added missing func.

- Skip systemd-resolved initialization on Android (ChromeOS crash fix)
- Skip system DNS discovery commands on iOS mobile (sandbox restrictions)
- Skip route-based DNS discovery on Android
- Skip systemd resolver on Android
- Add mobile platform checks to prevent sandbox access violations

These changes ensure ctrld works correctly in mobile sandboxed environments
where system commands and file access are restricted.

cmd/cli/cli.go

@@ -451,18 +451,7 @@ func run(appCallback *AppCallback, stopCh chan struct{}) {
 	p.onStopped = append(p.onStopped, func() {
 		// restore static DNS settings or DHCP
 		p.resetDNS(false, true)
-		// Iterate over all physical interfaces and restore static DNS if a saved static config exists.
-		withEachPhysicalInterfaces("", "restore static DNS", func(i *net.Interface) error {
-			file := savedStaticDnsSettingsFilePath(i)
-			if _, err := os.Stat(file); err == nil {
-				if err := restoreDNS(i); err != nil {
-					mainLog.Load().Error().Err(err).Msgf("Could not restore static DNS on interface %s", i.Name)
-				} else {
-					mainLog.Load().Debug().Msgf("Restored static DNS on interface %s successfully", i.Name)
-				}
-			}
-			return nil
-		})
+		restoreSavedStaticDNS("", false)
 	})
 
 	close(waitCh)
@@ -638,6 +627,19 @@ const defaultDeactivationPin = -1
 // cdDeactivationPin is used in cd mode to decide whether stop and uninstall commands can be run.
 var cdDeactivationPin atomic.Int64
 
+// Brute-force protection for the deactivation PIN endpoint on the control socket.
+// After deactivationMaxFailedAttempts consecutive wrong PINs, further attempts are
+// rejected for deactivationLockoutSeconds. Counter resets on a correct PIN.
+const (
+	deactivationMaxFailedAttempts = 5
+	deactivationLockoutSeconds    = 60
+)
+
+var (
+	deactivationFailedAttempts atomic.Int64
+	deactivationLockedUntil    atomic.Int64
+)
+
 func init() {
 	cdDeactivationPin.Store(defaultDeactivationPin)
 }
@@ -813,7 +815,7 @@ func processLogAndCacheFlags(v *viper.Viper, cfg *ctrld.Config) {
 }
 
 func netInterface(ifaceName string) (*net.Interface, error) {
-	if ifaceName == "auto" {
+	if ifaceName == autoIface {
 		ifaceName = defaultIfaceName()
 	}
 	var iface *net.Interface
@@ -1099,23 +1101,7 @@ func uninstall(p *prog, s service.Service) {
 		}
 		// restore static DNS settings or DHCP
 		p.resetDNS(false, true)
-
-		// Iterate over all physical interfaces and restore DNS if a saved static config exists.
-		withEachPhysicalInterfaces(p.runningIface, "restore static DNS", func(i *net.Interface) error {
-			file := savedStaticDnsSettingsFilePath(i)
-			if _, err := os.Stat(file); err == nil {
-				if err := restoreDNS(i); err != nil {
-					mainLog.Load().Error().Err(err).Msgf("Could not restore static DNS on interface %s", i.Name)
-				} else {
-					mainLog.Load().Debug().Msgf("Restored static DNS on interface %s successfully", i.Name)
-					err = os.Remove(file)
-					if err != nil {
-						mainLog.Load().Debug().Err(err).Msgf("Could not remove saved static DNS file for interface %s", i.Name)
-					}
-				}
-			}
-			return nil
-		})
+		restoreSavedStaticDNS(p.runningIface, true)
 
 		if router.Name() != "" {
 			mainLog.Load().Debug().Msg("Router cleanup")
@@ -1128,6 +1114,26 @@ func uninstall(p *prog, s service.Service) {
 	}
 }
 
+// restoreSavedStaticDNS restores DNS from saved static config files on physical interfaces.
+func restoreSavedStaticDNS(excludeIfaceName string, removeSaved bool) {
+	withEachPhysicalInterfaces(excludeIfaceName, "restore static DNS", func(i *net.Interface) error {
+		file := savedStaticDnsSettingsFilePath(i)
+		if _, err := os.Stat(file); err == nil {
+			if err := restoreDNS(i); err != nil {
+				mainLog.Load().Error().Err(err).Msgf("Could not restore static DNS on interface %s", i.Name)
+			} else {
+				mainLog.Load().Debug().Msgf("Restored static DNS on interface %s successfully", i.Name)
+				if removeSaved {
+					if err := os.Remove(file); err != nil {
+						mainLog.Load().Debug().Err(err).Msgf("Could not remove saved static DNS file for interface %s", i.Name)
+					}
+				}
+			}
+		}
+		return nil
+	})
+}
+
 func validateConfig(cfg *ctrld.Config) error {
 	if err := ctrld.ValidateConfig(validator.New(), cfg); err != nil {
 		var ve validator.ValidationErrors
@@ -1245,7 +1251,7 @@ func tryUpdateListenerConfigIntercept(cfg *ctrld.Config, notifyFunc func(), fata
 		return false, true
 	}
 
-	hasExplicitConfig := lc.IP != "" && lc.IP != "0.0.0.0" && lc.Port != 0
+	hasExplicitConfig := isExplicitInterceptListener(lc.IP, lc.Port)
 	if !hasExplicitConfig {
 		// Set defaults for intercept mode
 		if lc.IP == "" || lc.IP == "0.0.0.0" {
@@ -1303,6 +1309,16 @@ func tryUpdateListenerConfigIntercept(cfg *ctrld.Config, notifyFunc func(), fata
 	return updated, false
 }
 
+func isExplicitInterceptListener(ip string, port int) bool {
+	if ip == "" || ip == "0.0.0.0" || port == 0 {
+		return false
+	}
+	// 127.0.0.1:53 is the default macOS DNS-intercept listener. It can appear
+	// in generated/custom Control D configs, but it should still be allowed to
+	// fall back to 127.0.0.1:5354 when mDNSResponder already owns port 53.
+	return !(ip == "127.0.0.1" && port == 53)
+}
+
 // tryUpdateListenerConfig tries updating listener config with a working one.
 // If fatal is true, and there's listen address conflicted, the function do
 // fatal error.
@@ -1809,6 +1825,9 @@ var errInvalidDeactivationPin = errors.New("deactivation pin is invalid")
 // errRequiredDeactivationPin indicates that the deactivation pin is required but not provided by users.
 var errRequiredDeactivationPin = errors.New("deactivation pin is required to stop or uninstall the service")
 
+// errTooManyDeactivationPin represents an error indicating excessive deactivation PIN request attempts.
+var errTooManyDeactivationPin = errors.New("too many request attempts")
+
 // checkDeactivationPin validates if the deactivation pin matches one in ControlD config.
 func checkDeactivationPin(s service.Service, stopCh chan struct{}) error {
 	mainLog.Load().Debug().Msg("Checking deactivation pin")
@@ -1837,6 +1856,9 @@ func checkDeactivationPin(s service.Service, stopCh chan struct{}) error {
 		case http.StatusBadRequest:
 			mainLog.Load().Error().Msg(errRequiredDeactivationPin.Error())
 			return errRequiredDeactivationPin // pin is required
+		case http.StatusTooManyRequests:
+			mainLog.Load().Error().Msg(errTooManyDeactivationPin.Error())
+			return errTooManyDeactivationPin
 		case http.StatusOK:
 			return nil // valid pin
 		case http.StatusNotFound:
@@ -1849,7 +1871,9 @@ func checkDeactivationPin(s service.Service, stopCh chan struct{}) error {
 
 // isCheckDeactivationPinErr reports whether there is an error during check deactivation pin process.
 func isCheckDeactivationPinErr(err error) bool {
-	return errors.Is(err, errInvalidDeactivationPin) || errors.Is(err, errRequiredDeactivationPin)
+	return errors.Is(err, errInvalidDeactivationPin) ||
+		errors.Is(err, errRequiredDeactivationPin) ||
+		errors.Is(err, errTooManyDeactivationPin)
 }
 
 // ensureUninstall ensures that s.Uninstall will remove ctrld service from system completely.
@@ -2002,17 +2026,25 @@ func doValidateCdRemoteConfig(cdUID string, fatal bool) error {
 	} else {
 		if errors.As(cfgErr, &viper.ConfigParseError{}) {
 			if configStr, _ := base64.StdEncoding.DecodeString(rc.Ctrld.CustomConfig); len(configStr) > 0 {
-				tmpDir := os.TempDir()
-				tmpConfFile := filepath.Join(tmpDir, "ctrld.toml")
 				errorLogged := false
-				// Write remote config to a temporary file to get details error.
-				if we := os.WriteFile(tmpConfFile, configStr, 0600); we == nil {
+				// Write remote config to a uniquely named temporary file to get detailed error.
+				if tmpFile, tmpErr := os.CreateTemp("", "ctrld-*.toml"); tmpErr == nil {
+					tmpConfFile := tmpFile.Name()
+					if _, err := tmpFile.Write(configStr); err != nil {
+						mainLog.Load().Error().Err(err).Msg("failed to write temporary config file")
+					}
+					if err := tmpFile.Close(); err != nil {
+						mainLog.Load().Error().Err(err).Msg("failed to save temporary config file")
+
+					}
 					if de := decoderErrorFromTomlFile(tmpConfFile); de != nil {
 						row, col := de.Position()
 						mainLog.Load().Error().Msgf("failed to parse custom config at line: %d, column: %d, error: %s", row, col, de.Error())
 						errorLogged = true
 					}
-					_ = os.Remove(tmpConfFile)
+					if err := os.Remove(tmpConfFile); err != nil {
+						mainLog.Load().Error().Err(err).Msg("failed to remove temporary config file")
+					}
 				}
 				// If we could not log details error, emit what we have already got.
 				if !errorLogged {
@@ -2030,6 +2062,23 @@ func doValidateCdRemoteConfig(cdUID string, fatal bool) error {
 	return nil
 }
 
+// ensureRunningIfaceForInvalidUninstall populates p.runningIface before the
+// invalid-device self-uninstall resets DNS. This path can run early during
+// service startup (e.g. right after a reboot) before the running interface is
+// otherwise known. resetDNS, via resetDNSForRunningIface, silently skips DNS
+// restoration when p.runningIface is empty, which would leave the OS pointed at
+// ctrld's local listener after the service is removed. See issue-556.
+func ensureRunningIfaceForInvalidUninstall(p *prog, s service.Service) {
+	if iface == "" {
+		iface = autoIface
+	}
+	p.preRun()
+	if ir := runningIface(s); ir != nil {
+		p.runningIface = ir.Name
+		p.requiredMultiNICsConfig = ir.All
+	}
+}
+
 // uninstallInvalidCdUID performs self-uninstallation because the ControlD device does not exist.
 func uninstallInvalidCdUID(p *prog, logger zerolog.Logger, doStop bool) bool {
 	s, err := newService(p, svcConfig)
@@ -2037,8 +2086,13 @@ func uninstallInvalidCdUID(p *prog, logger zerolog.Logger, doStop bool) bool {
 		logger.Warn().Err(err).Msg("failed to create new service")
 		return false
 	}
+	ensureRunningIfaceForInvalidUninstall(p, s)
 	// restore static DNS settings or DHCP
 	p.resetDNS(false, true)
+	// The invalid-device path may run early during service startup before runningIface
+	// is known. Restore every saved static DNS file so uninstalling does not leave the
+	// OS pointed at ctrld's local listener after the service is removed.
+	restoreSavedStaticDNS("", true)
 
 	tasks := []task{{s.Uninstall, true, "Uninstall"}}
 	if doTasks(tasks) {

cmd/cli/cli_intercept_listener_test.go

@@ -0,0 +1,116 @@
+package cli
+
+import (
+	"testing"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+func TestIsExplicitInterceptListener(t *testing.T) {
+	tests := []struct {
+		name string
+		ip   string
+		port int
+		want bool
+	}{
+		{name: "empty", ip: "", port: 0, want: false},
+		{name: "wildcard", ip: "0.0.0.0", port: 53, want: false},
+		{name: "zero port", ip: "127.0.0.1", port: 0, want: false},
+		{name: "default intercept listener", ip: "127.0.0.1", port: 53, want: false},
+		{name: "fallback port explicit", ip: "127.0.0.1", port: 5354, want: true},
+		{name: "custom loopback explicit", ip: "127.0.0.2", port: 53, want: true},
+		{name: "custom address explicit", ip: "192.0.2.10", port: 53, want: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isExplicitInterceptListener(tt.ip, tt.port); got != tt.want {
+				t.Fatalf("isExplicitInterceptListener(%q, %d) = %v, want %v", tt.ip, tt.port, got, tt.want)
+			}
+		})
+	}
+}
+
+// TestPreserveBoundListeners is a regression test for #551: on reload, the on-disk
+// generated config still declares 127.0.0.1:53, but the running listener has fallen back
+// to 127.0.0.1:5354. preserveBoundListeners must keep the in-memory config on the actual
+// bound port so pf rdr rules and probes do not target the dead default port.
+func TestPreserveBoundListeners(t *testing.T) {
+	// cur = actual running listener (fell back to 5354); newCfg = freshly read from disk (53).
+	cur := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+	newListeners := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 53}}
+
+	preserveBoundListeners(newListeners, cur)
+
+	if got := newListeners["0"].Port; got != 5354 {
+		t.Errorf("listener port after reload = %d, want 5354 (actual bound port)", got)
+	}
+	if got := newListeners["0"].IP; got != "127.0.0.1" {
+		t.Errorf("listener IP after reload = %q, want 127.0.0.1", got)
+	}
+}
+
+// TestPreserveBoundListeners_NoChange verifies that when the on-disk config matches the
+// running listener, the config is left untouched (a legitimate reload with the same port).
+func TestPreserveBoundListeners_NoChange(t *testing.T) {
+	cur := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+	newListeners := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+
+	preserveBoundListeners(newListeners, cur)
+
+	if got := newListeners["0"].Port; got != 5354 {
+		t.Errorf("listener port = %d, want 5354", got)
+	}
+}
+
+// TestPreserveBoundListeners_MissingCurrent verifies that a listener present on disk but not
+// in the current running set (e.g. newly added) is left as configured.
+func TestPreserveBoundListeners_MissingCurrent(t *testing.T) {
+	cur := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+	newListeners := map[string]*ctrld.ListenerConfig{
+		"0": {IP: "127.0.0.1", Port: 53},
+		"1": {IP: "127.0.0.1", Port: 5355},
+	}
+
+	preserveBoundListeners(newListeners, cur)
+
+	if got := newListeners["0"].Port; got != 5354 {
+		t.Errorf("listener 0 port = %d, want 5354 (preserved)", got)
+	}
+	if got := newListeners["1"].Port; got != 5355 {
+		t.Errorf("listener 1 port = %d, want 5355 (unchanged, no current binding)", got)
+	}
+}
+
+// TestPreserveBoundListeners_ExplicitChangeNotMasked verifies that an explicit, non-default
+// listener in the reloaded config is applied rather than reverted to the old bound listener.
+// Reverting an explicit change would make the control-server reload comparison return 200
+// instead of 201, silently dropping the new listener. Regression guard for #551 review.
+func TestPreserveBoundListeners_ExplicitChangeNotMasked(t *testing.T) {
+	// Running listener fell back to 5354; user reloads with an explicit new listener.
+	cur := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+	newListeners := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.2", Port: 5399}}
+
+	preserveBoundListeners(newListeners, cur)
+
+	if got := newListeners["0"].IP; got != "127.0.0.2" {
+		t.Errorf("explicit listener IP = %q, want 127.0.0.2 (not reverted)", got)
+	}
+	if got := newListeners["0"].Port; got != 5399 {
+		t.Errorf("explicit listener port = %d, want 5399 (not reverted)", got)
+	}
+}
+
+// TestPreserveBoundListeners_ExplicitDefaultPreserved verifies that the default
+// 127.0.0.1:53 listener remains fallback-eligible: when it diverges from the running
+// fallback port it is still preserved (isExplicitInterceptListener treats :53 as non-explicit).
+func TestPreserveBoundListeners_ExplicitDefaultPreserved(t *testing.T) {
+	cur := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}
+	newListeners := map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 53}}
+
+	preserveBoundListeners(newListeners, cur)
+
+	if got := newListeners["0"].Port; got != 5354 {
+		t.Errorf("default listener port = %d, want 5354 (preserved fallback)", got)
+	}
+}

cmd/cli/commands.go

@@ -399,7 +399,7 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 			reportSetDnsOk := func(sockDir string) {
 				if cc := newSocketControlClient(ctx, s, sockDir); cc != nil {
 					if resp, _ := cc.post(ifacePath, nil); resp != nil && resp.StatusCode == http.StatusOK {
-						if iface == "auto" {
+						if iface == autoIface {
 							iface = defaultIfaceName()
 						}
 						res := &ifaceResponse{}
@@ -748,7 +748,7 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 			startCmd.Run(cmd, args)
 		},
 	}
-	startCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", "auto", `Update DNS setting for iface, "auto" means the default interface gateway`)
+	startCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", autoIface, `Update DNS setting for iface, "auto" means the default interface gateway`)
 	startCmdAlias.Flags().AddFlagSet(startCmd.Flags())
 	rootCmd.AddCommand(startCmdAlias)
 
@@ -833,7 +833,7 @@ func initStopCmd() *cobra.Command {
 			stopCmd.Run(cmd, args)
 		},
 	}
-	stopCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", "auto", `Reset DNS setting for iface, "auto" means the default interface gateway`)
+	stopCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", autoIface, `Reset DNS setting for iface, "auto" means the default interface gateway`)
 	stopCmdAlias.Flags().AddFlagSet(stopCmd.Flags())
 	rootCmd.AddCommand(stopCmdAlias)
 
@@ -865,7 +865,7 @@ func initRestartCmd() *cobra.Command {
 				return
 			}
 			if iface == "" {
-				iface = "auto"
+				iface = autoIface
 			}
 			p.preRun()
 			if ir := runningIface(s); ir != nil {
@@ -1111,7 +1111,7 @@ NOTE: Uninstalling will set DNS to values provided by DHCP.`,
 				return
 			}
 			if iface == "" {
-				iface = "auto"
+				iface = autoIface
 			}
 			p.preRun()
 			if ir := runningIface(s); ir != nil {
@@ -1207,7 +1207,7 @@ NOTE: Uninstalling will set DNS to values provided by DHCP.`,
 			uninstallCmd.Run(cmd, args)
 		},
 	}
-	uninstallCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", "auto", `Reset DNS setting for iface, "auto" means the default interface gateway`)
+	uninstallCmdAlias.Flags().StringVarP(&ifaceStartStop, "iface", "", autoIface, `Reset DNS setting for iface, "auto" means the default interface gateway`)
 	uninstallCmdAlias.Flags().AddFlagSet(uninstallCmd.Flags())
 	rootCmd.AddCommand(uninstallCmdAlias)
 
@@ -1400,7 +1400,7 @@ func initUpgradeCmd() *cobra.Command {
 				return
 			}
 			if iface == "" {
-				iface = "auto"
+				iface = autoIface
 			}
 			p.preRun()
 			if ir := runningIface(s); ir != nil {
@@ -1595,7 +1595,7 @@ func onlyInterceptFlags(args []string) bool {
 			} else {
 				return false
 			}
-		case arg == "--iface=auto" || arg == "--iface" || arg == "auto":
+		case arg == "--iface="+autoIface || arg == "--iface" || arg == autoIface:
 			// Auto-added by startCmdAlias or its value; safe to ignore.
 			continue
 		default:

cmd/cli/control_server.go

@@ -59,12 +59,18 @@ func newControlServer(addr string) (*controlServer, error) {
 func (s *controlServer) start() error {
 	_ = os.Remove(s.addr)
 	unixListener, err := net.Listen("unix", s.addr)
-	if l, ok := unixListener.(*net.UnixListener); ok {
-		l.SetUnlinkOnClose(true)
-	}
 	if err != nil {
 		return err
 	}
+	// Restrict socket permissions to owner-only (0600) so that only the
+	// process owner (typically root) can connect. Defense-in-depth since
+	// the control server endpoints carry no authentication of their own.
+	if err := os.Chmod(s.addr, 0600); err != nil {
+		return err
+	}
+	if l, ok := unixListener.(*net.UnixListener); ok {
+		l.SetUnlinkOnClose(true)
+	}
 	go s.server.Serve(unixListener)
 	return nil
 }
@@ -219,6 +225,12 @@ func (p *prog) registerControlServerHandler() {
 			return
 		}
 
+		// Reject further attempts while locked out due to repeated wrong PINs.
+		if now := time.Now().Unix(); now < deactivationLockedUntil.Load() {
+			w.WriteHeader(http.StatusTooManyRequests)
+			return
+		}
+
 		// Re-fetch pin code from API.
 		rcReq := &controld.ResolverConfigRequest{
 			RawUID:   cdUID,
@@ -252,6 +264,7 @@ func (p *prog) registerControlServerHandler() {
 		switch req.Pin {
 		case cdDeactivationPin.Load():
 			code = http.StatusOK
+			deactivationFailedAttempts.Store(0)
 			select {
 			case p.pinCodeValidCh <- struct{}{}:
 			default:
@@ -259,6 +272,11 @@ func (p *prog) registerControlServerHandler() {
 		case defaultDeactivationPin:
 			// If the pin code was set, but users do not provide --pin, return proper code to client.
 			code = http.StatusBadRequest
+		default:
+			if deactivationFailedAttempts.Add(1) >= deactivationMaxFailedAttempts {
+				deactivationLockedUntil.Store(time.Now().Unix() + deactivationLockoutSeconds)
+				deactivationFailedAttempts.Store(0)
+			}
 		}
 		w.WriteHeader(code)
 	}))

cmd/cli/dns_intercept_darwin.go

@@ -41,6 +41,11 @@ const (
 	// 2s re-check misses.
 	pfAnchorRecheckDelayLong = 4 * time.Second
 
+	// pfExecFailureBackoff suppresses repeated pfctl/scutil checks briefly after
+	// macOS reports local resource exhaustion. Without this, network-change storms
+	// can turn a pf ruleset race into a fork/file-descriptor exhaustion loop.
+	pfExecFailureBackoff = 5 * time.Second
+
 	// pfVPNInterfacePrefixes lists interface name prefixes that indicate VPN/tunnel
 	// interfaces on macOS. Used to add interface-specific DNS intercept rules so that
 	// VPN software with "pass out quick on <iface>" rules cannot bypass our intercept.
@@ -1207,7 +1212,6 @@ func stringSlicesEqual(a, b []string) bool {
 	return true
 }
 
-
 // pfStartStabilization enters stabilization mode, suppressing all pf restores
 // until the VPN's ruleset stops changing. This prevents a death spiral where
 // ctrld and the VPN repeatedly overwrite each other's pf rules.
@@ -1284,6 +1288,10 @@ func (p *prog) pfStabilizationLoop(ctx context.Context, stableRequired time.Dura
 			p.pfStabilizing.Store(false)
 			mainLog.Load().Info().Msgf("DNS intercept: pf stable for %s — restoring anchor rules", stableRequired)
 			p.ensurePFAnchorActive()
+			routes, domainlessServers, exemptions := p.refreshDNSAfterVPNSettle("pf_stabilized")
+			if routes == 0 && domainlessServers == 0 && exemptions == 0 {
+				p.scheduleDNSAfterVPNSettleRefresh("pf_stabilized_followup", pfAnchorRecheckDelayLong)
+			}
 			p.pfLastRestoreTime.Store(time.Now().UnixMilli())
 			return
 		}
@@ -1300,6 +1308,15 @@ func (p *prog) ensurePFAnchorActive() bool {
 	if p.dnsInterceptState == nil {
 		return false
 	}
+	if !p.pfEnsureRunning.CompareAndSwap(false, true) {
+		mainLog.Load().Debug().Msg("DNS intercept watchdog: check already running, skipping duplicate")
+		return false
+	}
+	defer p.pfEnsureRunning.Store(false)
+
+	if p.pfExecBackoffActive() {
+		return false
+	}
 
