The problem addressed here is the same as the previous two, except that the remote administrator won’t open up *any* ports in their firewall. Machines on the customer’s network can connect out onto the internet, but there is no direct way to connect to them from out here. This post describes how to set up a pair of tunnels that meet in the middle, allowing me access to machines that don’t even have a public IP address.
In a previous post I described how and why one might set up a very simple ssh tunnel in order to access a web server or a remote desktop server on a machine with a restrictive firewall. This post goes one step deeper, showing how to chain tunnels together.
A basic introduction to ssh tunnels: As a first puzzle, assume that a friendly administrator gave me an ssh account on her web server. However, her organization’s security scheme only allows me to get to port 22. I can ssh in, but I can’t really debug some problem with the web browser because port 80 is still blocked. Assume as well that we’ve already called up the network administrator, who offered to think about it – laughed maniacally – and hung up on us.
