SSH tunneling is a method for making a secure connection from inside a network, through the NAT & firewall to a specific location elsewhere on the internet. VPN is a common use of this technique.
However, there are times when you may be somewhere on the internet and need to get to a private box behind a NAT and/or firewall. Typically that cannot be done – that’s the whole purpose of a firewall and an NAT. However, these directions provided by HowToForge provide us with step-by-step directions for setting up the target box and the NAT to do just that: reverse SSH tunneling into a Linux box on a private network.