I have a server ("A") running behind a firewall, which serves HTTP and websockets. I have no control over the firewall, but do have an external server ("B") to which the internal server can connect (note that the reverse connection from B to A is not possible due to the firewall). How can I set up some sort of proxy on B such that an Internet client ("C") can access the resources on A?

I'd prefer something lightweight—even a Python program or an SSH tunnel (which I've tried without success)—rather than something more heavyweight but robust.