ipn/ipnlocal: add a C2N endpoint for fetching a netmap

For debugging purposes, add a new C2N endpoint returning the current
netmap. Optionally, coordination server can send a new "candidate" map
response, which the client will generate a separate netmap for.
Coordination server can later compare two netmaps, detecting unexpected
changes to the client state.

Updates tailscale/corp#32095

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/direct.go

@@ -1160,6 +1160,27 @@ func (c *Direct) sendMapRequest(ctx context.Context, isStreaming bool, nu Netmap
 	return nil
 }
 
+// NetmapFromMapResponseForDebug returns a NetworkMap from the given MapResponse.
+// It is intended for debugging only.
+func NetmapFromMapResponseForDebug(ctx context.Context, pr persist.PersistView, resp *tailcfg.MapResponse) (*netmap.NetworkMap, error) {
+	if resp == nil {
+		return nil, errors.New("nil MapResponse")
+	}
+	if resp.Node == nil {
+		return nil, errors.New("MapResponse lacks Node")
+	}
+
+	nu := &rememberLastNetmapUpdater{}
+	sess := newMapSession(pr.PrivateNodeKey(), nu, nil)
+	defer sess.Close()
+
+	if err := sess.HandleNonKeepAliveMapResponse(ctx, resp); err != nil {
+		return nil, fmt.Errorf("HandleNonKeepAliveMapResponse: %w", err)
+	}
+
+	return sess.netmap(), nil
+}
+
 func (c *Direct) handleDebugMessage(ctx context.Context, debug *tailcfg.Debug) error {
 	if code := debug.Exit; code != nil {
 		c.logf("exiting process with status %v per controlplane", *code)

control/controlclient/map_test.go

@@ -20,6 +20,7 @@ import (
 	"go4.org/mem"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/health"
+	"tailscale.com/ipn"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
@@ -27,6 +28,7 @@ import (
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
+	"tailscale.com/types/persist"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/eventbus/eventbustest"
 	"tailscale.com/util/mak"
@@ -1419,3 +1421,27 @@ func TestNetmapDisplayMessageIntegration(t *testing.T) {
 		t.Errorf("unexpected message contents (-want +got):\n%s", diff)
 	}
 }
+
+func TestNetmapForMapResponseForDebug(t *testing.T) {
+	mr := &tailcfg.MapResponse{
+		Node: &tailcfg.Node{
+			ID:   1,
+			Name: "foo.bar.ts.net.",
+		},
+		Peers: []*tailcfg.Node{
+			{ID: 2, Name: "peer1.bar.ts.net.", HomeDERP: 1},
+			{ID: 3, Name: "peer2.bar.ts.net.", HomeDERP: 1},
+		},
+	}
+	ms := newTestMapSession(t, nil)
+	nm1 := ms.netmapForResponse(mr)
+
+	prefs := &ipn.Prefs{Persist: &persist.Persist{PrivateNodeKey: ms.privateNodeKey}}
+	nm2, err := NetmapFromMapResponseForDebug(t.Context(), prefs.View().Persist(), mr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(nm1, nm2) {
+		t.Errorf("mismatch\nnm1: %s\nnm2: %s\n", logger.AsJSON(nm1), logger.AsJSON(nm2))
+	}
+}

ipn/ipnlocal/c2n.go

@@ -13,19 +13,23 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"strconv"
 	"strings"
 	"time"
 
 	"tailscale.com/clientupdate"
+	"tailscale.com/control/controlclient"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/posture"
 	"tailscale.com/tailcfg"
+	"tailscale.com/types/netmap"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/goroutines"
+	"tailscale.com/util/httpm"
 	"tailscale.com/util/set"
 	"tailscale.com/util/syspolicy/pkey"
 	"tailscale.com/util/syspolicy/ptype"
@@ -44,6 +48,7 @@ var c2nHandlers = map[methodAndPath]c2nHandler{
 	req("/debug/metrics"):           handleC2NDebugMetrics,
 	req("/debug/component-logging"): handleC2NDebugComponentLogging,
 	req("/debug/logheap"):           handleC2NDebugLogHeap,
+	req("/debug/netmap"):            handleC2NDebugNetMap,
 
