control,health,ipn: move IP forwarding check to health tracker (#19007)

Currently IP forwarding health check is done on sending MapRequests.

Move ip forwarding to the health service to gain the benefits
of the health tracker and perodic monitoring out of band from
the MapRequest path. ipnlocal now provides a closure to
the health service to provide the check if forwarding is broken.

Removed `skipIPForwardingCheck` from controlclient/direct.go,
it wasn't being used as the comments describe it, that check
has moved to ipnlocal for the closure to the health tracker.

Updates #18976

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

cmd/tailscaled/depaware-min.txt

@@ -101,7 +101,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/netknob                                    from tailscale.com/logpolicy+
         tailscale.com/net/netmon                                     from tailscale.com/cmd/tailscaled+
         tailscale.com/net/netns                                      from tailscale.com/cmd/tailscaled+
-        tailscale.com/net/netutil                                    from tailscale.com/control/controlclient+
+        tailscale.com/net/netutil                                    from tailscale.com/control/controlhttp+
         tailscale.com/net/netx                                       from tailscale.com/control/controlclient+
         tailscale.com/net/packet                                     from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/packet/checksum                            from tailscale.com/net/tstun

cmd/tailscaled/depaware-minbox.txt

@@ -118,7 +118,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/netknob                                    from tailscale.com/logpolicy+
         tailscale.com/net/netmon                                     from tailscale.com/cmd/tailscaled+
         tailscale.com/net/netns                                      from tailscale.com/cmd/tailscaled+
-        tailscale.com/net/netutil                                    from tailscale.com/control/controlclient+
+        tailscale.com/net/netutil                                    from tailscale.com/client/local+
         tailscale.com/net/netx                                       from tailscale.com/control/controlclient+
         tailscale.com/net/packet                                     from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/packet/checksum                            from tailscale.com/net/tstun

control/controlclient/auto.go

@@ -773,6 +773,15 @@ func (c *Auto) SetDiscoPublicKey(key key.DiscoPublic) {
 	c.updateControl()
 }
 
+// SetIPForwardingBroken updates the IP forwarding broken state and sends
+// a control update if the value changed.
+func (c *Auto) SetIPForwardingBroken(v bool) {
+	if !c.direct.SetIPForwardingBroken(v) {
+		return
+	}
+	c.updateControl()
+}
+
 func (c *Auto) Shutdown() {
 	c.mu.Lock()
 	if c.closed {

control/controlclient/client.go

@@ -87,6 +87,9 @@ type Client interface {
 	// future map requests. This should be called after rotating the discovery key.
 	// Note: the auto client uploads the new key to control immediately.
 	SetDiscoPublicKey(key.DiscoPublic)
+	// SetIPForwardingBroken updates the IP forwarding broken state
+	// and sends a control update if the value changed.
+	SetIPForwardingBroken(bool)
 	// ClientID returns the ClientID of a client. This ID is meant to
 	// distinguish one client from another.
 	ClientID() int64

control/controlclient/direct.go

@@ -39,7 +39,6 @@ import (
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/netmon"
-	"tailscale.com/net/netutil"
 	"tailscale.com/net/netx"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/net/tsdial"
@@ -80,7 +79,6 @@ type Direct struct {
 	controlTimePub    *eventbus.Publisher[ControlTime]
 	getMachinePrivKey func() (key.MachinePrivate, error)
 	debugFlags        []string
-	skipIPForwardingCheck bool
 	pinger            Pinger
 	popBrowser        func(url string)    // or nil
 	polc              policyclient.Client // always non-nil
@@ -95,6 +93,7 @@ type Direct struct {
 	serverLegacyKey key.MachinePublic // original ("legacy") nacl crypto_box-based public key; only used for signRegisterRequest on Windows now
 	serverNoiseKey  key.MachinePublic
 	discoPubKey     key.DiscoPublic // protected by mu; can be updated via [SetDiscoPublicKey]
+	ipForwardBroken bool            // protected by mu; can be updated via [SetIPForwardingBroken]
 
 	sfGroup     singleflight.Group[struct{}, *ts2021.Client] // protects noiseClient creation.
 	noiseClient *ts2021.Client                               // also protected by mu
@@ -159,11 +158,6 @@ type Options struct {
 	// If nil, no status updates are reported.
 	Observer Observer
 
-	// SkipIPForwardingCheck declares that the host's IP
-	// forwarding works and should not be double-checked by the
-	// controlclient package.
-	SkipIPForwardingCheck bool
-
 	// Pinger optionally specifies the Pinger to use to satisfy
 	// MapResponse.PingRequest queries from the control plane.
 	// If nil, PingRequest queries are not answered.
@@ -319,7 +313,6 @@ func NewDirect(opts Options) (*Direct, error) {
 		debugFlags:        opts.DebugFlags,
 		netMon:            netMon,
 		health:            opts.HealthTracker,
-		skipIPForwardingCheck: opts.SkipIPForwardingCheck,
 		pinger:            opts.Pinger,
 		polc:              cmp.Or(opts.PolicyClient, policyclient.Client(policyclient.NoPolicyClient{})),
 		popBrowser:        opts.PopBrowserURL,
@@ -861,6 +854,18 @@ func (c *Direct) SetDiscoPublicKey(key key.DiscoPublic) {
 	c.discoPubKey = key
 }
 
