Module lan_mdns_health 

Is our mDNS service really announced on an IPv4 interface?

mdns-sd builds one socket per interface when its daemon is created. For IPv4 that is a bind to 0.0.0.0:5353, a multicast-group join and a test packet; if any step fails the crate logs at debug! and skips that interface (mdns-sd’s service daemon, “bind a socket to {}: {}. Skipped.”). The daemon still starts and LAN discovery started (mDNS) is still logged, so a host can hear its peers (over IPv6) and never be heard. That is exactly what happened on Area54 on 2026-10-01 (SPEC_LAN_FIREWALL_SETUP_2026_10_01.md 8.1).

The daemon already says which interfaces it announced on: its monitor channel emits DaemonEvent::Announce(fullname, addresses) only for interfaces that have a working socket. This module turns that into a health verdict. It opens no socket of its own: a second receiver on UDP 5353 would compete with the daemon it is checking (Codex P1 on #4133), and mdns-sd always answers by multicast, so a legacy-unicast probe from an ephemeral port would never get an answer.

Structs

Watchdog

Decides, tick by tick, when an undiscoverable verdict justifies a rebuild.

Enums

Action

MdnsHealth

Constants

GRACE

How long after start the daemon gets to announce before “nothing announced” counts as a verdict. Probing plus the first announcement takes well under a second on a healthy host.

MAX_DOWN_RETRIES

Ticks the watchdog will keep trying to bring the daemon back when LAN is wanted but no daemon is running (a rebuild whose new daemon failed to start). Without this a transient failure inside a rebuild would leave LAN dead until the user toggled it (ReAgent P2 on #4148).

RETRY_AFTER_TICKS

Ticks to wait after rebuild attempt k (1-based) before attempt k+1; the last value repeats for ever. At the 15 s tick that is 30 s, 1 min, 2 min, 5 min, then every 10 min. There is NO cap on the number of attempts.

STRIKES_BEFORE_REBUILD

Consecutive undiscoverable verdicts, one per watchdog tick, required before the daemon is rebuilt. One miss can be a slow start, not a fault.

Functions

evaluate

The verdict. Only addresses we expected count: an announcement on an address we do not listen on proves nothing about the ones we do. missing names one address (the lowest) per uncovered interface.

expected_by_interface

The IPv4 addresses a LAN peer could reach us on, grouped by interface: not loopback, unspecified, link-local (169.254/16), multicast or broadcast.

ipv4_in

Every IPv4 address written in payload. mdns-sd 0.12 formats the address part of DaemonEvent::Announce two ways (a debug-formatted list at registration, host:addr on later announcements), so this scans for dotted quads and does not depend on either shape. IPv6 is ignored: only the IPv4 socket was ever missing.

should_publish

Whether this verdict should be sent to the windows on this tick.

verdict

The verdict for a daemon of a given age.