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
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.key, respectively, in the server's
data directory, but other names and locations can be specified
using the configuration parameters ssl_cert_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 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 “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
PostgreSQL reads the
configuration file. By default, this file is named
openssl.cnf and is located in the directory
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
It is possible to have authentication without encryption
overhead by using
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 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
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 34.18 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 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
is set. (See http://h41379.www4.hpe.com/doc/83final/ba554_90007/ch04s02.html
for diagrams showing SSL certificate usage.)
option is available for all authentication methods, but only in
pg_hba.conf lines specified as
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
method, so that the certificates control user authentication as
well as providing connection security. See Section 20.12
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 names. The locally configured names could be different.)
Table 18.2. SSL Server File Usage
||server certificate||sent to client to indicate server's identity|
||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.
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=
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.key should 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 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
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.
intermediate.key should be stored offline for
use in creating future certificates.
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