 	// PPROF - We only expose a subset of typical pprof endpoints for security.
 	req("/debug/pprof/heap"):   handleC2NPprof,
@@ -142,6 +147,66 @@ func handleC2NLogtailFlush(b *LocalBackend, w http.ResponseWriter, r *http.Reque
 	}
 }
 
+func handleC2NDebugNetMap(b *LocalBackend, w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	if r.Method != httpm.POST && r.Method != httpm.GET {
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	b.logf("c2n: %s /debug/netmap received", r.Method)
+
+	// redactAndMarshal redacts private keys from the given netmap, clears fields
+	// that should be omitted, and marshals it to JSON.
+	redactAndMarshal := func(nm *netmap.NetworkMap, omitFields []string) (json.RawMessage, error) {
+		for _, f := range omitFields {
+			field := reflect.ValueOf(nm).Elem().FieldByName(f)
+			if !field.IsValid() {
+				b.logf("c2n: /debug/netmap: unknown field %q in omitFields", f)
+				continue
+			}
+			field.SetZero()
+		}
+		nm, _ = redactNetmapPrivateKeys(nm)
+		return json.Marshal(nm)
+	}
+
+	var omitFields []string
+	resp := &tailcfg.C2NDebugNetmapResponse{}
+
+	if r.Method == httpm.POST {
+		var req tailcfg.C2NDebugNetmapRequest
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			http.Error(w, fmt.Sprintf("failed to decode request body: %v", err), http.StatusBadRequest)
+			return
+		}
+		omitFields = req.OmitFields
+
+		if req.Candidate != nil {
+			cand, err := controlclient.NetmapFromMapResponseForDebug(ctx, b.unsanitizedPersist(), req.Candidate)
+			if err != nil {
+				http.Error(w, fmt.Sprintf("failed to convert candidate MapResponse: %v", err), http.StatusBadRequest)
+				return
+			}
+			candJSON, err := redactAndMarshal(cand, omitFields)
+			if err != nil {
+				http.Error(w, fmt.Sprintf("failed to marshal candidate netmap: %v", err), http.StatusInternalServerError)
+				return
+			}
+			resp.Candidate = candJSON
+		}
+	}
+
+	var err error
+	resp.Current, err = redactAndMarshal(b.currentNode().netMapWithPeers(), omitFields)
+	if err != nil {
+		http.Error(w, fmt.Sprintf("failed to marshal current netmap: %v", err), http.StatusInternalServerError)
+		return
+	}
+
+	writeJSON(w, resp)
+}
+
 func handleC2NDebugGoroutines(_ *LocalBackend, w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "text/plain")
 	w.Write(goroutines.ScrubbedGoroutineDump(true))

ipn/ipnlocal/c2n_test.go

@@ -4,9 +4,11 @@
 package ipnlocal
 
 import (
+	"bytes"
 	"cmp"
 	"crypto/x509"
 	"encoding/json"
+	"fmt"
 	"net/http/httptest"
 	"net/url"
 	"os"
@@ -18,8 +20,15 @@ import (
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
+	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/netmap"
+	"tailscale.com/types/opt"
+	"tailscale.com/types/views"
 	"tailscale.com/util/must"
+
+	gcmp "github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
 )
 
 func TestHandleC2NTLSCertStatus(t *testing.T) {
@@ -132,3 +141,177 @@ func TestHandleC2NTLSCertStatus(t *testing.T) {
 	}
 
