OpenSSL configuration file. By default, this file is named `openssl.cnf` and is located in the directory reported by `openssl version -d`. This default can be overridden by setting environment variable `OPENSSL_CONF` to the name of the desired configuration file.

OpenSSL supports a wide range of ciphers and authentication algorithms, of varying strength. While a list of ciphers can be specified in the OpenSSL configuration file, you can specify ciphers specifically for use by the database server by modifying ssl_ciphers in `postgresql.conf`.

> Note: It is possible to have authentication without encryption overhead by using `NULL-SHA` or `NULL-MD5` ciphers. However, a man-in-the-middle could read and pass communications between client and server. Also, encryption overhead is minimal compared to the overhead of authentication. For these reasons NULL ciphers are not recommended.

To start in SSL mode, files containing the server certificate and private key must exist. By default, these files are expected to be named `server.crt` and `server.key`, respectively, in the server's data directory, but other names and locations can be specified using the configuration parameters ssl_cert_file and ssl_key_file. On Unix systems, the permissions on `server.key` must disallow any access to world or group; achieve this by the command `chmod 0600 server.key`. If the private key is protected with a passphrase, the server will prompt for the passphrase and will not start until it has been entered.

In some cases, the server certificate might be signed by an "intermediate" certificate authority, rather than one that is directly trusted by clients. To use such a certificate, append the certificate of the signing authority to the `server.crt` file, then its parent authority's certificate, and so on up to a certificate authority, "root" or "intermediate", that is trusted by clients, i.e. signed by a certificate in the clients' `root.crt` files.

## 17.9.1. Using Client Certificates

To require the client to supply a trusted certificate, place certificates of the certificate authorities (CAs) you trust in the file `root.crt` in the data directory, set the parameter ssl_ca_file in `postgresql.conf` to `root.crt`, and set the `clientcert` parameter to 1 on the appropriate `hostssl` line(s) in `pg_hba.conf`. A certificate will then be requested from the client during SSL connection startup. (See Section 31.18 for a description of how to set up certificates on the client.) The server will verify that the client's certificate is signed by one of the trusted certificate authorities. If intermediate CAs appear in `root.crt`, the file must also contain certificate chains to their root CAs. Certificate Revocation List (CRL) entries are also checked if the parameter ssl_crl_file is set. (See http://h71000.www7.hp.com/DOC/83final/BA554_90007/ch04s02.html for diagrams showing SSL certificate usage.)

The `clientcert` option in `pg_hba.conf` is available for all authentication methods, but only for rows specified as `hostssl`. When `clientcert` is not specified or is set to 0, the server will still verify presented client certificates against its CA list, if one is configured, — but it will not insist that a client certificate be presented.

Note that the server's `root.crt` lists the top-level CAs that are considered trusted for signing client certificates. In principle it need not list the CA that signed the server's certificate, though in most cases that CA would also be trusted for client certificates.

If you are setting up client certificates, you may wish to use the `cert` authentication method, so that the certificates control user authentication as well as providing connection security. See Section 19.3.9 for details.

## 17.9.2. SSL Server File Usage

Table 17-2 summarizes the files that are relevant to the SSL setup on the server. (The shown file names are default or typical names. The locally configured names could be different.)

Table 17-2. SSL Server File Usage

File | Contents | Effect
--- | --- | ---
ssl_cert_file (`$PGDATA/server.crt`) | server certificate | sent to client to indicate server's identity
ssl_key_file (`$PGDATA/server.key`) | server private key | proves server certificate was sent by the owner; does not indicate certificate owner is trustworthy
ssl_ca_file (`$PGDATA/root.crt`) | trusted certificate authorities | checks that client certificate is signed by a trusted certificate authority
ssl_crl_file (`$PGDATA/root.crl`) | certificates revoked by certificate authorities | client certificate must not be on this list

The files `server.key`, `server.crt`, `root.crt`, and `root.crl` (or their configured alternative names) are only examined during server start; so you must restart the server for changes in them to take effect.

## 17.9.3. Creating a Self-signed Certificate

To create a quick self-signed certificate for the server, use the following OpenSSL command:

    openssl req -new -text -out server.req

Fill out the information that openssl asks for. Make sure you enter the local host name as "Common Name"; the challenge password can be left blank. The program will generate a key that is passphrase protected; it will not accept a passphrase that is less than four characters long. To remove the passphrase (as you must if you want automatic start-up of the server), run the commands:

    openssl rsa -in privkey.pem -out server.key
    rm privkey.pem

Enter the old passphrase to unlock the existing key. Now do:

    openssl req -x509 -in server.req -text -key server.key -out server.crt

to turn the certificate into a self-signed certificate and to copy the key and certificate to where the server will look for them. Finally do:

    chmod og-rwx server.key

because the server will reject the file if its permissions are more liberal than this. For more details on how to create your server private key and certificate, refer to the OpenSSL documentation.

A self-signed certificate can be used for testing, but a certificate signed by a certificate authority (CA) (either one of the global CAs or a local one) should be used in production so that clients can verify the server's identity. If all the clients are local to the organization, using a local CA is recommended.

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