 	// While stabilizing (VPN connecting), suppress all restores.
 	// The stabilization loop will restore once pf settles.
@@ -1333,6 +1350,9 @@ func (p *prog) ensurePFAnchorActive() bool {
 	// Check 1: anchor references in the main ruleset.
 	natOut, err := exec.Command("pfctl", "-sn").CombinedOutput()
 	if err != nil {
+		if p.pfBackoffResourceExhaustion(err, natOut, "dump NAT rules") {
+			return false
+		}
 		mainLog.Load().Warn().Err(err).Msg("DNS intercept watchdog: could not dump NAT rules")
 		return false
 	}
@@ -1345,6 +1365,9 @@ func (p *prog) ensurePFAnchorActive() bool {
 	if !needsRestore {
 		filterOut, err := exec.Command("pfctl", "-sr").CombinedOutput()
 		if err != nil {
+			if p.pfBackoffResourceExhaustion(err, filterOut, "dump filter rules") {
+				return false
+			}
 			mainLog.Load().Warn().Err(err).Msg("DNS intercept watchdog: could not dump filter rules")
 			return false
 		}
@@ -1362,6 +1385,9 @@ func (p *prog) ensurePFAnchorActive() bool {
 	if !needsRestore {
 		anchorFilter, err := exec.Command("pfctl", "-a", pfAnchorName, "-sr").CombinedOutput()
 		if err != nil || len(strings.TrimSpace(string(anchorFilter))) == 0 {
+			if p.pfBackoffResourceExhaustion(err, anchorFilter, "dump anchor filter rules") {
+				return false
+			}
 			mainLog.Load().Warn().Msg("DNS intercept watchdog: anchor has no filter rules — content was flushed")
 			needsRestore = true
 		}
@@ -1369,6 +1395,9 @@ func (p *prog) ensurePFAnchorActive() bool {
 	if !needsRestore {
 		anchorNat, err := exec.Command("pfctl", "-a", pfAnchorName, "-sn").CombinedOutput()
 		if err != nil || len(strings.TrimSpace(string(anchorNat))) == 0 {
+			if p.pfBackoffResourceExhaustion(err, anchorNat, "dump anchor NAT rules") {
+				return false
+			}
 			mainLog.Load().Warn().Msg("DNS intercept watchdog: anchor has no rdr rules — translation was flushed (will cause packet loop on lo0)")
 			needsRestore = true
 		}
@@ -1402,6 +1431,7 @@ func (p *prog) ensurePFAnchorActive() bool {
 	// Restore: re-inject anchor references into the main ruleset.
 	mainLog.Load().Info().Msg("DNS intercept watchdog: restoring pf anchor references")
 	if err := p.ensurePFAnchorReference(); err != nil {
+		p.pfBackoffResourceExhaustion(err, nil, "restore anchor references")
 		mainLog.Load().Error().Err(err).Msg("DNS intercept watchdog: failed to restore anchor references")
 		return true
 	}
@@ -1418,6 +1448,7 @@ func (p *prog) ensurePFAnchorActive() bool {
 	if err := os.WriteFile(pfAnchorFile, []byte(rulesStr), 0644); err != nil {
 		mainLog.Load().Error().Err(err).Msg("DNS intercept watchdog: failed to write anchor file")
 	} else if out, err := exec.Command("pfctl", "-a", pfAnchorName, "-f", pfAnchorFile).CombinedOutput(); err != nil {
+		p.pfBackoffResourceExhaustion(err, out, "load rebuilt anchor")
 		mainLog.Load().Error().Err(err).Msgf("DNS intercept watchdog: failed to load rebuilt anchor (output: %s)", strings.TrimSpace(string(out)))
 	} else {
 		flushPFStates()
@@ -1446,6 +1477,58 @@ func (p *prog) ensurePFAnchorActive() bool {
 	return true
 }
 
+func (p *prog) scheduleDNSAfterVPNSettleRefresh(reason string, delay time.Duration) {
+	time.AfterFunc(delay, func() {
+		if p.dnsInterceptState == nil {
+			return
+		}
+		p.refreshDNSAfterVPNSettle(reason)
+	})
+}
+
+func (p *prog) pfExecBackoffActive() bool {
+	until := p.pfExecBackoffUntil.Load()
+	if until == 0 {
+		return false
+	}
+	remaining := time.Until(time.UnixMilli(until))
+	if remaining <= 0 {
+		p.pfExecBackoffUntil.CompareAndSwap(until, 0)
+		return false
+	}
+	mainLog.Load().Debug().Dur("remaining", remaining).Msg("DNS intercept watchdog: suppressed during pf exec backoff")
+	return true
+}
+
+func (p *prog) pfBackoffResourceExhaustion(err error, output []byte, operation string) bool {
+	if !isResourceExhaustion(err, output) {
+		return false
+	}
+	until := time.Now().Add(pfExecFailureBackoff)
+	p.pfExecBackoffUntil.Store(until.UnixMilli())
+	mainLog.Load().Warn().Err(err).Dur("backoff", pfExecFailureBackoff).Str("operation", operation).
+		Msg("DNS intercept watchdog: backing off after local exec resource exhaustion")
+	return true
+}
+
+func isResourceExhaustion(err error, output []byte) bool {
+	if err == nil && len(output) == 0 {
+		return false
+	}
+	var msg string
+	if err != nil {
+		msg = err.Error()
+	}
+	if len(output) > 0 {
+		msg += "\n" + string(output)
+	}
+	msg = strings.ToLower(msg)
+	return strings.Contains(msg, "resource temporarily unavailable") ||
+		strings.Contains(msg, "too many open files") ||
+		strings.Contains(msg, "too many processes") ||
+		strings.Contains(msg, "cannot allocate memory")
+}
+
 // pfWatchdog periodically checks that our pf anchor is still active.
 // Other programs (e.g., Windscribe desktop app, macOS configd) can replace
 // scheduleDelayedRechecks schedules delayed re-checks after a network change event.
@@ -1458,8 +1541,19 @@ func (p *prog) ensurePFAnchorActive() bool {
 //
 // Two delays (2s and 4s) cover both fast and slow VPN teardowns.
 func (p *prog) scheduleDelayedRechecks() {
+	p.pfDelayedRecheckMu.Lock()
+	defer p.pfDelayedRecheckMu.Unlock()
+
+	for _, timer := range p.pfDelayedRecheckTimers {
+		if timer != nil {
+			timer.Stop()
+		}
+	}
+	p.pfDelayedRecheckTimers = p.pfDelayedRecheckTimers[:0]
+
 	for _, delay := range []time.Duration{pfAnchorRecheckDelay, pfAnchorRecheckDelayLong} {
-		time.AfterFunc(delay, func() {
+		delay := delay
+		timer := time.AfterFunc(delay, func() {
 			if p.dnsInterceptState == nil || p.pfStabilizing.Load() {
 				return
 			}
@@ -1473,6 +1567,7 @@ func (p *prog) scheduleDelayedRechecks() {
 				p.vpnDNS.Refresh(true)
 			}
 		})
+		p.pfDelayedRecheckTimers = append(p.pfDelayedRecheckTimers, timer)
 	}
 }
 
@@ -1811,7 +1906,14 @@ func (p *prog) forceReloadPFMainRuleset() {
 		mainLog.Load().Error().Err(err).Msgf("DNS intercept: force reload — failed to load anchor (output: %s)", strings.TrimSpace(string(out)))
 	}
 
-	// Reset upstream transports — pf reload flushes state table, killing DoH connections.
+	// Flush stale rdr/reply states after the forced ruleset + anchor reload.
+	// Without this, macOS can keep using pre-reload state and try to send
+	// redirected DNS replies directly from loopback to tunnel client addresses
+	// (for example, 127.0.0.1:<listener> -> 100.64.0.0/10), which fails with
+	// "sendmsg: can't assign requested address".
+	flushPFStates()
+
+	// Reset upstream transports — pf reload/state flush kills existing DoH connections.
 	p.resetUpstreamTransports()
 
 	mainLog.Load().Info().Msg("DNS intercept: force reload — pf ruleset and anchor reloaded successfully")

cmd/cli/dns_intercept_darwin_test.go

@@ -3,6 +3,7 @@
 package cli
 
 import (
+	"errors"
 	"strings"
 	"testing"
 
@@ -122,6 +123,35 @@ func TestPFBuildAnchorRules_Ordering(t *testing.T) {
 	}
 }
 
+// TestPFBuildAnchorRules_FallbackPort verifies that when the listener falls back
+// to an alternate local port (e.g. 5354 because mDNSResponder owns *:53), the pf
+// rdr rules redirect DNS to the ACTUAL bound port, not the configured default 53.
+// Regression test for #551: pf redirected to a dead port after listener fallback.
+func TestPFBuildAnchorRules_FallbackPort(t *testing.T) {
+	// Configured/generated listener is 127.0.0.1:53, but the runtime bound port is 5354.
+	p := &prog{cfg: &ctrld.Config{Listener: map[string]*ctrld.ListenerConfig{"0": {IP: "127.0.0.1", Port: 5354}}}}
+	rules := p.buildPFAnchorRules(nil)
+
+	// rdr must redirect to the actual bound port 5354.
+	if !strings.Contains(rules, "rdr on lo0 inet proto udp from any to ! 127.0.0.1 port 53 -> 127.0.0.1 port 5354") {
+		t.Errorf("UDP rdr must redirect to bound port 5354, got:\n%s", rules)
+	}
+	if !strings.Contains(rules, "rdr on lo0 inet proto tcp from any to ! 127.0.0.1 port 53 -> 127.0.0.1 port 5354") {
+		t.Errorf("TCP rdr must redirect to bound port 5354, got:\n%s", rules)
+	}
+
+	// The rdr redirect target must NOT point at the dead default port 53.
+	// Match the exact port at line end so "port 5354" is not a false positive.
+	if strings.Contains(rules, "-> 127.0.0.1 port 53\n") {
+		t.Errorf("rdr must not redirect to dead port 53 after fallback, got:\n%s", rules)
+	}
+
+	// The inbound accept rule must also target the actual bound port.
+	if !strings.Contains(rules, "127.0.0.1 port 5354") {
+		t.Errorf("pass in rule must reference bound port 5354, got:\n%s", rules)
+	}
+}
+
 // TestPFAddressFamily tests the pfAddressFamily helper.
 func TestPFAddressFamily(t *testing.T) {
 	tests := []struct {
@@ -141,3 +171,47 @@ func TestPFAddressFamily(t *testing.T) {
 		}
 	}
 }
+
+func TestIsResourceExhaustion(t *testing.T) {
+	tests := []struct {
+		name   string
+		err    error
+		output []byte
+		want   bool
+	}{
+		{
+			name: "exec start failure",
+			err:  errors.New("fork/exec /sbin/pfctl: resource temporarily unavailable"),
+			want: true,
+		},
+		{
+			name:   "fd exhaustion from stderr output",
+			err:    errors.New("exit status 1"),
+			output: []byte("pfctl: Pipe: Too many open files"),
+			want:   true,
+		},
+		{
+			name: "process exhaustion from wrapped restore error",
+			err:  errors.New("failed to dump running filter rules: exit status 1 (output: too many processes)"),
+			want: true,
+		},
+		{
+			name:   "ordinary pf syntax failure",
+			err:    errors.New("exit status 1"),
+			output: []byte("pfctl: syntax error"),
+			want:   false,
+		},
+		{
+			name: "nil error and empty output",
+			want: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isResourceExhaustion(tt.err, tt.output); got != tt.want {
+				t.Fatalf("isResourceExhaustion() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

cmd/cli/dns_intercept_settle.go

@@ -0,0 +1,50 @@
+package cli
+
+import "github.com/Control-D-Inc/ctrld"
+
+var initializeOsResolver = ctrld.InitializeOsResolver
+
+func (p *prog) refreshDNSAfterVPNSettle(reason string) (routes, domainlessServers, exemptions int) {
+	mainLog.Load().Info().Msgf("DNS intercept: refreshing OS/VPN DNS route state after VPN settle (%s)", reason)
+	ns := initializeOsResolver(true)
+	mainLog.Load().Debug().Msgf("DNS intercept: post-settle OS resolver nameservers: %v", ns)
+
+	if p.vpnDNS == nil {
+		mainLog.Load().Debug().Msg("DNS intercept: post-settle VPN DNS route refresh skipped — manager unavailable")
+		return 0, 0, 0
+	}
+
+	beforeExemptions := p.vpnDNS.CurrentExemptions()
+	routes, domainlessServers, exemptions = p.vpnDNS.RefreshRoutesOnly()
+	afterExemptions := p.vpnDNS.CurrentExemptions()
+
+	if vpnDNSExemptionsEqual(beforeExemptions, afterExemptions) {
+		mainLog.Load().Info().Msgf("DNS intercept: post-settle VPN DNS route refresh completed — %d routes, %d domainless servers, %d exemptions (pf unchanged)",
+			routes, domainlessServers, exemptions)
+		return routes, domainlessServers, exemptions
+	}
+
+	if err := p.exemptVPNDNSServers(afterExemptions); err != nil {
+		mainLog.Load().Warn().Err(err).Msg("DNS intercept: post-settle VPN DNS exemption update failed")
+	} else {
+		mainLog.Load().Info().Msgf("DNS intercept: post-settle VPN DNS exemptions changed — updated pf/WFP with %d exemptions", len(afterExemptions))
+	}
+	return routes, domainlessServers, exemptions
+}
+
+func vpnDNSExemptionsEqual(a, b []vpnDNSExemption) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	seen := make(map[vpnDNSExemption]int, len(a))
+	for _, ex := range a {
+		seen[ex]++
+	}
+	for _, ex := range b {
+		if seen[ex] == 0 {
+			return false
+		}
+		seen[ex]--
+	}
+	return true
+}

cmd/cli/dns_intercept_settle_test.go

@@ -0,0 +1,49 @@
+package cli
+
+import (
+	"context"
+	"testing"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+func TestRefreshDNSAfterVPNSettleRefreshesOSResolverAndVPNRoutes(t *testing.T) {
+	oldInitialize := initializeOsResolver
+	defer func() { initializeOsResolver = oldInitialize }()
+
+	var initialized []bool
+	initializeOsResolver = func(force bool) []string {
+		initialized = append(initialized, force)
+		return []string{"10.102.26.10:53"}
+	}
+
+	var exemptionUpdates [][]vpnDNSExemption
+	p := &prog{}
+	p.vpnDNS = newVPNDNSManager(func(exemptions []vpnDNSExemption) error {
+		exemptionUpdates = append(exemptionUpdates, append([]vpnDNSExemption{}, exemptions...))
+		return nil
+	})
+	p.vpnDNS.discoverVPNDNS = func(context.Context) []ctrld.VPNDNSConfig {
+		return []ctrld.VPNDNSConfig{{
+			InterfaceName: "utun4",
+			Servers:       []string{"10.102.26.10"},
+			Domains:       []string{"bmwgroup.net"},
+		}}
+	}
+
+	routes, domainlessServers, exemptions := p.refreshDNSAfterVPNSettle("test")
+
+	if routes != 1 || domainlessServers != 0 || exemptions != 1 {
+		t.Fatalf("expected 1 route, 0 domainless servers, 1 exemption, got routes=%d domainless=%d exemptions=%d",
+			routes, domainlessServers, exemptions)
+	}
+	if len(initialized) != 1 || !initialized[0] {
+		t.Fatalf("expected forced OS resolver refresh once, got %v", initialized)
+	}
+	if got := p.vpnDNS.UpstreamForDomain("jira.cc.bmwgroup.net."); len(got) != 1 || got[0] != "10.102.26.10" {
+		t.Fatalf("expected refreshed VPN DNS route, got %v", got)
+	}
+	if len(exemptionUpdates) != 0 {
+		t.Fatalf("expected route-only refresh to avoid pf exemption updates, got %+v", exemptionUpdates)
+	}
+}

cmd/cli/dns_intercept_windows.go

@@ -278,6 +278,9 @@ type wfpState struct {
 	loopbackProtectActive bool
 	// loopbackPermitIDs stores the filter IDs for the loopback protect permits.
 	loopbackPermitIDs []uint64
+	// nrptRecoveryLimiter prevents repeated Windows policy/Dnscache signaling
+	// when another agent keeps putting NRPT back into a broken state.
+	nrptRecoveryLimiter nrptRecoveryLimiter
 }
 
 // Lazy-loaded WFP DLL procedures.
@@ -343,9 +346,10 @@ const (
 //   - GP path: SOFTWARE\Policies\...\DnsPolicyConfig (Group Policy)
 //   - Local path: SYSTEM\CurrentControlSet\...\DnsPolicyConfig (service store)
 //
-// If ANY rules exist in the GP path (from IT policy, VPN, MDM, etc.), DNS Client
-// enters "GP mode" and ignores ALL local-path rules entirely. Conversely, if the
-// GP path is empty/absent, DNS Client reads from the local path only.
+// If the GP path contains real rules (from IT policy, VPN, MDM, etc.), DNS
+// Client enters "GP mode" and ignores ALL local-path rules entirely. An empty GP
+// parent key is worse: it still puts DNS Client in GP mode, but contributes no
+// usable rule, so our local catch-all is hidden until that empty parent is gone.
 //
 // Strategy (matching Tailscale's approach):
 //   - Always write to the local path (baseline for non-domain machines).
@@ -395,18 +399,20 @@ func otherGPRulesExist() bool {
 	return false
 }
 
-// cleanGPPath removes our CtrldCatchAll rule from the GP path and deletes
-// the GP DnsPolicyConfig parent key if no other rules remain. Removing the
-// empty GP key is critical: its mere existence forces DNS Client into "GP mode"
-// where local-path rules are ignored.
-func cleanGPPath() {
+// cleanGPPath removes only ctrld's GP-path rule and deletes the GP parent when
+// no rules remain. The return value tells callers whether the parent key was
+// actually deleted, which means DNS Client should be signaled once.
+//
+// Do not leave an empty GP parent behind: Windows treats the parent key itself
+// as the policy store boundary, so an empty key can still hide local-path rules.
+func cleanGPPath() bool {
 	// Delete our specific rule.
 	registry.DeleteKey(registry.LOCAL_MACHINE, nrptBaseKey+`\`+nrptRuleName)
 
 	// If the GP parent key is now empty, delete it entirely to exit "GP mode".
 	k, err := registry.OpenKey(registry.LOCAL_MACHINE, nrptBaseKey, registry.ENUMERATE_SUB_KEYS)
 	if err != nil {
-		return // Key doesn't exist — clean state.
+		return false // Key doesn't exist — clean state.
 	}
 	names, err := k.ReadSubKeyNames(-1)
 	k.Close()
@@ -414,12 +420,14 @@ func cleanGPPath() {
 		if len(names) > 0 {
 			mainLog.Load().Debug().Strs("remaining", names).Msg("DNS intercept: GP path has other rules, leaving parent key")
 		}
-		return
+		return false
 	}
 	// Empty — delete it to exit "GP mode".
 	if err := registry.DeleteKey(registry.LOCAL_MACHINE, nrptBaseKey); err == nil {
 		mainLog.Load().Info().Msg("DNS intercept: deleted empty GP DnsPolicyConfig key (exits GP mode)")
+		return true
 	}
+	return false
 }
 
 // writeNRPTRule writes a single NRPT catch-all rule at the given registry keyPath.
@@ -542,10 +550,11 @@ func refreshNRPTPolicy() {
 // Group Policy refresh so NRPT changes take effect immediately.
 // Uses DnsFlushResolverCache from dnsapi.dll + RefreshPolicyEx from userenv.dll.
 func flushDNSCache() {
-	// Step 1: Refresh GP so DNS Client loads the new NRPT rules from registry.
 	refreshNRPTPolicy()
+	flushDNSCacheOnly()
+}
 
-	// Step 2: Flush the DNS cache so stale entries from pre-NRPT resolution are cleared.
+func flushDNSCacheOnly() {
 	if err := dnsapiDLL.Load(); err == nil {
 		if err := procDnsFlushResolverCache.Find(); err == nil {
 			ret, _, _ := procDnsFlushResolverCache.Call()
@@ -555,7 +564,6 @@ func flushDNSCache() {
 			}
 		}
 	}
-	// Fallback: use ipconfig /flushdns.
 	if out, err := exec.Command("ipconfig", "/flushdns").CombinedOutput(); err != nil {
 		mainLog.Load().Debug().Msgf("DNS intercept: ipconfig /flushdns failed: %v: %s", err, string(out))
 	} else {
@@ -563,6 +571,12 @@ func flushDNSCache() {
 	}
 }
 
+func signalNRPTChange() {
+	refreshNRPTPolicy()
+	sendParamChange()
+	flushDNSCacheOnly()
+}
+
 // startDNSIntercept activates WFP-based DNS interception on Windows.
 // It creates a WFP sublayer and adds filters that block all outbound DNS (port 53)
 // traffic except to localhost (127.0.0.1/::1), ensuring all DNS queries must go
@@ -598,21 +612,19 @@ func (p *prog) startDNSIntercept() error {
 	mainLog.Load().Info().Msgf("DNS intercept: initializing (mode: %s)", interceptMode)
 	logNRPTParentKeyState("pre-write")
 
-	// Two-phase empty parent key recovery: if the GP DnsPolicyConfig key exists
-	// but is empty, DNS Client has cached a "no rules" state and won't accept
-	// new rules even after they're written. Delete the empty key and signal DNS
-	// Client to reset before writing our rule.
-	// Two-phase recovery handles its own 2s signaling burst internally.
-	cleanEmptyNRPTParent()
+	// Empty parent key recovery: if the GP DnsPolicyConfig key exists but is
+	// empty, DNS Client enters GP mode and hides local rules. Delete empty
+	// parents first, then send one change signal so DNS Client drops stale state.
+	if cleanEmptyNRPTParent() {
+		signalNRPTChange()
+	}
 
 	if err := addNRPTCatchAllRule(listenerIP); err != nil {
 		return fmt.Errorf("dns intercept: failed to add NRPT catch-all rule: %w", err)
 	}
 	logNRPTParentKeyState("post-write")
 	state.nrptActive = true
-	refreshNRPTPolicy()
-	sendParamChange()
-	flushDNSCache()
+	signalNRPTChange()
 	mainLog.Load().Info().Msgf("DNS intercept: NRPT catch-all rule active — all DNS queries directed to %s", listenerIP)
 
 	// Step 2: In hard mode, also set up WFP filters to block non-local DNS.
@@ -1555,7 +1567,7 @@ func (p *prog) scheduleDelayedRechecks() {
 					mainLog.Load().Error().Err(err).Msg("DNS intercept: failed to re-add NRPT catch-all rule")
 					state.nrptActive = false
 				} else {
-					flushDNSCache()
+					signalNRPTChange()
 					mainLog.Load().Info().Msg("DNS intercept: NRPT catch-all rule restored")
 				}
 			}
@@ -1593,14 +1605,22 @@ func (p *prog) nrptHealthMonitor(state *wfpState) {
 
 			// Step 1: Check registry key exists.
 			if !nrptCatchAllRuleExists() {
+				now := time.Now()
+				if ok, wait := state.nrptRecoveryLimiter.allow(now, p.cfg); !ok {
+					if state.nrptRecoveryLimiter.shouldLogSkip(now) {
+						mainLog.Load().Warn().Dur("remaining", wait).
+							Msg("DNS intercept: NRPT rule restore suppressed after repeated recovery flows")
+					}
+					continue
+				}
 				mainLog.Load().Warn().Msg("DNS intercept: NRPT health check — catch-all rule missing, restoring")
 				if err := addNRPTCatchAllRule(state.listenerIP); err != nil {
 					mainLog.Load().Error().Err(err).Msg("DNS intercept: failed to restore NRPT catch-all rule")
 					state.nrptActive = false
 					continue
 				}
-				refreshNRPTPolicy()
-				flushDNSCache()
+				signalNRPTChange()
+				state.nrptRecoveryLimiter.recordRecoveryFlow(time.Now(), p.cfg)
 				mainLog.Load().Info().Msg("DNS intercept: NRPT catch-all rule restored by health monitor")
 				// After restoring, verify it's actually working.
 				go p.nrptProbeAndHeal()
@@ -1612,6 +1632,8 @@ func (p *prog) nrptHealthMonitor(state *wfpState) {
 			if !p.probeNRPT() {
 				mainLog.Load().Warn().Msg("DNS intercept: NRPT health check — rule present but probe failed, running heal cycle")
 				go p.nrptProbeAndHeal()
+			} else {
+				state.nrptRecoveryLimiter.recordStableSuccess()
 			}
 
 			// Step 3: In hard mode, also verify WFP sublayer.
@@ -1710,43 +1732,39 @@ func sendParamChange() {
 }
 
 // cleanEmptyNRPTParent removes empty NRPT parent keys that block activation.
-// An empty DnsPolicyConfig key (exists but no subkeys) causes DNS Client to
-// cache "no rules" and ignore subsequently-added rules.
+// Empty GP and local parents have different failure shapes:
+//   - empty GP parent: DNS Client is in GP mode and ignores local-path rules;
+//   - empty local parent: DNS Client can cache an empty local policy store.
 //
-// Also cleans the GP path entirely if it has no non-ctrld rules, since the GP
-// path's existence forces DNS Client into "GP mode" where it ignores the local
-// service store path.
+// This helper only changes registry state. The caller sends the single
+// RefreshPolicyEx/paramchange/flush signal after it knows cleanup occurred.
 //
-// Returns true if cleanup was performed (caller should add a delay).
+// Returns true if cleanup was performed (caller should signal DNS Client).
 func cleanEmptyNRPTParent() bool {
-	cleaned := false
-
 	// Always clean the GP path — its existence blocks local path activation.
-	cleanGPPath()
+	cleaned := cleanGPPath()
 
