18.9. 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 16).

18.9.1. Basic Setup

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.

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 data directory allows group read access then certificate files may need to be located outside of the data directory in order to conform to the security requirements outlined above. Generally, group access is enabled to allow an unprivileged user to backup the database, and in that case the backup software will not be able to read the certificate files and will likely error.

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 by default disables the ability to change the server’s SSL configuration without a server restart, but see ssl_passphrase_command_supports_reload. 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 “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 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.

18.9.2. OpenSSL Configuration

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.

18.9.3. Using Client Certificates

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=verify-ca or clientcert=verify-full 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.)

For a hostssl entry with clientcert=verify-ca, the server will verify that the client’s certificate is signed by one of the trusted certificate authorities. If clientcert=verify-full is specified, the server will not only verify the certificate chain, but it will also check whether the username or its mapping matches the cn (Common Name) of the provided certificate. Note that certificate chain validation is always ensured when the cert authentication method is used (see Section 20.12).

Intermediate certificates that chain up to existing root certificates can also appear in the ssl_ca_file file 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. (See http://h41379.www4.hpe.com/doc/83final/ba554_90007/ch04s02.html for diagrams showing SSL certificate usage.)

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 no-verify, 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.

There are two approaches to enforce that users provide a certificate during login.

The first approach makes use of the cert authentication method for hostssl entries in pg_hba.conf, such that the certificate itself is used for authentication while also providing ssl connection security. See Section 20.12 for details. (It is not necessary to specify any clientcert options explicitly when using the cert authentication method.) In this case, the cn (Common Name) provided in the certificate is checked against the user name or an applicable mapping.

The second approach combines any authentication method for hostssl entries with the verification of client certificates by setting the clientcert authentication option to verify-ca or verify-full. The former option only enforces that the certificate is valid, while the latter also ensures that the cn (Common Name) in the certificate matches the user name or an applicable mapping.

18.9.4. SSL Server File Usage

Table 18.2 summarizes the files that are relevant to the SSL setup on the server. (The shown file names are default names. The locally configured names could be different.)

Table 18.2. 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 trusted certificate authorities checks that client certificate is signed by a trusted certificate authority
ssl_crl_file certificates revoked by certificate authorities client certificate must not be on this list

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.

18.9.5. Creating Certificates

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:

  1. openssl req -new -x509 -days 365 -nodes -text -out server.crt \
  2.   -keyout server.key -subj "/CN=dbhost.yourdomain.com"

Then do:

  1. 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.

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:

  1. openssl req -new -nodes -text -out root.csr \
  2.   -keyout root.key -subj "/CN=root.yourdomain.com"
  3. chmod og-rwx root.key

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

  1. openssl x509 -req -in root.csr -text -days 3650 \
  2.   -extfile /etc/ssl/openssl.cnf -extensions v3_ca \
  3.   -signkey root.key -out root.crt

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

  1. openssl req -new -nodes -text -out server.csr \
  2.   -keyout server.key -subj "/CN=dbhost.yourdomain.com"
  3. chmod og-rwx server.key
  5. openssl x509 -req -in server.csr -text -days 365 \
  6.   -CA root.crt -CAkey root.key -CAcreateserial \
  7.   -out server.crt

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:

  1. # root
  2. openssl req -new -nodes -text -out root.csr \
  3.   -keyout root.key -subj "/CN=root.yourdomain.com"
  4. chmod og-rwx root.key
  5. openssl x509 -req -in root.csr -text -days 3650 \
  6.   -extfile /etc/ssl/openssl.cnf -extensions v3_ca \
  7.   -signkey root.key -out root.crt
  9. # intermediate
  10. openssl req -new -nodes -text -out intermediate.csr \
  11.   -keyout intermediate.key -subj "/CN=intermediate.yourdomain.com"
  12. chmod og-rwx intermediate.key
  13. openssl x509 -req -in intermediate.csr -text -days 1825 \
  14.   -extfile /etc/ssl/openssl.cnf -extensions v3_ca \
  15.   -CA root.crt -CAkey root.key -CAcreateserial \
  16.   -out intermediate.crt
  18. # leaf
  19. openssl req -new -nodes -text -out server.csr \
  20.   -keyout server.key -subj "/CN=dbhost.yourdomain.com"
  21. chmod og-rwx server.key
  22. openssl x509 -req -in server.csr -text -days 365 \
  23.   -CA intermediate.crt -CAkey intermediate.key -CAcreateserial \
  24.   -out server.crt

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.