PostgreSQL

18.9.1. Using Client Certificates

18.9.2. SSL Server File Usage

18.9.3. Creating Certificates

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 16).

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 20.1 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.

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. Alternatively, the file can be owned by root and have group read access (that is, 0640 permissions). That setup is intended for installations where certificate and key files are managed by the operating system. The user under which the PostgreSQL server runs should then be made a member of the group that has access to those certificate and key files.

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. Using a passphrase also disables the ability to change the server’s SSL configuration without a server restart. Furthermore, passphrase-protected private keys cannot be used at all on Windows.

The first certificate in server.crt must be the server’s certificate because it must match the server’s private key. The certificates of “[.quote]#intermediate”# certificate authorities can also be appended to the file. Doing this avoids the necessity of storing intermediate certificates on clients, assuming the root and intermediate certificates were created with v3_ca extensions. (This sets the certificate’s basic constraint of CA to true.) This allows easier expiration of intermediate certificates.

It is not necessary to add the root certificate to server.crt. Instead, clients must have the root certificate of the server’s certificate chain.

To require the client to supply a trusted certificate, place certificates of the root certificate authorities (CAs) you trust in a file in the data directory, set the parameter ssl_ca_file in postgresql.conf to the new file name, and add the authentication option clientcert=1 to the appropriate hostssl line(s) in pg_hba.conf. A certificate will then be requested from the client during SSL connection startup. (See Section 33.18 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.

Intermediate certificates that chain up to existing root certificates can also appear in the file root.crt if you wish to avoid storing them on clients (assuming the root and intermediate certificates were created with v3_ca extensions). Certificate Revocation List (CRL) entries are also checked if the parameter ssl_crl_file is set.

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

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 20.3.9 for details. (It is not necessary to specify clientcert=1 explicitly when using the cert authentication method.)

Table 18.2 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 18.2. SSL Server File Usage

The server reads these files at server start and whenever the server configuration is reloaded. On Windows systems, they are also re-read whenever a new backend process is spawned for a new client connection.

If an error in these files is detected at server start, the server will refuse to start. But if an error is detected during a configuration reload, the files are ignored and the old SSL configuration continues to be used. On Windows systems, if an error in these files is detected at backend start, that backend will be unable to establish an SSL connection. In all these cases, the error condition is reported in the server log.

To create a simple self-signed certificate for the server, valid for 365 days, use the following OpenSSL command, replacing `dbhost.yourdomain.com` with the server’s host name:

Then do:

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.

While a self-signed certificate can be used for testing, a certificate signed by a certificate authority (CA) (usually an enterprise-wide root CA) should be used in production.

To create a server certificate whose identity can be validated by clients, first create a certificate signing request (CSR) and a public/private key file:

Then, sign the request with the key to create a root certificate authority (using the default OpenSSL configuration file location on Linux):

Finally, create a server certificate signed by the new root certificate authority:

server.crt and server.key should be stored on the server, and root.crt should be stored on the client so the client can verify that the server’s leaf certificate was signed by its trusted root certificate. root.key should be stored offline for use in creating future certificates.

It is also possible to create a chain of trust that includes intermediate certificates:

server.crt and intermediate.crt should be concatenated into a certificate file bundle and stored on the server. server.key should also be stored on the server. root.crt should be stored on the client so the client can verify that the server’s leaf certificate was signed by a chain of certificates linked to its trusted root certificate. root.key and intermediate.key should be stored offline for use in creating future certificates.

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