 	// Clean empty local/direct path parent key.
-	k, err := registry.OpenKey(registry.LOCAL_MACHINE, nrptDirectKey, registry.ENUMERATE_SUB_KEYS)
-	if err != nil {
-		return false
-	}
-	names, err := k.ReadSubKeyNames(-1)
-	k.Close()
-	if err != nil || len(names) > 0 {
-		return false
+	if !nrptParentKeyEmpty(nrptDirectKey) {
+		return cleaned
 	}
 
 	mainLog.Load().Warn().Msg("DNS intercept: found empty NRPT local parent key (blocks activation) — removing")
 	if err := registry.DeleteKey(registry.LOCAL_MACHINE, nrptDirectKey); err != nil {
 		mainLog.Load().Warn().Err(err).Msg("DNS intercept: failed to delete empty NRPT local parent key")
+		return cleaned
+	}
+	return true
+}
+
+func nrptParentKeyEmpty(keyPath string) bool {
+	k, err := registry.OpenKey(registry.LOCAL_MACHINE, keyPath, registry.ENUMERATE_SUB_KEYS)
+	if err != nil {
 		return false
 	}
-	cleaned = true
-
-	// Signal DNS Client to process the deletion and reset its internal cache.
-	mainLog.Load().Info().Msg("DNS intercept: empty NRPT parent key removed — signaling DNS Client")
-	sendParamChange()
-	flushDNSCache()
-	return cleaned
+	names, err := k.ReadSubKeyNames(-1)
+	k.Close()
+	return err == nil && len(names) == 0
 }
 
 // logNRPTParentKeyState logs the state of both NRPT registry paths for diagnostics.
@@ -1783,18 +1801,39 @@ func logNRPTParentKeyState(context string) {
 // nrptProbeAndHeal runs the NRPT probe with retries and escalating remediation.
 // Called asynchronously after startup and from the health monitor.
 //
-// Retry sequence (each attempt: GP refresh + paramchange + flush → sleep → probe):
-//  1. Immediate probe
-//  2. GP refresh + paramchange + flush → 1s → probe
-//  3. GP refresh + paramchange + flush → 2s → probe
-//  4. GP refresh + paramchange + flush → 4s → probe
+// Retry sequence:
+//  1. Immediate probe.
+//  2. If the GP parent is empty, clean it immediately, signal once, then probe.
+//     This is intentionally before the normal retry loop: policy refresh and
+//     Dnscache paramchange cannot make local rules visible while GP mode is
+//     selected by an empty GP parent.
+//  3. Otherwise, signal DNS Client with increasing backoff between probes.
 func (p *prog) nrptProbeAndHeal() {
+	state, _ := p.dnsInterceptState.(*wfpState)
+	if state != nil {
+		now := time.Now()
+		if ok, wait := state.nrptRecoveryLimiter.allow(now, p.cfg); !ok {
+			if state.nrptRecoveryLimiter.shouldLogSkip(now) {
+				mainLog.Load().Warn().Dur("remaining", wait).
+					Msg("DNS intercept: NRPT recovery suppressed after repeated failed recovery flows")
+			}
+			return
+		}
+	}
+
 	if !nrptProbeRunning.CompareAndSwap(false, true) {
 		mainLog.Load().Debug().Msg("DNS intercept: NRPT probe already running, skipping")
 		return
 	}
 	defer nrptProbeRunning.Store(false)
 
+	remediated := false
+	defer func() {
+		if remediated && state != nil {
+			state.nrptRecoveryLimiter.recordRecoveryFlow(time.Now(), p.cfg)
+		}
+	}()
+
 	mainLog.Load().Info().Msg("DNS intercept: starting NRPT verification probe sequence")
 
 	// Log parent key state for diagnostics.
@@ -1805,17 +1844,37 @@ func (p *prog) nrptProbeAndHeal() {
 		mainLog.Load().Info().Msg("DNS intercept: NRPT verified working")
 		return
 	}
+	remediated = true
 
-	// Attempts 2-4: GP refresh + paramchange + flush with increasing backoff
+	// If the GP parent exists but is empty, do not burn retries on Windows
+	// signaling. Those retries create SIEM noise but cannot succeed because DNS
+	// Client is still reading the empty GP store instead of the populated local
+	// store. Delete the blocker, send one notification, then re-probe.
+	if nrptParentKeyEmpty(nrptBaseKey) {
+		mainLog.Load().Warn().Msg("DNS intercept: NRPT probe failed with empty GP parent — cleaning before retry signaling")
+		if cleanEmptyNRPTParent() {
+			signalNRPTChange()
+			time.Sleep(1 * time.Second)
+			logNRPTParentKeyState("empty-gp-after-clean")
+			if p.probeNRPT() {
+				mainLog.Load().Info().Msg("DNS intercept: NRPT verified working after empty GP parent cleanup")
+				return
+			}
+		}
+		if nrptParentKeyEmpty(nrptBaseKey) {
+			mainLog.Load().Warn().Msg("DNS intercept: empty GP NRPT parent still present after cleanup; skipping redundant policy refresh retries")
+			return
+		}
+	}
+
+	// Attempts 2-4: signal DNS Client with increasing backoff between probes.
 	delays := []time.Duration{1 * time.Second, 2 * time.Second, 4 * time.Second}
 	for i, delay := range delays {
 		attempt := i + 2
 		mainLog.Load().Info().Int("attempt", attempt).Dur("delay", delay).
-			Msg("DNS intercept: NRPT probe failed, retrying with GP refresh + paramchange")
+			Msg("DNS intercept: NRPT probe failed, retrying with policy refresh + paramchange")
 		logNRPTParentKeyState(fmt.Sprintf("probe-attempt-%d", attempt))
-		refreshNRPTPolicy()
-		sendParamChange()
-		flushDNSCache()
+		signalNRPTChange()
 		time.Sleep(delay)
 		if p.probeNRPT() {
 			mainLog.Load().Info().Int("attempt", attempt).
@@ -1829,7 +1888,7 @@ func (p *prog) nrptProbeAndHeal() {
 	// signal DNS Client to forget it, wait, then re-add and signal again.
 	mainLog.Load().Warn().Msg("DNS intercept: all probes failed — attempting two-phase NRPT recovery (delete → signal → re-add)")
 	listenerIP := "127.0.0.1"
-	if state, ok := p.dnsInterceptState.(*wfpState); ok {
+	if state != nil {
 		listenerIP = state.listenerIP
 	}
 
@@ -1837,9 +1896,7 @@ func (p *prog) nrptProbeAndHeal() {
 	_ = removeNRPTCatchAllRule()
 	// If parent key is now empty after removing our rule, delete it too.
 	cleanEmptyNRPTParent()
-	refreshNRPTPolicy()
-	sendParamChange()
-	flushDNSCache()
+	signalNRPTChange()
 	logNRPTParentKeyState("nuclear-after-delete")
 
 	// Wait for DNS Client to process the deletion.
@@ -1850,9 +1907,7 @@ func (p *prog) nrptProbeAndHeal() {
 		mainLog.Load().Error().Err(err).Msg("DNS intercept: failed to re-add NRPT after nuclear recovery")
 		return
 	}
-	refreshNRPTPolicy()
-	sendParamChange()
-	flushDNSCache()
+	signalNRPTChange()
 	logNRPTParentKeyState("nuclear-after-readd")
 
 	// Final probe after recovery.

cmd/cli/dns_proxy.go

@@ -736,6 +736,17 @@ func (p *prog) proxy(ctx context.Context, req *proxyRequest) *proxyResponse {
 			}
 			continue
 		}
+		// Reject an answer whose question does not match the request before it
+		// can be served or cached. A mismatched question means the upstream
+		// answered a different name/type than asked; caching it would poison
+		// the shared cache with wrong-domain records for the requested name.
+		// See github.com/Control-D-Inc/ctrld/issues/322.
+		if !sameQuestion(req.msg, answer) {
+			ctrld.Log(ctx, mainLog.Load().Debug(),
+				"discarding answer from %s: question mismatch (asked %q, got %q)",
+				upstreams[n], questionString(req.msg), questionString(answer))
+			continue
+		}
 		// We are doing LAN/PTR lookup using private resolver, so always process next one.
 		// Except for the last, we want to send response instead of saying all upstream failed.
 		if answer.Rcode != dns.RcodeSuccess && isLanOrPtrQuery && n != len(upstreamConfigs)-1 {
@@ -899,6 +910,33 @@ func containRcode(rcodes []int, rcode int) bool {
 	return false
 }
 
+// sameQuestion reports whether the upstream answer echoes the request's
+// question. A well-behaved resolver always copies the question section from
+// the query (RFC 1035 section 4.1.2); names are compared case-insensitively
+// because DNS names are case-insensitive. A mismatch means the upstream
+// answered a different name/type than asked - malformed or malicious - and the
+// answer must not be served or cached, or it would poison the shared cache with
+// wrong-domain records. See github.com/Control-D-Inc/ctrld/issues/322.
+func sameQuestion(req, answer *dns.Msg) bool {
+	if req == nil || answer == nil {
+		return false
+	}
+	if len(req.Question) == 0 || len(answer.Question) == 0 {
+		return false
+	}
+	rq, aq := req.Question[0], answer.Question[0]
+	return rq.Qtype == aq.Qtype && rq.Qclass == aq.Qclass && strings.EqualFold(rq.Name, aq.Name)
+}
+
+// questionString renders a message's first question as "name/type" for logging.
+func questionString(msg *dns.Msg) string {
+	if msg == nil || len(msg.Question) == 0 {
+		return "<none>"
+	}
+	q := msg.Question[0]
+	return q.Name + "/" + dns.TypeToString[q.Qtype]
+}
+
 func setCachedAnswerTTL(answer *dns.Msg, now, expiredTime time.Time) {
 	ttlSecs := expiredTime.Sub(now).Seconds()
 	if ttlSecs < 0 {
@@ -1299,7 +1337,8 @@ func isPrivatePtrLookup(m *dns.Msg) bool {
 			return addr.IsPrivate() ||
 				addr.IsLoopback() ||
 				addr.IsLinkLocalUnicast() ||
-				tsaddr.CGNATRange().Contains(addr)
+				tsaddr.CGNATRange().Contains(addr) ||
+				isServiceContinuityAddr(addr)
 		}
 	}
 	return false
@@ -1337,6 +1376,20 @@ func isLanHostname(name string) bool {
 		strings.HasSuffix(name, ".local")
 }
 
+// ipv4ServiceContinuityPrefix is the RFC 7335 IPv4 Service Continuity Prefix
+// (192.0.0.0/29), used by the CLAT in 464XLAT/DS-Lite transition setups. On such
+// networks (common on IPv6-only cellular carriers and iPhone hotspots) the local
+// machine's DNS queries reach ctrld with a source in this range (e.g. 192.0.0.2),
+// so they must be treated as local, not WAN. Go's netip.IsPrivate does not cover
+// this range — the same reason the CGNAT range is special-cased below. See #552.
+var ipv4ServiceContinuityPrefix = netip.MustParsePrefix("192.0.0.0/29")
+
+// isServiceContinuityAddr reports whether ip is in the RFC 7335 IPv4 Service
+// Continuity Prefix (464XLAT/DS-Lite CLAT).
+func isServiceContinuityAddr(ip netip.Addr) bool {
+	return ipv4ServiceContinuityPrefix.Contains(ip)
+}
+
 // isWanClient reports whether the input is a WAN address.
 func isWanClient(na net.Addr) bool {
 	var ip netip.Addr
@@ -1347,7 +1400,8 @@ func isWanClient(na net.Addr) bool {
 		!ip.IsPrivate() &&
 		!ip.IsLinkLocalUnicast() &&
 		!ip.IsLinkLocalMulticast() &&
-		!tsaddr.CGNATRange().Contains(ip)
+		!tsaddr.CGNATRange().Contains(ip) &&
+		!isServiceContinuityAddr(ip)
 }
 
 // isIPv6LoopbackListener reports whether the listener address is [::1].
@@ -1733,6 +1787,15 @@ func (p *prog) checkUpstreamOnce(upstream string, uc *ctrld.UpstreamConfig) erro
 			mainLog.Load().Debug().Err(err).Msgf("Upstream %s check failed after %v (WFP loopback protect active)", upstream, duration)
 			return errOsHealthcheckSuppressed
 		}
+		// A no-route/network-unreachable failure means the endpoint's address
+		// family is available locally but unroutable (e.g. an IPv6 DoH endpoint
+		// while IPv6 is up but has no route). These repeat until the route
+		// returns and are handled by bounded backoff in the recovery loop, so
+		// keep them at debug to avoid sustained error-log spam.
+		if ctrldnet.IsUnreachable(err) {
+			mainLog.Load().Debug().Err(err).Msgf("Upstream %s check failed after %v (network unreachable)", upstream, duration)
+			return err
+		}
 		mainLog.Load().Error().Err(err).Msgf("Upstream %s check failed after %v", upstream, duration)
 		return err
 	}
@@ -1937,6 +2000,9 @@ func (p *prog) handleRecovery(reason RecoveryReason) {
 // waitForUpstreamRecovery checks the provided upstreams concurrently until one recovers.
 // It returns the name of the recovered upstream or an error if the check times out.
 func (p *prog) waitForUpstreamRecovery(ctx context.Context, upstreams map[string]*ctrld.UpstreamConfig) (string, error) {
+	recoveryCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
 	recoveredCh := make(chan string, 1)
 	var wg sync.WaitGroup
 
@@ -1948,9 +2014,10 @@ func (p *prog) waitForUpstreamRecovery(ctx context.Context, upstreams map[string
 			defer wg.Done()
 			mainLog.Load().Debug().Msgf("Starting recovery check loop for upstream: %s", name)
 			attempts := 0
+			unreachableStreak := 0
 			for {
 				select {
-				case <-ctx.Done():
+				case <-recoveryCtx.Done():
 					mainLog.Load().Debug().Msgf("Context canceled for upstream %s", name)
 					return
 				default:
@@ -1962,13 +2029,30 @@ func (p *prog) waitForUpstreamRecovery(ctx context.Context, upstreams map[string
 						select {
 						case recoveredCh <- name:
 							mainLog.Load().Debug().Msgf("Sent recovery notification for upstream %s", name)
+							cancel()
 						default:
 							mainLog.Load().Debug().Msg("Recovery channel full, another upstream already recovered")
 						}
 						return
 					}
-					mainLog.Load().Debug().Msgf("Upstream %s check failed, sleeping before retry", name)
-					time.Sleep(checkUpstreamBackoffSleep)
+					// Back off the retry cadence for an unroutable endpoint so a
+					// host with IPv6 up but no route to the IPv6 DoH endpoint does
+					// not re-bootstrap/re-check every checkUpstreamBackoffSleep and
+					// spam the log. The backoff is bounded (checkUpstreamUnreachableBackoffMax)
+					// so the endpoint is still re-probed and recovers when the route
+					// returns; any other failure resets to the base cadence.
+					sleep := checkUpstreamBackoffSleep
+					if ctrldnet.IsUnreachable(err) {
+						unreachableStreak++
+						sleep = unreachableRecoveryBackoff(unreachableStreak)
+						mainLog.Load().Debug().Msgf("Upstream %s unreachable (streak %d), backing off %s before retry", name, unreachableStreak, sleep)
+					} else {
+						unreachableStreak = 0
+						mainLog.Load().Debug().Msgf("Upstream %s check failed, sleeping before retry", name)
+					}
+					if !sleepWithContext(recoveryCtx, sleep) {
+						return
+					}
 
 					// if this is the upstreamOS and it's the 3rd attempt (or multiple of 3),
 					// we should try to reinit the OS resolver to ensure we can recover
@@ -1996,6 +2080,17 @@ func (p *prog) waitForUpstreamRecovery(ctx context.Context, upstreams map[string
 	return recovered, nil
 }
 
+func sleepWithContext(ctx context.Context, d time.Duration) bool {
+	timer := time.NewTimer(d)
+	defer timer.Stop()
+	select {
+	case <-timer.C:
+		return true
+	case <-ctx.Done():
+		return false
+	}
+}
+
 // buildRecoveryUpstreams constructs the map of upstream configurations to test.
 // For OS failures we supply the manual OS resolver upstream configuration.
 // For network change or regular failure we use the upstreams defined in p.cfg (ignoring OS).

cmd/cli/dns_proxy_test.go

@@ -405,6 +405,8 @@ func Test_isPrivatePtrLookup(t *testing.T) {
 		{"CGNAT", newDnsMsgPtr("100.66.27.28", t), true},
 		{"Loopback", newDnsMsgPtr("127.0.0.1", t), true},
 		{"Link Local Unicast", newDnsMsgPtr("fe80::69f6:e16e:8bdb:433f", t), true},
+		// RFC 7335 IPv4 Service Continuity Prefix (464XLAT/DS-Lite CLAT), see #552.
+		{"464XLAT CLAT host", newDnsMsgPtr("192.0.0.2", t), true},
 		{"Public IP", newDnsMsgPtr("8.8.8.8", t), false},
 	}
 	for _, tc := range tests {
@@ -452,6 +454,11 @@ func Test_isWanClient(t *testing.T) {
 		{"CGNAT", &net.UDPAddr{IP: net.ParseIP("100.66.27.28")}, false},
 		{"Loopback", &net.UDPAddr{IP: net.ParseIP("127.0.0.1")}, false},
 		{"Link Local Unicast", &net.UDPAddr{IP: net.ParseIP("fe80::69f6:e16e:8bdb:433f")}, false},
+		// RFC 7335 IPv4 Service Continuity Prefix (464XLAT/DS-Lite CLAT), see #552.
+		{"464XLAT PLAT side", &net.UDPAddr{IP: net.ParseIP("192.0.0.1")}, false},
+		{"464XLAT CLAT host", &net.UDPAddr{IP: net.ParseIP("192.0.0.2")}, false},
+		// Outside the /29 but inside 192.0.0.0/24: still WAN (fix is scoped to /29).
+		{"192.0.0.0/24 outside /29", &net.UDPAddr{IP: net.ParseIP("192.0.0.100")}, true},
 		{"Public", &net.UDPAddr{IP: net.ParseIP("8.8.8.8")}, true},
 	}
 	for _, tc := range tests {
@@ -474,3 +481,33 @@ func Test_prog_queryFromSelf(t *testing.T) {
 		p.queryFromSelf("foo")
 	})
 }
+
+func Test_sameQuestion(t *testing.T) {
+	mk := func(name string, qtype uint16) *dns.Msg {
+		m := new(dns.Msg)
+		m.SetQuestion(name, qtype)
+		return m
+	}
+	tests := []struct {
+		name   string
+		req    *dns.Msg
+		answer *dns.Msg
+		want   bool
+	}{
+		{"identical", mk("example.com.", dns.TypeA), mk("example.com.", dns.TypeA), true},
+		{"case insensitive", mk("Example.COM.", dns.TypeA), mk("example.com.", dns.TypeA), true},
+		{"different name", mk("victim.example.", dns.TypeA), mk("attacker.example.", dns.TypeA), false},
+		{"different type", mk("example.com.", dns.TypeA), mk("example.com.", dns.TypeAAAA), false},
+		{"nil req", nil, mk("example.com.", dns.TypeA), false},
+		{"nil answer", mk("example.com.", dns.TypeA), nil, false},
+		{"empty answer question", mk("example.com.", dns.TypeA), new(dns.Msg), false},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			if got := sameQuestion(tc.req, tc.answer); got != tc.want {
+				t.Errorf("sameQuestion() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}

cmd/cli/log_writer.go

@@ -318,6 +318,12 @@ func (p *prog) initInternalLogging(writers []io.Writer) {
 
 // needInternalLogging reports whether prog needs to run internal logging.
 func (p *prog) needInternalLogging() bool {
+	// Do not run in silent mode: the user explicitly asked for no logging, so
+	// ctrld must not create or write the persisted internal log file (nor reset
+	// the global level back to debug). See https://github.com/Control-D-Inc/ctrld/issues/320.
+	if silent {
+		return false
+	}
 	// Do not run in non-cd mode.
 	if cdUID == "" {
 		return false

cmd/cli/log_writer_silent_test.go

@@ -0,0 +1,66 @@
+package cli
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+// Test_needInternalLogging_silent is a regression test for
+// https://github.com/Control-D-Inc/ctrld/issues/320: running with --silent must
+// not enable internal logging, otherwise ctrld creates and writes
+// <homedir>/ctrld.log (and, when verbose==0, resets the global level back to
+// debug) despite the user asking for silence.
+func Test_needInternalLogging_silent(t *testing.T) {
+	origSilent, origCdUID := silent, cdUID
+	t.Cleanup(func() { silent, cdUID = origSilent, origCdUID })
+
+	tests := []struct {
+		name    string
+		silent  bool
+		cdUID   string
+		logPath string
+		want    bool
+	}{
+		{"silent suppresses internal logging in cd mode", true, "test-uid", "", false},
+		{"cd mode enables internal logging", false, "test-uid", "", true},
+		{"non-cd mode disabled", false, "", "", false},
+		{"explicit log path disables internal logging", false, "test-uid", "/var/log/ctrld.log", false},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			silent = tt.silent
+			cdUID = tt.cdUID
+			p := &prog{cfg: &ctrld.Config{}}
+			p.cfg.Service.LogPath = tt.logPath
+			if got := p.needInternalLogging(); got != tt.want {
+				t.Fatalf("needInternalLogging() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+// Test_initInternalLogging_silentCreatesNoFile drives the real initInternalLogging
+// path and asserts that a --silent --cd run does not create <homedir>/ctrld.log,
+// which is the observable failure reported in
+// https://github.com/Control-D-Inc/ctrld/issues/320.
+func Test_initInternalLogging_silentCreatesNoFile(t *testing.T) {
+	origSilent, origCdUID, origHomedir := silent, cdUID, homedir
+	t.Cleanup(func() { silent, cdUID, homedir = origSilent, origCdUID, origHomedir })
+
+	dir := t.TempDir()
+	homedir = dir
+	cdUID = "test-uid" // cd mode, which would otherwise enable internal logging
+	silent = true
+
+	p := &prog{cfg: &ctrld.Config{}}
+	p.initInternalLogging(nil)
+
+	logPath := filepath.Join(dir, logFileName)
+	if _, err := os.Stat(logPath); !os.IsNotExist(err) {
+		t.Fatalf("silent mode must not create %s (stat err = %v)", logPath, err)
+	}
+}

cmd/cli/main.go

@@ -56,6 +56,9 @@ const (
 	cdOrgFlagName          = "cd-org"
 	customHostnameFlagName = "custom-hostname"
 	nextdnsFlagName        = "nextdns"
+
+	// autoIface is the sentinel --iface value meaning "use the default gateway interface".
+	autoIface = "auto"
 )
 
 func init() {

cmd/cli/main_test.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"os"
+	"os/exec"
 	"strings"
 	"testing"
 
