FreeSWITCH supports two client-side NAT-busting protocols by default called NAT-PMP and UPnP. Both of these protocols use slightly different methods but the basic gist is the same.

Both methods use a network protocol to discover your NAT router and communicate with it, so rather than making the NAT mappings on-the-fly, it asks the router to open a port and actually learns the details of the port mapping, so when FreeSWITCH talks to another server it's putting the correct information in the packets for both the SIP and the media. That is cool! Beware though, now we're opening the door to the second, third, and fourth pitfalls.

We have now disguised the fact that we are behind NAT so the other side cannot detect it. An ALG might be hiding in our midst and mess with the packet thinking that it still needs to when it doesn't. The other side might be using something similar to aggressive NAT detection and spring a double anti-NAT trap. The great thing is, if you have some very strict NAT routers or firewalls, this feature will not only solve the address mapping dilemma, but also unlock otherwise blocked ports held tight by the firewall.