dkg's blog

i suspect a lot of people are used to forwarding TCP sockets with SSH -- for example, to connect locally to a mysql daemon that runs only on the loopback interface of a remote machine (this is debian's default mysql-server configuration):

ssh -N -T -oExitOnForwardFailure=yes -L 3306:localhost:3306 remoteuser@mysqlserver.example

But sometimes, the remote service runs on a UNIX-domain socket, not on a TCP socket -- for example, debian's default configuration for postgresql is to have it listen only on a UNIX domain socket in /var/run/postgresql, and use SO_PEERCRED with a simple system account == psql account mapping scheme to authenticate users without needing any extra credentials. This is not quite as simple to forward over ssh, but it's doable as long as socat is installed on both your local host and on the remote postgres server.

Here's one way to do it if $SOCKET_DIR points to the full path of a directory under the user's control (this is all one command, split across lines for easier reading):

socat "UNIX-LISTEN:$SOCKET_DIR/.s.PGSQL.5432,reuseaddr,fork" \   EXEC:'ssh remoteuser@psqlserver.example socat STDIO UNIX-CONNECT\:/var/run/postgresql/.s.PGSQL.5432'

Then, you'd connect with something like:

psql "user=remoteuser host=$SOCKET_DIR"

Each such psql connection will trigger an ssh connection to be made. Of course, this won't work well if ssh has to prompt for passwords, but you should be using ssh-agent anyway, right?

There are at least a couple nice features of being able to use postgresql from a local client like this:

• your psql client can load files from your local machine, and can dump/export files to the local machine.
• your ~/.psql_history stays local, so you can review what you did even when you're offline
• you can run local RDBMS administrative GUIs like pgadmin3 with minimal network traffic and no extra packages installed on the server.
• unlike forwarding TCP ports (where any other user account on the machine can hop onto your connection), you can control access to your local UNIX-domain socket with standard filesystem permissions on $SOCKET_DIR.

Of course, postgresql itself already comes with a nice range of high-quality network-capable authentication mechanisms you could use. But many of them (like GSSAPI or X.509 mutual key-based authentication over TLS) require additional infrastructure setup; and you probably already have sshd up and running on that machine -- so why not make use of it?

Tags: postgresql, socat, ssh, tip, unix-domain socket