How do I make a TCP server work behind a router (NAT) without any redirection configuration needed

Question

The scenario is the following. I have two machines A and B: A: Client (behind NAT) B: Server (behind NAT)

I want B to be able to listen on any given port, so that A can send packets to B through that specific TCP port and receive any response. If both machines are not behind a NAT it is pretty straight foward process. However how do I make it work so that it works even when B is behind a router, without him having to go change the router configuration enable some port forwarding etc...

For example, how do peer-to-peer programs like torrent clients work without the user having anything to configure?

Answer 1

An alternative to a proxy server is a match-making server. Instead of proxying all of the traffic, the match maker just negotiates until the peers can talk to each other. This involves finding the external public IPs of the peers and talking to each one so that the firewall/router knows that the peers wish to communicate.

This is called hole punching and it often has to be done by the match maker rather than the peers themselves. Once the hole are punched though, the match maker can tell the peers about each other and they can communicate directly.