@@ -13,5 +14,20 @@ var logOutput strings.Builder
 func TestMain(m *testing.M) {
 	l := zerolog.New(&logOutput)
 	mainLog.Store(&l)
+
+	// Stub the self-upgrade command builder for the whole test binary. The real
+	// builder execs os.Executable() — which under `go test` IS this test binary
+	// — with positional args ("upgrade", ...). `go test` stops flag parsing at
+	// the first positional arg and ignores the rest, so the child just re-runs
+	// the entire suite, hits the upgrade tests again, and spawns more children:
+	// a fork bomb of detached processes that stalls the host and (on Windows)
+	// holds the test binary's image locked, breaking CI artifact cleanup.
+	// Point it at the test binary with a no-match -test.run so any test that
+	// reaches performUpgrade still exercises the cmd.Start() success path while
+	// the child exits immediately without recursing.
+	newUpgradeCmd = func(exe string) *exec.Cmd {
+		return exec.Command(exe, "-test.run=^$")
+	}
+
 	os.Exit(m.Run())
 }

cmd/cli/nrpt_recovery_limiter_windows.go

@@ -0,0 +1,101 @@
+//go:build windows
+
+package cli
+
+import (
+	"sync"
+	"time"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+const (
+	// Default to current behavior: keep recovering indefinitely unless configured.
+	defaultNRPTRecoveryMaxAttempts = 0
+	defaultNRPTRecoveryCooldown    = 30 * time.Minute
+
+	// Require more than one good health tick before clearing the circuit. A probe can
+	// pass briefly after delete/re-add even when another agent recreates broken NRPT state.
+	nrptRecoveryStableSuccessesToReset = 2
+)
+
+type nrptRecoveryLimiter struct {
+	mu              sync.Mutex
+	attempts        int
+	stableSuccesses int
+	cooldownUntil   time.Time
+	lastSkipLog     time.Time
+}
+
+func nrptRecoveryMaxAttempts(cfg *ctrld.Config) int {
+	if cfg != nil && cfg.Service.NRPTRecoveryMaxAttempts != nil {
+		return *cfg.Service.NRPTRecoveryMaxAttempts
+	}
+	return defaultNRPTRecoveryMaxAttempts
+}
+
+func nrptRecoveryCooldown(cfg *ctrld.Config) time.Duration {
+	if cfg != nil && cfg.Service.NRPTRecoveryCooldown != nil {
+		return *cfg.Service.NRPTRecoveryCooldown
+	}
+	return defaultNRPTRecoveryCooldown
+}
+
+func (l *nrptRecoveryLimiter) allow(now time.Time, cfg *ctrld.Config) (bool, time.Duration) {
+	maxAttempts := nrptRecoveryMaxAttempts(cfg)
+	if maxAttempts <= 0 {
+		return true, 0
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	if now.Before(l.cooldownUntil) {
+		return false, l.cooldownUntil.Sub(now)
+	}
+	return true, 0
+}
+
+func (l *nrptRecoveryLimiter) recordRecoveryFlow(now time.Time, cfg *ctrld.Config) {
+	maxAttempts := nrptRecoveryMaxAttempts(cfg)
+	if maxAttempts <= 0 {
+		return
+	}
+
+	cooldown := nrptRecoveryCooldown(cfg)
+	if cooldown <= 0 {
+		cooldown = defaultNRPTRecoveryCooldown
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	l.stableSuccesses = 0
+	l.attempts++
+	if l.attempts >= maxAttempts {
+		l.cooldownUntil = now.Add(cooldown)
+	}
+}
+
+func (l *nrptRecoveryLimiter) recordStableSuccess() {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	l.stableSuccesses++
+	if l.stableSuccesses >= nrptRecoveryStableSuccessesToReset {
+		l.attempts = 0
+		l.cooldownUntil = time.Time{}
+		l.lastSkipLog = time.Time{}
+	}
+}
+
+func (l *nrptRecoveryLimiter) shouldLogSkip(now time.Time) bool {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	if l.lastSkipLog.IsZero() || now.Sub(l.lastSkipLog) >= 5*time.Minute {
+		l.lastSkipLog = now
+		return true
+	}
+	return false
+}

cmd/cli/nrpt_recovery_limiter_windows_test.go

@@ -0,0 +1,74 @@
+//go:build windows
+
+package cli
+
+import (
+	"testing"
+	"time"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+func TestNRPTRecoveryLimiterCooldownAndStableReset(t *testing.T) {
+	maxAttempts := 2
+	cooldown := 10 * time.Minute
+	cfg := &ctrld.Config{}
+	cfg.Service.NRPTRecoveryMaxAttempts = &maxAttempts
+	cfg.Service.NRPTRecoveryCooldown = &cooldown
+
+	limiter := &nrptRecoveryLimiter{}
+	now := time.Unix(100, 0)
+
+	if ok, wait := limiter.allow(now, cfg); !ok || wait != 0 {
+		t.Fatalf("initial allow = %v, %v; want true, 0", ok, wait)
+	}
+
+	limiter.recordRecoveryFlow(now, cfg)
+	if ok, wait := limiter.allow(now.Add(time.Second), cfg); !ok || wait != 0 {
+		t.Fatalf("allow after first flow = %v, %v; want true, 0", ok, wait)
+	}
+
+	limiter.recordRecoveryFlow(now.Add(2*time.Second), cfg)
+	if ok, wait := limiter.allow(now.Add(3*time.Second), cfg); ok || wait <= 0 {
+		t.Fatalf("allow after max flows = %v, %v; want false, positive wait", ok, wait)
+	}
+
+	limiter.recordStableSuccess()
+	if ok, _ := limiter.allow(now.Add(4*time.Second), cfg); ok {
+		t.Fatal("one stable success cleared cooldown; want cooldown to remain")
+	}
+
+	limiter.recordStableSuccess()
+	if ok, wait := limiter.allow(now.Add(5*time.Second), cfg); !ok || wait != 0 {
+		t.Fatalf("allow after stable reset = %v, %v; want true, 0", ok, wait)
+	}
+}
+
+func TestNRPTRecoveryLimiterDefaultIsUnlimited(t *testing.T) {
+	cfg := &ctrld.Config{}
+	limiter := &nrptRecoveryLimiter{}
+	now := time.Unix(100, 0)
+
+	for i := 0; i < 10; i++ {
+		limiter.recordRecoveryFlow(now.Add(time.Duration(i)*time.Second), cfg)
+	}
+	if ok, wait := limiter.allow(now.Add(time.Hour), cfg); !ok || wait != 0 {
+		t.Fatalf("default allow after recovery flows = %v, %v; want true, 0", ok, wait)
+	}
+}
+
+func TestNRPTRecoveryLimiterUnlimited(t *testing.T) {
+	maxAttempts := 0
+	cfg := &ctrld.Config{}
+	cfg.Service.NRPTRecoveryMaxAttempts = &maxAttempts
+
+	limiter := &nrptRecoveryLimiter{}
+	now := time.Unix(100, 0)
+
+	for i := 0; i < 10; i++ {
+		limiter.recordRecoveryFlow(now.Add(time.Duration(i)*time.Second), cfg)
+	}
+	if ok, wait := limiter.allow(now.Add(time.Hour), cfg); !ok || wait != 0 {
+		t.Fatalf("unlimited allow = %v, %v; want true, 0", ok, wait)
+	}
+}

cmd/cli/prog.go

@@ -34,6 +34,7 @@ import (
 	"github.com/Control-D-Inc/ctrld/internal/clientinfo"
 	"github.com/Control-D-Inc/ctrld/internal/controld"
 	"github.com/Control-D-Inc/ctrld/internal/dnscache"
+	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
 	"github.com/Control-D-Inc/ctrld/internal/router"
 	"github.com/Control-D-Inc/ctrld/internal/router/dnsmasq"
 )
@@ -188,6 +189,21 @@ type prog struct {
 	// interception with exponential backoff and auto-heals if broken.
 	pfMonitorRunning atomic.Bool //lint:ignore U1000 used on darwin
 
+	// pfEnsureRunning ensures only one pf anchor validation/restoration runs at a time.
+	// Network-change callbacks, delayed rechecks, and the periodic watchdog can all
+	// converge during macOS interface churn; concurrent pfctl/scutil exec storms can
+	// exhaust process/file limits and make the outage worse.
+	pfEnsureRunning atomic.Bool //lint:ignore U1000 used on darwin
+
+	// pfExecBackoffUntil suppresses pf anchor validation after pfctl/scutil execs
+	// fail due host resource exhaustion (fork unavailable, too many open files).
+	pfExecBackoffUntil atomic.Int64 //lint:ignore U1000 used on darwin
+
+	// pfDelayedRecheckTimers coalesces delayed DNS-intercept rechecks after noisy
+	// network changes. Protected by pfDelayedRecheckMu.
+	pfDelayedRecheckMu     sync.Mutex    //lint:ignore U1000 used on darwin
+	pfDelayedRecheckTimers []*time.Timer //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
@@ -321,6 +337,18 @@ func (p *prog) runWait() {
 
 		p.mu.Lock()
 		*p.cfg = *newCfg
+		// In DNS-intercept mode on macOS, the DNS listener is bound once at startup and is
+		// NOT re-bound on reload (see prog.run: serveDNS is started only when !reload). When
+		// the configured/generated port (e.g. 127.0.0.1:53) is unavailable at startup because
+		// mDNSResponder owns *:53, ctrld falls back to an alternate local port (e.g. 5354).
+		// The on-disk config still declares 53, so adopting it here would revert p.cfg to a
+		// port nothing is listening on, and the pf rdr rules/probes rebuilt from p.cfg would
+		// target a dead port. Since a reload cannot move the running listener anyway, keep
+		// p.cfg pointing at the actual bound listener. The on-disk config (written above) is
+		// left unchanged. See #551.
+		if dnsIntercept && runtime.GOOS == "darwin" {
+			preserveBoundListeners(p.cfg.Listener, curListener)
+		}
 		p.mu.Unlock()
 
 		logger.Notice().Msg("reloading config successfully")
@@ -332,8 +360,41 @@ func (p *prog) runWait() {
 	}
 }
 
+// preserveBoundListeners overrides the IP/Port of each listener in newListeners with the
+// actual bound address from curListeners when they differ, logging the divergence. It is used
+// on config reload in DNS-intercept mode where the running listener is never re-bound, so a
+// port change on disk (e.g. reverting a fallback 5354 back to the generated 53) must not be
+// applied to the in-memory config that drives pf rdr rules and probes.
+//
+// Preservation is limited to fallback-eligible (default/unset, i.e. 127.0.0.1:53) listeners.
+// An explicit, non-default listener in the reloaded config is an intentional change that must
+// be applied: tryUpdateListenerConfigIntercept binds explicit listeners exactly (no fallback),
+// and the control-server reload handler detects the IP/port diff to trigger a restart that
+// re-binds. Reverting an explicit change here would make that comparison return 200 instead of
+// 201, silently dropping the new listener. See #551.
+func preserveBoundListeners(newListeners, curListeners map[string]*ctrld.ListenerConfig) {
+	for n, curLc := range curListeners {
+		newLc := newListeners[n]
+		if newLc == nil || curLc == nil {
+			continue
+		}
+		if newLc.IP == curLc.IP && newLc.Port == curLc.Port {
+			continue
+		}
+		if isExplicitInterceptListener(newLc.IP, newLc.Port) {
+			continue
+		}
+		mainLog.Load().Info().
+			Str("configured", net.JoinHostPort(newLc.IP, strconv.Itoa(newLc.Port))).
+			Str("actual", net.JoinHostPort(curLc.IP, strconv.Itoa(curLc.Port))).
+			Msg("DNS intercept: preserving actual bound listener across reload; on-disk config port not applied to running listener")
+		newLc.IP = curLc.IP
+		newLc.Port = curLc.Port
+	}
+}
+
 func (p *prog) preRun() {
-	if iface == "auto" {
+	if iface == autoIface {
 		iface = defaultIfaceName()
 		p.requiredMultiNICsConfig = requiredMultiNICsConfig()
 	}
@@ -1328,13 +1389,14 @@ func errAddrInUse(err error) bool {
 
 var _ = errAddrInUse
 
+// The unreachable winsock errnos (ENETUNREACH/EHOSTUNREACH) are matched via
+// ctrldnet.IsUnreachable, which owns their definitions.
+//
 // https://learn.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
 var (
 	windowsECONNREFUSED = syscall.Errno(10061)
-	windowsENETUNREACH  = syscall.Errno(10051)
 	windowsEINVAL       = syscall.Errno(10022)
 	windowsEADDRINUSE   = syscall.Errno(10048)
-	windowsEHOSTUNREACH = syscall.Errno(10065)
 )
 
 func errUrlNetworkError(err error) bool {
@@ -1351,14 +1413,14 @@ func errNetworkError(err error) bool {
 		if opErr.Temporary() {
 			return true
 		}
+		if ctrldnet.IsUnreachable(err) {
+			return true
+		}
 		switch {
 		case errors.Is(opErr.Err, syscall.ECONNREFUSED),
 			errors.Is(opErr.Err, syscall.EINVAL),
-			errors.Is(opErr.Err, syscall.ENETUNREACH),
-			errors.Is(opErr.Err, windowsENETUNREACH),
 			errors.Is(opErr.Err, windowsEINVAL),
-			errors.Is(opErr.Err, windowsECONNREFUSED),
-			errors.Is(opErr.Err, windowsEHOSTUNREACH):
+			errors.Is(opErr.Err, windowsECONNREFUSED):
 			return true
 		}
 	}
@@ -1651,6 +1713,19 @@ func shouldUpgrade(vt string, cv *semver.Version, logger *zerolog.Logger) bool {
 	return true
 }
 
+// newUpgradeCmd builds the detached command used to self-upgrade. It is a
+// package-level variable so tests can stub it. With the real implementation a
+// *test* binary would re-exec itself — os.Executable() is the test binary, and
+// because `go test` stops flag parsing at the first positional arg ("upgrade")
+// it ignores the args and re-runs the entire suite. That child hits the same
+// upgrade test and spawns another child, recursively: a fork bomb of detached
+// processes that pins the host and locks the test binary's image file.
+var newUpgradeCmd = func(exe string) *exec.Cmd {
+	cmd := exec.Command(exe, "upgrade", "prod", "-vv")
+	cmd.SysProcAttr = sysProcAttrForDetachedChildProcess()
+	return cmd
+}
+
 // performUpgrade executes the self-upgrade command.
 // Returns true if upgrade was initiated successfully, false otherwise.
 func performUpgrade(vt string) bool {
@@ -1659,8 +1734,7 @@ func performUpgrade(vt string) bool {
 		mainLog.Load().Error().Err(err).Msg("failed to get executable path, skipped self-upgrade")
 		return false
 	}
-	cmd := exec.Command(exe, "upgrade", "prod", "-vv")
-	cmd.SysProcAttr = sysProcAttrForDetachedChildProcess()
+	cmd := newUpgradeCmd(exe)
 	if err := cmd.Start(); err != nil {
 		mainLog.Load().Error().Err(err).Msg("failed to start self-upgrade")
 		return false

cmd/cli/prog_linux.go

@@ -14,6 +14,9 @@ import (
 )
 
 func init() {
+	if isAndroid() {
+		return
+	}
 	if r, err := newLoopbackOSConfigurator(); err == nil {
 		useSystemdResolved = r.Mode() == "systemd-resolved"
 	}

cmd/cli/prog_test.go

@@ -1,7 +1,11 @@
 package cli
 
 import (
+	"context"
+	"net"
+	"net/url"
 	"runtime"
+	"syscall"
 	"testing"
 	"time"
 
@@ -12,6 +16,32 @@ import (
 	"github.com/Control-D-Inc/ctrld"
 )
 
+func TestErrNetworkErrorTreatsNoRouteAsNetworkError(t *testing.T) {
+	err := &net.OpError{Op: "dial", Net: "tcp", Err: syscall.EHOSTUNREACH}
+	assert.True(t, errNetworkError(err))
+	assert.True(t, errUrlNetworkError(&url.Error{Op: "Get", URL: "https://dns.controld.com", Err: err}))
+}
+
+func TestSleepWithContext(t *testing.T) {
+	assert.True(t, sleepWithContext(context.Background(), time.Millisecond))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	start := time.Now()
+	assert.False(t, sleepWithContext(ctx, time.Minute))
+	assert.Less(t, time.Since(start), 100*time.Millisecond)
+}
+
+func TestUnreachableRecoveryBackoff(t *testing.T) {
+	// Streak starts at the base cadence and doubles each attempt, capped at the max.
+	assert.Equal(t, checkUpstreamBackoffSleep, unreachableRecoveryBackoff(0))
+	assert.Equal(t, checkUpstreamBackoffSleep, unreachableRecoveryBackoff(1))
+	assert.Equal(t, 2*checkUpstreamBackoffSleep, unreachableRecoveryBackoff(2))
+	assert.Equal(t, 4*checkUpstreamBackoffSleep, unreachableRecoveryBackoff(3))
+	assert.Equal(t, checkUpstreamUnreachableBackoffMax, unreachableRecoveryBackoff(100))
+}
+
 func Test_prog_dnsWatchdogEnabled(t *testing.T) {
 	p := &prog{cfg: &ctrld.Config{}}
 
@@ -262,6 +292,8 @@ func Test_performUpgrade(t *testing.T) {
 		},
 	}
 
+	// newUpgradeCmd is stubbed in TestMain so performUpgrade does not re-exec
+	// (and fork-bomb) the test binary; see the comment there.
 	for _, tc := range tests {
 		tc := tc
 		t.Run(tc.name, func(t *testing.T) {

cmd/cli/uninstall_invalid_test.go

@@ -0,0 +1,61 @@
+package cli
+
+import "testing"
+
+// Test_ensureRunningIfaceForInvalidUninstall is a regression test for issue-556:
+// after a reboot, the invalid-device self-uninstall path could run before the
+// running interface was known. Because resetDNS (via resetDNSForRunningIface)
+// silently skips DNS restoration when p.runningIface is empty, the OS was left
+// pointed at ctrld's local listener with no internet after the service was
+// removed. ensureRunningIfaceForInvalidUninstall must populate p.runningIface
+// before resetDNS runs.
+func Test_ensureRunningIfaceForInvalidUninstall(t *testing.T) {
+	// preRun mutates the package-level iface global; restore it after the test.
+	origIface := iface
+	t.Cleanup(func() { iface = origIface })
+
+	// newService needs the package service config; it is safe to build here
+	// because ensureRunningIfaceForInvalidUninstall only queries the (absent)
+	// control socket via runningIface, which returns nil when ctrld is not
+	// running, and performs no DNS or service mutation.
+	s, err := newService(&prog{}, svcConfig)
+	if err != nil {
+		t.Fatalf("newService: %v", err)
+	}
+
+	t.Run("iface flag already resolved but not yet copied", func(t *testing.T) {
+		iface = "eth-test"
+		p := &prog{}
+
+		// Precondition mirrors the buggy post-reboot state: an empty running
+		// interface would make resetDNS skip restoration entirely.
+		if p.runningIface != "" {
+			t.Fatalf("precondition: runningIface = %q, want empty", p.runningIface)
+		}
+
+		ensureRunningIfaceForInvalidUninstall(p, s)
+
+		if p.runningIface == "" {
+			t.Fatal("runningIface still empty after prepare: resetDNS would skip DNS " +
+				"restoration and leave the OS pointed at ctrld's local listener")
+		}
+		if p.runningIface != "eth-test" {
+			t.Fatalf("runningIface = %q, want the resolved iface %q", p.runningIface, "eth-test")
+		}
+	})
+
+	t.Run("iface unset falls back to auto-detected interface", func(t *testing.T) {
+		iface = ""
+		p := &prog{}
+
+		ensureRunningIfaceForInvalidUninstall(p, s)
+
+		// With iface unset the prep resolves "auto" to the default interface
+		// (defaultIfaceName never returns empty on the supported platforms), so
+		// resetDNS has a concrete interface to restore.
+		if p.runningIface == "" {
+			t.Fatal("runningIface still empty after prepare with iface unset: " +
+				"resetDNS would skip DNS restoration")
+		}
+	})
+}

cmd/cli/upstream_monitor.go

@@ -12,8 +12,27 @@ const (
 	maxFailureRequest = 50
 	// checkUpstreamBackoffSleep is the time interval between each upstream checks.
 	checkUpstreamBackoffSleep = 2 * time.Second
+	// checkUpstreamUnreachableBackoffMax caps the recovery retry interval for an
+	// endpoint that keeps failing with a network-unreachable error. It bounds
+	// the backoff so an unroutable endpoint is still re-probed periodically and
+	// recovers once the route returns.
+	checkUpstreamUnreachableBackoffMax = 60 * time.Second
 )
 
+// unreachableRecoveryBackoff returns the retry interval for the given streak of
+// consecutive network-unreachable failures. It starts at checkUpstreamBackoffSleep
+// and doubles each attempt, capped at checkUpstreamUnreachableBackoffMax.
+func unreachableRecoveryBackoff(streak int) time.Duration {
+	d := checkUpstreamBackoffSleep
+	for i := 1; i < streak; i++ {
+		d *= 2
+		if d >= checkUpstreamUnreachableBackoffMax {
+			return checkUpstreamUnreachableBackoffMax
+		}
+	}
+	return d
+}
+
 // upstreamMonitor performs monitoring upstreams health.
 type upstreamMonitor struct {
 	cfg *ctrld.Config

cmd/cli/vpn_dns.go

@@ -6,7 +6,9 @@ import (
 	"runtime"
 	"strings"
 	"sync"
+	"sync/atomic"
 
+	"github.com/rs/zerolog"
 	"tailscale.com/net/netmon"
 
 	"github.com/Control-D-Inc/ctrld"
@@ -49,6 +51,9 @@ type vpnDNSManager struct {
 	// discoverVPNDNS is injected for tests so Refresh does not depend on the
 	// runner host's real VPN/virtual adapter state.
 	discoverVPNDNS func(context.Context) []ctrld.VPNDNSConfig
+	// refreshRunning keeps noisy network-change storms from running overlapping
+	// scutil/networksetup VPN DNS discovery work.
+	refreshRunning atomic.Bool
 	// Called when VPN DNS server list changes, to update intercept exemptions.
 	onServersChanged vpnDNSExemptFunc
 }
@@ -68,6 +73,11 @@ func newVPNDNSManager(exemptFunc vpnDNSExemptFunc) *vpnDNSManager {
 // Called on network change events.
 func (m *vpnDNSManager) Refresh(guardAgainstNoNameservers bool) {
 	logger := mainLog.Load()
+	if !m.refreshRunning.CompareAndSwap(false, true) {
+		logger.Debug().Msg("VPN DNS refresh already running, skipping duplicate")
+		return
+	}
+	defer m.refreshRunning.Store(false)
 
 	logger.Debug().Msg("Refreshing VPN DNS configurations")
 	discoverVPNDNS := m.discoverVPNDNS
@@ -94,6 +104,8 @@ func (m *vpnDNSManager) Refresh(guardAgainstNoNameservers bool) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
+	previousExemptions := m.currentExemptionsLocked()
+
 	if vpnDNSSettlingEnabled && len(configs) == 0 && guardAgainstNoNameservers && m.hasVPNDNSStateLocked() {
 		if !m.retainedAfterEmptyDiscovery {
 			exemptions := m.currentExemptionsLocked()
@@ -180,14 +192,85 @@ func (m *vpnDNSManager) Refresh(guardAgainstNoNameservers bool) {
 	logger.Debug().Msgf("VPN DNS refresh completed: %d configs, %d routes, %d domainless servers, %d unique exemptions",
 		len(m.configs), len(m.routes), len(m.domainlessServers), len(exemptions))
 
-	// Update intercept rules to permit VPN DNS traffic.
-	// Always call onServersChanged — including when exemptions is empty — so that
-	// stale exemptions from a previous VPN session get cleared on disconnect.
-	if m.onServersChanged != nil {
-		if err := m.onServersChanged(exemptions); err != nil {
-			logger.Error().Err(err).Msg("Failed to update intercept exemptions for VPN DNS servers")
+	// Update intercept rules to permit VPN DNS traffic only when the exemption set
+	// actually changes. Network-change events can fire repeatedly while macOS/VPN
+	// state is otherwise identical; rewriting pf for identical exemptions can feed
+	// a self-triggering network-change loop. Empty exemptions are still applied
+	// when they differ from the previous set, so stale VPN exemptions are cleared
+	// on disconnect.
+	m.updateInterceptExemptionsIfChanged(logger, previousExemptions, exemptions, "VPN DNS")
+}
+
+func (m *vpnDNSManager) updateInterceptExemptionsIfChanged(logger *zerolog.Logger, before, after []vpnDNSExemption, reason string) {
+	if m.onServersChanged == nil {
+		return
+	}
+	if vpnDNSExemptionsEqual(before, after) {
+		logger.Debug().Msgf("VPN DNS exemptions unchanged after %s refresh; skipping intercept rule update", reason)
+		return
+	}
+	if err := m.onServersChanged(after); err != nil {
+		logger.Error().Err(err).Msg("Failed to update intercept exemptions for VPN DNS servers")
+	}
+}
+
+// RefreshRoutesOnly re-discovers VPN DNS configs and updates only ctrld's
+// in-memory split-DNS routes. It intentionally does not call onServersChanged,
+// so it does not rewrite/reload pf/WFP rules. Use this for post-settle discovery
+// checks where we only need to learn late-published VPN search domains.
+func (m *vpnDNSManager) RefreshRoutesOnly() (routes, domainlessServers, exemptions int) {
+	logger := mainLog.Load()
+
+	logger.Debug().Msg("Refreshing VPN DNS route state only")
+	discoverVPNDNS := m.discoverVPNDNS
+	if discoverVPNDNS == nil {
+		discoverVPNDNS = ctrld.DiscoverVPNDNS
+	}
+	configs := discoverVPNDNS(context.Background())
+
+	if dri, err := netmon.DefaultRouteInterface(); err == nil && dri != "" {
+		for i := range configs {
+			if configs[i].InterfaceName == dri {
+				configs[i].IsExitMode = true
+			}
 		}
 	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.retainedAfterEmptyDiscovery = false
+	m.configs = configs
+	m.routes = make(map[string][]string)
+
+	for _, config := range configs {
+		for _, domain := range config.Domains {
+			domain = strings.TrimPrefix(domain, "~")
+			domain = strings.TrimPrefix(domain, ".")
+			domain = strings.ToLower(domain)
+			if domain != "" {
+				m.routes[domain] = append([]string{}, config.Servers...)
+			}
+		}
+	}
+
+	var domainless []string
+	seenDomainless := make(map[string]bool)
+	for _, config := range configs {
+		if len(config.Domains) == 0 && len(config.Servers) > 0 {
+			for _, server := range config.Servers {
+				if !seenDomainless[server] {
+					seenDomainless[server] = true
+					domainless = append(domainless, server)
+				}
+			}
+		}
+	}
+	m.domainlessServers = domainless
+
+	logger.Debug().Msgf("VPN DNS route-only refresh completed: %d configs, %d routes, %d domainless servers, %d exemptions",
+		len(m.configs), len(m.routes), len(m.domainlessServers), len(m.currentExemptionsLocked()))
+	return len(m.routes), len(m.domainlessServers), len(m.currentExemptionsLocked())
 }
 
 func (m *vpnDNSManager) hasVPNDNSStateLocked() bool {

cmd/cli/vpn_dns_test.go

@@ -2,6 +2,8 @@ package cli
 
 import (
 	"context"
+	"sync"
+	"sync/atomic"
 	"testing"
 
 	"github.com/Control-D-Inc/ctrld"
@@ -14,6 +16,36 @@ func withVPNDNSSettlingEnabled(t *testing.T) {
 	t.Cleanup(func() { vpnDNSSettlingEnabled = old })
 }
 
