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18.9.1. Basic Setup

18.9.2. OpenSSL Configuration

18.9.3. Using Client Certificates

18.9.4. SSL Server File Usage

18.9.5. Creating Certificates

WithSSL support compiled in, thePostgreSQL server can be started with support for encrypted connections usingTLS protocols enabled by setting the parameterssl toon inpostgresql.conf. The server will listen for both normal andSSL connections on the same TCP port, and will negotiate with any connecting client on whether to useSSL. By default, this is at the client's option; seeSection 20.1 about how to set up the server to require use ofSSL for some or all connections.

To start inSSL mode, files containing the server certificate and private key must exist. By default, these files are expected to be namedserver.crt andserver.key, respectively, in the server's data directory, but other names and locations can be specified using the configuration parametersssl_cert_file andssl_key_file.

On Unix systems, the permissions onserver.key must disallow any access to world or group; achieve this by the commandchmod 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 thePostgreSQL 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 seessl_passphrase_command_supports_reload. Furthermore, passphrase-protected private keys cannot be used at all on Windows.

The first certificate inserver.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 withv3_ca extensions. (This sets the certificate's basic constraint ofCA totrue.) This allows easier expiration of intermediate certificates.

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

PostgreSQL reads the system-wideOpenSSL configuration file. By default, this file is namedopenssl.cnf and is located in the directory reported byopenssl version -d. This default can be overridden by setting environment variableOPENSSL_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 theOpenSSL configuration file, you can specify ciphers specifically for use by the database server by modifyingssl_ciphers inpostgresql.conf.

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 parameterssl_ca_file inpostgresql.conf to the new file name, and add the authentication optionclientcert=verify-ca orclientcert=verify-full to the appropriatehostssl line(s) inpg_hba.conf. A certificate will then be requested from the client during SSL connection startup. (SeeSection 32.19 for a description of how to set up certificates on the client.)

For ahostssl entry withclientcert=verify-ca, the server will verify that the client's certificate is signed by one of the trusted certificate authorities. Ifclientcert=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 thecn (Common Name) of the provided certificate. Note that certificate chain validation is always ensured when thecert authentication method is used (seeSection 20.12).

Intermediate certificates that chain up to existing root certificates can also appear in thessl_ca_file file if you wish to avoid storing them on clients (assuming the root and intermediate certificates were created withv3_ca extensions). Certificate Revocation List (CRL) entries are also checked if the parameterssl_crl_file orssl_crl_dir is set.

Theclientcert authentication option is available for all authentication methods, but only inpg_hba.conf lines specified ashostssl. Whenclientcert is not specified, the server verifies the client certificate against its CA file only if a client certificate is presented and the CA is configured.

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

The first approach makes use of thecert authentication method forhostssl entries inpg_hba.conf, such that the certificate itself is used for authentication while also providing ssl connection security. SeeSection 20.12 for details. (It is not necessary to specify anyclientcert options explicitly when using thecert authentication method.) In this case, thecn (Common Name) provided in the certificate is checked against the user name or an applicable mapping.

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

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.)

The server reads these files at server start and whenever the server configuration is reloaded. OnWindows 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. OnWindows 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 followingOpenSSL command, replacingdbhost.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 theOpenSSL documentation.

While a self-signed certificate can be used for testing, a certificate signed by a certificate authority (CA) (usually an enterprise-wide rootCA) 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 defaultOpenSSL configuration file location onLinux):

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

server.crt andserver.key should be stored on the server, androot.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:

# rootopenssl req -new -nodes -text -out root.csr \  -keyout root.key -subj "/CN=root.yourdomain.com"chmod og-rwx root.keyopenssl x509 -req -in root.csr -text -days 3650 \  -extfile /etc/ssl/openssl.cnf -extensions v3_ca \  -signkey root.key -out root.crt# intermediateopenssl req -new -nodes -text -out intermediate.csr \  -keyout intermediate.key -subj "/CN=intermediate.yourdomain.com"chmod og-rwx intermediate.keyopenssl 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# leafopenssl req -new -nodes -text -out server.csr \  -keyout server.key -subj "/CN=dbhost.yourdomain.com"chmod og-rwx server.keyopenssl x509 -req -in server.csr -text -days 365 \  -CA intermediate.crt -CAkey intermediate.key -CAcreateserial \  -out server.crt

server.crt andintermediate.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 andintermediate.key should be stored offline for use in creating future certificates.