NAT traversal with Java

I want to connect to computers, each one of them beh开发者_开发技巧ind a NAT router. I read that STUN only works with one computer behind a NAT router. Is that true? If so, how can I solve that double-NAT problem?

Thanks, Thomas

UDP hole punching and TCP hole punching

TURN is a set of extensions to STUN that help to solve the double-NAT problem. You still need a server in the cloud, and the clients need to register. Source for some free servers is available.

From the TURN internet draft:

As described in [RFC5128] and [RFC4787], hole punching techniques
will fail if both hosts are behind NATs that are not well-behaved. For example, if both hosts are behind NATs that have a mapping behavior of "address dependent mapping" or "address and port dependent mapping", then hole punching techniques generally fail.

If you dó not have a central server, you May consider using upnp if available to create a hole in the router. Azureus/vuze does this.