+func TestVPNDNSRefreshSkipsConcurrentDuplicate(t *testing.T) {
+	m := newVPNDNSManager(nil)
+	started := make(chan struct{})
+	release := make(chan struct{})
+	done := make(chan struct{})
+	var once sync.Once
+	var calls atomic.Int32
+
+	m.discoverVPNDNS = func(context.Context) []ctrld.VPNDNSConfig {
+		calls.Add(1)
+		once.Do(func() { close(started) })
+		<-release
+		return nil
+	}
+
+	go func() {
+		defer close(done)
+		m.Refresh(true)
+	}()
+
+	<-started
+	m.Refresh(true)
+	close(release)
+	<-done
+
+	if calls.Load() != 1 {
+		t.Fatalf("expected overlapping refresh to be skipped, got %d discovery calls", calls.Load())
+	}
+}
+
 func TestVPNDNSRefreshRetainsStateForOneGuardedEmptyDiscovery(t *testing.T) {
 	withVPNDNSSettlingEnabled(t)
 	var gotExemptions []vpnDNSExemption
@@ -69,6 +101,31 @@ func TestVPNDNSRefreshClearsOnSecondGuardedEmptyDiscovery(t *testing.T) {
 	}
 }
 
+func TestVPNDNSRefreshSkipsUnchangedInterceptExemptions(t *testing.T) {
+	var updates [][]vpnDNSExemption
+	m := newVPNDNSManager(func(exemptions []vpnDNSExemption) error {
+		updates = append(updates, append([]vpnDNSExemption{}, exemptions...))
+		return nil
+	})
+	m.discoverVPNDNS = func(context.Context) []ctrld.VPNDNSConfig {
+		return []ctrld.VPNDNSConfig{{
+			InterfaceName: "utun-test",
+			Servers:       []string{"10.102.26.10"},
+			Domains:       []string{"example.internal"},
+		}}
+	}
+
+	m.Refresh(true)
+	m.Refresh(true)
+
+	if len(updates) != 1 {
+		t.Fatalf("expected exactly one intercept exemption update for unchanged VPN DNS state, got %d", len(updates))
+	}
+	if len(updates[0]) != 1 || updates[0][0].Server != "10.102.26.10" || updates[0][0].Interface != "utun-test" {
+		t.Fatalf("unexpected exemption update: %+v", updates[0])
+	}
+}
+
 func TestVPNDNSTransportFailureSuppressesFallbackOnlyWhileRetainingState(t *testing.T) {
 	withVPNDNSSettlingEnabled(t)
 	m := newVPNDNSManager(nil)

config.go

@@ -241,6 +241,8 @@ type ServiceConfig struct {
 	ForceRefetchWaitTime    *int           `mapstructure:"force_refetch_wait_time" toml:"force_refetch_wait_time,omitempty"`
 	LeakOnUpstreamFailure   *bool          `mapstructure:"leak_on_upstream_failure" toml:"leak_on_upstream_failure,omitempty"`
 	InterceptMode           string         `mapstructure:"intercept_mode" toml:"intercept_mode,omitempty" validate:"omitempty,oneof=off dns hard"`
+	NRPTRecoveryMaxAttempts *int           `mapstructure:"nrpt_recovery_max_attempts" toml:"nrpt_recovery_max_attempts,omitempty" validate:"omitempty,gte=0"`
+	NRPTRecoveryCooldown    *time.Duration `mapstructure:"nrpt_recovery_cooldown" toml:"nrpt_recovery_cooldown,omitempty"`
 	Daemon                  bool           `mapstructure:"-" toml:"-"`
 	AllocateIP              bool           `mapstructure:"-" toml:"-"`
 }
@@ -640,6 +642,7 @@ func (uc *UpstreamConfig) newDOHTransport(addrs []string) *http.Transport {
 	transport.TLSClientConfig = &tls.Config{
 		RootCAs:            uc.certPool,
 		ClientSessionCache: tls.NewLRUClientSessionCache(0),
+		MinVersion:         tls.VersionTLS12,
 	}
 
 	// Prevent bad tcp connection hanging the requests for too long.

config_quic.go

@@ -18,7 +18,7 @@ func (uc *UpstreamConfig) newDOH3Transport(addrs []string) http.RoundTripper {
 		return nil
 	}
 	rt := &http3.Transport{}
-	rt.TLSClientConfig = &tls.Config{RootCAs: uc.certPool}
+	rt.TLSClientConfig = &tls.Config{RootCAs: uc.certPool, MinVersion: tls.VersionTLS12}
 	rt.Dial = func(ctx context.Context, addr string, tlsCfg *tls.Config, cfg *quic.Config) (*quic.Conn, error) {
 		_, port, _ := net.SplitHostPort(addr)
 		// if we have a bootstrap ip set, use it to avoid DNS lookup
@@ -77,7 +77,17 @@ type parallelDialerResult struct {
 	err  error
 }
 
-type quicParallelDialer struct{}
+// quicParallelDialer races DialEarly across a list of remote addresses and
+// returns the first successful connection. When transport is non-nil, all
+// dials share that transport's UDP socket, which removes both the per-dial
+// socket allocation and the winner-path socket leak that an owner-of-the-conn
+// receiver cannot clean up. When transport is nil, the dialer falls back to a
+// fresh UDP socket per attempt (compat path used where no shared transport is
+// available yet); the loser paths close their sockets, and the winner path's
+// socket is owned by quic.DialEarly's internal transport.
+type quicParallelDialer struct {
+	transport *quic.Transport
+}
 
 // Dial performs parallel dialing to the given address list.
 func (d *quicParallelDialer) Dial(ctx context.Context, addrs []string, tlsCfg *tls.Config, cfg *quic.Config) (*quic.Conn, error) {
@@ -105,12 +115,24 @@ func (d *quicParallelDialer) Dial(ctx context.Context, addrs []string, tlsCfg *t
 				ch <- &parallelDialerResult{conn: nil, err: err}
 				return
 			}
-			udpConn, err := net.ListenUDP("udp", nil)
-			if err != nil {
-				ch <- &parallelDialerResult{conn: nil, err: err}
-				return
+			var (
+				conn    *quic.Conn
+				udpConn *net.UDPConn
+			)
+			if d.transport != nil {
+				conn, err = d.transport.DialEarly(ctx, remoteAddr, tlsCfg, cfg)
+			} else {
+				udpConn, err = net.ListenUDP("udp", nil)
+				if err != nil {
+					ch <- &parallelDialerResult{conn: nil, err: err}
+					return
+				}
+				conn, err = quic.DialEarly(ctx, udpConn, remoteAddr, tlsCfg, cfg)
+				if err != nil {
+					udpConn.Close()
+					udpConn = nil
+				}
 			}
-			conn, err := quic.DialEarly(ctx, udpConn, remoteAddr, tlsCfg, cfg)
 			select {
 			case ch <- &parallelDialerResult{conn: conn, err: err}:
 			case <-done:

dns.go

@@ -14,6 +14,12 @@ func SetCacheReply(answer, msg *dns.Msg, code int) {
 		// See https://datatracker.ietf.org/doc/html/rfc7873#section-4
 		sCookie.Cookie = cCookie.Cookie[:16] + sCookie.Cookie[16:]
 	}
+	// NOTE: the answer's EDNS Client Subnet (ECS) is intentionally left as the
+	// upstream returned it. Correctness across clients is guaranteed by
+	// partitioning the cache and singleflight keys by ECS (see
+	// dnscache.CanonicalECS), so a cache hit only ever serves an answer that was
+	// resolved for the requester's own subnet. Rewriting the ECS option here
+	// without re-scoping the Answer records would violate RFC 7871 §7.3.
 }
 
 // getEdns0Cookie returns Edns0 cookie from *dns.OPT if present.

dns_test.go

@@ -0,0 +1,57 @@
+package ctrld
+
+import (
+	"net"
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+// Test_SetCacheReply_DoesNotRewriteECS documents the post-#564 contract: cross-client
+// correctness is guaranteed by partitioning the cache/singleflight keys by ECS
+// (dnscache.CanonicalECS), NOT by rewriting the cached answer's ECS option. Rewriting the
+// ECS metadata while leaving the Answer records scoped to another subnet would violate
+// RFC 7871 §7.3 and make forwarders accept a wrong-subnet answer. SetCacheReply must
+// therefore leave the answer's ECS untouched.
+func Test_SetCacheReply_DoesNotRewriteECS(t *testing.T) {
+	answer := new(dns.Msg)
+	answer.SetQuestion(dns.Fqdn("controld.com"), dns.TypeA)
+	answer.SetEdns0(4096, false)
+	cachedSubnet := &dns.EDNS0_SUBNET{
+		Code:          dns.EDNS0SUBNET,
+		Family:        2,
+		SourceNetmask: 64,
+		SourceScope:   64,
+		Address:       net.ParseIP("2001:db8:1::"),
+	}
+	answer.IsEdns0().Option = append(answer.IsEdns0().Option, cachedSubnet)
+
+	req := new(dns.Msg)
+	req.SetQuestion(dns.Fqdn("controld.com"), dns.TypeA)
+	req.SetEdns0(4096, true)
+	req.IsEdns0().Option = append(req.IsEdns0().Option, &dns.EDNS0_SUBNET{
+		Code:          dns.EDNS0SUBNET,
+		Family:        2,
+		SourceNetmask: 64,
+		Address:       net.ParseIP("2001:db8:2::"),
+	})
+
+	SetCacheReply(answer, req, dns.RcodeSuccess)
+
+	var got *dns.EDNS0_SUBNET
+	for _, o := range answer.IsEdns0().Option {
+		if e, ok := o.(*dns.EDNS0_SUBNET); ok {
+			got = e
+			break
+		}
+	}
+	if got == nil {
+		t.Fatal("SetCacheReply dropped the answer's ECS option")
+	}
+	if want := net.ParseIP("2001:db8:1::"); !got.Address.Equal(want) {
+		t.Fatalf("SetCacheReply rewrote the answer ECS to the requester's subnet: got %v, want %v (unchanged)", got.Address, want)
+	}
+	if got.SourceScope != 64 {
+		t.Fatalf("SetCacheReply altered the answer ECS scope: got %d, want 64 (unchanged)", got.SourceScope)
+	}
+}

docker/Dockerfile

@@ -1,4 +1,4 @@
-# Using Debian bullseye for building regular image.
+# Using Debian bookworm for building regular image.
 # Using scratch image for minimal image size.
 # The final image has:
 #
@@ -8,11 +8,12 @@
 # - Non-cgo ctrld binary.
 #
 # CI_COMMIT_TAG is used to set the version of ctrld binary.
-FROM golang:1.20-bullseye as base
+FROM golang:1.25-bookworm AS base
 
 WORKDIR /app
 
-RUN apt-get update && apt-get install -y upx-ucl
+RUN echo "deb http://deb.debian.org/debian bookworm-backports main" | tee /etc/apt/sources.list.d/backports.list
+RUN apt update && apt install -t bookworm-backports upx-ucl
 
 COPY . .
 

docker/Dockerfile.debug

@@ -1,4 +1,4 @@
-# Using Debian bullseye for building regular image.
+# Using Debian bookworm for building regular image.
 # Using scratch image for minimal image size.
 # The final image has:
 #
@@ -8,11 +8,12 @@
 # - Non-cgo ctrld binary.
 #
 # CI_COMMIT_TAG is used to set the version of ctrld binary.
-FROM golang:bullseye as base
+FROM golang:1.25-bookworm AS base
 
 WORKDIR /app
 
-RUN apt-get update && apt-get install -y upx-ucl
+RUN echo "deb http://deb.debian.org/debian bookworm-backports main" | tee /etc/apt/sources.list.d/backports.list
+RUN apt update && apt install -t bookworm-backports upx-ucl
 
 COPY . .
 

docs/config.md

@@ -295,6 +295,22 @@ If a remote upstream fails to resolve a query or is unreachable, `ctrld` will fo
 - Required: no
 - Default: true on Windows, MacOS and non-router Linux.
 
+### nrpt_recovery_max_attempts
+Windows DNS intercept mode uses NRPT health probes and recovery when Windows stops routing queries to the local `ctrld` listener. This limits how many consecutive recovery flows can run before `ctrld` enters a cooldown and stops making policy/Dnscache changes.
+
+Set to `0` to disable this circuit breaker and keep retrying indefinitely.
+
+- Type: integer
+- Required: no
+- Default: 0 (unlimited, current behavior)
+
+### nrpt_recovery_cooldown
+Cooldown duration after `nrpt_recovery_max_attempts` consecutive Windows NRPT recovery flows. During cooldown, `ctrld` logs the suppressed recovery and avoids additional `RefreshPolicyEx`, Dnscache `paramchange`, and DNS cache flush calls.
+
+- Type: time duration string
+- Required: no
+- Default: 30m
+
 ## Upstream
 The `[upstream]` section specifies the DNS upstream servers that `ctrld` will forward DNS requests to.
 

docs/wfp-dns-intercept.md

@@ -91,6 +91,20 @@ ctrld uses an adaptive strategy (matching [Tailscale's approach](https://github.
    the empty GP parent key. This ensures DNS Client stays in "local mode" where
    the local-path rule activates immediately via `paramchange`.
 
+### Reproducing the Empty GP Parent Case
+
+This is a production code reference, so the temporary repro script is not kept in
+the repository. For MR !942 review, the test script and exact before/after steps
+are posted in the MR discussion. The scenario to compare is:
+
+1. Run the same approved PowerShell repro script against a pre-fix build and this
+   branch with the same ctrld config.
+2. Create an empty GP NRPT parent key while ctrld is running in DNS intercept mode.
+3. Confirm pre-fix logs can spend policy refresh/paramchange retries while the GP
+   parent remains empty.
+4. Confirm post-fix logs clean the empty GP parent, send one NRPT-change signal,
+   and re-probe before normal retries.
+
 ### VPN Coexistence
 
 NRPT uses most-specific-match. VPN NRPT rules for specific domains (e.g.,

doh.go

@@ -25,6 +25,16 @@ const (
 	dohOsHeader           = "x-cd-os"
 	dohClientIDPrefHeader = "x-cd-cpref"
 	headerApplicationDNS  = "application/dns-message"
+
+	// dohMaxResponseSize caps the response body read from a DoH/DoH3
+	// upstream. A DNS message is bounded by the protocol's 16-bit length
+	// field; anything larger cannot be a valid response. The cap stops a
+	// malicious or compromised upstream from driving ctrld into unbounded
+	// memory growth via io.ReadAll on attacker-controlled bytes.
+	dohMaxResponseSize = dns.MaxMsgSize
+	// dohMaxErrorBodySize bounds how much of a non-200 response body is
+	// read for inclusion in the returned error.
+	dohMaxErrorBodySize = 1024
 )
 
 // EncodeOsNameMap provides mapping from OS name to a shorter string, used for encoding x-cd-os value.
@@ -130,13 +140,17 @@ func (r *dohResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 	}
 	defer resp.Body.Close()
 
-	buf, err := io.ReadAll(resp.Body)
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(io.LimitReader(resp.Body, dohMaxErrorBodySize))
+		return nil, fmt.Errorf("wrong response from DOH server, got: %s, status: %d", string(body), resp.StatusCode)
+	}
+
+	buf, err := io.ReadAll(io.LimitReader(resp.Body, dohMaxResponseSize+1))
 	if err != nil {
 		return nil, fmt.Errorf("could not read message from response: %w", err)
 	}
-
-	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("wrong response from DOH server, got: %s, status: %d", string(buf), resp.StatusCode)
+	if len(buf) > dohMaxResponseSize {
+		return nil, fmt.Errorf("DoH response exceeds %d-byte maximum DNS message size", dohMaxResponseSize)
 	}
 
 	answer := new(dns.Msg)

doh_test.go

@@ -196,6 +196,7 @@ func testTLSServer(t *testing.T, handler http.Handler) (*httptest.Server, *x509.
 	server := httptest.NewUnstartedServer(handler)
 	server.TLS = &tls.Config{
 		Certificates: []tls.Certificate{testCert.tlsCert},
+		MinVersion:   tls.VersionTLS12,
 	}
 	server.StartTLS()
 
@@ -232,6 +233,7 @@ func newTestHTTP3Server(t *testing.T, handler http.Handler) *testHTTP3Server {
 	tlsConfig := &tls.Config{
 		Certificates: []tls.Certificate{testCert.tlsCert},
 		NextProtos:   []string{"h3"}, // HTTP/3 protocol identifier
+		MinVersion:   tls.VersionTLS12,
 	}
 
 	// Create HTTP/3 server
@@ -264,3 +266,215 @@ func newTestHTTP3Server(t *testing.T, handler http.Handler) *testHTTP3Server {
 
 	return h3Server
 }
+
+// blockingBodyHandler writes exactly nbytes of body with the given status,
+// flushes them, then blocks until release is closed WITHOUT ever returning.
+// Because the handler does not return, the response stream is never terminated
+// (no EOF/FIN). A client that stops after a bounded prefix therefore completes,
+// while a client that reads to EOF blocks. Tests set nbytes to the exact read
+// cap so the client consumes the whole written body (no half-written frame is
+// left blocking on flow control) yet still never sees EOF.
+func blockingBodyHandler(status, nbytes int, release <-chan struct{}) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", headerApplicationDNS)
+		w.WriteHeader(status)
+		if _, err := w.Write(make([]byte, nbytes)); err != nil {
+			return
+		}
+		if f, ok := w.(http.Flusher); ok {
+			f.Flush()
+		}
+		<-release
+	}
+}
+
+// requireBoundedResolve asserts that r.Resolve returns the expected size/status
+// error while the server is still withholding EOF (the handler is blocked in
+// blockingBodyHandler). Returning under those conditions proves ctrld read only
+// a bounded prefix of the body: a resolver that instead read to EOF would block
+// on the withheld stream and trip the deadline. This is the deterministic
+// regression guard for the issue-312 OOM protections, replacing the earlier
+// flaky server-side byte counter (issue-561).
+func requireBoundedResolve(t *testing.T, r Resolver, msg *dns.Msg, wantErrSubstr string) {
+	t.Helper()
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	type result struct {
+		answer *dns.Msg
+		err    error
+	}
+	done := make(chan result, 1)
+	go func() {
+		answer, err := r.Resolve(ctx, msg)
+		done <- result{answer, err}
+	}()
+
+	select {
+	case res := <-done:
+		if res.err == nil {
+			t.Fatalf("Resolve unexpectedly succeeded; answer=%v", res.answer)
+		}
+		if !strings.Contains(res.err.Error(), wantErrSubstr) {
+			t.Fatalf("error %q does not contain %q", res.err, wantErrSubstr)
+		}
+		if res.answer != nil {
+			t.Fatalf("Resolve returned non-nil answer alongside error: %v", res.answer)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("Resolve did not return while the server withheld EOF: the body is being read to EOF instead of a bounded prefix (issue-312 OOM protection missing)")
+	}
+}
+
+// dohUpstreamForTLSServer wires an UpstreamConfig at a local httptest TLS
+// server, trusting its self-signed certificate. BootstrapIP is set so no
+// real DNS lookup runs.
+func dohUpstreamForTLSServer(t *testing.T, srv *httptest.Server) *UpstreamConfig {
+	t.Helper()
+	pool := x509.NewCertPool()
+	pool.AddCert(srv.Certificate())
+	u, err := url.Parse(srv.URL)
+	if err != nil {
+		t.Fatalf("parse server URL: %v", err)
+	}
+	uc := &UpstreamConfig{
+		Name:        "doh-oversize",
+		Type:        ResolverTypeDOH,
+		Endpoint:    srv.URL + "/dns-query",
+		BootstrapIP: u.Hostname(),
+		Timeout:     2000,
+	}
+	uc.SetCertPool(pool)
+	uc.Init()
+	return uc
+}
+
+// doh3UpstreamForAddr wires an UpstreamConfig at a local HTTP/3 server,
+// trusting its self-signed certificate.
+func doh3UpstreamForAddr(t *testing.T, addr string, cert *x509.Certificate) *UpstreamConfig {
+	t.Helper()
+	pool := x509.NewCertPool()
+	pool.AddCert(cert)
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		t.Fatalf("split host/port %q: %v", addr, err)
+	}
+	uc := &UpstreamConfig{
+		Name:        "doh3-oversize",
+		Type:        ResolverTypeDOH3,
+		Endpoint:    "h3://" + addr + "/dns-query",
+		BootstrapIP: host,
+		Timeout:     5000,
+	}
+	uc.SetCertPool(pool)
+	uc.Init()
+	return uc
+}
+
+// TestDoHResolve_OversizedBody_Rejected locks in the fix for
+// github.com/Control-D-Inc/ctrld/issues/312: a malicious DoH upstream
+// returning a body larger than the DNS protocol allows must be rejected
+// with an explicit size error rather than buffered into ctrld memory.
+func TestDoHResolve_OversizedBody_Rejected(t *testing.T) {
+	// Write exactly the LimitReader cap, then withhold EOF. ctrld's bounded
+	// read (io.LimitReader of dohMaxResponseSize+1) returns after this prefix;
+	// an unbounded read would block on the missing EOF and trip the deadline.
+	release := make(chan struct{})
+	defer close(release)
+	srv := httptest.NewUnstartedServer(blockingBodyHandler(http.StatusOK, dohMaxResponseSize+1, release))
+	testCert := generateTestCertificate(t)
+	srv.TLS = &tls.Config{
+		Certificates: []tls.Certificate{testCert.tlsCert},
+		NextProtos:   []string{"h2", "http/1.1"},
+		MinVersion:   tls.VersionTLS12,
+	}
+	srv.StartTLS()
+	t.Cleanup(srv.Close)
+
+	uc := dohUpstreamForTLSServer(t, srv)
+	r, err := NewResolver(uc)
+	if err != nil {
+		t.Fatalf("NewResolver: %v", err)
+	}
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	requireBoundedResolve(t, r, msg, "maximum DNS message size")
+}
+
+// TestDoHResolve_NonOKStatus_BoundedErrorBody locks in that a non-200
+// response with a huge body does not pull the body fully into ctrld
+// memory just to format an error string.
+func TestDoHResolve_NonOKStatus_BoundedErrorBody(t *testing.T) {
+	// Same synchronization as the oversized-body test, but at the error-body
+	// cap: the non-200 path reads through an io.LimitReader of
+	// dohMaxErrorBodySize, so it must return after this prefix without EOF.
+	release := make(chan struct{})
+	defer close(release)
+	srv := httptest.NewUnstartedServer(blockingBodyHandler(http.StatusBadGateway, dohMaxErrorBodySize, release))
+	testCert := generateTestCertificate(t)
+	srv.TLS = &tls.Config{
+		Certificates: []tls.Certificate{testCert.tlsCert},
+		NextProtos:   []string{"h2", "http/1.1"},
+		MinVersion:   tls.VersionTLS12,
+	}
+	srv.StartTLS()
+	t.Cleanup(srv.Close)
+
+	uc := dohUpstreamForTLSServer(t, srv)
+	r, err := NewResolver(uc)
+	if err != nil {
+		t.Fatalf("NewResolver: %v", err)
+	}
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	requireBoundedResolve(t, r, msg, "status: 502")
+}
+
+// TestDoHResolve_OversizedBody_DoH3 mirrors the DoH oversized-body check
+// on the HTTP/3 transport, since github-312 specifically reproduced the
+// OOM via DoH3.
+func TestDoHResolve_OversizedBody_DoH3(t *testing.T) {
+	release := make(chan struct{})
+	defer close(release)
+	testCert := generateTestCertificate(t)
+	udpConn, err := net.ListenUDP("udp", &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: 0})
+	if err != nil {
+		t.Fatalf("udp listen: %v", err)
+	}
+	h3 := &http3.Server{
+		Handler: blockingBodyHandler(http.StatusOK, dohMaxResponseSize+1, release),
+		TLSConfig: &tls.Config{
+			Certificates: []tls.Certificate{testCert.tlsCert},
+			NextProtos:   []string{"h3"},
+			MinVersion:   tls.VersionTLS12,
+		},
+	}
+	go func() {
+		if err := h3.Serve(udpConn); err != nil && !errors.Is(err, http.ErrServerClosed) {
+			t.Logf("h3 server: %v", err)
+		}
+	}()
+	t.Cleanup(func() {
+		_ = h3.Close()
+		_ = udpConn.Close()
+	})
+	time.Sleep(100 * time.Millisecond)
+
+	uc := doh3UpstreamForAddr(t, udpConn.LocalAddr().String(), testCert.cert)
+	r, err := NewResolver(uc)
+	if err != nil {
+		t.Fatalf("NewResolver: %v", err)
+	}
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	requireBoundedResolve(t, r, msg, "maximum DNS message size")
+}

doq.go

@@ -6,15 +6,23 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"runtime"
+	"sync"
 	"time"
 
 	"github.com/miekg/dns"
 	"github.com/quic-go/quic-go"
 )
 