 }
+
+// reflectNonzero returns a non-zero value for a given reflect.Value.
+func reflectNonzero(t reflect.Type) reflect.Value {
+	switch t.Kind() {
+	case reflect.Bool:
+		return reflect.ValueOf(true)
+	case reflect.String:
+		if reflect.TypeFor[opt.Bool]() == t {
+			return reflect.ValueOf("true").Convert(t)
+		}
+		return reflect.ValueOf("foo").Convert(t)
+	case reflect.Int64:
+		return reflect.ValueOf(int64(1)).Convert(t)
+	case reflect.Slice:
+		return reflect.MakeSlice(t, 1, 1)
+	case reflect.Ptr:
+		return reflect.New(t.Elem())
+	case reflect.Map:
+		return reflect.MakeMap(t)
+	case reflect.Struct:
+		switch t {
+		case reflect.TypeFor[key.NodePrivate]():
+			return reflect.ValueOf(key.NewNode())
+		}
+	}
+	panic(fmt.Sprintf("unhandled %v", t))
+}
+
+// setFieldsToRedact sets fields in the given netmap to non-zero values
+// according to the fieldMap, which maps field names to whether they
+// should be reset (true) or not (false).
+func setFieldsToRedact(t *testing.T, nm *netmap.NetworkMap, fieldMap map[string]bool) {
+	t.Helper()
+	v := reflect.ValueOf(nm).Elem()
+	for i := range v.NumField() {
+		name := v.Type().Field(i).Name
+		f := v.Field(i)
+		if !f.CanSet() {
+			continue
+		}
+		shouldReset, ok := fieldMap[name]
+		if !ok {
+			t.Errorf("fieldMap missing field %q", name)
+		}
+		if shouldReset {
+			f.Set(reflectNonzero(f.Type()))
+		}
+	}
+}
+
+func TestRedactNetmapPrivateKeys(t *testing.T) {
+	fieldMap := map[string]bool{
+		// Private fields (should be redacted):
+		"PrivateKey": true,
+
+		// Public fields (should not be redacted):
+		"AllCaps":           false,
+		"CollectServices":   false,
+		"DERPMap":           false,
+		"DNS":               false,
+		"DisplayMessages":   false,
+		"Domain":            false,
+		"DomainAuditLogID":  false,
+		"Expiry":            false,
+		"MachineKey":        false,
+		"Name":              false,
+		"NodeKey":           false,
+		"PacketFilter":      false,
+		"PacketFilterRules": false,
+		"Peers":             false,
+		"SSHPolicy":         false,
+		"SelfNode":          false,
+		"TKAEnabled":        false,
+		"TKAHead":           false,
+		"UserProfiles":      false,
+	}
+
+	nm := &netmap.NetworkMap{}
+	setFieldsToRedact(t, nm, fieldMap)
+
+	got, _ := redactNetmapPrivateKeys(nm)
+	if !reflect.DeepEqual(got, &netmap.NetworkMap{}) {
+		t.Errorf("redacted netmap is not empty: %+v", got)
+	}
+}
+
+func TestHandleC2NDebugNetmap(t *testing.T) {
+	nm := &netmap.NetworkMap{
+		Name: "myhost",
+		SelfNode: (&tailcfg.Node{
+			ID:       100,
+			Name:     "myhost",
+			StableID: "deadbeef",
+			Key:      key.NewNode().Public(),
+			Hostinfo: (&tailcfg.Hostinfo{Hostname: "myhost"}).View(),
+		}).View(),
+		Peers: []tailcfg.NodeView{
+			(&tailcfg.Node{
+				ID:       101,
+				Name:     "peer1",
+				StableID: "deadbeef",
+				Key:      key.NewNode().Public(),
+				Hostinfo: (&tailcfg.Hostinfo{Hostname: "peer1"}).View(),
+			}).View(),
+		},
+		PrivateKey: key.NewNode(),
+	}
+	withoutPrivateKey := *nm
+	withoutPrivateKey.PrivateKey = key.NodePrivate{}
+
+	for _, tt := range []struct {
+		name string
+		req  *tailcfg.C2NDebugNetmapRequest
+		want *netmap.NetworkMap
+	}{
+		{
+			name: "simple_get",
+			want: &withoutPrivateKey,
+		},
+		{
+			name: "post_no_omit",
+			req:  &tailcfg.C2NDebugNetmapRequest{},
+			want: &withoutPrivateKey,
+		},
+		{
+			name: "post_omit_peers_and_name",
+			req:  &tailcfg.C2NDebugNetmapRequest{OmitFields: []string{"Peers", "Name"}},
+			want: &netmap.NetworkMap{
+				SelfNode: nm.SelfNode,
+			},
+		},
+		{
+			name: "post_omit_nonexistent_field",
+			req:  &tailcfg.C2NDebugNetmapRequest{OmitFields: []string{"ThisFieldDoesNotExist"}},
+			want: &withoutPrivateKey,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			b := newTestLocalBackend(t)
+			b.currentNode().SetNetMap(nm)
+
+			rec := httptest.NewRecorder()
+			req := httptest.NewRequest("GET", "/debug/netmap", nil)
+			if tt.req != nil {
+				b, err := json.Marshal(tt.req)
+				if err != nil {
+					t.Fatalf("json.Marshal: %v", err)
+				}
+				req = httptest.NewRequest("POST", "/debug/netmap", bytes.NewReader(b))
+			}
+			handleC2NDebugNetMap(b, rec, req)
+			res := rec.Result()
+			wantStatus := 200
+			if res.StatusCode != wantStatus {
+				t.Fatalf("status code = %v; want %v. Body: %s", res.Status, wantStatus, rec.Body.Bytes())
+			}
+			var resp tailcfg.C2NDebugNetmapResponse
+			if err := json.Unmarshal(rec.Body.Bytes(), &resp); err != nil {
+				t.Fatalf("bad JSON: %v", err)
+			}
+			got := &netmap.NetworkMap{}
+			if err := json.Unmarshal(resp.Current, got); err != nil {
+				t.Fatalf("bad JSON: %v", err)
+			}
+
+			if diff := gcmp.Diff(tt.want, got,
+				gcmp.AllowUnexported(netmap.NetworkMap{}, key.NodePublic{}, views.Slice[tailcfg.FilterRule]{}),
+				cmpopts.EquateComparable(key.MachinePublic{}),
+			); diff != "" {
+				t.Errorf("netmap mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}

