ipn/ipnlocal: warn incompatibility between no-snat-routes and exitnode (#19023)

* ipn/ipnlocal: warn incompatibility between no-snat-routes and exitnode

This commit adds a warning to health check when the --snat-subnet-routes=false flag for subnet router is
set alone side --advertise-exit-node=true. These two would conflict with each other and result internet-bound
traffic from peers using this exit node no masqueraded to the node's source IP and fail to route return
packets back. The described combination is not valid until we figure out a way to separate exitnode masquerade rule and skip it for subnet routes.

Updates #18725

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

* use date instead of for now to clarify effectivness

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

---------

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

cmd/tailscale/cli/set.go

@@ -265,6 +265,11 @@ func runSet(ctx context.Context, args []string) (retErr error) {
 
 	checkPrefs := curPrefs.Clone()
 	checkPrefs.ApplyEdits(maskedPrefs)
+	// We want to make sure user is aware setting --snat-subnet-routes=false with --advertise-exit-node would break exitnode,
+	// but we won't prevent them from doing it since there are current dependencies on that combination. (as of 2026-03-25)
+	if checkPrefs.NoSNAT && checkPrefs.AdvertisesExitNode() {
+		warnf("--snat-subnet-routes=false is set with --advertise-exit-node; internet traffic through this exit node may not work as expected")
+	}
 	if err := localClient.CheckPrefs(ctx, checkPrefs); err != nil {
 		return err
 	}

cmd/tailscale/cli/up.go

@@ -357,6 +357,11 @@ func prefsFromUpArgs(upArgs upArgsT, warnf logger.Logf, st *ipnstate.Status, goo
 
 	if goos == "linux" {
 		prefs.NoSNAT = !upArgs.snat
+		// We want to make sure user is aware setting --snat-subnet-routes=false with --advertise-exit-node would break exitnode,
+		// but we won't prevent them from doing it since there are current dependencies on that combination. (as of 2026-03-25)
+		if prefs.NoSNAT && prefs.AdvertisesExitNode() {
+			warnf("--snat-subnet-routes=false is set with --advertise-exit-node; internet traffic through this exit node may not work as expected")
+		}
 
 		// Backfills for NoStatefulFiltering occur when loading a profile; just set it explicitly here.
 		prefs.NoStatefulFiltering.Set(!upArgs.statefulFiltering)

ipn/ipnlocal/local.go

@@ -2576,6 +2576,7 @@ func (b *LocalBackend) startLocked(opts ipn.Options) error {
 	// Reset the always-on override whenever Start is called.
 	b.resetAlwaysOnOverrideLocked()
 	b.setAtomicValuesFromPrefsLocked(prefs)
+	b.updateNoSNATExitNodeWarning(prefs)
 
 	wantRunning := prefs.WantRunning()
 	if wantRunning {
@@ -4745,6 +4746,7 @@ func (b *LocalBackend) setPrefsLocked(newp *ipn.Prefs) ipn.PrefsView {
 
 	b.pauseOrResumeControlClientLocked() // for prefs.Sync changes
 	b.updateWarnSync(prefs)
+	b.updateNoSNATExitNodeWarning(prefs)
 
 	if oldp.ShieldsUp() != newp.ShieldsUp || hostInfoChanged {
 		b.doSetHostinfoFilterServicesLocked()
@@ -6984,6 +6986,18 @@ func (b *LocalBackend) sshServerOrInit() (_ SSHServer, err error) {
 	Text:     health.StaticMessage("SELinux is enabled; Tailscale SSH may not work. See https://tailscale.com/s/ssh-selinux"),
 })
 