+// doqMaxResponseSize caps the bytes read from a DoQ stream: a 2-byte
+// length prefix plus a DNS message bounded by dns.MaxMsgSize. Anything
+// larger cannot be a valid response and is rejected before buffering more
+// data from the upstream.
+const doqMaxResponseSize = 2 + dns.MaxMsgSize
+
 type doqResolver struct {
 	uc *UpstreamConfig
 }
@@ -41,6 +49,10 @@ func (r *doqResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 const doqPoolSize = 16
 
 // doqConnPool manages a pool of QUIC connections for DoQ queries using a buffered channel.
+// A single quic.Transport (and its UDP socket) is shared by every connection in the pool,
+// so the OS socket lifecycle is tied to the pool rather than to each dial. Without this
+// ownership model, a strict DoQ upstream that triggers reconnect churn would leak one
+// caller-owned UDP socket per dial — see github.com/Control-D-Inc/ctrld/issues/309.
 type doqConnPool struct {
 	uc         *UpstreamConfig
 	addrs      []string
@@ -48,6 +60,13 @@ type doqConnPool struct {
 	tlsConfig  *tls.Config
 	quicConfig *quic.Config
 	conns      chan *doqConn
+
+	transportMu   sync.Mutex
+	transport     *quic.Transport
+	transportConn *net.UDPConn
+	transportErr  error
+	transportInit bool
+	closed        bool
 }
 
 type doqConn struct {
@@ -64,6 +83,7 @@ func newDOQConnPool(uc *UpstreamConfig, addrs []string) *doqConnPool {
 		NextProtos: []string{"doq"},
 		RootCAs:    uc.certPool,
 		ServerName: uc.Domain,
+		MinVersion: tls.VersionTLS12,
 	}
 
 	quicConfig := &quic.Config{
@@ -167,29 +187,70 @@ func (p *doqConnPool) doResolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, er
 		return nil, err
 	}
 
-	// Read response
-	buf, err := io.ReadAll(stream)
-	stream.Close()
-
-	// Return connection to pool (mark as potentially bad if error occurred)
-	isGood := err == nil && len(buf) > 0
-	p.putConn(conn, isGood)
-
-	if err != nil {
+	// RFC 9250 section 4.2 requires the client to indicate end-of-request by
+	// closing the send side of the stream (STREAM FIN). Servers may defer
+	// processing until FIN arrives, so the close must happen before reading.
+	// Stream.Close closes only the send direction; the receive direction
+	// remains open for the response.
+	if err := stream.Close(); err != nil {
+		p.putConn(conn, false)
 		return nil, err
 	}
 
-	// io.ReadAll hides io.EOF error, so check for empty buffer
+	// A DoQ response is a 2-byte length prefix followed by a DNS message.
+	// The DNS message is bounded by the protocol at dns.MaxMsgSize, so a
+	// well-formed response is at most doqMaxResponseSize bytes. Read one
+	// byte past that cap to distinguish "at limit" from "over limit" and
+	// reject oversized responses before they can drive memory growth from
+	// a malicious or compromised upstream.
+	buf, err := io.ReadAll(io.LimitReader(stream, doqMaxResponseSize+1))
+	if err != nil {
+		p.putConn(conn, false)
+		return nil, err
+	}
+
+	// io.ReadAll hides io.EOF error, so check for empty buffer.
 	if len(buf) == 0 {
+		p.putConn(conn, false)
 		return nil, io.EOF
 	}
 
-	// Unpack DNS response (skip 2-byte length prefix)
+	if len(buf) > doqMaxResponseSize {
+		p.putConn(conn, false)
+		return nil, fmt.Errorf("DoQ response exceeds %d-byte maximum", doqMaxResponseSize)
+	}
+
+	// RFC 9250: each DoQ DNS message is encoded as a 2-octet length field
+	// followed by the DNS message. Reject responses that are shorter than
+	// the prefix or whose prefix declares more bytes than were received,
+	// and retire the misbehaving connection. Without this guard, buf[2:]
+	// would panic when len(buf) < 2.
+	if len(buf) < 2 {
+		p.putConn(conn, false)
+		return nil, fmt.Errorf("malformed DoQ response: %d byte(s), need >= 2 for length prefix", len(buf))
+	}
+	respLen := int(buf[0])<<8 | int(buf[1])
+	if 2+respLen > len(buf) {
+		p.putConn(conn, false)
+		return nil, fmt.Errorf("malformed DoQ response: length prefix %d exceeds payload %d", respLen, len(buf)-2)
+	}
+
+	p.putConn(conn, true)
+
+	// Unpack DNS response (skip 2-byte length prefix).
 	answer := new(dns.Msg)
-	if err := answer.Unpack(buf[2:]); err != nil {
+	if err := answer.Unpack(buf[2 : 2+respLen]); err != nil {
 		return nil, err
 	}
-	answer.SetReply(msg)
+	// RFC 9250 section 4.2.1 requires the DNS Message ID to be 0 on the wire,
+	// so restore the downstream transaction ID for the client. Do NOT use
+	// SetReply here: it rewrites the RCODE to NOERROR and overwrites the
+	// Question with the request's, which would mask upstream failures from the
+	// failover logic (a SERVFAIL would look like success) and let a
+	// wrong-question answer pass validation and poison the cache. Preserve the
+	// upstream RCODE, Question, and answer sections untouched so the proxy can
+	// evaluate them. See github.com/Control-D-Inc/ctrld/issues/322.
+	answer.Id = msg.Id
 	return answer, nil
 }
 
@@ -233,25 +294,26 @@ func (p *doqConnPool) putConn(conn *quic.Conn, isGood bool) {
 }
 
 // dialConn creates a new QUIC connection using parallel dialing like DoH3.
+// All connections from the pool multiplex on a single pool-owned UDP socket,
+// so reconnect churn cannot grow the host's FD count.
 func (p *doqConnPool) dialConn(ctx context.Context) (string, *quic.Conn, error) {
 	logger := ProxyLogger.Load()
 
+	tr, err := p.getOrInitTransport()
+	if err != nil {
+		return "", nil, err
+	}
+
 	// If we have a bootstrap IP, use it directly
 	if p.uc.BootstrapIP != "" {
 		addr := net.JoinHostPort(p.uc.BootstrapIP, p.port)
 		Log(ctx, logger.Debug(), "Sending DoQ request to: %s", addr)
-		udpConn, err := net.ListenUDP("udp", nil)
-		if err != nil {
-			return "", nil, err
-		}
 		remoteAddr, err := net.ResolveUDPAddr("udp", addr)
 		if err != nil {
-			udpConn.Close()
 			return "", nil, err
 		}
-		conn, err := quic.DialEarly(ctx, udpConn, remoteAddr, p.tlsConfig, p.quicConfig)
+		conn, err := tr.DialEarly(ctx, remoteAddr, p.tlsConfig, p.quicConfig)
 		if err != nil {
-			udpConn.Close()
 			return "", nil, err
 		}
 		return addr, conn, nil
@@ -263,7 +325,7 @@ func (p *doqConnPool) dialConn(ctx context.Context) (string, *quic.Conn, error)
 		dialAddrs[i] = net.JoinHostPort(p.addrs[i], p.port)
 	}
 
-	pd := &quicParallelDialer{}
+	pd := &quicParallelDialer{transport: tr}
 	conn, err := pd.Dial(ctx, dialAddrs, p.tlsConfig, p.quicConfig)
 	if err != nil {
 		return "", nil, err
@@ -274,9 +336,35 @@ func (p *doqConnPool) dialConn(ctx context.Context) (string, *quic.Conn, error)
 	return addr, conn, nil
 }
 
-// CloseIdleConnections closes all connections in the pool.
-// Connections currently checked out (in use) are not closed.
+// getOrInitTransport returns the pool's shared quic.Transport, initialising it
+// on first call. Once the pool has been closed it permanently returns an error
+// so that callers cannot resurrect a dead pool.
+func (p *doqConnPool) getOrInitTransport() (*quic.Transport, error) {
+	p.transportMu.Lock()
+	defer p.transportMu.Unlock()
+	if p.closed {
+		return nil, errors.New("doq pool closed")
+	}
+	if p.transportInit {
+		return p.transport, p.transportErr
+	}
+	p.transportInit = true
+	udpConn, err := net.ListenUDP("udp", nil)
+	if err != nil {
+		p.transportErr = err
+		return nil, err
+	}
+	p.transportConn = udpConn
+	p.transport = &quic.Transport{Conn: udpConn}
+	return p.transport, nil
+}
+
+// CloseIdleConnections closes all idle connections, the shared quic.Transport,
+// and the pool's UDP socket. Connections currently checked out (in use) get
+// terminated by the transport close as well — without that, the OS socket
+// would remain bound to a goroutine that the caller cannot reach to clean up.
 func (p *doqConnPool) CloseIdleConnections() {
+drain:
 	for {
 		select {
 		case dc := <-p.conns:
@@ -284,7 +372,22 @@ func (p *doqConnPool) CloseIdleConnections() {
 				dc.conn.CloseWithError(quic.ApplicationErrorCode(quic.NoError), "")
 			}
 		default:
-			return
+			break drain
 		}
 	}
+	p.transportMu.Lock()
+	if p.closed {
+		p.transportMu.Unlock()
+		return
+	}
+	p.closed = true
+	tr := p.transport
+	udpConn := p.transportConn
+	p.transportMu.Unlock()
+	if tr != nil {
+		_ = tr.Close()
+	}
+	if udpConn != nil {
+		_ = udpConn.Close()
+	}
 }

doq_test.go

@@ -1,4 +1,3 @@
-// test_helpers.go
 package ctrld
 
 import (
@@ -8,8 +7,11 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"io"
 	"math/big"
 	"net"
+	"os"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -99,6 +101,7 @@ func newTestQUICServer(t *testing.T) *testQUICServer {
 	tlsConfig := &tls.Config{
 		Certificates: []tls.Certificate{testCert.tlsCert},
 		NextProtos:   []string{"doq"},
+		MinVersion:   tls.VersionTLS12,
 	}
 
 	// Create QUIC listener
@@ -221,3 +224,595 @@ func (s *testQUICServer) handleStream(t *testing.T, stream *quic.Stream) {
 		return
 	}
 }
+
+// malformedDoQServer is a test QUIC server that drains the client's DoQ
+// request and writes caller-supplied raw bytes back. The bytes are not
+// required to be a well-framed DoQ response, which is what lets the
+// regression tests exercise malformed-response handling.
+type malformedDoQServer struct {
+	listener *quic.Listener
+	cert     *x509.Certificate
+	addr     string
+	response []byte
+}
+
+func newMalformedDoQServer(t *testing.T, response []byte) *malformedDoQServer {
+	t.Helper()
+
+	testCert := generateTestCertificate(t)
+	tlsConfig := &tls.Config{
+		Certificates: []tls.Certificate{testCert.tlsCert},
+		NextProtos:   []string{"doq"},
+	}
+
+	listener, err := quic.ListenAddr("127.0.0.1:0", tlsConfig, nil)
+	if err != nil {
+		t.Fatalf("failed to create QUIC listener: %v", err)
+	}
+
+	s := &malformedDoQServer{
+		listener: listener,
+		cert:     testCert.cert,
+		addr:     listener.Addr().String(),
+		response: response,
+	}
+
+	go s.serve()
+	t.Cleanup(func() { _ = listener.Close() })
+	return s
+}
+
+func (s *malformedDoQServer) serve() {
+	for {
+		conn, err := s.listener.Accept(context.Background())
+		if err != nil {
+			return
+		}
+		go s.handleConn(conn)
+	}
+}
+
+func (s *malformedDoQServer) handleConn(conn *quic.Conn) {
+	for {
+		stream, err := conn.AcceptStream(context.Background())
+		if err != nil {
+			return
+		}
+		go s.handleStream(stream)
+	}
+}
+
+func (s *malformedDoQServer) handleStream(stream *quic.Stream) {
+	defer stream.Close()
+
+	// Drain the client's DoQ-framed request so the client's writes complete
+	// cleanly before we reply with our attacker-controlled bytes. Using
+	// io.ReadFull because a single Read on a QUIC stream may return short.
+	lenBuf := make([]byte, 2)
+	if _, err := io.ReadFull(stream, lenBuf); err != nil {
+		return
+	}
+	msgLen := uint16(lenBuf[0])<<8 | uint16(lenBuf[1])
+	if msgLen > 0 {
+		discard := make([]byte, msgLen)
+		if _, err := io.ReadFull(stream, discard); err != nil {
+			return
+		}
+	}
+
+	if len(s.response) > 0 {
+		_, _ = stream.Write(s.response)
+	}
+}
+
+// newMalformedDoQUpstream builds an UpstreamConfig wired to a local
+// malformed test server with the test certificate trusted via a custom
+// cert pool. We bypass SetupBootstrapIP by setting BootstrapIP directly,
+// so the pool dials 127.0.0.1 without any DNS lookup.
+func newMalformedDoQUpstream(t *testing.T, cert *x509.Certificate, addr string) *UpstreamConfig {
+	t.Helper()
+
+	pool := x509.NewCertPool()
+	pool.AddCert(cert)
+
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		t.Fatalf("split host/port %q: %v", addr, err)
+	}
+
+	uc := &UpstreamConfig{
+		Name:        "doq-malformed",
+		Type:        ResolverTypeDOQ,
+		Endpoint:    addr,
+		Domain:      host,
+		BootstrapIP: host,
+		Timeout:     2000,
+	}
+	uc.SetCertPool(pool)
+	return uc
+}
+
+// TestDoQResolve_MalformedResponse verifies that DoQ upstream
+// responses violating RFC 9250 framing — fewer than 2 bytes, or a
+// length prefix declaring more payload than was received — return a
+// handled error instead of panicking on the length-prefix slice.
+func TestDoQResolve_MalformedResponse(t *testing.T) {
+	tests := []struct {
+		name     string
+		response []byte
+	}{
+		// Empty stream is already handled via io.EOF; locked in so a
+		// future change that drops that branch is caught.
+		{"empty response", nil},
+
+		// One byte: too short to hold the 2-octet length prefix.
+		{"single byte response", []byte{0x00}},
+
+		// Length prefix declares 16 bytes; payload is absent.
+		{"length prefix only", []byte{0x00, 0x10}},
+
+		// Length prefix declares 65535 bytes; only 1 byte of payload
+		// arrived.
+		{"length prefix larger than payload", []byte{0xFF, 0xFF, 0x00}},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			server := newMalformedDoQServer(t, tt.response)
+			uc := newMalformedDoQUpstream(t, server.cert, server.addr)
+
+			ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+			defer cancel()
+
+			pool := newDOQConnPool(uc, []string{"127.0.0.1"})
+			t.Cleanup(pool.CloseIdleConnections)
+
+			msg := new(dns.Msg)
+			msg.SetQuestion("example.com.", dns.TypeA)
+			msg.RecursionDesired = true
+
+			answer, err := pool.Resolve(ctx, msg)
+			if err == nil {
+				t.Fatalf("Resolve unexpectedly succeeded for malformed response %v; answer=%v", tt.response, answer)
+			}
+			if answer != nil {
+				t.Fatalf("Resolve returned non-nil answer alongside error: answer=%v err=%v", answer, err)
+			}
+		})
+	}
+}
+
+// strictDoQServer accepts DoQ queries but defers the response until the
+// client signals end-of-request with STREAM FIN, as required by RFC 9250
+// section 4.2. It exists to lock in the fix for
+// github.com/Control-D-Inc/ctrld/issues/309 where a client
+// that never closes its send side caused the server to wait forever and the
+// client to churn through reconnects.
+type strictDoQServer struct {
+	listener *quic.Listener
+	cert     *x509.Certificate
+	addr     string
+}
+
+func newStrictDoQServer(t *testing.T) *strictDoQServer {
+	t.Helper()
+
+	testCert := generateTestCertificate(t)
+	tlsConfig := &tls.Config{
+		Certificates: []tls.Certificate{testCert.tlsCert},
+		NextProtos:   []string{"doq"},
+		MinVersion:   tls.VersionTLS12,
+	}
+
+	listener, err := quic.ListenAddr("127.0.0.1:0", tlsConfig, nil)
+	if err != nil {
+		t.Fatalf("failed to create QUIC listener: %v", err)
+	}
+
+	s := &strictDoQServer{
+		listener: listener,
+		cert:     testCert.cert,
+		addr:     listener.Addr().String(),
+	}
+	go s.serve()
+	t.Cleanup(func() { _ = listener.Close() })
+	return s
+}
+
+func (s *strictDoQServer) serve() {
+	for {
+		conn, err := s.listener.Accept(context.Background())
+		if err != nil {
+			return
+		}
+		go s.handleConn(conn)
+	}
+}
+
+func (s *strictDoQServer) handleConn(conn *quic.Conn) {
+	for {
+		stream, err := conn.AcceptStream(context.Background())
+		if err != nil {
+			return
+		}
+		go s.handleStream(stream)
+	}
+}
+
+func (s *strictDoQServer) handleStream(stream *quic.Stream) {
+	defer stream.Close()
+
+	// Drain until the client closes the send side. This is the behaviour
+	// that triggered the bug: if the client never sends STREAM FIN, this
+	// read blocks until the stream's deadline fires.
+	body, err := io.ReadAll(stream)
+	if err != nil {
+		return
+	}
+	if len(body) < 2 {
+		return
+	}
+	msgLen := uint16(body[0])<<8 | uint16(body[1])
+	if int(msgLen) != len(body)-2 {
+		return
+	}
+
+	msg := new(dns.Msg)
+	if err := msg.Unpack(body[2:]); err != nil {
+		return
+	}
+
+	response := new(dns.Msg)
+	response.SetReply(msg)
+	response.Authoritative = true
+	if len(msg.Question) > 0 && msg.Question[0].Qtype == dns.TypeA {
+		response.Answer = append(response.Answer, &dns.A{
+			Hdr: dns.RR_Header{
+				Name:   msg.Question[0].Name,
+				Rrtype: dns.TypeA,
+				Class:  dns.ClassINET,
+				Ttl:    300,
+			},
+			A: net.ParseIP("192.0.2.1"),
+		})
+	}
+
+	respBytes, err := response.Pack()
+	if err != nil {
+		return
+	}
+	respLen := uint16(len(respBytes))
+	if _, err := stream.Write([]byte{byte(respLen >> 8), byte(respLen & 0xFF)}); err != nil {
+		return
+	}
+	if _, err := stream.Write(respBytes); err != nil {
+		return
+	}
+}
+
+func newStrictDoQUpstream(t *testing.T, cert *x509.Certificate, addr string, useBootstrap bool) *UpstreamConfig {
+	t.Helper()
+
+	pool := x509.NewCertPool()
+	pool.AddCert(cert)
+
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		t.Fatalf("split host/port %q: %v", addr, err)
+	}
+
+	uc := &UpstreamConfig{
+		Name:     "doq-strict",
+		Type:     ResolverTypeDOQ,
+		Endpoint: addr,
+		Domain:   host,
+		Timeout:  3000,
+	}
+	if useBootstrap {
+		uc.BootstrapIP = host
+	}
+	uc.SetCertPool(pool)
+	return uc
+}
+
+// TestDoQResolve_StrictServerWaitsForFIN exercises the RFC 9250 client-FIN
+// requirement. With the bug present, the server's io.ReadAll blocks until
+// the stream deadline expires and the client sees a timeout, so a successful
+// resolve here proves that the client now sends STREAM FIN before reading.
+func TestDoQResolve_StrictServerWaitsForFIN(t *testing.T) {
+	t.Parallel()
+
+	server := newStrictDoQServer(t)
+	uc := newStrictDoQUpstream(t, server.cert, server.addr, true)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	host, _, _ := net.SplitHostPort(server.addr)
+	pool := newDOQConnPool(uc, []string{host})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	answer, err := pool.Resolve(ctx, msg)
+	if err != nil {
+		t.Fatalf("Resolve failed against strict DoQ server: %v", err)
+	}
+	if answer == nil || len(answer.Answer) == 0 {
+		t.Fatalf("Resolve returned no answer records: %+v", answer)
+	}
+	a, ok := answer.Answer[0].(*dns.A)
+	if !ok || !a.A.Equal(net.ParseIP("192.0.2.1")) {
+		t.Fatalf("unexpected answer: %+v", answer.Answer[0])
+	}
+}
+
+// TestDoQResolve_ParallelDialPathStrictFIN exercises the parallel-dial path
+// (no BootstrapIP) against the same FIN-strict server, so that both the
+// single-dial branch and the parallel-dial branch are covered.
+func TestDoQResolve_ParallelDialPathStrictFIN(t *testing.T) {
+	t.Parallel()
+
+	server := newStrictDoQServer(t)
+	uc := newStrictDoQUpstream(t, server.cert, server.addr, false)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	host, _, _ := net.SplitHostPort(server.addr)
+	pool := newDOQConnPool(uc, []string{host})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	answer, err := pool.Resolve(ctx, msg)
+	if err != nil {
+		t.Fatalf("Resolve (parallel-dial path) failed against strict DoQ server: %v", err)
+	}
+	if answer == nil || len(answer.Answer) == 0 {
+		t.Fatalf("Resolve (parallel-dial path) returned no answer records: %+v", answer)
+	}
+}
+
+// TestDoQPool_ChurnDoesNotGrowFDs exercises the reconnect-churn scenario
+// described in github.com/Control-D-Inc/ctrld/issues/309: repeated dials
+// against a server that closes existing connections must not grow the process
+// FD count, because the pool now shares one UDP socket via quic.Transport instead
+// of allocating one per dial. Linux-only because /proc/self/fd is the cheapest
+// portable proxy for "what's still open."
+func TestDoQPool_ChurnDoesNotGrowFDs(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("FD accounting via /proc/self/fd is linux-only")
+	}
+	t.Parallel()
+
+	server := newStrictDoQServer(t)
+	uc := newStrictDoQUpstream(t, server.cert, server.addr, true)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	host, _, _ := net.SplitHostPort(server.addr)
+	pool := newDOQConnPool(uc, []string{host})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	makeQuery := func(i int) *dns.Msg {
+		msg := new(dns.Msg)
+		// Vary the question so any caching layer cannot short-circuit.
+		msg.SetQuestion(dns.Fqdn(strings.Repeat("a", 1+i%8)+".example.com"), dns.TypeA)
+		msg.RecursionDesired = true
+		return msg
+	}
+
+	// Warm the pool so the steady-state transport and at least one
+	// connection are open. Without this, the first resolve in the measured
+	// loop would inflate the baseline.
+	if _, err := pool.Resolve(ctx, makeQuery(0)); err != nil {
+		t.Fatalf("warm-up Resolve failed: %v", err)
+	}
+
+	baseline := countOpenFDs(t)
+
+	// Force reconnect churn by closing the connection between each query.
+	// Without the fix this would leak one UDP socket per round; with the
+	// fix the pool's shared transport keeps a single socket open.
+	const rounds = 20
+	for i := 1; i <= rounds; i++ {
+		// Drain any pooled connection so the next Resolve has to redial.
+		drainPooledConns(pool)
+
+		if _, err := pool.Resolve(ctx, makeQuery(i)); err != nil {
+			t.Fatalf("Resolve in churn loop iteration %d failed: %v", i, err)
+		}
+	}
+
+	// Give quic-go a moment to drop any background goroutines that hold
+	// references to closed sockets.
+	time.Sleep(200 * time.Millisecond)
+
+	after := countOpenFDs(t)
+
+	// Allow a small slack for transient FDs (goroutine wake-ups, qlog,
+	// etc.) but reject anything that scales with the number of rounds.
+	const slack = 5
+	if after > baseline+slack {
+		t.Fatalf("FD count grew under DoQ churn: baseline=%d after=%d rounds=%d (slack=%d)", baseline, after, rounds, slack)
+	}
+}
+
+// drainPooledConns removes any idle pooled connections so the next Resolve
+// is forced to dial a fresh one. It does not close the pool's transport.
+func drainPooledConns(p *doqConnPool) {
+	for {
+		select {
+		case dc := <-p.conns:
+			if dc.conn != nil {
+				dc.conn.CloseWithError(quic.ApplicationErrorCode(quic.NoError), "")
+			}
+		default:
+			return
+		}
+	}
+}
+
+func countOpenFDs(t *testing.T) int {
+	t.Helper()
+	entries, err := os.ReadDir("/proc/self/fd")
+	if err != nil {
+		t.Fatalf("read /proc/self/fd: %v", err)
+	}
+	return len(entries)
+}
+
+// TestDoQResolve_OversizedResponse_Rejected locks in the fix for
+// github.com/Control-D-Inc/ctrld/issues/312 on the DoQ transport: a
+// malicious upstream that writes a response larger than the DNS protocol
+// allows must be rejected with an explicit size error, not buffered
+// without bound into ctrld memory.
+func TestDoQResolve_OversizedResponse_Rejected(t *testing.T) {
+	t.Parallel()
+
+	// doqMaxResponseSize is 2 + dns.MaxMsgSize. Send something well past
+	// that. 256 KiB is enough to exceed the cap while keeping the test
+	// fast on loopback.
+	response := make([]byte, 256*1024)
+	// A well-formed length prefix isn't required: the size cap should
+	// fire before any framing check runs. Use a non-zero prefix so the
+	// test also documents that the order of validation is "size first,
+	// framing later."
+	response[0] = 0xFF
+	response[1] = 0xFF
+
+	server := newMalformedDoQServer(t, response)
+	uc := newMalformedDoQUpstream(t, server.cert, server.addr)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	pool := newDOQConnPool(uc, []string{"127.0.0.1"})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	answer, err := pool.Resolve(ctx, msg)
+	if err == nil {
+		t.Fatalf("Resolve unexpectedly succeeded for oversized response; answer=%v", answer)
+	}
+	if !strings.Contains(err.Error(), "exceeds") {
+		t.Fatalf("error %q does not surface the size cap", err)
+	}
+	if answer != nil {
+		t.Fatalf("Resolve returned non-nil answer alongside error: %v", answer)
+	}
+}
+
+// frameDoQResponse packs msg and prepends the RFC 9250 2-octet length prefix,
+// producing the exact bytes a DoQ server writes on the wire.
+func frameDoQResponse(t *testing.T, msg *dns.Msg) []byte {
+	t.Helper()
+	b, err := msg.Pack()
+	if err != nil {
+		t.Fatalf("pack response: %v", err)
+	}
+	n := uint16(len(b))
+	return append([]byte{byte(n >> 8), byte(n & 0xFF)}, b...)
+}
+
+// TestDoQResolve_PreservesRcode locks in the fix for
+// github.com/Control-D-Inc/ctrld/issues/322: the DoQ resolver must not rewrite
+// an upstream response with SetReply, which would clobber a SERVFAIL into
+// NOERROR and hide the failure from the proxy's failover logic. The upstream
+// RCODE must survive; only the transaction ID is restored for the client.
+func TestDoQResolve_PreservesRcode(t *testing.T) {
+	t.Parallel()
+
+	// RFC 9250 puts the DNS Message ID at 0 on the wire.
+	resp := new(dns.Msg)
+	resp.SetQuestion("example.com.", dns.TypeA)
+	resp.Response = true
+	resp.Id = 0
+	resp.Rcode = dns.RcodeServerFailure
+
+	server := newMalformedDoQServer(t, frameDoQResponse(t, resp))
+	uc := newMalformedDoQUpstream(t, server.cert, server.addr)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	pool := newDOQConnPool(uc, []string{"127.0.0.1"})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	answer, err := pool.Resolve(ctx, msg)
+	if err != nil {
+		t.Fatalf("Resolve failed: %v", err)
+	}
+	if answer.Rcode != dns.RcodeServerFailure {
+		t.Fatalf("upstream SERVFAIL was rewritten to %s; failover would be bypassed",
+			dns.RcodeToString[answer.Rcode])
+	}
+	if answer.Id != msg.Id {
+		t.Fatalf("transaction ID not restored: got %d, want %d", answer.Id, msg.Id)
+	}
+}
+
+// TestDoQResolve_PreservesWrongQuestion locks in the fix for
+// github.com/Control-D-Inc/ctrld/issues/322: when an upstream answers a
+// different name than asked, the resolver must preserve the upstream's
+// question rather than rewriting it to the request's question (as SetReply
+// did). Rewriting would hide the mismatch and let wrong-domain records poison
+// the shared cache.
+func TestDoQResolve_PreservesWrongQuestion(t *testing.T) {
+	t.Parallel()
+
+	resp := new(dns.Msg)
+	resp.SetQuestion("attacker.example.", dns.TypeA)
+	resp.Response = true
+	resp.Id = 0
+	resp.Answer = append(resp.Answer, &dns.A{
+		Hdr: dns.RR_Header{
+			Name:   "attacker.example.",
+			Rrtype: dns.TypeA,
+			Class:  dns.ClassINET,
+			Ttl:    300,
+		},
+		A: net.ParseIP("192.0.2.1"),
+	})
+
+	server := newMalformedDoQServer(t, frameDoQResponse(t, resp))
+	uc := newMalformedDoQUpstream(t, server.cert, server.addr)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	pool := newDOQConnPool(uc, []string{"127.0.0.1"})
+	t.Cleanup(pool.CloseIdleConnections)
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("victim.example.", dns.TypeA)
+	msg.RecursionDesired = true
+
+	answer, err := pool.Resolve(ctx, msg)
+	if err != nil {
+		t.Fatalf("Resolve failed: %v", err)
+	}
+	if len(answer.Question) == 0 || !strings.EqualFold(answer.Question[0].Name, "attacker.example.") {
+		t.Fatalf("upstream question was rewritten; got %v, want the upstream's attacker.example.",
+			answer.Question)
+	}
+	if answer.Id != msg.Id {
+		t.Fatalf("transaction ID not restored: got %d, want %d", answer.Id, msg.Id)
+	}
+}

