cmd/cli: decouple reset DNS task from ctrld status

So it can be run regardless of ctrld current status. This prevents a
racy behavior when reset DNS task restores DNS settings of the system,
but current running ctrld process may revert it immediately.

cmd/cli/cli.go

@@ -194,11 +194,15 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 			isCtrldRunning := status == service.StatusRunning
 			isCtrldInstalled := !errors.Is(err, service.ErrNotInstalled)
 
+			// Get current running iface, if any.
+			var currentIface string
+
 			// If pin code was set, do not allow running start command.
 			if isCtrldRunning {
 				if err := checkDeactivationPin(s, nil); isCheckDeactivationPinErr(err) {
 					os.Exit(deactivationPinInvalidExitCode)
 				}
+				currentIface = runningIface(s)
 			}
 
 			if !startOnly {
@@ -213,12 +217,15 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 
 				initLogging()
 				tasks := []task{
-					resetDnsTask(p, s),
 					{s.Stop, false},
+					resetDnsTask(p, s, isCtrldInstalled, currentIface),
 					{func() error {
 						// Save current DNS so we can restore later.
-						withEachPhysicalInterfaces("", "save DNS settings", func(i *net.Interface) error {
-							return saveCurrentStaticDNS(i)
+						withEachPhysicalInterfaces("", "", func(i *net.Interface) error {
+							if err := saveCurrentStaticDNS(i); !errors.Is(err, errSaveCurrentStaticDNSNotSupported) && err != nil {
+								return err
+							}
+							return nil
 						})
 						return nil
 					}, false},
@@ -334,14 +341,17 @@ NOTE: running "ctrld start" without any arguments will start already installed c
 			}
 
 			tasks := []task{
-				resetDnsTask(p, s),
 				{s.Stop, false},
 				{func() error { return doGenerateNextDNSConfig(nextdns) }, true},
 				{func() error { return ensureUninstall(s) }, false},
+				resetDnsTask(p, s, isCtrldInstalled, currentIface),
 				{func() error {
 					// Save current DNS so we can restore later.
-					withEachPhysicalInterfaces("", "save DNS settings", func(i *net.Interface) error {
-						return saveCurrentStaticDNS(i)
+					withEachPhysicalInterfaces("", "", func(i *net.Interface) error {
+						if err := saveCurrentStaticDNS(i); !errors.Is(err, errSaveCurrentStaticDNSNotSupported) && err != nil {
+							return err
+						}
+						return nil
 					})
 					return nil
 				}, false},
@@ -1340,9 +1350,7 @@ func run(appCallback *AppCallback, stopCh chan struct{}) {
 	close(waitCh)
 	<-stopCh
 
-	// Wait goroutines which watches/manipulates DNS settings terminated,
-	// ensuring that changes to DNS since here won't be reverted.
-	p.dnsWg.Wait()
+	p.stopDnsWatchers()
 	for _, f := range p.onStopped {
 		f()
 	}
@@ -2642,17 +2650,20 @@ func runningIface(s service.Service) string {
 
 // resetDnsNoLog performs resetting DNS with logging disable.
 func resetDnsNoLog(p *prog) {
-	lvl := zerolog.GlobalLevel()
-	zerolog.SetGlobalLevel(zerolog.Disabled)
+	// Normally, disable log to prevent annoying users.
+	if verbose < 3 {
+		lvl := zerolog.GlobalLevel()
+		zerolog.SetGlobalLevel(zerolog.Disabled)
+		p.resetDNS()
+		zerolog.SetGlobalLevel(lvl)
+		return
+	}
+	// For debugging purpose, still emit log.
 	p.resetDNS()
-	zerolog.SetGlobalLevel(lvl)
 }
 
 // resetDnsTask returns a task which perform reset DNS operation.
-func resetDnsTask(p *prog, s service.Service) task {
-	status, err := s.Status()
-	isCtrldInstalled := !errors.Is(err, service.ErrNotInstalled)
-	isCtrldRunning := status == service.StatusRunning
+func resetDnsTask(p *prog, s service.Service, isCtrldInstalled bool, currentRunningIface string) task {
 	return task{func() error {
 		if iface == "" {
 			return nil
@@ -2662,11 +2673,14 @@ func resetDnsTask(p *prog, s service.Service) task {
 		// process to reset what setDNS has done properly.
 		oldIface := iface
 		iface = "auto"
-		if isCtrldRunning {
-			iface = runningIface(s)
+		if currentRunningIface != "" {
+			iface = currentRunningIface
 		}
 		if isCtrldInstalled {
 			mainLog.Load().Debug().Msg("restore system DNS settings")
+			if status, _ := s.Status(); status == service.StatusRunning {
+				mainLog.Load().Fatal().Msg("reset DNS while ctrld still running is not safe")
+			}
 			resetDnsNoLog(p)
 		}
 		iface = oldIface

