[pzmarzly]

Most clients try to set up port forwarding using UPnP IGD, NAT PMP, or PCP protocol, a lot of residential routers support one of them.

If you enable WebTorrent as a transport protocol (enabled in gotorrent, disabled by default in libtorrent), it should be possible to use existing public STUN/TURN infrastructure, but I don't know if any client does it yet.

In practice, you just have to accept that many connections will simply fail, and make your client move on to try a different peer.

[anacrolix]

Yeah I believe WebRTC connections use stun/turn. The feature was contributed and just has a static list somewhere that works fine.