On macOS DNS-intercept mode, when mDNSResponder owns *:53 ctrld falls back
to listening on 127.0.0.1:5354, and the pf rdr rules correctly redirect DNS
to the bound port at startup. However, DNS resolution later breaks with an
endless watchdog "anchor intact but probe FAILED -> force reload" loop and
`dig @127.0.0.1` timeouts.
Root cause is config reload. In CD mode, apiConfigReload refetches the
generated config (which always declares port 53) every hour and the reload
merge in runWait only inherits the running port when the new port is 0. The
generated config explicitly says 53, so `*p.cfg = *newCfg` reverts p.cfg to
port 53. The DNS listener goroutines are started only when !reload, so they
are never re-bound and stay on 5354. Every subsequent pf rebuild reads p.cfg
and targets the dead port 53.
Fix: after applying the reloaded config in DNS-intercept mode on darwin,
restore the actual bound listener IP/Port into the in-memory config via the
new preserveBoundListeners helper, logging the configured-vs-actual
divergence. A reload cannot move the running listener anyway, so this keeps
p.cfg consistent with reality; all pf rdr rules and the watchdog probe then
target the live port. The on-disk generated config is intentionally left
unchanged (still 53), so no generated-config change is required.
Implement DNS interception on macOS using pf (packet filter):
- Anchor injection into running ruleset (not /etc/pf.conf)
- route-to lo0 + rdr rules for locally-originated DNS capture
- _ctrld group exemption so ctrld's own queries bypass interception
- Watchdog to detect and restore wiped anchor rules
- Probe-based auto-heal for Parallels VM pf corruption
- IPv6 DNS blocking and block-return for clean timeouts
- Interface-specific tunnel detection for VPN coexistence
- Port 5354 fallback in intercept mode
Includes pf technical reference docs and test scripts.
Squashed from intercept mode development on v1.0 branch (#497).
Cuong Manh Le