Pwnat - NAT to NAT Client-server Communication
pwnat, pronounced “poe-nat”, is a tool that allows any number of clients behind NATs to communicate with a server behind a separate NAT with *no* port forwarding and *no* DMZ setup on any routers in order to directly communicate with each other. The server does not need to know anything about the clients trying to connect.
Simply put, this is a proxy server that works behind a NAT, even when the client is behind a NAT, without any 3rd party.
This seems really useful for new p2p projects.