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
*no* DMZ setup, and *no* 3rd party involvement. 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 different NAT, without any
3rd party or network changes.

There is no middle man, no proxy, no 3rd party,
no UPnP/STUN/ICE required, no spoofing, and no DNS tricks.

More importantly, the client can then connect to any
host or port on any remote host or to a fixed host and
port decided by the server.

WWW: https://samy.pl/pwnat