+// warnNoSNATWithExitNode is a warnable for when a node is advertising as an
+// exit node but has SNAT disabled. In this configuration internet-bound traffic
+// from peers using this exit node will not be masqueraded to the node's own
+// source IP, so return packets cannot be routed back, causing the exit node to
+// not work as expected.
+var warnNoSNATWithExitNode = health.Register(&health.Warnable{
+	Code:     "nosnat-with-advertised-exit-node",
+	Title:    "Exit node advertising may not work correctly",
+	Severity: health.SeverityMedium,
+	Text:     health.StaticMessage("snat-subnet-routes is disabled while advertising as an exit node; internet traffic through this exit node may not work as expected"),
+})
+
 func (b *LocalBackend) updateSELinuxHealthWarning() {
 	if hostinfo.IsSELinuxEnforcing() {
 		b.health.SetUnhealthy(warnSSHSELinuxWarnable, nil)
@@ -7000,6 +7014,17 @@ func (b *LocalBackend) updateWarnSync(prefs ipn.PrefsView) {
 	}
 }
 
+func (b *LocalBackend) updateNoSNATExitNodeWarning(prefs ipn.PrefsView) {
+	if !buildfeatures.HasAdvertiseExitNode {
+		return
+	}
+	if prefs.NoSNAT() && prefs.AdvertisesExitNode() {
+		b.health.SetUnhealthy(warnNoSNATWithExitNode, nil)
+	} else {
+		b.health.SetHealthy(warnNoSNATWithExitNode)
+	}
+}
+
 func (b *LocalBackend) handleSSHConn(c net.Conn) (err error) {
 	s, err := b.sshServerOrInit()
 	if err != nil {

ipn/ipnlocal/local_test.go

@@ -7878,3 +7878,75 @@ func TestEditPrefs_InvalidAdvertiseRoutes(t *testing.T) {
 		})
 	}
 }
+
+func TestNoSNATWithAdvertisedExitNodeWarning(t *testing.T) {
+	exitRoutes := []netip.Prefix{
+		netip.MustParsePrefix("0.0.0.0/0"),
+		netip.MustParsePrefix("::/0"),
+	}
+	warnCode := health.WarnableCode("nosnat-with-advertised-exit-node")
+
+	tests := []struct {
+		name        string
+		prefs       *ipn.Prefs
+		wantWarning bool
+	}{
+		{
+			name: "no-snat-without-exit-node",
+			prefs: &ipn.Prefs{
+				NoSNAT:          true,
+				AdvertiseRoutes: []netip.Prefix{netip.MustParsePrefix("10.0.0.0/24")},
+			},
+			wantWarning: false,
+		},
+		{
+			name: "snat-enabled-with-exit-node",
+			prefs: &ipn.Prefs{
+				NoSNAT:          false,
+				AdvertiseRoutes: exitRoutes,
+			},
+			wantWarning: false,
+		},
+		{
+			name: "no-snat-with-exit-node",
+			prefs: &ipn.Prefs{
+				NoSNAT:          true,
+				AdvertiseRoutes: exitRoutes,
+			},
+			wantWarning: true,
+		},
+		{
+			name: "no-snat-with-exit-node-and-subnet",
+			prefs: &ipn.Prefs{
+				NoSNAT: true,
+				AdvertiseRoutes: append([]netip.Prefix{
+					netip.MustParsePrefix("10.0.0.0/24"),
+				}, exitRoutes...),
+			},
+			wantWarning: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			b := newTestLocalBackend(t)
+			b.SetPrefsForTest(tt.prefs)
+			_, hasWarning := b.HealthTracker().CurrentState().Warnings[warnCode]
+			if hasWarning != tt.wantWarning {
+				t.Errorf("warning present = %v, want %v", hasWarning, tt.wantWarning)
+			}
+		})
+	}
+
+	// Verify that the warning clears when the conflicting combination is resolved.
+	t.Run("warning-clears-on-fix", func(t *testing.T) {
+		b := newTestLocalBackend(t)
+		b.SetPrefsForTest(&ipn.Prefs{NoSNAT: true, AdvertiseRoutes: exitRoutes})
+		if _, ok := b.HealthTracker().CurrentState().Warnings[warnCode]; !ok {
+			t.Fatal("expected warning to be set")
+		}
+		b.SetPrefsForTest(&ipn.Prefs{NoSNAT: false, AdvertiseRoutes: exitRoutes})
+		if _, ok := b.HealthTracker().CurrentState().Warnings[warnCode]; ok {
+			t.Fatal("expected warning to be cleared after enabling SNAT")
+		}
+	})
+}