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

the client doesn't work on a particular host when the server is on the same host

In some corner cases, NATs only accept traffic from the target IP address:port used to punch the hole. Since it is not the one of the remote NAT-ed peer, they block the traffic from it. This could explain your issue.