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Reverse SSH tunneling

2014-08-27

Live demo in BSD Now Episode 052. | Originally written by TJ for bsdnow.tv | Last updated: 2014/08/27

NOTE: the author/maintainer of the tutorial(s) is no longer with the show, so the information below may be outdated or incorrect.

We've done a number of SSH tutorials in the past, but most of them rely on the fact that you have a certain level of control on the network. In some cases, you need to be able to access a system that's behind a firewall. This guide will show you how to do just that - reversing the connection and accessing an internal system from the outside. The only requirement in this case is that the firewall allows outbound SSH traffic. You'll have to have access to the machine behind the firewall at some point for this to work. The -R switch will play a key role in this (very short) tutorial. Since we'll be reversing the connection, be sure your client system has a publicly-accessible sshd running. On the system behind the firewall, run the following:

$ ssh -fN -R 9000:localhost:22 user@clientip

Replace "clientip" with the IP of your system and "22" with the port on which you run sshd. It's recommended to run that command in tmux so it doesn't get lost. You might want to consider running sshd on port 443, so it looks similar to normal SSL traffic. See our stunnel tutorial for more ideas there. Now move back to the client system, and we'll make the reverse connection like so:

$ ssh -p 9000 user@127.0.0.1

While it may appear to be connecting on the loopback device, it's actually using the already-established connection made by the internal machine. You'll need to use the username and password/key that you normally would for the internal system. Some recommended settings to have in the client's /etc/ssh/sshd_config:

ClientAliveInterval 300
TCPKeepAlive yes

With these, the internal system will send a packet to the client every five minutes to keep the connection from dying due to inactivity. One problem with this setup is, of course, if the first connection dies. Another possible issue is if your client's IP changes. While not much can be done about the first one (aside from maybe a cron job to try and re-establish the connection), there is a good way to handle the second situation. If you use something like SSH chaining, you can leave the internal system connected to a dedicated server whose IP doesn't ever change. From there, connect to the server, then to localhost.

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