tailcfg: add Hostinfo.ExitNodeID to report the selected exit node (#16625)

When a client selects a particular exit node, Control may use that as
a signal for deciding other routes.

This patch causes the client to report whenever the current exit node
changes, through tailcfg.Hostinfo.ExitNodeID. It relies on a properly
set ipn.Prefs.ExitNodeID, which should already be resolved by
`tailscale set`.

Updates tailscale/corp#30536

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnlocal/local.go

@@ -5612,6 +5612,11 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	// WireIngress.
 	hi.WireIngress = b.shouldWireInactiveIngressLocked()
 	hi.AppConnector.Set(prefs.AppConnector().Advertise)
+
+	// The [tailcfg.Hostinfo.ExitNodeID] field tells control which exit node
+	// was selected, if any. Since [LocalBackend.resolveExitNodeIPLocked]
+	// has already run, there is no need to consult [ipn.Prefs.ExitNodeIP].
+	hi.ExitNodeID = prefs.ExitNodeID()
 }
 
 // enterState transitions the backend into newState, updating internal
@@ -6136,6 +6141,10 @@ func (b *LocalBackend) resolveExitNode() (changed bool) {
 	}); err != nil {
 		b.logf("failed to save exit node changes: %v", err)
 	}
+
+	// Send the resolved exit node to Control via Hostinfo.
+	b.hostinfo.ExitNodeID = prefs.ExitNodeID
+
 	b.sendToLocked(ipn.Notify{Prefs: ptr.To(prefs.View())}, allClients)
 	return true
 }

