[sholladay]

There are plenty of free public STUN servers and ways to share the information they return.

The real problem is the port randomization if any client is behind a symmetric NAT. The search space for randomly trying port numbers is too large.

There are some ways to reduce the search space, like port prediction. But ultimately, a large dose of port scanning is the only way I know of to make the connection reliably. And there’s only so much of that you can do before triggering IPS or overwhelming the NAT.

IPv6 makes this way easier, of course!