pwnat, pronounced
Simply put, this is a proxy server that works behind a NAT, even when the client is behind a NAT, without any 3rd party. 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.

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