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Karl Matthias

DevOps, sysadmin, coder. Principal Systems Engineer at Nitro. Dublin, Ireland.

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A Works-Anywhere Config For SSH Tunneling

TL;DR: This article proposes a simple solution to DNS and jump hosts that allows you to use short names on the ssh command line without impacting accessing external servers.

This is not a new topic but I think this is a novel way of handling it. I’ve had a number of solutions for SSH’ing through a jump host over the years. Some have worked better than others.  I recently built a setup that seems to work very well and that I am happy with so I thought it was worth sharing.

Generally you have a jump box if you have a remote network of isolated hosts and you want to have a security pinch point on inbound SSH sessions.  You generally fortify the jump host as a bastion host and only allow SSH sessions on the remote internal network when they are inbound from the jump box. This is a fairly common architecture and I’ve seen it at a score of companies.

What becomes a pain is initiating two ssh sessions every time you want to get to a host inside the remote network, or if you want to scp a file, you end up copying it twice.  So people often set up scripts for getting around this and connecting directly to the remote internal host over some kind of SSH tunnel.  Now you either need a config entry for each host, or a remote domain name that you can use to wildcard a <code>Host</code> entry in your SSH config. But then you are not just typing the hostname each time you connect, you are also typing the domain name.  It’s annoying.

What I’ve set up is, I think, much better.  It connects a backgrounded tunnel to the jump host, running a SOCKS proxy.  All future connections are then tested to see if they would work directly, and if so they are connected.  Otherwise they are proxied over the SOCKS tunnel to the jump host.  It’s simple, and having used it in production now for awhile, it seems to work pretty well.  I don’t have to use the domain name for the host on the other side of the tunnel, and I don’t have to manage ssh config records.  Here’s the only entry you need in your SSH config:

You’ll want to replace username and jumphost with your actual remote username and jump host name. You may also want an SSH config entry for that host to make sure it uses your RSA key for authentication. And here is the script that runs it all, to be installed in ~bin/ssh-proxy.sh:

Enjoy!


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