ipn/ipnlocal/local_test.go

@@ -614,19 +614,20 @@ func TestConfigureExitNode(t *testing.T) {
 	}
 
 	tests := []struct {
-		name                  string
+		name                   string
-		prefs                 ipn.Prefs
+		prefs                  ipn.Prefs
-		netMap                *netmap.NetworkMap
+		netMap                 *netmap.NetworkMap
-		report                *netcheck.Report
+		report                 *netcheck.Report
-		changePrefs           *ipn.MaskedPrefs
+		changePrefs            *ipn.MaskedPrefs
-		useExitNodeEnabled    *bool
+		useExitNodeEnabled     *bool
-		exitNodeIDPolicy      *tailcfg.StableNodeID
+		exitNodeIDPolicy       *tailcfg.StableNodeID
-		exitNodeIPPolicy      *netip.Addr
+		exitNodeIPPolicy       *netip.Addr
-		exitNodeAllowedIDs    []tailcfg.StableNodeID // nil if all IDs are allowed for auto exit nodes
+		exitNodeAllowedIDs     []tailcfg.StableNodeID // nil if all IDs are allowed for auto exit nodes
-		exitNodeAllowOverride bool                   // whether [syspolicy.AllowExitNodeOverride] should be set to true
+		exitNodeAllowOverride  bool                   // whether [syspolicy.AllowExitNodeOverride] should be set to true
-		wantChangePrefsErr    error                  // if non-nil, the error we expect from [LocalBackend.EditPrefsAs]
+		wantChangePrefsErr     error                  // if non-nil, the error we expect from [LocalBackend.EditPrefsAs]
-		wantPrefs             ipn.Prefs
+		wantPrefs              ipn.Prefs
-		wantExitNodeToggleErr error // if non-nil, the error we expect from [LocalBackend.SetUseExitNodeEnabled]
+		wantExitNodeToggleErr  error // if non-nil, the error we expect from [LocalBackend.SetUseExitNodeEnabled]
+		wantHostinfoExitNodeID *tailcfg.StableNodeID
 	}{
 		{
 			name: "exit-node-id-via-prefs", // set exit node ID via prefs
@@ -643,6 +644,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode1.StableID(),
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "exit-node-ip-via-prefs", // set exit node IP via prefs (should be resolved to an ID)
@@ -659,6 +661,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode1.StableID(),
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-exit-node-via-prefs/any", // set auto exit node via prefs
@@ -676,6 +679,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(),
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-exit-node-via-prefs/set-exit-node-id-via-prefs", // setting exit node ID explicitly should disable auto exit node
@@ -695,6 +699,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode2.StableID(),
 				AutoExitNode: "", // should be unset
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-exit-node-via-prefs/any/no-report", // set auto exit node via prefs, but no report means we can't resolve the exit node ID
@@ -711,6 +716,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   unresolvedExitNodeID, // cannot resolve; traffic will be dropped
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(unresolvedExitNodeID),
 		},
 		{
 			name: "auto-exit-node-via-prefs/any/no-netmap", // similarly, but without a netmap (no exit node should be selected)
@@ -727,6 +733,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   unresolvedExitNodeID, // cannot resolve; traffic will be dropped
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(unresolvedExitNodeID),
 		},
 		{
 			name: "auto-exit-node-via-prefs/foo", // set auto exit node via prefs with an unknown/unsupported expression
@@ -744,6 +751,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(), // unknown exit node expressions should work as "any"
 				AutoExitNode: "foo",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-exit-node-via-prefs/off", // toggle the exit node off after it was set to "any"
@@ -763,6 +771,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode:          "",
 				InternalExitNodePrior: "auto:any",
 			},
+			wantHostinfoExitNodeID: ptr.To(tailcfg.StableNodeID("")),
 		},
 		{
 			name: "auto-exit-node-via-prefs/on", // toggle the exit node on
@@ -779,6 +788,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode:          "any",
 				InternalExitNodePrior: "auto:any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "id-via-policy", // set exit node ID via syspolicy
@@ -791,6 +801,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode1.StableID(),
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "id-via-policy/cannot-override-via-prefs/by-id", // syspolicy should take precedence over prefs
@@ -809,7 +820,8 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode1.StableID(),
 			},
-			wantChangePrefsErr: errManagedByPolicy,
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
+			wantChangePrefsErr:     errManagedByPolicy,
 		},
 		{
 			name: "id-via-policy/cannot-override-via-prefs/by-ip", // syspolicy should take precedence over prefs
@@ -828,7 +840,8 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode1.StableID(),
 			},
-			wantChangePrefsErr: errManagedByPolicy,
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
+			wantChangePrefsErr:     errManagedByPolicy,
 		},
 		{
 			name: "id-via-policy/cannot-override-via-prefs/by-auto-expr", // syspolicy should take precedence over prefs
@@ -860,6 +873,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ControlURL: controlURL,
 				ExitNodeID: exitNode2.StableID(),
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-any-via-policy", // set auto exit node via syspolicy (an exit node should be selected)
@@ -874,6 +888,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(),
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/no-report", // set auto exit node via syspolicy without a netcheck report (no exit node should be selected)
@@ -888,6 +903,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   unresolvedExitNodeID,
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(unresolvedExitNodeID),
 		},
 		{
 			name: "auto-any-via-policy/no-netmap", // similarly, but without a netmap (no exit node should be selected)
@@ -902,6 +918,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   unresolvedExitNodeID,
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(unresolvedExitNodeID),
 		},
 		{
 			name: "auto-any-via-policy/no-netmap/with-existing", // set auto exit node via syspolicy without a netmap, but with a previously set exit node ID
@@ -918,6 +935,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode2.StableID(),
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/no-netmap/with-allowed-existing", // same, but now with a syspolicy setting that explicitly allows the existing exit node ID
@@ -936,6 +954,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode2.StableID(),
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/no-netmap/with-disallowed-existing", // same, but now with a syspolicy setting that does not allow the existing exit node ID
@@ -954,6 +973,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   unresolvedExitNodeID, // we don't have a netmap yet, and the current exit node ID is not allowed; block traffic
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(unresolvedExitNodeID),
 		},
 		{
 			name: "auto-any-via-policy/with-netmap/with-allowed-existing", // same, but now with a syspolicy setting that does not allow the existing exit node ID
@@ -972,6 +992,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode2.StableID(), // we have a netmap; switch to the best allowed exit node
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/with-netmap/switch-to-better", // if all exit nodes are allowed, switch to the best one once we have a netmap
@@ -987,6 +1008,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(), // switch to the best exit node
 				AutoExitNode: "any",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-foo-via-policy", // set auto exit node via syspolicy with an unknown/unsupported expression
@@ -1001,6 +1023,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(), // unknown exit node expressions should work as "any"
 				AutoExitNode: "foo",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-foo-via-edit-prefs", // set auto exit node via EditPrefs with an unknown/unsupported expression
@@ -1018,6 +1041,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode1.StableID(), // unknown exit node expressions should work as "any"
 				AutoExitNode: "foo",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/toggle-off", // cannot toggle off the exit node if it was set via syspolicy
@@ -1035,6 +1059,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode:          "any",
 				InternalExitNodePrior: "",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/allow-override/change", // changing the exit node is allowed by [syspolicy.AllowExitNodeOverride]
@@ -1056,6 +1081,7 @@ func TestConfigureExitNode(t *testing.T) {
 				ExitNodeID:   exitNode2.StableID(), // overridden by user
 				AutoExitNode: "",                   // cleared, as we are setting the exit node ID explicitly
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode2.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/allow-override/clear", // clearing the exit node ID is not allowed by [syspolicy.AllowExitNodeOverride]
@@ -1079,6 +1105,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode:          "any",
 				InternalExitNodePrior: "",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-any-via-policy/allow-override/toggle-off", // similarly, toggling off the exit node is not allowed even with [syspolicy.AllowExitNodeOverride]
@@ -1097,6 +1124,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode:          "any",
 				InternalExitNodePrior: "",
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 		{
 			name: "auto-any-via-initial-prefs/no-netmap/clear-auto-exit-node",
@@ -1117,6 +1145,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode: "", // cleared
 				ExitNodeID:   "", // has never been resolved, so it should be cleared as well
 			},
+			wantHostinfoExitNodeID: ptr.To(tailcfg.StableNodeID("")),
 		},
 		{
 			name: "auto-any-via-initial-prefs/with-netmap/clear-auto-exit-node",
@@ -1137,6 +1166,7 @@ func TestConfigureExitNode(t *testing.T) {
 				AutoExitNode: "",                   // cleared
 				ExitNodeID:   exitNode1.StableID(), // a resolved exit node ID should be retained
 			},
+			wantHostinfoExitNodeID: ptr.To(exitNode1.StableID()),
 		},
 	}
 	syspolicy.RegisterWellKnownSettingsForTest(t)
@@ -1197,6 +1227,13 @@ func TestConfigureExitNode(t *testing.T) {
 			if diff := cmp.Diff(&tt.wantPrefs, lb.Prefs().AsStruct(), opts...); diff != "" {
 				t.Errorf("Prefs(+got -want): %v", diff)
 			}
+
+			// And check Hostinfo.
+			if tt.wantHostinfoExitNodeID != nil {
+				if got := lb.hostinfo.ExitNodeID; got != *tt.wantHostinfoExitNodeID {
+					t.Errorf("Hostinfo.ExitNodeID got %v, want %v", got, *tt.wantHostinfoExitNodeID)
+				}
+			}
 		})
 	}
 }

