windows: adopt GP-managed NRPT catch-all

cmd/cli/prog.go

@@ -172,6 +172,22 @@ type prog struct {
 	// On Windows: *wfpState, on macOS: *pfState, nil on other platforms.
 	dnsInterceptState any
 
+	// dnsInterceptMu serializes DNS intercept lifecycle transitions - start, stop and
+	// the health monitor's rebuild - and guards every write to dnsInterceptState, so a
+	// service stop can never interleave with a monitor-driven rebuild.
+	dnsInterceptMu sync.Mutex //lint:ignore U1000 used on windows
+
+	// dnsInterceptStopRequested is set while a stop waits for dnsInterceptMu. The
+	// health and recovery flows read it as a shutdown signal and abandon their work,
+	// rather than making the stop wait out their probe backoffs.
+	dnsInterceptStopRequested atomic.Bool //lint:ignore U1000 used on windows
+
+	// nrptTransitionMu makes one NRPT ownership transition - observe, mutate, signal,
+	// record owner - atomic against shutdown and against another transition. It is
+	// deliberately finer-grained than dnsInterceptMu: it is taken for the duration of a
+	// single transition, never across the recovery flows' probe backoffs.
+	nrptTransitionMu sync.Mutex //lint:ignore U1000 used on windows
+
 	// lastTunnelIfaces tracks the tunnel set included in the last successfully loaded
 	// pf anchor. Pending tunnel state is kept separately so failed PF work is retried
 	// instead of being mistaken for an applied update. Protected by mu.
@@ -220,15 +236,22 @@ type prog struct {
 	// existing delayed checks provide a trailing reconciliation after churn.
 	pfIgnoredChangeLastReconcile atomic.Int64 //lint:ignore U1000 used on darwin
 
-	// pfProbeExpected holds the domain name of a pending pf interception probe.
-	// When non-empty, the DNS handler checks incoming queries against this value
-	// and signals pfProbeCh if matched. The probe verifies that pf's rdr rules
-	// are actually translating packets (not just present in rule text).
-	pfProbeExpected atomic.Value // string
-
-	// pfProbeCh is signaled when the DNS handler receives the expected probe query.
-	// The channel is created by probePFIntercept() and closed when the probe arrives.
-	pfProbeCh atomic.Value // *chan struct{}
+	// interceptProbes maps the domain of each pending interception probe to the channel
+	// that probe waits on. A probe verifies that interception is actually translating or
+	// redirecting packets, not merely present in rule text: the DNS handler looks up
+	// incoming queries here and signals the matching waiter.
+	//
+	// It holds one entry per in-flight probe rather than a single slot, because probes do
+	// overlap - the health monitor, a handback and a heal cycle can each have one out at
+	// the same time - and a single slot means the last registration wins and the loser
+	// waits out its timeout for a query that was answered. A false failure then triggers
+	// recovery work that was not needed.
+	//
+	// Registrations are rare and lookups happen on every query, so the map is stored as
+	// an immutable snapshot behind an atomic: readers never take a lock, writers copy
+	// under interceptProbeMu.
+	interceptProbes  atomic.Value // map[string]chan struct{}
+	interceptProbeMu sync.Mutex   //lint:ignore U1000 written only by registerInterceptProbe, used on darwin/windows
 
 	// VPN DNS manager for split DNS routing when intercept mode is active.
 	vpnDNS *vpnDNSManager
@@ -424,7 +447,12 @@ func (p *prog) postRun() {
 			p.runningOnDomainController = isDC
 			mainLog.Load().Debug().Msgf("running on domain controller: %t, role: %d", p.runningOnDomainController, roleInt)
 		}
-		p.resetDNS(false, false)
+		// A Windows organization can install a GP-owned NRPT catch-all before
+		// starting ctrld. Detect that policy before resetDNS touches adapter DNS;
+		// startDNSIntercept will then prove the rule functionally before adopting it.
+		if !p.skipInitialDNSReset() {
+			p.resetDNS(false, false)
+		}
 		ns := ctrld.InitializeOsResolver(false)
 		mainLog.Load().Debug().Msgf("initialized OS resolver with nameservers: %v", ns)
 		p.setDNS()
@@ -907,7 +935,7 @@ func (p *prog) setDNS() {
 		mainLog.Load().Fatal().Msgf("invalid --intercept-mode value %q: must be 'off', 'dns', or 'hard'", interceptMode)
 	}
 	if interceptMode == "" || interceptMode == "off" {
-		interceptMode = cfg.Service.InterceptMode
+		interceptMode = p.configuredInterceptMode()
 		if interceptMode != "" && interceptMode != "off" {
 			mainLog.Load().Info().Msgf("Intercept mode enabled via config (intercept_mode = %q)", interceptMode)
 		}
@@ -927,6 +955,30 @@ func (p *prog) setDNS() {
 	// software that also manages DNS. See issue #489.
 	if dnsIntercept {
 		if err := startDNSInterceptFn(p); err != nil {
+			// This check comes first: it is the one failure where DNS already works
+			// without ctrld touching anything else, so neither the refusal below nor the
+			// fallback applies.
+			//
+			// An externally managed rule was proved - by probe, not by registry shape -
+			// to be routing DNS to this listener. Falling through would rewrite adapter
+			// DNS after explicitly preserving it, and DNS still works, so stop here.
+			//
+			// Only a verified route earns this. A rule that merely exists does not: if it
+			// is not actually routing and intercept failed too, the machine would be left
+			// with no NRPT, no WFP and no adapter fallback - that is, unfiltered - so
+			// every other failure takes the paths below.
+			if interceptFailedUnderExternalDNSPolicy(err) {
+				if interceptFailedWithVerifiedExternalDNS(err) {
+					mainLog.Load().Error().Err(err).Msg("DNS intercept mode failed but externally managed DNS policy is verified routing to ctrld — not falling back to interface DNS settings")
+				} else {
+					// Owned by external policy but not proved to route: DNS is not
+					// reaching ctrld. Adapter DNS still stays as the organization set it,
+					// and setDnsOK stays false, so this start reports as failed until a
+					// probe succeeds.
+					mainLog.Load().Error().Err(err).Msg("DNS intercept mode failed and externally managed DNS policy is not routing to ctrld — leaving interface DNS settings untouched; the service is not ready")
+				}
+				return
+			}
 			// Interface DNS cannot express a port: macOS interface settings and Windows
 			// NRPT rules both name a resolver by IP alone. So it is only a usable
 			// fallback when the listener actually bound :53. When something else owns
@@ -1037,6 +1089,17 @@ func (p *prog) setDNS() {
 	}
 }
 
+// configuredInterceptMode resolves the service's effective intercept mode without
+// mutating package state. Platform startup preflights use the same precedence as
+// setDNS so they do not make adapter-DNS decisions from a different mode value.
+func (p *prog) configuredInterceptMode() string {
+	im := interceptMode
+	if im == "" || im == "off" {
+		im = p.cfg.Service.InterceptMode
+	}
+	return im
+}
+
 func (p *prog) setDnsForRunningIface(nameservers []string) (runningIface *net.Interface) {
 	if p.runningIface == "" {
 		return