ipn/ipnlocal/local.go

@@ -1223,6 +1223,13 @@ func (b *LocalBackend) sanitizedPrefsLocked() ipn.PrefsView {
 	return stripKeysFromPrefs(b.pm.CurrentPrefs())
 }
 
+// unsanitizedPersist returns the current PersistView, including any private keys.
+func (b *LocalBackend) unsanitizedPersist() persist.PersistView {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.pm.CurrentPrefs().Persist()
+}
+
 // Status returns the latest status of the backend and its
 // sub-components.
 func (b *LocalBackend) Status() *ipnstate.Status {
@@ -3257,21 +3264,34 @@ func (b *LocalBackend) WatchNotificationsAs(ctx context.Context, actor ipnauth.A
 // listener.
 func filterPrivateKeys(fn func(roNotify *ipn.Notify) (keepGoing bool)) func(*ipn.Notify) bool {
 	return func(n *ipn.Notify) bool {
-		if n.NetMap == nil || n.NetMap.PrivateKey.IsZero() {
+		redacted, changed := redactNetmapPrivateKeys(n.NetMap)
+		if !changed {
 			return fn(n)
 		}
 
 		// The netmap in n is shared across all watchers, so to mutate it for a
 		// single watcher we have to clone the notify and the netmap. We can
 		// make shallow clones, at least.
-		nm2 := *n.NetMap
 		n2 := *n
-		n2.NetMap = &nm2
-		n2.NetMap.PrivateKey = key.NodePrivate{}
+		n2.NetMap = redacted
 		return fn(&n2)
 	}
 }
 
