Hand-rolled RFC 5389/5780 STUN client (stdlib only) that exercises the
server's stun-5780 capability: one socket, three binding requests
(primary, OTHER-ADDRESS alternate IP, CHANGE-REQUEST port) — the
comparison classifies NAT mapping and filtering behavior.
Verified on the OnePlus: local 10.13.102.124 -> mapped
178.191.120.247:53259 (behind_nat true), alternate address answered from
the server's second IP, mapping endpoint-independent, filtering
address/port-dependent. Finding nat.symmetric (medium) for the
P2P-hostile case.
Two real bugs found by running it: port preservation was misread as "no
NAT" (compare addresses, not ports), and an unbound socket reports the
wildcard local address (resolve via a throwaway connected socket).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
