Helping someone behind NAT with SSH Tunnels

While helping a friend set up a wireless card, I needed to log into his machine. He's behind the typical NAT+router, and describing how to do port forwarding is beyond my interests, here's a simpler way.

Ingredients

A server running SSH on the publically accessible internet, or behind a NAT with port-forwarding so that internet addressable SSH works. (polis.nbtsc.org in my case)
Ability to log in on that server from both places (Check!)
A machine trapped behind a NAT that SSH can be installed on
Ability to log into that machine

You'll need to have SSH installed on the machine you're trying to log into.

In Ubuntu, you can do this by running in a terminal:

sudo apt-get install openssh-server openssh-client

In PLD, you can do this by running:

poldek -i openssh-server openssh-clients sudo service sshd start

In a terminal on the machine behind the NAT, type ssh -R9022:localhost:22 -l username server, where 9022 is a number over 1024 that's not being used — you might have to try several — and where username is the username on the server and server is the name of the server you can log in to. For me, it looks like this:

ssh -R 9022:localhost:22 -l aredridel polis.nbtsc.org

From the helping side, log in to that server from where you're trying to help from. For me:

ssh -l aredridel polis.nbtsc.org

From there, ssh localhost -p 9022 -l username, where 9022 is the port number chosen above that worked, and username is the username on the machine you're ultimately trying to log into. For me:

ssh -p 9022 -l bailey localhost

If everything worked, you will have logged in to the machine and can install or fix things to your hearts content.