dot.go

@@ -64,7 +64,8 @@ func newDOTClientPool(uc *UpstreamConfig, addrs []string) *dotConnPool {
 	dialer := newDialer(net.JoinHostPort(controldPublicDns, "53"))
 
 	tlsConfig := &tls.Config{
-		RootCAs: uc.certPool,
+		RootCAs:    uc.certPool,
+		MinVersion: tls.VersionTLS12,
 	}
 
 	if uc.BootstrapIP != "" {

go.mod

@@ -29,16 +29,16 @@ require (
 	github.com/prometheus/client_golang v1.19.1
 	github.com/prometheus/client_model v0.5.0
 	github.com/prometheus/prom2json v1.3.3
-	github.com/quic-go/quic-go v0.57.1
+	github.com/quic-go/quic-go v0.59.1
 	github.com/rs/zerolog v1.28.0
 	github.com/spf13/cobra v1.9.1
 	github.com/spf13/pflag v1.0.6
 	github.com/spf13/viper v1.16.0
 	github.com/stretchr/testify v1.11.1
 	github.com/vishvananda/netlink v1.3.1
-	golang.org/x/net v0.55.0
-	golang.org/x/sync v0.20.0
-	golang.org/x/sys v0.45.0
+	golang.org/x/net v0.56.0
+	golang.org/x/sync v0.22.0
+	golang.org/x/sys v0.46.0
 	golang.zx2c4.com/wireguard/windows v0.5.3
 	tailscale.com v1.74.0
 )
@@ -92,11 +92,11 @@ require (
 	github.com/yusufpapurcu/wmi v1.2.4 // indirect
 	go4.org/mem v0.0.0-20220726221520-4f986261bf13 // indirect
 	go4.org/netipx v0.0.0-20231129151722-fdeea329fbba // indirect
-	golang.org/x/crypto v0.51.0 // indirect
+	golang.org/x/crypto v0.53.0 // indirect
 	golang.org/x/exp v0.0.0-20240119083558-1b970713d09a // indirect
-	golang.org/x/mod v0.35.0 // indirect
-	golang.org/x/text v0.37.0 // indirect
-	golang.org/x/tools v0.44.0 // indirect
+	golang.org/x/mod v0.37.0 // indirect
+	golang.org/x/text v0.40.0 // indirect
+	golang.org/x/tools v0.47.0 // indirect
 	google.golang.org/protobuf v1.33.0 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect

go.sum

@@ -271,8 +271,8 @@ github.com/prometheus/prom2json v1.3.3 h1:IYfSMiZ7sSOfliBoo89PcufjWO4eAR0gznGcET
 github.com/prometheus/prom2json v1.3.3/go.mod h1:Pv4yIPktEkK7btWsrUTWDDDrnpUrAELaOCj+oFwlgmc=
 github.com/quic-go/qpack v0.6.0 h1:g7W+BMYynC1LbYLSqRt8PBg5Tgwxn214ZZR34VIOjz8=
 github.com/quic-go/qpack v0.6.0/go.mod h1:lUpLKChi8njB4ty2bFLX2x4gzDqXwUpaO1DP9qMDZII=
-github.com/quic-go/quic-go v0.57.1 h1:25KAAR9QR8KZrCZRThWMKVAwGoiHIrNbT72ULHTuI10=
-github.com/quic-go/quic-go v0.57.1/go.mod h1:ly4QBAjHA2VhdnxhojRsCUOeJwKYg+taDlos92xb1+s=
+github.com/quic-go/quic-go v0.59.1 h1:0Gmua0HW1Tv7ANR7hUYwRyD0MG5OJfgvYSZasGZzBic=
+github.com/quic-go/quic-go v0.59.1/go.mod h1:upnsH4Ju1YkqpLXC305eW3yDZ4NfnNbmQRCMWS58IKU=
 github.com/rivo/uniseg v0.2.0/go.mod h1:J6wj4VEh+S6ZtnVlnTBMWIodfgj8LQOQFoIToxlJtxc=
 github.com/rivo/uniseg v0.4.4 h1:8TfxU8dW6PdqD27gjM8MVNuicgxIjxpm4K7x4jp8sis=
 github.com/rivo/uniseg v0.4.4/go.mod h1:FN3SvrM+Zdj16jyLfmOkMNblXMcoc8DfTHruCPUcx88=
@@ -349,8 +349,8 @@ golang.org/x/crypto v0.0.0-20210421170649-83a5a9bb288b/go.mod h1:T9bdIzuCu7OtxOm
 golang.org/x/crypto v0.0.0-20211209193657-4570a0811e8b/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
 golang.org/x/crypto v0.0.0-20211215153901-e495a2d5b3d3/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
 golang.org/x/crypto v0.0.0-20220722155217-630584e8d5aa/go.mod h1:IxCIyHEi3zRg3s0A5j5BB6A9Jmi73HwBIUl50j+osU4=
-golang.org/x/crypto v0.51.0 h1:IBPXwPfKxY7cWQZ38ZCIRPI50YLeevDLlLnyC5wRGTI=
-golang.org/x/crypto v0.51.0/go.mod h1:8AdwkbraGNABw2kOX6YFPs3WM22XqI4EXEd8g+x7Oc8=
+golang.org/x/crypto v0.53.0 h1:QZ4Muo8THX6CizN2vPPd5fBGHyogrdK9fG4wLPFUsto=
+golang.org/x/crypto v0.53.0/go.mod h1:DNLU434OwVakk9PzuwV8w62mAJpRJL3vsgcfp4Qnsio=
 golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190306152737-a1d7652674e8/go.mod h1:CJ0aWSM057203Lf6IL+f9T1iT9GByDxfZKAQTCR3kQA=
 golang.org/x/exp v0.0.0-20190510132918-efd6b22b2522/go.mod h1:ZjyILWgesfNpC6sMxTJOJm9Kp84zZh5NQWvqDGG3Qr8=
@@ -386,8 +386,8 @@ golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.3.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.4.1/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
-golang.org/x/mod v0.35.0 h1:Ww1D637e6Pg+Zb2KrWfHQUnH2dQRLBQyAtpr/haaJeM=
-golang.org/x/mod v0.35.0/go.mod h1:+GwiRhIInF8wPm+4AoT6L0FA1QWAad3OMdTRx4tFYlU=
+golang.org/x/mod v0.37.0 h1:vF1DjpVEshcIqoEaauuHebaLk1O1forxjxBaVn884JQ=
+golang.org/x/mod v0.37.0/go.mod h1:m8S8VeM9r4dzDwjrKO0a1sZP3YjeMamRRlD+fmR2Q/0=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20190108225652-1e06a53dbb7e/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
@@ -420,8 +420,8 @@ golang.org/x/net v0.0.0-20201209123823-ac852fbbde11/go.mod h1:m0MpNAwzfU5UDzcl9v
 golang.org/x/net v0.0.0-20201224014010-6772e930b67b/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/net v0.0.0-20211112202133-69e39bad7dc2/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
-golang.org/x/net v0.55.0 h1:bcvxaJn3e1U6InsFWt1JUq1aSjnRxLzT2rtD2KfkDF8=
-golang.org/x/net v0.55.0/go.mod h1:L5U2KuzuOe1lY7Z+aWVIKK6qEeJXnXV9yzGA+WCHJww=
+golang.org/x/net v0.56.0 h1:Rw8j/hFzGvJUZwNBXnAtf5sVDVt+65SK2C7IxCxZt5o=
+golang.org/x/net v0.56.0/go.mod h1:D3Ku6r+V6JROoZK144D2XfMHFcMq/0zSfLelVTCFKec=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
 golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
 golang.org/x/oauth2 v0.0.0-20190604053449-0f29369cfe45/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
@@ -441,8 +441,8 @@ golang.org/x/sync v0.0.0-20200317015054-43a5402ce75a/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20200625203802-6e8e738ad208/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
-golang.org/x/sync v0.20.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
+golang.org/x/sync v0.22.0 h1:SZjpbeLmrCk4xhRSZFNZW5gFUeCeFgjekvI/+gfScek=
+golang.org/x/sync v0.22.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190312061237-fead79001313/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
@@ -492,8 +492,8 @@ golang.org/x/sys v0.4.1-0.20230131160137-e7d7f63158de/go.mod h1:oPkhp1MJrh7nUepC
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.10.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.12.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.45.0 h1:dO4czNzziLiiXplLQgBCEpCvXQ3dnkn0SdaZSYdQ+FY=
-golang.org/x/sys v0.45.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/sys v0.46.0 h1:noSf2Fq6F8DBgS+LysIkx7rIExoNHJsxOAtPp4rthXw=
+golang.org/x/sys v0.46.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
 golang.org/x/term v0.0.0-20201117132131-f5c789dd3221/go.mod h1:Nr5EML6q2oocZ2LXRh80K7BxOlk5/8JxuGnuhpl+muw=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
@@ -504,13 +504,11 @@ golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.4/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
-golang.org/x/text v0.37.0 h1:Cqjiwd9eSg8e0QAkyCaQTNHFIIzWtidPahFWR83rTrc=
-golang.org/x/text v0.37.0/go.mod h1:a5sjxXGs9hsn/AJVwuElvCAo9v8QYLzvavO5z2PiM38=
+golang.org/x/text v0.40.0 h1:Ub2Z6/xjgF1WrYQz2nuITOEegKFtiIy+rieRJ5lHZKs=
+golang.org/x/text v0.40.0/go.mod h1:hpnzDAfGV753zIKo+wk3u1bVKCGPbrnF7+7LBF/UHVY=
 golang.org/x/time v0.0.0-20181108054448-85acf8d2951c/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/time v0.0.0-20190308202827-9d24e82272b4/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/time v0.0.0-20191024005414-555d28b269f0/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
-golang.org/x/time v0.12.0 h1:ScB/8o8olJvc+CQPWrK3fPZNfh7qgwCrY0zJmoEQLSE=
-golang.org/x/time v0.12.0/go.mod h1:CDIdPxbZBQxdj6cxyCIdrNogrJKMJ7pr37NYpMcMDSg=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190226205152-f727befe758c/go.mod h1:9Yl7xja0Znq3iFh3HoIrodX9oNMXvdceNzlUR8zjMvY=
@@ -558,8 +556,8 @@ golang.org/x/tools v0.0.0-20201208233053-a543418bbed2/go.mod h1:emZCQorbCU4vsT4f
 golang.org/x/tools v0.0.0-20210105154028-b0ab187a4818/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.0.0-20210108195828-e2f9c7f1fc8e/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.1.0/go.mod h1:xkSsbof2nBLbhDlRMhhhyNLN/zl3eTqcnHD5viDpcZ0=
-golang.org/x/tools v0.44.0 h1:UP4ajHPIcuMjT1GqzDWRlalUEoY+uzoZKnhOjbIPD2c=
-golang.org/x/tools v0.44.0/go.mod h1:KA0AfVErSdxRZIsOVipbv3rQhVXTnlU6UhKxHd1seDI=
+golang.org/x/tools v0.47.0 h1:7Kn5x/d1svx/PzryTsqeoZN4TZwqeH5pGWjefhLi/1Q=
+golang.org/x/tools v0.47.0/go.mod h1:dFHnyTvFWY212G+h7ZY4Vsp/K3U4/7W9TyVaAul8uCA=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=

internal/certs/root_ca_test.go

@@ -11,7 +11,8 @@ func TestCACertPool(t *testing.T) {
 	c := &http.Client{
 		Transport: &http.Transport{
 			TLSClientConfig: &tls.Config{
-				RootCAs: CACertPool(),
+				RootCAs:    CACertPool(),
+				MinVersion: tls.VersionTLS12,
 			},
 		},
 		Timeout: 2 * time.Second,

internal/controld/config.go

@@ -293,7 +293,7 @@ func apiTransport(cdDev bool) *http.Transport {
 		return dial(ctx, "tcp6", addrsFromPort(apiIpsV6, port))
 	}
 	if router.Name() == ddwrt.Name || runtime.GOOS == "android" {
-		transport.TLSClientConfig = &tls.Config{RootCAs: certs.CACertPool()}
+		transport.TLSClientConfig = &tls.Config{RootCAs: certs.CACertPool(), MinVersion: tls.VersionTLS12}
 	}
 	return transport
 }

internal/dnscache/cache.go

@@ -1,6 +1,8 @@
 package dnscache
 
 import (
+	"fmt"
+	"net"
 	"strings"
 	"time"
 
@@ -16,11 +18,17 @@ type Cacher interface {
 }
 
 // Key is the caching key for DNS message.
+//
+// ECS partitions the cache by EDNS Client Subnet so an answer resolved for one
+// subnet is never served to a client in a different subnet. Answer records are
+// scoped to the network that generated them (RFC 7871 §7.3), so they must not
+// be shared across subnets even when the question is otherwise identical.
 type Key struct {
 	Qtype    uint16
 	Qclass   uint16
 	Name     string
 	Upstream string
+	ECS      string
 }
 
 type Value struct {
@@ -60,7 +68,58 @@ func NewLRUCache(size int) (*LRUCache, error) {
 // NewKey creates a new cache key for given DNS message.
 func NewKey(msg *dns.Msg, upstream string) Key {
 	q := msg.Question[0]
-	return Key{Qtype: q.Qtype, Qclass: q.Qclass, Name: normalizeQname(q.Name), Upstream: upstream}
+	return Key{Qtype: q.Qtype, Qclass: q.Qclass, Name: normalizeQname(q.Name), Upstream: upstream, ECS: CanonicalECS(msg)}
+}
+
+// CanonicalECS returns a canonical string form of the EDNS Client Subnet (ECS,
+// EDNS option 8) carried by msg, suitable for partitioning cache and
+// singleflight keys. A request with no ECS option returns "", so all ECS-less
+// queries share one partition and behave as before.
+//
+// A request that DOES carry an ECS option is never mapped to "", even at SOURCE
+// PREFIX-LENGTH 0: the /0 query is forwarded with an ECS option, may draw
+// different upstream data than an ECS-less query, and its answer (with the
+// echoed option) must not be served to a client that sent no ECS — RFC 7871
+// §7.3.1 requires /0-cached data to remain distinguishable. The family is part
+// of the token, so an IPv4 /0 and an IPv6 /0 stay distinct too.
+//
+// The address is masked to its source prefix length so only the significant
+// subnet bits contribute to the key: two clients in the same subnet share a
+// partition, while different subnets (or address families) never do. The
+// response-only SCOPE PREFIX-LENGTH is deliberately excluded — it is not part of
+// what the client asked for.
+func CanonicalECS(msg *dns.Msg) string {
+	opt := msg.IsEdns0()
+	if opt == nil {
+		return ""
+	}
+	for _, o := range opt.Option {
+		e, ok := o.(*dns.EDNS0_SUBNET)
+		if !ok {
+			continue
+		}
+		bits := int(e.SourceNetmask)
+		total := 128
+		zero := net.IPv6zero
+		if e.Family == 1 {
+			total = 32
+			zero = net.IPv4zero
+		}
+		addr := e.Address
+		if len(addr) == 0 {
+			// A /0 request commonly carries an empty address; normalize it to
+			// the family zero so its token is stable regardless of encoding.
+			addr = zero
+		}
+		masked := addr
+		if bits <= total {
+			if m := addr.Mask(net.CIDRMask(bits, total)); m != nil {
+				masked = m
+			}
+		}
+		return fmt.Sprintf("%d/%d/%s", e.Family, e.SourceNetmask, masked.String())
+	}
+	return ""
 }
 
 // NewValue creates a new cache value for given DNS message.

internal/dnscache/cache_test.go

@@ -0,0 +1,186 @@
+package dnscache
+
+import (
+	"net"
+	"testing"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// msgWithECS builds an A query for name carrying an EDNS Client Subnet option, or none
+// when family == 0.
+func msgWithECS(name string, family uint16, prefix uint8, addr string) *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion(dns.Fqdn(name), dns.TypeA)
+	if family == 0 {
+		return m
+	}
+	m.SetEdns0(4096, true)
+	m.IsEdns0().Option = append(m.IsEdns0().Option, &dns.EDNS0_SUBNET{
+		Code:          dns.EDNS0SUBNET,
+		Family:        family,
+		SourceNetmask: prefix,
+		Address:       net.ParseIP(addr),
+	})
+	return m
+}
+
+// answerWithA builds a cached answer holding a single A record with the given address.
+func answerWithA(name, a string) *dns.Msg {
+	m := new(dns.Msg)
+	m.SetQuestion(dns.Fqdn(name), dns.TypeA)
+	rr, err := dns.NewRR(dns.Fqdn(name) + " 300 IN A " + a)
+	if err != nil {
+		panic(err)
+	}
+	m.Answer = []dns.RR{rr}
+	return m
+}
+
+func firstA(msg *dns.Msg) string {
+	for _, rr := range msg.Answer {
+		if a, ok := rr.(*dns.A); ok {
+			return a.A.String()
+		}
+	}
+	return ""
+}
+
+// TestCanonicalECS covers the key-partitioning helper: only a request with no ECS option
+// collapses to the shared empty partition, while a carried /0 keeps its own family-scoped
+// token (distinct from no-ECS, per RFC 7871 §7.3.1); host bits below the source prefix do
+// not fragment the key, and different subnets / families produce different keys.
+func TestCanonicalECS(t *testing.T) {
+	tests := []struct {
+		name string
+		msg  *dns.Msg
+		want string
+	}{
+		{"no ecs", msgWithECS("controld.com", 0, 0, ""), ""},
+		// A carried ECS option is never "" (RFC 7871 §7.3.1), and IPv4 /0 vs
+		// IPv6 /0 stay distinct; the address encoding must not change the token.
+		{"ipv4 /0", msgWithECS("controld.com", 1, 0, "0.0.0.0"), "1/0/0.0.0.0"},
+		{"ipv4 /0 empty addr", msgWithECS("controld.com", 1, 0, ""), "1/0/0.0.0.0"},
+		{"ipv6 /0", msgWithECS("controld.com", 2, 0, "::"), "2/0/::"},
+		{"ipv6 /0 empty addr", msgWithECS("controld.com", 2, 0, ""), "2/0/::"},
+		{"ipv4 /24", msgWithECS("controld.com", 1, 24, "203.0.113.0"), "1/24/203.0.113.0"},
+		{"ipv4 host bits masked", msgWithECS("controld.com", 1, 24, "203.0.113.7"), "1/24/203.0.113.0"},
+		{"ipv6 /64", msgWithECS("controld.com", 2, 64, "2001:db8:1::"), "2/64/2001:db8:1::"},
+		{"ipv6 host bits masked", msgWithECS("controld.com", 2, 64, "2001:db8:1::dead:beef"), "2/64/2001:db8:1::"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := CanonicalECS(tt.msg); got != tt.want {
+				t.Fatalf("CanonicalECS = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestNewKey_ECSPartition verifies the cache key distinguishes subnets while collapsing
+// same-subnet requests, so the LRU cache cannot serve one subnet's records to another.
+func TestNewKey_ECSPartition(t *testing.T) {
+	const up = "https://dns.example/dns-query"
+	subnetA := NewKey(msgWithECS("controld.com", 2, 64, "2001:db8:1::"), up)
+	subnetB := NewKey(msgWithECS("controld.com", 2, 64, "2001:db8:2::"), up)
+	subnetAHost := NewKey(msgWithECS("controld.com", 2, 64, "2001:db8:1::5"), up)
+	ipv4 := NewKey(msgWithECS("controld.com", 1, 24, "203.0.113.0"), up)
+	noECS := NewKey(msgWithECS("controld.com", 0, 0, ""), up)
+
+	if subnetA == subnetB {
+		t.Fatal("different subnets must not share a cache key")
+	}
+	if subnetA != subnetAHost {
+		t.Fatal("same subnet (different host bits) must share a cache key")
+	}
+	if subnetA == ipv4 || subnetA == noECS || ipv4 == noECS {
+		t.Fatal("different families / no-ECS must not collide")
+	}
+}
+
+// TestLRUCache_ECSZeroPrefixDistinctFromNoECS is the regression test for the /0-vs-no-ECS
+// collision (RFC 7871 §7.3.1). A query carrying an ECS /0 option is forwarded WITH ECS and
+// may draw different upstream data, so its cached answer must never be served to a client
+// that sent no ECS at all, nor may IPv4 /0 and IPv6 /0 cross-serve each other.
+func TestLRUCache_ECSZeroPrefixDistinctFromNoECS(t *testing.T) {
+	c, err := NewLRUCache(16)
+	if err != nil {
+		t.Fatalf("NewLRUCache: %v", err)
+	}
+	const up = "https://dns.example/dns-query"
+	expire := time.Now().Add(time.Minute)
+
+	noECS := msgWithECS("controld.com", 0, 0, "")
+	zeroV4 := msgWithECS("controld.com", 1, 0, "0.0.0.0")
+	zeroV6 := msgWithECS("controld.com", 2, 0, "::")
+
+	// Only the IPv4 /0 query's answer is cached.
+	c.Add(NewKey(zeroV4, up), NewValue(answerWithA("controld.com", "192.0.2.1"), expire))
+
+	// A no-ECS client must NOT receive the ECS /0 cached answer.
+	if got := c.Get(NewKey(noECS, up)); got != nil {
+		t.Fatalf("no-ECS client received the ECS /0 cached record %q", firstA(got.Msg))
+	}
+	// An IPv6 /0 client must NOT receive the IPv4 /0 answer.
+	if got := c.Get(NewKey(zeroV6, up)); got != nil {
+		t.Fatalf("IPv6 /0 client received the IPv4 /0 cached record %q", firstA(got.Msg))
+	}
+	// The IPv4 /0 client still hits its own entry.
+	if got := c.Get(NewKey(zeroV4, up)); got == nil || firstA(got.Msg) != "192.0.2.1" {
+		t.Fatalf("IPv4 /0 lost its own cached record: %v", got)
+	}
+
+	// The no-ECS partition is independent and does not corrupt the /0 entry.
+	c.Add(NewKey(noECS, up), NewValue(answerWithA("controld.com", "198.51.100.1"), expire))
+	if got := c.Get(NewKey(noECS, up)); got == nil || firstA(got.Msg) != "198.51.100.1" {
+		t.Fatalf("no-ECS partition wrong record: %v", got)
+	}
+	if got := c.Get(NewKey(zeroV4, up)); got == nil || firstA(got.Msg) != "192.0.2.1" {
+		t.Fatalf("IPv4 /0 record corrupted by no-ECS insert: %v", got)
+	}
+}
+
+// TestLRUCache_ECSNoCrossSubnetServe is the real cache-path regression test for #564:
+// an entry populated for subnet A must never be returned to a client in subnet B, even
+// though the question (name/type/class/upstream) is identical. The two subnets carry
+// different A records; the second client must get its own record or a miss, never A's.
+func TestLRUCache_ECSNoCrossSubnetServe(t *testing.T) {
+	c, err := NewLRUCache(16)
+	if err != nil {
+		t.Fatalf("NewLRUCache: %v", err)
+	}
+	const up = "https://dns.example/dns-query"
+	expire := time.Now().Add(time.Minute)
+
+	reqA := msgWithECS("controld.com", 2, 64, "2001:db8:1::")
+	reqB := msgWithECS("controld.com", 2, 64, "2001:db8:2::")
+
+	// Only subnet A's answer (A record 192.0.2.1) is cached.
+	c.Add(NewKey(reqA, up), NewValue(answerWithA("controld.com", "192.0.2.1"), expire))
+
+	// A client in subnet B must NOT hit subnet A's entry.
+	if got := c.Get(NewKey(reqB, up)); got != nil {
+		t.Fatalf("subnet B received subnet A's cached record %q; cache is not ECS-partitioned", firstA(got.Msg))
+	}
+
+	// Subnet A still hits its own entry.
+	if got := c.Get(NewKey(reqA, up)); got == nil || firstA(got.Msg) != "192.0.2.1" {
+		t.Fatalf("subnet A lost its own cached record: %+v", got)
+	}
+
+	// Now cache subnet B's distinct answer and confirm the two never cross.
+	c.Add(NewKey(reqB, up), NewValue(answerWithA("controld.com", "198.51.100.1"), expire))
+	if got := c.Get(NewKey(reqB, up)); got == nil || firstA(got.Msg) != "198.51.100.1" {
+		t.Fatalf("subnet B got the wrong record: %v", got)
+	}
+	if got := c.Get(NewKey(reqA, up)); got == nil || firstA(got.Msg) != "192.0.2.1" {
+		t.Fatalf("subnet A record corrupted by subnet B insert: %v", got)
+	}
+
+	// A second host within subnet A shares the partition (cache hit with A's record).
+	reqAHost := msgWithECS("controld.com", 2, 64, "2001:db8:1::9")
+	if got := c.Get(NewKey(reqAHost, up)); got == nil || firstA(got.Msg) != "192.0.2.1" {
+		t.Fatalf("same-subnet host missed the shared cache entry: %v", got)
+	}
+}