tailcfg/tailcfg.go

@@ -166,7 +166,8 @@ type CapabilityVersion int
 //   - 119: 2025-07-10: Client uses Hostinfo.Location.Priority to prioritize one route over another.
 //   - 120: 2025-07-15: Client understands peer relay disco messages, and implements peer client and relay server functions
 //   - 121: 2025-07-19: Client understands peer relay endpoint alloc with [disco.AllocateUDPRelayEndpointRequest] & [disco.AllocateUDPRelayEndpointResponse]
-const CurrentCapabilityVersion CapabilityVersion = 121
+//   - 122: 2025-07-21: Client sends Hostinfo.ExitNodeID to report which exit node it has selected, if any.
+const CurrentCapabilityVersion CapabilityVersion = 122
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.
@@ -875,6 +876,7 @@ type Hostinfo struct {
 	UserspaceRouter opt.Bool       `json:",omitempty"` // if the client's subnet router is running in userspace (netstack) mode
 	AppConnector    opt.Bool       `json:",omitempty"` // if the client is running the app-connector service
 	ServicesHash    string         `json:",omitempty"` // opaque hash of the most recent list of tailnet services, change in hash indicates config should be fetched via c2n
+	ExitNodeID      StableNodeID   `json:",omitzero"`  // the client’s selected exit node, empty when unselected.
 
 	// Location represents geographical location data about a
 	// Tailscale host. Location is optional and only set if

tailcfg/tailcfg_clone.go

@@ -186,6 +186,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo
 	StateEncrypted  opt.Bool

tailcfg/tailcfg_test.go

@@ -67,6 +67,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"UserspaceRouter",
 		"AppConnector",
 		"ServicesHash",
+		"ExitNodeID",
 		"Location",
 		"TPM",
 		"StateEncrypted",
@@ -273,6 +274,21 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{IngressEnabled: true},
 			false,
 		},
+		{
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{},
+			true,
+		},
+		{
+			&Hostinfo{ExitNodeID: ""},
+			&Hostinfo{ExitNodeID: "stable-exit"},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)

tailcfg/tailcfg_view.go

@@ -300,6 +300,7 @@ func (v HostinfoView) Userspace() opt.Bool                    { return v.ж.User
 func (v HostinfoView) UserspaceRouter() opt.Bool              { return v.ж.UserspaceRouter }
 func (v HostinfoView) AppConnector() opt.Bool                 { return v.ж.AppConnector }
 func (v HostinfoView) ServicesHash() string                   { return v.ж.ServicesHash }
+func (v HostinfoView) ExitNodeID() StableNodeID               { return v.ж.ExitNodeID }
 func (v HostinfoView) Location() LocationView                 { return v.ж.Location.View() }
 func (v HostinfoView) TPM() views.ValuePointer[TPMInfo]       { return views.ValuePointerOf(v.ж.TPM) }
 
@@ -345,6 +346,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo
 	StateEncrypted  opt.Bool