+// redactNetmapPrivateKeys returns a copy of nm with private keys zeroed out.
+// If no change was needed, it returns nm unmodified.
+func redactNetmapPrivateKeys(nm *netmap.NetworkMap) (redacted *netmap.NetworkMap, changed bool) {
+	if nm == nil || nm.PrivateKey.IsZero() {
+		return nm, false
+	}
+
+	// The netmap might be shared across watchers, so make at least a shallow
+	// clone before mutating it.
+	nm2 := *nm
+	nm2.PrivateKey = key.NodePrivate{}
+	return &nm2, true
+}
+
 // appendHealthActions returns an IPN listener func that wraps the supplied IPN
 // listener func and transforms health messages passed to the wrapped listener.
 // If health messages with PrimaryActions are present, it appends the label &

tailcfg/c2ntypes.go

@@ -5,7 +5,10 @@
 
 package tailcfg
 
-import "net/netip"
+import (
+	"encoding/json"
+	"net/netip"
+)
 
 // C2NSSHUsernamesRequest is the request for the /ssh/usernames.
 // A GET request without a request body is equivalent to the zero value of this type.
@@ -117,3 +120,29 @@ type C2NVIPServicesResponse struct {
 	// changes. This value matches what is reported in latest [Hostinfo.ServicesHash].
 	ServicesHash string
 }
+
+// C2NDebugNetmapRequest is the request (from control to node) for the
+// /debug/netmap handler.
+type C2NDebugNetmapRequest struct {
+	// Candidate is an optional full MapResponse to be used for generating a candidate
+	// network map. If unset, only the current network map is returned.
+	Candidate *MapResponse `json:"candidate,omitzero"`
+
+	// OmitFields is an optional list of netmap fields to omit from the response.
+	// If unset, no fields are omitted.
+	OmitFields []string `json:"omitFields,omitzero"`
+}
+
+// C2NDebugNetmapResponse is the response (from node to control) from the
+// /debug/netmap handler. It contains the current network map and, if a
+// candidate full MapResponse was provided in the request, a candidate network
+// map generated from it.
+// To avoid import cycles, and reflect the non-stable nature of
+// netmap.NetworkMap values, they are returned as json.RawMessage.
+type C2NDebugNetmapResponse struct {
+	// Current is the current network map (netmap.NetworkMap).
+	Current json.RawMessage `json:"current"`
+
+	// Candidate is a network map produced based on the candidate MapResponse.
+	Candidate json.RawMessage `json:"candidate,omitzero"`
+}

tailcfg/tailcfg.go

@@ -171,7 +171,8 @@ type CapabilityVersion int
 //   - 124: 2025-08-08: removed NodeAttrDisableMagicSockCryptoRouting support, crypto routing is now mandatory
 //   - 125: 2025-08-11: dnstype.Resolver adds UseWithExitNode field.
 //   - 126: 2025-09-17: Client uses seamless key renewal unless disabled by control (tailscale/corp#31479)
-const CurrentCapabilityVersion CapabilityVersion = 126
+//   - 127: 2025-09-19: can handle C2N /debug/netmap.
+const CurrentCapabilityVersion CapabilityVersion = 127
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.

