ipnlocal: support automatic exit node disablement when captive portal detected

control/controlknobs/controlknobs.go

@@ -103,6 +103,10 @@ type Knobs struct {
 	// DisableCaptivePortalDetection is whether the node should not perform captive portal detection
 	// automatically when the network state changes.
 	DisableCaptivePortalDetection atomic.Bool
+
+	// DisableExitNodeBehindCaptivePortal is whether the node should temporarily disable exit nodes
+	// whenever a captive portal is detected.
+	DisableExitNodeBehindCaptivePortal atomic.Bool
 }
 
 // UpdateFromNodeAttributes updates k (if non-nil) based on the provided self
@@ -132,6 +136,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 		disableLocalDNSOverrideViaNRPT       = has(tailcfg.NodeAttrDisableLocalDNSOverrideViaNRPT)
 		disableCryptorouting                 = has(tailcfg.NodeAttrDisableMagicSockCryptoRouting)
 		disableCaptivePortalDetection        = has(tailcfg.NodeAttrDisableCaptivePortalDetection)
+		disableExitNodeBehindCaptivePortal   = has(tailcfg.NodeAttrDisableExitNodeBehindCaptivePortal)
 	)
 
 	if has(tailcfg.NodeAttrOneCGNATEnable) {
@@ -159,6 +164,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 	k.DisableLocalDNSOverrideViaNRPT.Store(disableLocalDNSOverrideViaNRPT)
 	k.DisableCryptorouting.Store(disableCryptorouting)
 	k.DisableCaptivePortalDetection.Store(disableCaptivePortalDetection)
+	k.DisableExitNodeBehindCaptivePortal.Store(disableExitNodeBehindCaptivePortal)
 }
 
 // AsDebugJSON returns k as something that can be marshalled with json.Marshal
@@ -187,5 +193,6 @@ func (k *Knobs) AsDebugJSON() map[string]any {
 		"DisableLocalDNSOverrideViaNRPT":       k.DisableLocalDNSOverrideViaNRPT.Load(),
 		"DisableCryptorouting":                 k.DisableCryptorouting.Load(),
 		"DisableCaptivePortalDetection":        k.DisableCaptivePortalDetection.Load(),
+		"DisableExitNodeBehindCaptivePortal":   k.DisableExitNodeBehindCaptivePortal.Load(),
 	}
 }

ipn/ipnlocal/local.go

@@ -362,8 +362,13 @@ type LocalBackend struct {
 	// captiveCtx will always be non-nil, though it might be a canceled
 	// context. captiveCancel is non-nil if checkCaptivePortalLoop is
 	// running, and is set to nil after being canceled.
-	captiveCtx    context.Context
-	captiveCancel context.CancelFunc
+	//
+	// captiveDetected is true if the backend is currently behind a captive
+	// portal, and is used to temporarily disable exit nodes and other
+	// features that require connectivity.
+	captiveCtx      context.Context
+	captiveCancel   context.CancelFunc
+	captiveDetected bool
 	// needsCaptiveDetection is a channel that is used to signal either
 	// that captive portal detection is required (sending true) or that the
 	// backend is healthy and captive portal detection is not required
@@ -812,7 +817,7 @@ func (b *LocalBackend) onHealthChange(w *health.Warnable, us *health.UnhealthySt
 	} else {
 		// If connectivity is not impacted, we know for sure we're not behind a captive portal,
 		// so drop any warning, and signal that we don't need captive portal detection.
-		b.health.SetHealthy(captivePortalWarnable)
+		b.setCaptivePortalDetected(false)
 		select {
 		case b.needsCaptiveDetection <- false:
 		case <-ctx.Done():
@@ -820,6 +825,33 @@ func (b *LocalBackend) onHealthChange(w *health.Warnable, us *health.UnhealthySt
 	}
 }
 
+func (b *LocalBackend) setCaptivePortalDetected(found bool) {
+	b.mu.Lock()
+	prev := b.captiveDetected
+	b.captiveDetected = found
+	b.mu.Unlock()
+	b.logf("b.captiveDetected = %v, was %v", found, prev)
+
+	if found {
+		b.health.SetUnhealthy(captivePortalWarnable, nil)
+	} else {
+		b.health.SetHealthy(captivePortalWarnable)
+	}
+
+	if b.wantsExitNodeDisablementBehindCaptivePortal() && prev != found {
+		b.logf("b.captiveDetected changed to %v (was %v), calling authReconfig()", found, prev)
+		b.authReconfig()
+	}
+}
+
+func (b *LocalBackend) wantsExitNodeDisablementBehindCaptivePortal() bool {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	res := b.ControlKnobs().DisableExitNodeBehindCaptivePortal.Load()
+	b.logf("wantsExitNodeDisablementBehindCaptivePortal = %v", res)
+	return res
+}
+
 // Shutdown halts the backend and all its sub-components. The backend
 // can no longer be used after Shutdown returns.
 func (b *LocalBackend) Shutdown() {
@@ -2318,11 +2350,7 @@ func (b *LocalBackend) performCaptiveDetection() {
 	netMon := b.NetMon()
 	b.mu.Unlock()
 	found := d.Detect(ctx, netMon, dm, preferredDERP)
-	if found {
-		b.health.SetUnhealthy(captivePortalWarnable, health.Args{})
-	} else {
-		b.health.SetHealthy(captivePortalWarnable)
-	}
+	b.setCaptivePortalDetected(found)
 }
 
 // shouldRunCaptivePortalDetection reports whether captive portal detection
@@ -4673,6 +4701,22 @@ func (b *LocalBackend) routerConfig(cfg *wgcfg.Config, prefs ipn.PrefsView, oneC
 				b.logf("warning: ExitNodeAllowLANAccess has no effect on " + runtime.GOOS)
 			}
 		}
+
+		b.logf("routerConfig: b.captiveDetected is %v", b.captiveDetected)
+		if b.captiveDetected && b.wantsExitNodeDisablementBehindCaptivePortal() {
+			isZeroRouteFunc := func(p netip.Prefix) bool {
+				return p == ipv4Default || p == ipv6Default
+			}
+			hasZeroRoutes := slices.ContainsFunc(rs.Routes, isZeroRouteFunc)
+			if hasZeroRoutes {
+				b.logf("captive portal detected, dropping zero routes")
+				// If a captive portal is present, remove the zero routes (ipv4Default and ipv6Default)
+				// to allow the user to authenticate with the captive portal.
+				rs.Routes = slices.DeleteFunc(rs.Routes, isZeroRouteFunc)
+			} else {
+				b.logf("captive portal detected, but no zero routes to drop")
+			}
+		}
 	}
 
 	if slices.ContainsFunc(rs.LocalAddrs, tsaddr.PrefixIs4) {

tailcfg/tailcfg.go

@@ -2346,6 +2346,10 @@ const (
 	// NodeAttrSSHEnvironmentVariables enables logic for handling environment variables sent
 	// via SendEnv in the SSH server and applying them to the SSH session.
 	NodeAttrSSHEnvironmentVariables NodeCapability = "ssh-env-vars"
+
+	// NodeAttrDisableExitNodeBehindCaptivePortal instructs the client to temporarily disable exit nodes when
+	// behind a captive portal.
+	NodeAttrDisableExitNodeBehindCaptivePortal NodeCapability = "disable-exit-node-behind-captive-portal"
 )
 
 // SetDNSRequest is a request to add a DNS record.