+// SetIPForwardingBroken updates the IP forwarding broken state.
+// It reports whether the value changed.
+func (c *Direct) SetIPForwardingBroken(v bool) bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.ipForwardBroken == v {
+		return false
+	}
+	c.ipForwardBroken = v
+	return true
+}
+
 // ClientID returns the controlClientID of the controlClient.
 func (c *Direct) ClientID() int64 {
 	return c.controlClientID
@@ -991,10 +996,6 @@ func (c *Direct) sendMapRequest(ctx context.Context, isStreaming bool, nu Netmap
 	}
 
 	var extraDebugFlags []string
-	if buildfeatures.HasAdvertiseRoutes && hi != nil && c.netMon != nil && !c.skipIPForwardingCheck &&
-		ipForwardingBroken(hi.RoutableIPs, c.netMon.InterfaceState()) {
-		extraDebugFlags = append(extraDebugFlags, "warn-ip-forwarding-off")
-	}
 	if c.health.RouterHealth() != nil {
 		extraDebugFlags = append(extraDebugFlags, "warn-router-unhealthy")
 	}
@@ -1413,24 +1414,6 @@ func initDevKnob() devKnobs {
 
 var clock tstime.Clock = tstime.StdClock{}
 
-// ipForwardingBroken reports whether the system's IP forwarding is disabled
-// and will definitely not work for the routes provided.
-//
-// It should not return false positives.
-//
-// TODO(bradfitz): Change controlclient.Options.SkipIPForwardingCheck into a
-// func([]netip.Prefix) error signature instead.
-func ipForwardingBroken(routes []netip.Prefix, state *netmon.State) bool {
-	warn, err := netutil.CheckIPForwarding(routes, state)
-	if err != nil {
-		// Oh well, we tried. This is just for debugging.
-		// We don't want false positives.
-		// TODO: maybe we want a different warning for inability to check?
-		return false
-	}
-	return warn != nil
-}
-
 // isUniquePingRequest reports whether pr contains a new PingRequest.URL
 // not already handled, noting its value when returning true.
 func (c *Direct) isUniquePingRequest(pr *tailcfg.PingRequest) bool {

health/health.go

@@ -132,6 +132,11 @@ type Tracker struct {
 	localLogConfigErr           error
 	tlsConnectionErrors         map[string]error // map[ServerName]error
 	metricHealthMessage         any              // nil or *metrics.MultiLabelMap[metricHealthMessageLabel]
+
+	// IP forwarding check
+	// If non-nil, called periodically to check if IP forwarding is broken.
+	// Should return true if broken, false if healthy.
+	isIPForwardingBroken func() bool
 }
 
 // NewTracker contructs a new [Tracker] and attaches the given eventbus.
@@ -1097,6 +1102,8 @@ func (t *Tracker) updateBuiltinWarnablesLocked() {
 		t.setHealthyLocked(NetworkStatusWarnable)
 	}
 
+	t.updateIPForwardingWarnableLocked()
+
 	if t.localLogConfigErr != nil {
 		t.setUnhealthyLocked(localLogWarnable, Args{
 			ArgError: t.localLogConfigErr.Error(),
@@ -1389,3 +1396,29 @@ func (t *Tracker) LastNoiseDialWasRecent() bool {
 	t.lastNoiseDial = now
 	return dur < 2*time.Minute
 }
+
+// SetIPForwardingCheck sets the function to check if IP forwarding is broken.
+// The function should return true if IP forwarding is broken, false if healthy.
+// Pass nil to disable IP forwarding checks.
+func (t *Tracker) SetIPForwardingCheck(checkFunc func() bool) {
+	if t.nil() {
+		return
+	}
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	t.isIPForwardingBroken = checkFunc
+
+	// Run an immediate check to set initial state
+	t.updateIPForwardingWarnableLocked()
+}
+
+// updateIPForwardingWarnableLocked checks the IP forwarding state and
+// sets or clears the ipForwardingWarnable accordingly.
+func (t *Tracker) updateIPForwardingWarnableLocked() {
+	if t.isIPForwardingBroken != nil && t.isIPForwardingBroken() {
+		t.setUnhealthyLocked(ipForwardingWarnable, Args{})
+	} else {
+		t.setHealthyLocked(ipForwardingWarnable)
+	}
+}

