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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, the files server.crt and server.key must exist in the server's data directory. These files should contain the server certificate and private key, respectively. 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 "root" authority that is trusted by the clients. The root certificate should be included in every case where server.crt contains more than one certificate.

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, 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 31.17 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. Certificate Revocation List (CRL) entries are also checked if the file root.crl exists. (See 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 root.crt if that file exists — but it will not insist that a client certificate be presented.

Note that 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 19.3.10 for details.

17.9.2. SSL Server File Usage

Table 17-3 summarizes the files that are relevant to the SSL setup on the server.

Table 17-3. SSL Server File Usage

File Contents Effect
$PGDATA/server.crt server certificate sent to client to indicate server's identity
$PGDATA/server.key server private key proves server certificate was sent by the owner; does not indicate certificate owner is trustworthy
$PGDATA/root.crt trusted certificate authorities checks that client certificate is signed by a trusted certificate authority
$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 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.