tstest/integration/integration_test.go

@@ -27,6 +27,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/miekg/dns"
 	"go4.org/mem"
 	"tailscale.com/client/local"
@@ -41,6 +42,7 @@ import (
 	"tailscale.com/tstest"
 	"tailscale.com/tstest/integration/testcontrol"
 	"tailscale.com/types/key"
+	"tailscale.com/types/netmap"
 	"tailscale.com/types/opt"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/must"
@@ -1623,3 +1625,146 @@ func TestPeerRelayPing(t *testing.T) {
 		}
 	}
 }
+
+func TestC2NDebugNetmap(t *testing.T) {
+	tstest.Shard(t)
+	tstest.Parallel(t)
+	env := NewTestEnv(t)
+
+	var testNodes []*TestNode
+	var nodes []*tailcfg.Node
+	for i := range 2 {
+		n := NewTestNode(t, env)
+		d := n.StartDaemon()
+		defer d.MustCleanShutdown(t)
+
+		n.AwaitResponding()
+		n.MustUp()
+		n.AwaitRunning()
+		testNodes = append(testNodes, n)
+
+		controlNodes := env.Control.AllNodes()
+		if len(controlNodes) != i+1 {
+			t.Fatalf("expected %d nodes, got %d nodes", i+1, len(controlNodes))
+		}
+		for _, cn := range controlNodes {
+			if n.MustStatus().Self.PublicKey == cn.Key {
+				nodes = append(nodes, cn)
+				break
+			}
+		}
+	}
+
+	// getC2NNetmap fetches the current netmap. If a candidate map response is provided,
+	// a candidate netmap is also fetched and compared to the current netmap.
+	getC2NNetmap := func(node key.NodePublic, cand *tailcfg.MapResponse) *netmap.NetworkMap {
+		t.Helper()
+		ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+		defer cancel()
+
+		var req *http.Request
+		if cand != nil {
+			body := must.Get(json.Marshal(&tailcfg.C2NDebugNetmapRequest{Candidate: cand}))
+			req = must.Get(http.NewRequestWithContext(ctx, "POST", "/debug/netmap", bytes.NewReader(body)))
+		} else {
+			req = must.Get(http.NewRequestWithContext(ctx, "GET", "/debug/netmap", nil))
+		}
+		httpResp := must.Get(env.Control.NodeRoundTripper(node).RoundTrip(req))
+		defer httpResp.Body.Close()
+
+		if httpResp.StatusCode != 200 {
+			t.Errorf("unexpected status code: %d", httpResp.StatusCode)
+			return nil
+		}
+
+		respBody := must.Get(io.ReadAll(httpResp.Body))
+		var resp tailcfg.C2NDebugNetmapResponse
+		must.Do(json.Unmarshal(respBody, &resp))
+
+		var current netmap.NetworkMap
+		must.Do(json.Unmarshal(resp.Current, &current))
+
+		if !current.PrivateKey.IsZero() {
+			t.Errorf("current netmap has non-zero private key: %v", current.PrivateKey)
+		}
+		// Check candidate netmap if we sent a map response.
+		if cand != nil {
+			var candidate netmap.NetworkMap
+			must.Do(json.Unmarshal(resp.Candidate, &candidate))
+			if !candidate.PrivateKey.IsZero() {
+				t.Errorf("candidate netmap has non-zero private key: %v", candidate.PrivateKey)
+			}
+			if diff := cmp.Diff(current.SelfNode, candidate.SelfNode); diff != "" {
+				t.Errorf("SelfNode differs (-current +candidate):\n%s", diff)
+			}
+			if diff := cmp.Diff(current.Peers, candidate.Peers); diff != "" {
+				t.Errorf("Peers differ (-current +candidate):\n%s", diff)
+			}
+		}
+		return &current
+	}
+
+	for _, n := range nodes {
+		mr := must.Get(env.Control.MapResponse(&tailcfg.MapRequest{NodeKey: n.Key}))
+		nm := getC2NNetmap(n.Key, mr)
+
+		// Make sure peers do not have "testcap" initially (we'll change this later).
+		if len(nm.Peers) != 1 || nm.Peers[0].CapMap().Contains("testcap") {
+			t.Fatalf("expected 1 peer without testcap, got: %v", nm.Peers)
+		}
+
+		// Make sure nodes think each other are offline initially.
+		if nm.Peers[0].Online().Get() {
+			t.Fatalf("expected 1 peer to be offline, got: %v", nm.Peers)
+		}
+	}
+
+	// Send a delta update to n0, setting "testcap" on node 1.
+	env.Control.AddRawMapResponse(nodes[0].Key, &tailcfg.MapResponse{
+		PeersChangedPatch: []*tailcfg.PeerChange{{
+			NodeID: nodes[1].ID, CapMap: tailcfg.NodeCapMap{"testcap": []tailcfg.RawMessage{}},
+		}},
+	})
+
+	// node 0 should see node 1 with "testcap".
+	must.Do(tstest.WaitFor(5*time.Second, func() error {
+		st := testNodes[0].MustStatus()
+		p, ok := st.Peer[nodes[1].Key]
+		if !ok {
+			return fmt.Errorf("node 0 (%s) doesn't see node 1 (%s) as peer\n%v", nodes[0].Key, nodes[1].Key, st)
+		}
+		if _, ok := p.CapMap["testcap"]; !ok {
+			return fmt.Errorf("node 0 (%s) sees node 1 (%s) as peer but without testcap\n%v", nodes[0].Key, nodes[1].Key, p)
+		}
+		return nil
+	}))
+
+	// Check that node 0's current netmap has "testcap" for node 1.
+	nm := getC2NNetmap(nodes[0].Key, nil)
+	if len(nm.Peers) != 1 || !nm.Peers[0].CapMap().Contains("testcap") {
+		t.Errorf("current netmap missing testcap: %v", nm.Peers[0].CapMap())
+	}
+
+	// Send a delta update to n1, marking node 0 as online.
+	env.Control.AddRawMapResponse(nodes[1].Key, &tailcfg.MapResponse{
+		PeersChangedPatch: []*tailcfg.PeerChange{{
+			NodeID: nodes[0].ID, Online: ptr.To(true),
+		}},
+	})
+
+	// node 1 should see node 0 as online.
+	must.Do(tstest.WaitFor(5*time.Second, func() error {
+		st := testNodes[1].MustStatus()
+		p, ok := st.Peer[nodes[0].Key]
+		if !ok || !p.Online {
+			return fmt.Errorf("node 0 (%s) doesn't see node 1 (%s) as an online peer\n%v", nodes[0].Key, nodes[1].Key, st)
+		}
+		return nil
+	}))
+
+	// The netmap from node 1 should show node 0 as online.
+	nm = getC2NNetmap(nodes[1].Key, nil)
+	if len(nm.Peers) != 1 || !nm.Peers[0].Online().Get() {
+		t.Errorf("expected peer to be online; got %+v", nm.Peers[0].AsStruct())
+	}
+}

