ssh tunnels part 2

I've written before about using ssh tunnels to access servers behind a firewall, but recently I've been amazed again by the utility of ssh tunnels.

To access a web server behind a firewall, the ssh client running on the computer you are sitting at listens on a given port number, and the ssh server on the machine you connect to connects to a given host and port. Any connection made to the port on the local machine is captured by ssh and forwarded to the machine the ssh server has connected to. The -L switch accomplishes this, so for a command line like ssh -L 6789:192.168.3.134:80 remote.machine.com, the ssh client listens on port 6789, and the ssh server on remote.machine.com forwards connections to 192.168.3.134:80, which is some machine inside a private network. Now pointing your web browser at http://localhost:6789/ connects you to 192.168.3.134:80.

I recently had to help a friend with a problem they were having, and I wanted to ssh into thier machine so I could see what was going on. Unfortunatly, their machine is behind a firewall, and they didn't have the passwords to open a port. Tunnels to the rescue! I created an account for my friend on spacemonkeys.ca, so that they could ssh in. Thier machine was running an ssh server already, so they used this command to log in to spacemonkeys.ca: ssh -R 6789:localhost:22 spacemonkeys.ca. This opens port 6789 on spacemonkeys.ca, and forwards any connections to the ssh port on my friends firewalled computer. I could then ssh into thier machine from spacemonkeys.ca: ssh -p 6789 localhost.

Pretty spiffy!