Centos – pg_hba.conf entry for host

Check if the remote host allow tunneling. You can check with the client if the tunnel is refused with:

ssh -v -L 3333:myHost:5432 myUser@myHost

The answer from DerfK above is wrong. You can use PostgreSQL using Unix socket with roundcube provided you're configuring it well. In the db.inc.php, it should read:

$rcmail_config['db_dsnw'] = 'pgsql://roundcube:*password*@unix(/tmp)/roundcube';

Provided you've created a "roundcube" user in pgsql for the database "roundcube" with password "password" In your main postgresql.conf, you should prevent listening on the IP layer, by changing:

listen_addresses = ''
unix_socket_directory = '/tmp'
ssl = false  # There's no point in using SSL on a local UNIX socket except wasting CPU

Also, and that is the most important part, you must change in pg_hba.conf to add this line:

local   all         roundcube                        md5

Restart everything, and it's working perfectly well and faster than using TCP connections (since you avoid all the IP encapsulation).