The 2026-07-04 CGNAT smoke test found MTU 1420 silently black-holes bulk TCP
on sub-~1480 paths (mobile ~1400, DS-Lite ~1452): handshake+ping stay healthy,
PBS TLS page (and at S4 the backup itself) drops. Set a fleet-wide, permanent,
family-agnostic client MTU of 1280 (RFC 8200 IPv6-minimum floor; outer 1340 v4
/ 1360 v6 fits every realistic path).
Client-only by construction — interface MTU caps box→PBS, advertised MSS caps
PBS→box; the endpoint's wg0 is untouched (zero live-endpoint risk). New const
clientMTU=1280 as the single home; golden pins exact "MTU = 1280" (red-proofed
against a 1420 flip). Stale report.go comment updated. No wire/JSON change.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6
admin
312fd5ee29