cmd/cli/dns_proxy.go

@@ -915,6 +915,8 @@ func (p *prog) performCaptivePortalDetection() {
 		if found {
 			resetDnsOnce.Do(func() {
 				mainLog.Load().Warn().Msg("found captive portal, leaking query to OS resolver")
+				// Store the result once here, so changes made below won't be reverted by DNS watchers.
+				p.captivePortalDetected.Store(found)
 				p.resetDNS()
 			})
 		}

cmd/cli/os_windows.go

@@ -119,6 +119,7 @@ func resetDNS(iface *net.Interface) error {
 			if len(ns) == 0 {
 				continue
 			}
+			mainLog.Load().Debug().Msgf("setting static DNS for interface %q", iface.Name)
 			if err := setDNS(iface, ns); err != nil {
 				return err
 			}

cmd/cli/prog.go

@@ -69,21 +69,21 @@ var svcConfig = &service.Config{
 var useSystemdResolved = false
 
 type prog struct {
-	mu                  sync.Mutex
-	waitCh              chan struct{}
-	stopCh              chan struct{}
-	reloadCh            chan struct{} // For Windows.
-	reloadDoneCh        chan struct{}
-	apiReloadCh         chan *ctrld.Config
-	apiForceReloadCh    chan struct{}
-	apiForceReloadGroup singleflight.Group
-	logConn             net.Conn
-	cs                  *controlServer
-	csSetDnsDone        chan struct{}
-	csSetDnsOk          bool
-	dnsWatchDogOnce     sync.Once
-	dnsWg               sync.WaitGroup
-	dnsWatcherStopCh    chan struct{}
+	mu                   sync.Mutex
+	waitCh               chan struct{}
+	stopCh               chan struct{}
+	reloadCh             chan struct{} // For Windows.
+	reloadDoneCh         chan struct{}
+	apiReloadCh          chan *ctrld.Config
+	apiForceReloadCh     chan struct{}
+	apiForceReloadGroup  singleflight.Group
+	logConn              net.Conn
+	cs                   *controlServer
+	csSetDnsDone         chan struct{}
+	csSetDnsOk           bool
+	dnsWg                sync.WaitGroup
+	dnsWatcherClosedOnce sync.Once
+	dnsWatcherStopCh     chan struct{}
 
 	cfg                  *ctrld.Config
 	localUpstreams       []string
@@ -512,6 +512,8 @@ func (p *prog) metricsEnabled() bool {
 }
 
 func (p *prog) Stop(s service.Service) error {
+	p.stopDnsWatchers()
+	mainLog.Load().Debug().Msg("dns watchers stopped")
 	mainLog.Load().Info().Msg("Service stopped")
 	close(p.stopCh)
 	if err := p.deAllocateIP(); err != nil {
@@ -521,6 +523,15 @@ func (p *prog) Stop(s service.Service) error {
 	return nil
 }
 
+func (p *prog) stopDnsWatchers() {
+	// Ensure all DNS watchers goroutine are terminated,
+	// so it won't mess up with other DNS changes.
+	p.dnsWatcherClosedOnce.Do(func() {
+		close(p.dnsWatcherStopCh)
+	})
+	p.dnsWg.Wait()
+}
+
 func (p *prog) allocateIP(ip string) error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
@@ -611,6 +622,11 @@ func (p *prog) setDNS() {
 	}
 	setDnsOK = true
 	logger.Debug().Msg("setting DNS successfully")
+	if allIfaces {
+		withEachPhysicalInterfaces(netIface.Name, "set DNS", func(i *net.Interface) error {
+			return setDnsIgnoreUnusableInterface(i, nameservers)
+		})
+	}
 	if shouldWatchResolvconf() {
 		servers := make([]netip.Addr, len(nameservers))
 		for i := range nameservers {
@@ -622,11 +638,6 @@ func (p *prog) setDNS() {
 			p.watchResolvConf(netIface, servers, setResolvConf)
 		}()
 	}
-	if allIfaces {
-		withEachPhysicalInterfaces(netIface.Name, "set DNS", func(i *net.Interface) error {
-			return setDnsIgnoreUnusableInterface(i, nameservers)
-		})
-	}
 	if p.dnsWatchdogEnabled() {
 		p.dnsWg.Add(1)
 		go func() {
@@ -661,41 +672,42 @@ func (p *prog) dnsWatchdog(iface *net.Interface, nameservers []string, allIfaces
 		return
 	}
 
