## 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.

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 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.

### 17.9.1. 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=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 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.

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` 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.

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 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:

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

Then 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.

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:

    openssl req -new -nodes -text -out root.csr \
      -keyout root.key -subj "/CN=root.yourdomain.com"
    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):

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

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

    openssl req -new -nodes -text -out server.csr \
      -keyout server.key -subj "/CN=dbhost.yourdomain.com"
    chmod og-rwx server.key

    openssl x509 -req -in server.csr -text -days 365 \
      -CA root.crt -CAkey root.key -CAcreateserial \
      -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:

    # root
    openssl req -new -nodes -text -out root.csr \
      -keyout root.key -subj "/CN=root.yourdomain.com"
    chmod og-rwx root.key
    openssl x509 -req -in root.csr -text -days 3650 \
      -extfile /etc/ssl/openssl.cnf -extensions v3_ca \
      -signkey root.key -out root.crt

    # intermediate
    openssl req -new -nodes -text -out intermediate.csr \
      -keyout intermediate.key -subj "/CN=intermediate.yourdomain.com"
    chmod og-rwx intermediate.key
    openssl x509 -req -in intermediate.csr -text -days 1825 \
      -extfile /etc/ssl/openssl.cnf -extensions v3_ca \
      -CA root.crt -CAkey root.key -CAcreateserial \
      -out intermediate.crt

    # leaf
    openssl req -new -nodes -text -out server.csr \
      -keyout server.key -subj "/CN=dbhost.yourdomain.com"
    chmod og-rwx server.key
    openssl x509 -req -in server.csr -text -days 365 \
      -CA intermediate.crt -CAkey intermediate.key -CAcreateserial \
      -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.