resolved external address?? (new in .v0.14.40)

[calmh (Jakob Borg)]

@calmh

NAT support (in our code) is about using NAT traversal protocols to set up port mappings in routers.

STUN is about figuring out the NAT type and external address when lacking port mappings, so explicitly for the situation where NAT port mappings don’t work or aren’t enabled.