internal/net/net.go

@@ -157,6 +157,101 @@ type parallelDialerResult struct {
 	err  error
 }
 
+const (
+	// unreachableBackoffBase is the initial suppression window applied to a
+	// dial address after it returns a network-unreachable error (e.g.
+	// "connect: no route to host"). The window grows exponentially up to
+	// unreachableBackoffMax on repeated failures, and is cleared as soon as
+	// the address dials successfully.
+	unreachableBackoffBase = 5 * time.Second
+	// unreachableBackoffMax caps the suppression window so an address is
+	// always re-probed within a bounded interval, preserving recovery when
+	// the route comes back.
+	unreachableBackoffMax = 60 * time.Second
+)
+
+// Windows winsock codes for the unreachable errnos. A failing connect on
+// Windows surfaces these raw WSA codes (WSAENETUNREACH/WSAEHOSTUNREACH), whereas
+// syscall.ENETUNREACH/EHOSTUNREACH are Go's portable "invented" values
+// (APPLICATION_ERROR + iota) that never equal them. Matching these explicitly is
+// therefore required for the classifier to detect unreachable errors on Windows;
+// errors.Is against the syscall.* constants alone would not.
+//
+// https://learn.microsoft.com/en-us/windows/win32/winsock/windows-sockets-error-codes-2
+var (
+	windowsENETUNREACH  = syscall.Errno(10051)
+	windowsEHOSTUNREACH = syscall.Errno(10065)
+)
+
+// IsUnreachable reports whether err indicates the destination network or host
+// has no route (ENETUNREACH/EHOSTUNREACH). These are the errors produced when
+// an endpoint's address family is available locally but unroutable, e.g. an
+// IPv6 DoH endpoint while the host has IPv6 but no route to it.
+func IsUnreachable(err error) bool {
+	if err == nil {
+		return false
+	}
+	var opErr *net.OpError
+	if errors.As(err, &opErr) {
+		return errors.Is(opErr.Err, syscall.ENETUNREACH) ||
+			errors.Is(opErr.Err, syscall.EHOSTUNREACH) ||
+			errors.Is(opErr.Err, windowsENETUNREACH) ||
+			errors.Is(opErr.Err, windowsEHOSTUNREACH)
+	}
+	return false
+}
+
+type unreachableEntry struct {
+	until   time.Time
+	backoff time.Duration
+}
+
+// unreachableTracker records dial addresses that recently failed with a
+// network-unreachable error so ParallelDialer can temporarily stop hammering
+// them. This prevents an unroutable endpoint from generating a sustained dial
+// /health-check storm, while still re-probing each address once its bounded
+// backoff window expires so genuine recovery is never permanently blocked.
+type unreachableTracker struct {
+	mu      sync.Mutex
+	entries map[string]unreachableEntry
+}
+
+var unreachable = &unreachableTracker{entries: make(map[string]unreachableEntry)}
+
+// suppressed reports whether addr is currently within its unreachable backoff
+// window and should be skipped.
+func (t *unreachableTracker) suppressed(addr string, now time.Time) bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	e, ok := t.entries[addr]
+	return ok && now.Before(e.until)
+}
+
+// markUnreachable extends the suppression window for addr using a bounded
+// exponential backoff.
+func (t *unreachableTracker) markUnreachable(addr string, now time.Time) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	e := t.entries[addr]
+	if e.backoff == 0 {
+		e.backoff = unreachableBackoffBase
+	} else {
+		e.backoff *= 2
+		if e.backoff > unreachableBackoffMax {
+			e.backoff = unreachableBackoffMax
+		}
+	}
+	e.until = now.Add(e.backoff)
+	t.entries[addr] = e
+}
+
+// markReachable clears any suppression for addr after a successful dial.
+func (t *unreachableTracker) markReachable(addr string) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	delete(t.entries, addr)
+}
+
 type ParallelDialer struct {
 	net.Dialer
 }
@@ -165,26 +260,63 @@ func (d *ParallelDialer) DialContext(ctx context.Context, network string, addrs
 	if len(addrs) == 0 {
 		return nil, errors.New("empty addresses")
 	}
+
+	// Skip addresses that recently returned a network-unreachable error so an
+	// unroutable endpoint (e.g. an IPv6 DoH address while the host has IPv6
+	// but no route to it) does not generate a sustained dial storm. Suppression
+	// is bounded: once an address's backoff window expires it is re-probed, so
+	// genuine recovery is preserved.
+	now := time.Now()
+	live := make([]string, 0, len(addrs))
+	var suppressed int
+	for _, addr := range addrs {
+		if unreachable.suppressed(addr, now) {
+			suppressed++
+			continue
+		}
+		live = append(live, addr)
+	}
+	if len(live) == 0 {
+		// Every candidate is within its unreachable backoff window. Fail fast
+		// and quietly instead of re-dialing known-unroutable addresses; the
+		// windows expire and re-probe shortly, so recovery still happens.
+		logger.Debug().Msgf("skipping %d unreachable address(es), all in backoff", suppressed)
+		// TODO: the errno here is hardcoded to EHOSTUNREACH, but the actual
+		// failure that triggered suppression may have been ENETUNREACH. This is
+		// harmless today (IsUnreachable treats both the same and nothing else
+		// inspects the errno), but if these errors are ever recorded/reported we
+		// should retain the real error in unreachableEntry and surface it here.
+		return nil, &net.OpError{Op: "dial", Net: network, Err: syscall.EHOSTUNREACH}
+	}
+	if suppressed > 0 {
+		logger.Debug().Msgf("skipping %d unreachable address(es) in backoff", suppressed)
+	}
+
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
 	done := make(chan struct{})
 	defer close(done)
-	ch := make(chan *parallelDialerResult, len(addrs))
+	ch := make(chan *parallelDialerResult, len(live))
 	var wg sync.WaitGroup
-	wg.Add(len(addrs))
+	wg.Add(len(live))
 	go func() {
 		wg.Wait()
 		close(ch)
 	}()
 
-	for _, addr := range addrs {
+	for _, addr := range live {
 		go func(addr string) {
 			defer wg.Done()
 			logger.Debug().Msgf("dialing to %s", addr)
 			conn, err := d.Dialer.DialContext(ctx, network, addr)
 			if err != nil {
 				logger.Debug().Msgf("failed to dial %s: %v", addr, err)
+				if IsUnreachable(err) {
+					unreachable.markUnreachable(addr, time.Now())
+				}
+			} else {
+				unreachable.markReachable(addr)
 			}
 			select {
 			case ch <- &parallelDialerResult{conn: conn, err: err}:

internal/net/net_test.go

@@ -2,10 +2,77 @@ package net
 
 import (
 	"context"
+	"net"
+	"syscall"
 	"testing"
 	"time"
 )
 
+func TestIsUnreachable(t *testing.T) {
+	tests := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{"nil", nil, false},
+		{"enetunreach", &net.OpError{Op: "dial", Net: "tcp", Err: syscall.ENETUNREACH}, true},
+		{"ehostunreach", &net.OpError{Op: "dial", Net: "tcp", Err: syscall.EHOSTUNREACH}, true},
+		{"windows enetunreach", &net.OpError{Op: "dial", Net: "tcp", Err: windowsENETUNREACH}, true},
+		{"windows ehostunreach", &net.OpError{Op: "dial", Net: "tcp", Err: windowsEHOSTUNREACH}, true},
+		{"connection refused", &net.OpError{Op: "dial", Net: "tcp", Err: syscall.ECONNREFUSED}, false},
+		{"not an opError", syscall.ENETUNREACH, false},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			if got := IsUnreachable(tc.err); got != tc.want {
+				t.Errorf("IsUnreachable(%v) = %v, want %v", tc.err, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestUnreachableTracker(t *testing.T) {
+	tr := &unreachableTracker{entries: make(map[string]unreachableEntry)}
+	const addr = "[2606:1a40::22]:443"
+	now := time.Unix(0, 0)
+
+	// Not suppressed before any failure.
+	if tr.suppressed(addr, now) {
+		t.Fatal("addr suppressed before any failure")
+	}
+
+	// First failure suppresses for the base window.
+	tr.markUnreachable(addr, now)
+	if !tr.suppressed(addr, now.Add(unreachableBackoffBase-time.Millisecond)) {
+		t.Fatal("addr not suppressed within base backoff window")
+	}
+	if tr.suppressed(addr, now.Add(unreachableBackoffBase)) {
+		t.Fatal("addr still suppressed at end of base backoff window")
+	}
+
+	// Backoff grows exponentially and is capped at the max.
+	tr.markUnreachable(addr, now)
+	if got := tr.entries[addr].backoff; got != 2*unreachableBackoffBase {
+		t.Fatalf("backoff after second failure = %v, want %v", got, 2*unreachableBackoffBase)
+	}
+	for i := 0; i < 10; i++ {
+		tr.markUnreachable(addr, now)
+	}
+	if got := tr.entries[addr].backoff; got != unreachableBackoffMax {
+		t.Fatalf("backoff not capped: got %v, want %v", got, unreachableBackoffMax)
+	}
+
+	// A successful dial clears suppression entirely.
+	tr.markReachable(addr)
+	if tr.suppressed(addr, now) {
+		t.Fatal("addr still suppressed after markReachable")
+	}
+	if _, ok := tr.entries[addr]; ok {
+		t.Fatal("entry not removed after markReachable")
+	}
+}
+
 func TestProbeStackTimeout(t *testing.T) {
 	done := make(chan struct{})
 	started := make(chan struct{})

internal/router/dnsmasq/dnsmasq.go

@@ -2,11 +2,11 @@ package dnsmasq
 
 import (
 	"errors"
-	"html/template"
 	"net"
 	"os"
 	"path/filepath"
 	"strings"
+	"text/template"
 
 	"github.com/Control-D-Inc/ctrld"
 )

nameservers.go

@@ -1,7 +1,14 @@
 package ctrld
 
+import "runtime"
+
 type dnsFn func() []string
 
+// isMobile reports whether the current OS is a mobile platform.
+func isMobile() bool {
+	return runtime.GOOS == "android" || runtime.GOOS == "ios"
+}
+
 // nameservers returns DNS nameservers from system settings.
 func nameservers() []string {
 	var dns []string

nameservers_darwin.go

@@ -25,6 +25,12 @@ func dnsFns() []dnsFn {
 func getDNSFromScutil() []string {
 	logger := *ProxyLogger.Load()
 
+	// Skip scutil on mobile platforms - not available in sandbox
+	if isMobile() {
+		Log(context.Background(), logger.Debug(), "skipping scutil DNS discovery on mobile platform")
+		return nil
+	}
+
 	const (
 		maxRetries    = 10
 		retryInterval = 100 * time.Millisecond
@@ -89,6 +95,11 @@ func getDNSFromScutil() []string {
 }
 
 func getDHCPNameservers(iface string) ([]string, error) {
+	// Skip ipconfig on mobile platforms - not available in sandbox
+	if isMobile() {
+		return nil, fmt.Errorf("ipconfig not available on mobile")
+	}
+
 	// Run the ipconfig command for the given interface.
 	cmd := exec.Command("ipconfig", "getpacket", iface)
 	output, err := cmd.Output()
@@ -201,6 +212,11 @@ func getAllDHCPNameservers() []string {
 }
 
 func patchNetIfaceName(iface *net.Interface) (bool, error) {
+	// Skip networksetup on mobile platforms - not available in sandbox
+	if isMobile() {
+		return false, nil
+	}
+
 	b, err := exec.Command("networksetup", "-listnetworkserviceorder").Output()
 	if err != nil {
 		return false, err

nameservers_linux.go

@@ -7,6 +7,7 @@ import (
 	"net"
 	"net/netip"
 	"os"
+	"runtime"
 	"strings"
 
 	"tailscale.com/net/netmon"
@@ -24,6 +25,11 @@ func dnsFns() []dnsFn {
 }
 
 func dns4() []string {
+	// Skip route-based DNS discovery on Android
+	if runtime.GOOS == "android" {
+		return nil
+	}
+
 	f, err := os.Open(v4RouteFile)
 	if err != nil {
 		return nil
@@ -64,6 +70,11 @@ func dns4() []string {
 }
 
 func dns6() []string {
+	// Skip route-based DNS discovery on Android
+	if runtime.GOOS == "android" {
+		return nil
+	}
+
 	f, err := os.Open(v6RouteFile)
 	if err != nil {
 		return nil
@@ -98,6 +109,11 @@ func dns6() []string {
 }
 
 func dnsFromSystemdResolver() []string {
+	// Skip systemd resolver on Android
+	if runtime.GOOS == "android" {
+		return nil
+	}
+
 	c, err := resolvconffile.ParseFile("/run/systemd/resolve/resolv.conf")
 	if err != nil {
 		return nil

resolver.go

@@ -19,6 +19,8 @@ import (
 	"golang.org/x/sync/singleflight"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
+
+	"github.com/Control-D-Inc/ctrld/internal/dnscache"
 )
 
 const (
@@ -45,8 +47,12 @@ const (
 
 const controldPublicDns = "76.76.2.0"
 
+const maxConcurrentOSResolverExchanges = 128
+
 var controldPublicDnsWithPort = net.JoinHostPort(controldPublicDns, "53")
 
+var osResolverExchangeSem = make(chan struct{}, maxConcurrentOSResolverExchanges)
+
 var localResolver Resolver
 
 func init() {
@@ -136,14 +142,15 @@ func availableNameservers() []string {
 // It's the caller's responsibility to ensure the system DNS is in a clean state before
 // calling this function.
 func InitializeOsResolver(guardAgainstNoNameservers bool) []string {
+	resolverMutex.Lock()
+	defer resolverMutex.Unlock()
+
 	nameservers := availableNameservers()
 	// if no nameservers, return empty slice so we dont remove all nameservers
 	if len(nameservers) == 0 && guardAgainstNoNameservers {
 		return []string{}
 	}
 	ns := initializeOsResolver(nameservers)
-	resolverMutex.Lock()
-	defer resolverMutex.Unlock()
 	or = newResolverWithNameserver(ns)
 	return ns
 }
@@ -373,8 +380,10 @@ func (o *osResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error
 	domain := strings.TrimSuffix(msg.Question[0].Name, ".")
 	qtype := msg.Question[0].Qtype
 
-	// Unique key for the singleflight group.
-	key := fmt.Sprintf("%s:%d:", domain, qtype)
+	// Unique key for the singleflight group. The EDNS Client Subnet is part of
+	// the key so subnet-specific answers are neither coalesced nor hot-cached
+	// across different subnets (RFC 7871 §7.3).
+	key := fmt.Sprintf("%s:%d:%s", domain, qtype, dnscache.CanonicalECS(msg))
 
 	// Checking the cache first.
 	if val, ok := o.cache.Load(key); ok {
@@ -466,6 +475,13 @@ func (o *osResolver) resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error
 		for _, server := range servers {
 			go func(server string) {
 				defer wg.Done()
+				release, ok := acquireOSResolverExchangeSlot(ctx)
+				if !ok {
+					ch <- &osResolverResult{err: ctx.Err(), server: server, lan: isLan}
+					return
+				}
+				defer release()
+
 				var answer *dns.Msg
 				var err error
 				var localOSResolverIP net.IP
@@ -576,6 +592,15 @@ func (o *osResolver) resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error
 	return nil, errors.Join(errs...)
 }
 
+func acquireOSResolverExchangeSlot(ctx context.Context) (func(), bool) {
+	select {
+	case osResolverExchangeSem <- struct{}{}:
+		return func() { <-osResolverExchangeSem }, true
+	case <-ctx.Done():
+		return nil, false
+	}
+}
+
 func (o *osResolver) removeCache(key string) {
 	o.cache.Delete(key)
 }

resolver_test.go

@@ -282,6 +282,93 @@ func Test_Edns0_CacheReply(t *testing.T) {
 	}
 }
 
+// ecsAnswerHandler returns a distinct A record per EDNS Client Subnet, so a test can
+// prove one subnet never receives another subnet's cached record. It counts upstream
+// calls to confirm the hot cache/singleflight is partitioned by ECS rather than shared.
+func ecsAnswerHandler(call *atomic.Int64) dns.HandlerFunc {
+	return func(w dns.ResponseWriter, msg *dns.Msg) {
+		call.Add(1)
+		a := "203.0.113.1" // no/other subnet
+		if opt := msg.IsEdns0(); opt != nil {
+			for _, o := range opt.Option {
+				if e, ok := o.(*dns.EDNS0_SUBNET); ok {
+					switch {
+					case e.Address.Equal(net.ParseIP("2001:db8:1::")):
+						a = "192.0.2.1"
+					case e.Address.Equal(net.ParseIP("2001:db8:2::")):
+						a = "198.51.100.1"
+					}
+				}
+			}
+		}
+		m := new(dns.Msg)
+		m.SetReply(msg)
+		rr, _ := dns.NewRR(msg.Question[0].Name + " 300 IN A " + a)
+		m.Answer = []dns.RR{rr}
+		w.WriteMsg(m)
+	}
+}
+
+// Test_osResolver_HotCache_ECSPartition is the real cache-path regression test for #564 on
+// the osResolver hot cache / singleflight path: the upstream returns a different A record
+// per subnet, and a client in subnet B must never be served subnet A's hot-cached record.
+func Test_osResolver_HotCache_ECSPartition(t *testing.T) {
+	lanPC, err := net.ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("failed to listen on LAN address: %v", err)
+	}
+	call := &atomic.Int64{}
+	lanServer, lanAddr, err := runLocalPacketConnTestServer(t, lanPC, ecsAnswerHandler(call))
+	if err != nil {
+		t.Fatalf("failed to run LAN test server: %v", err)
+	}
+	defer lanServer.Shutdown()
+
+	or := newResolverWithNameserver([]string{lanAddr})
+	query := func(subnet string) string {
+		m := new(dns.Msg)
+		m.SetQuestion(dns.Fqdn("controld.com"), dns.TypeA)
+		m.RecursionDesired = true
+		m.SetEdns0(4096, true)
+		m.IsEdns0().Option = append(m.IsEdns0().Option, &dns.EDNS0_SUBNET{
+			Code:          dns.EDNS0SUBNET,
+			Family:        2,
+			SourceNetmask: 64,
+			Address:       net.ParseIP(subnet),
+		})
+		answer, err := or.Resolve(context.Background(), m)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, rr := range answer.Answer {
+			if a, ok := rr.(*dns.A); ok {
+				return a.A.String()
+			}
+		}
+		return ""
+	}
+
+	// Subnet A populates the hot cache; a repeat hits it (upstream called once).
+	if got := query("2001:db8:1::"); got != "192.0.2.1" {
+		t.Fatalf("subnet A: got %q, want 192.0.2.1", got)
+	}
+	if got := query("2001:db8:1::"); got != "192.0.2.1" {
+		t.Fatalf("subnet A repeat: got %q, want 192.0.2.1", got)
+	}
+	if call.Load() != 1 {
+		t.Fatalf("subnet A repeat did not hit the hot cache: %d upstream calls", call.Load())
+	}
+
+	// Subnet B must get ITS OWN record, not subnet A's hot-cached one, and this
+	// requires a fresh upstream call (the cache is partitioned, not shared).
+	if got := query("2001:db8:2::"); got != "198.51.100.1" {
+		t.Fatalf("subnet B was served the wrong record %q (want 198.51.100.1); hot cache is not ECS-partitioned", got)
+	}
+	if call.Load() != 2 {
+		t.Fatalf("subnet B unexpectedly served from subnet A's cache: %d upstream calls, want 2", call.Load())
+	}
+}
+
 // https://github.com/Control-D-Inc/ctrld/issues/255
 func Test_legacyResolverWithBigExtraSection(t *testing.T) {
 	lanPC, err := net.ListenPacket("udp", "127.0.0.1:0") // 127.0.0.1 is considered LAN (loopback)
@@ -383,6 +470,11 @@ func nonSuccessHandlerWithRcode(rcode int) dns.HandlerFunc {
 
 func countHandler(call *atomic.Int64) dns.HandlerFunc {
 	return func(w dns.ResponseWriter, msg *dns.Msg) {
+		// Count the call before writing the reply. The client returns as soon
+		// as it receives the response, so a caller that reads this counter right
+		// after Resolve returns would race an increment done after WriteMsg and
+		// could observe a stale zero.
+		call.Add(1)
 		m := new(dns.Msg)
 		m.SetRcode(msg, dns.RcodeSuccess)
 		if cookie := getEdns0Cookie(msg.IsEdns0()); cookie != nil {
@@ -395,7 +487,6 @@ func countHandler(call *atomic.Int64) dns.HandlerFunc {
 			m.IsEdns0().Option = append(m.IsEdns0().Option, cookieOption)
 		}
 		w.WriteMsg(m)
-		call.Add(1)
 	}
 }