UDP hole punching host-specific failure

Question

I wrote a program that sets-up peer-to-peer links. The programm, which can be found at http://basyl.co.uk/code/punch/doc/files/Readme-txt.html, is in two parts: a server that r

Answer 1

I can't quite follow all that, but it sounds like you want to use an intermediary server to discover the source UDP ports for Clients A and B so that A and B can simultaneously send UDP datagrams at each-other, thereby opening NAT rules and (eventually) allowing the traffic through.

Here's the problem: NAT can map the source port to whatever it wants. When B sends a datagram to the server, there's no guarantee that the source port seen by the server will be the same one that is used when B sends a datagram to A.

There are a lot of reasons why the NAT might change the port number, and a security conscious one will randomize just to prevent what you are trying to do. So while you may be able to make double punching (NAT to NAT) work sometimes, you cannot do so every time.