The STUN (Simple Traversal of UDP through NATs (Network Address Translation))
server is an implementation of the STUN protocol that enables STUN
functionality in SIP-based systems. The STUN server tar ball also include a
client API to enable STUN functionality in SIP endpoints. In addition there
is a command line UNIX client and a graphical Windows client that check what
type of NAT the user is using.

STUN is an application-layer protocol that can determine the public IP and
nature of a NAT device that sits between the STUN client and STUN server.

The current version of the code supports most of RFC 3489 except the ability
to get OTPs from the server.

WWW: http://www.vovida.org/applications/downloads/stun/