method to find my UDP socket's “real” port?

Question

Is there any free service to which I could send a UDP packet and it would respond telling me what is my "real" UDP port? (my application is behind a NAT)

This kind of service could allow me to make a p2p coordination server with normal php hosting.

• The p2p clients would know their UDP ports from this service
• They will then contact my server over HTTP (which is what regular web hosting allows) and tell it their port (and ip, which is normally supplied to cgi scripts)
• My server will give the clients the IP addresses and ports of the other clients.

Answer 1

Such service is called STUN. If you query a search engine for public stun server you should get a bunch of results. There is an RFC on STUN servers, but I would be surprised if any public server would conform to that.