health/health_test.go

@@ -999,3 +999,86 @@ func TestCurrentStateETagWarnable(t *testing.T) {
 		}
 	})
 }
+
+func TestIPForwardingState(t *testing.T) {
+	tests := []struct {
+		name          string
+		checkFunc     func() bool // nil means no check function
+		wantUnhealthy bool
+	}{
+		{
+			name:          "broken",
+			checkFunc:     func() bool { return true },
+			wantUnhealthy: true,
+		},
+		{
+			name:          "healthy",
+			checkFunc:     func() bool { return false },
+			wantUnhealthy: false,
+		},
+		{
+			name:          "no_check_function",
+			checkFunc:     nil,
+			wantUnhealthy: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			bus := eventbus.New()
+			tr := NewTracker(bus)
+			defer bus.Close()
+
+			tr.SetIPNState("Running", true)
+			tr.SetIPForwardingCheck(tt.checkFunc)
+
+			tr.mu.Lock()
+			tr.updateBuiltinWarnablesLocked()
+			tr.mu.Unlock()
+
+			got := tr.IsUnhealthy(ipForwardingWarnable)
+			if got != tt.wantUnhealthy {
+				t.Errorf("IsUnhealthy(ipForwardingWarnable) = %v, want %v", got, tt.wantUnhealthy)
+			}
+		})
+	}
+
+	// Test state transitions
+	t.Run("transitions", func(t *testing.T) {
+		bus := eventbus.New()
+		tr := NewTracker(bus)
+		defer bus.Close()
+
+		tr.SetIPNState("Running", true)
+
+		// Start broken
+		tr.SetIPForwardingCheck(func() bool { return true })
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if !tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be unhealthy initially")
+		}
+
+		// Transition to healthy
+		tr.SetIPForwardingCheck(func() bool { return false })
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be healthy after transition")
+		}
+
+		// Transition to nil (should stay healthy)
+		tr.SetIPForwardingCheck(nil)
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be healthy after clearing check")
+		}
+	})
+}

health/warnings.go

@@ -298,3 +298,16 @@ var warmingUpWarnable = condRegister(func() *Warnable {
 		Text:     StaticMessage("Tailscale is starting. Please wait."),
 	}
 })
+
+// ipForwardingWarnable is a Warnable that warns the user that IP forwarding is disabled
+// but subnet routing or exit node functionality is being used.
+var ipForwardingWarnable = condRegister(func() *Warnable {
+	return &Warnable{
+		Code:                "ip-forwarding-off",
+		Title:               "IP forwarding is off",
+		Severity:            SeverityMedium,
+		MapDebugFlag:        "warn-ip-forwarding-off",
+		Text:                StaticMessage("Subnet routing is enabled, but IP forwarding is disabled. Check that IP forwarding is enabled on your machine."),
+		ImpactsConnectivity: true,
+	}
+})

ipn/ipnlocal/local.go

@@ -1072,6 +1072,14 @@ func (b *LocalBackend) onHealthChange(change health.Change) {
 		Health: state,
 	})
 
+	// Update control if IP forwarding state changed
+	_, broken := state.Warnings["ip-forwarding-off"]
+	b.mu.Lock()
+	if b.cc != nil {
+		b.cc.SetIPForwardingBroken(broken)
+	}
+	b.mu.Unlock()
+
 	if f, ok := hookCaptivePortalHealthChange.GetOk(); ok {
 		f(b, state)
 	}
@@ -2633,10 +2641,6 @@ func (b *LocalBackend) startLocked(opts ipn.Options) error {
 		Shutdown:             ccShutdown,
 		Bus:                  b.sys.Bus.Get(),
 		StartPaused:          prefs.Sync().EqualBool(false),
-
-		// Don't warn about broken Linux IP forwarding when
-		// netstack is being used.
-		SkipIPForwardingCheck: b.sys.IsNetstackRouter(),
 	})
 	if err != nil {
 		return err
@@ -5639,6 +5643,22 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 
 	if buildfeatures.HasAdvertiseRoutes {
 		b.metrics.advertisedRoutes.Set(float64(tsaddr.WithoutExitRoute(prefs.AdvertiseRoutes()).Len()))
+
+		// Set up IP forwarding check when routes change
+		if len(hi.RoutableIPs) > 0 && b.NetMon() != nil && !b.sys.IsNetstackRouter() {
+			routes := hi.RoutableIPs
+			netMon := b.NetMon()
+			b.health.SetIPForwardingCheck(func() bool {
+				warn, err := netutil.CheckIPForwarding(routes, netMon.InterfaceState())
+				if err != nil {
+					metricIPForwardingCheckError.Add(1)
+					return false // don't want false positives
+				}
+				return warn != nil // true if broken
+			})
+		} else {
+			b.health.SetIPForwardingCheck(nil)
+		}
 	}
 
 	var sshHostKeys []string
@@ -7918,6 +7938,7 @@ func maybeUsernameOf(actor ipnauth.Actor) string {
 
 var (
 	metricCurrentWatchIPNBus     = clientmetric.NewGauge("localbackend_current_watch_ipn_bus")
+	metricIPForwardingCheckError = clientmetric.NewCounter("localbackend_ip_forwarding_check_error")
 )
 
 func (b *LocalBackend) stateEncrypted() opt.Bool {

ipn/ipnlocal/state_test.go

@@ -336,6 +336,8 @@ func (cc *mockControl) ClientID() int64 {
 	return cc.controlClientID
 }
 
+func (cc *mockControl) SetIPForwardingBroken(bool) {}
+
 func (b *LocalBackend) nonInteractiveLoginForStateTest() {
 	b.mu.Lock()
 	if b.cc == nil {