tstest/integration/testcontrol/testcontrol.go

@@ -336,7 +336,7 @@ func (s *Server) serveUnhandled(w http.ResponseWriter, r *http.Request) {
 func (s *Server) serveC2N(w http.ResponseWriter, r *http.Request) {
 	if err := func() error {
 		if r.Method != httpm.POST {
-			return fmt.Errorf("POST required")
+			return errors.New("POST required")
 		}
 		token, ok := strings.CutPrefix(r.URL.Path, "/c2n/")
 		if !ok {
@@ -1148,18 +1148,25 @@ func (s *Server) canGenerateAutomaticMapResponseFor(nk key.NodePublic) bool {
 func (s *Server) hasPendingRawMapMessage(nk key.NodePublic) bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	_, ok := s.msgToSend[nk].(*tailcfg.MapResponse)
+	_, ok := s.msgToSend[nk]
 	return ok
 }
 
 func (s *Server) takeRawMapMessage(nk key.NodePublic) (mapResJSON []byte, ok bool) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	mr, ok := s.msgToSend[nk].(*tailcfg.MapResponse)
+	mr, ok := s.msgToSend[nk]
 	if !ok {
 		return nil, false
 	}
 	delete(s.msgToSend, nk)
+
+	// If it's a bare PingRequest, wrap it in a MapResponse.
+	switch pr := mr.(type) {
+	case *tailcfg.PingRequest:
+		mr = &tailcfg.MapResponse{PingRequest: pr}
+	}
+
 	var err error
 	mapResJSON, err = json.Marshal(mr)
 	if err != nil {