-	p.dnsWatchDogOnce.Do(func() {
-		mainLog.Load().Debug().Msg("start DNS settings watchdog")
-		ns := nameservers
-		slices.Sort(ns)
-		ticker := time.NewTicker(p.dnsWatchdogDuration())
-		logger := mainLog.Load().With().Str("iface", iface.Name).Logger()
-		for {
-			select {
-			case <-p.dnsWatcherStopCh:
+	mainLog.Load().Debug().Msg("start DNS settings watchdog")
+	ns := nameservers
+	slices.Sort(ns)
+	ticker := time.NewTicker(p.dnsWatchdogDuration())
+	logger := mainLog.Load().With().Str("iface", iface.Name).Logger()
+	for {
+		select {
+		case <-p.dnsWatcherStopCh:
+			return
+		case <-p.stopCh:
+			mainLog.Load().Debug().Msg("stop dns watchdog")
+			return
+		case <-ticker.C:
+			if p.captivePortalDetected.Load() {
 				return
-			case <-p.stopCh:
-				mainLog.Load().Debug().Msg("stop dns watchdog")
-				return
-			case <-ticker.C:
-				if dnsChanged(iface, ns) {
-					logger.Debug().Msg("DNS settings were changed, re-applying settings")
-					if err := setDNS(iface, ns); err != nil {
-						mainLog.Load().Error().Err(err).Str("iface", iface.Name).Msgf("could not re-apply DNS settings")
-					}
-				}
-				if allIfaces {
-					withEachPhysicalInterfaces(iface.Name, "", func(i *net.Interface) error {
-						if dnsChanged(i, ns) {
-							if err := setDnsIgnoreUnusableInterface(i, nameservers); err != nil {
-								mainLog.Load().Error().Err(err).Str("iface", i.Name).Msgf("could not re-apply DNS settings")
-							} else {
-								mainLog.Load().Debug().Msgf("re-applying DNS for interface %q successfully", i.Name)
-							}
-						}
-						return nil
-					})
+			}
+			if dnsChanged(iface, ns) {
+				logger.Debug().Msg("DNS settings were changed, re-applying settings")
+				if err := setDNS(iface, ns); err != nil {
+					mainLog.Load().Error().Err(err).Str("iface", iface.Name).Msgf("could not re-apply DNS settings")
 				}
 			}
+			if allIfaces {
+				withEachPhysicalInterfaces(iface.Name, "", func(i *net.Interface) error {
+					if dnsChanged(i, ns) {
+						if err := setDnsIgnoreUnusableInterface(i, nameservers); err != nil {
+							mainLog.Load().Error().Err(err).Str("iface", i.Name).Msgf("could not re-apply DNS settings")
+						} else {
+							mainLog.Load().Debug().Msgf("re-applying DNS for interface %q successfully", i.Name)
+						}
+					}
+					return nil
+				})
+			}
 		}
-	})
+	}
 }
 
 func (p *prog) resetDNS() {
@@ -965,11 +977,13 @@ func saveCurrentStaticDNS(iface *net.Interface) error {
 	if err := os.Remove(file); err != nil && !errors.Is(err, fs.ErrNotExist) {
 		mainLog.Load().Warn().Err(err).Msg("could not remove old static DNS settings file")
 	}
-	mainLog.Load().Debug().Msgf("DNS settings for %s is static, saving ...", iface.Name)
-	if err := os.WriteFile(file, []byte(strings.Join(ns, ",")), 0600); err != nil {
+	nss := strings.Join(ns, ",")
+	mainLog.Load().Debug().Msgf("DNS settings for %q is static: %v, saving ...", iface.Name, nss)
+	if err := os.WriteFile(file, []byte(nss), 0600); err != nil {
 		mainLog.Load().Err(err).Msgf("could not save DNS settings for iface: %s", iface.Name)
 		return err
 	}
+	mainLog.Load().Debug().Msgf("save DNS settings for interface %q successfully", iface.Name)
 	return nil
 }
 
@@ -1005,9 +1019,7 @@ func dnsChanged(iface *net.Interface, nameservers []string) bool {
 func selfUninstallCheck(uninstallErr error, p *prog, logger zerolog.Logger) {
 	var uer *controld.UtilityErrorResponse
 	if errors.As(uninstallErr, &uer) && uer.ErrorField.Code == controld.InvalidConfigCode {
-		// Ensure all DNS watchers goroutine are terminated, so it won't mess up with self-uninstall.
-		close(p.dnsWatcherStopCh)
-		p.dnsWg.Wait()
+		p.stopDnsWatchers()
 
 		// Perform self-uninstall now.
 		selfUninstall(p, logger)

cmd/cli/resolvconf.go

@@ -40,6 +40,9 @@ func (p *prog) watchResolvConf(iface *net.Interface, ns []netip.Addr, setDnsFn f
 			mainLog.Load().Debug().Msgf("stopping watcher for %s", resolvConfPath)
 			return
 		case event, ok := <-watcher.Events:
+			if p.captivePortalDetected.Load() {
+				return
+			}
 			if !ok {
 				return
 			}