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, Installation from Source Code).
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 the section called “The pg_hba.conf
File” 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.
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.
In some cases, the server certificate might be signed by an
“intermediate” certificate authority, rather than one that is
directly trusted by clients. To use such a certificate, append the
certificate of the signing authority to the server.crt
file,
then its parent authority's certificate, and so on up to a certificate
authority, “root” or “intermediate”, that is trusted by
clients, i.e. signed by a certificate in the clients'
root.crt
files.
To require the client to supply a trusted certificate, place
certificates of the certificate authorities (CAs)
you trust in the file root.crt
in the data
directory, set the parameter ssl_ca_file in
postgresql.conf
to root.crt
,
and set the clientcert
parameter
to 1 on the appropriate hostssl
line(s) in
pg_hba.conf
.
A certificate will then be requested from the client during
SSL connection startup. (See the section called “SSL Support” 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. If intermediate CAs appear in
root.crt
, the file must also contain certificate
chains to their root CAs. Certificate Revocation List
(CRL) entries
are also checked if the parameter ssl_crl_file is set.
(See http://h71000.www7.hp.com/DOC/83final/BA554_90007/ch04s02.html
for diagrams showing SSL certificate usage.)
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.
Note that the server's root.crt
lists the top-level
CAs that are considered trusted for signing client certificates.
In principle it need
not list the CA that signed the server's certificate, though in most cases
that CA would also be trusted for client certificates.
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 the section called “Certificate Authentication” for details.
Table 17.2, “SSL Server File Usage” 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.
To create a quick self-signed certificate for the server, use the following OpenSSL™ command:
openssl req -new -text -out server.req
Fill out the information that openssl asks for. Make sure you enter the local host name as “Common Name”; the challenge password can be left blank. The program will generate a key that is passphrase protected; it will not accept a passphrase that is less than four characters long. To remove the passphrase (as you must if you want automatic start-up of the server), run the commands:
openssl rsa -in privkey.pem -out server.key rm privkey.pem
Enter the old passphrase to unlock the existing key. Now do:
openssl req -x509 -in server.req -text -key server.key -out server.crt
to turn the certificate into a self-signed certificate and to copy the key and certificate to where the server will look for them. Finally 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.
A self-signed certificate can be used for testing, but a certificate signed by a certificate authority (CA) (either one of the global CAs or a local one) should be used in production so that clients can verify the server's identity. If all the clients are local to the organization, using a local CA is recommended.