control/controlknobs,wgengine/magicsock: disable TSMP disco advert if netmap caching is disabled

Updates #20081

Signed-off-by: Jordan Whited <jordan@tailscale.com>

control/controlknobs/controlknobs.go

@@ -135,6 +135,11 @@ type Knobs struct {
 	// underlay UDP packet TX path on Linux. Applies to magicsock and peer relay
 	// UDP sockets. See [tailcfg.NodeAttrNeverGSOEqualTail].
 	NeverGSOEqualTail atomic.Bool
+
+	// CacheNetworkMaps is whether the node should persistently cache network
+	// maps and use them to establish peer connectivity on start, if doing so
+	// is supported by the client and storage is available.
+	CacheNetworkMaps atomic.Bool
 }
 
 // UpdateFromNodeAttributes updates k (if non-nil) based on the provided self
@@ -170,6 +175,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 		disableTUNUDPGRO                     = has(tailcfg.NodeAttrDisableTUNUDPGRO)
 		disableTUNTCPGRO                     = has(tailcfg.NodeAttrDisableTUNTCPGRO)
 		neverGSOEqualTail                    = has(tailcfg.NodeAttrNeverGSOEqualTail)
+		cacheNetworkMaps                     = has(tailcfg.NodeAttrCacheNetworkMaps)
 	)
 
 	if has(tailcfg.NodeAttrOneCGNATEnable) {
@@ -203,6 +209,7 @@ func (k *Knobs) UpdateFromNodeAttributes(capMap tailcfg.NodeCapMap) {
 	k.DisableTUNUDPGRO.Store(disableTUNUDPGRO)
 	k.DisableTUNTCPGRO.Store(disableTUNTCPGRO)
 	k.NeverGSOEqualTail.Store(neverGSOEqualTail)
+	k.CacheNetworkMaps.Store(cacheNetworkMaps)
 }
 
 // AsDebugJSON returns k as something that can be marshalled with json.Marshal

wgengine/magicsock/magicsock.go

@@ -4526,8 +4526,10 @@ type NewDiscoKeyAvailable struct {
 
 // maybeSendTSMPDiscoAdvert conditionally emits an event indicating that we
 // should send our DiscoKey to the first node address of the magicksock endpoint.
-// The event is only emitted if we are not already communicating directly and
-// more than 60 seconds has passed since the last DiscoKey was sent.
+//
+// The event is suppressed if we are already communicating directly or
+// less than 60 seconds has passed since the last DiscoKey was sent, or netmap
+// caching is disabled on this node.
 //
 // We do not need the Conn to be locked, but the endpoint should be.
 func (c *Conn) maybeSendTSMPDiscoAdvert(de *endpoint) {
@@ -4535,6 +4537,17 @@ func (c *Conn) maybeSendTSMPDiscoAdvert(de *endpoint) {
 		return
 	}
 
+	// Disable TSMP disco advert by default, unless network map caching is
+	// enabled for the local node. Caching network maps on the remote node is
+	// what really matters in terms of handling a TSMP disco advert and applying
+	// it in a useful way, but the TSMP disco advert implementation as it exists
+	// here has pathological behaviors. Therefore, it should be disabled for
+	// almost all tailnets, and we lean on the network map caching control knob
+	// for this purpose. See #20081.
+	if c.controlKnobs == nil || !c.controlKnobs.CacheNetworkMaps.Load() {
+		return
+	}
+
 	de.mu.Lock()
 	defer de.mu.Unlock()
 

wgengine/magicsock/magicsock_test.go

@@ -4515,6 +4515,10 @@ func TestSendingTSMPDiscoTimer(t *testing.T) {
 	tw := eventbustest.NewWatcher(t, conn.eventBus)
 	t.Cleanup(func() { conn.Close() })
 
+	// maybeSendTSMPDiscoAdvert only advertises when netmap caching is enabled.
+	conn.controlKnobs = new(controlknobs.Knobs)
+	conn.controlKnobs.CacheNetworkMaps.Store(true)
+
 	peerKey := key.NewNode().Public()
 	ep := &endpoint{
 		nodeID:    1,
@@ -4576,3 +4580,43 @@ func TestSendingTSMPDiscoTimer(t *testing.T) {
 		t.Errorf("expected only one event, got: %s", err)
 	}
 }
+
+// TestSendingTSMPDiscoCachingDisabled verifies that maybeSendTSMPDiscoAdvert
+// early-returns (sends no advert) when netmap caching is not enabled via the
+// CacheNetworkMaps control knob, including when no knobs are present at all.
+func TestSendingTSMPDiscoCachingDisabled(t *testing.T) {
+	tests := []struct {
+		name  string
+		knobs *controlknobs.Knobs
+	}{
+		{name: "no-knobs", knobs: nil},
+		// Knobs present but CacheNetworkMaps left at its false default.
+		{name: "caching-disabled", knobs: new(controlknobs.Knobs)},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			conn := newTestConn(t)
+			t.Cleanup(func() { conn.Close() })
+			conn.controlKnobs = tt.knobs
+
+			ep := &endpoint{
+				nodeID:    1,
+				publicKey: key.NewNode().Public(),
+				nodeAddr:  netip.MustParseAddr("100.64.0.1"),
+			}
+			ep.c = conn
+
+			// A fresh endpoint with a zero lastDiscoKeyAdvertisement and no
+			// direct bestAddr would otherwise advertise; the only thing
+			// suppressing it here is the disabled caching knob. On early
+			// return the timestamp is left untouched (zero).
+			conn.maybeSendTSMPDiscoAdvert(ep)
+
+			ep.mu.Lock()
+			defer ep.mu.Unlock()
+			if !ep.lastDiscoKeyAdvertisement.IsZero() {
+				t.Errorf("lastDiscoKeyAdvertisement = %v; want zero (advert should have been suppressed)", ep.lastDiscoKeyAdvertisement)
+			}
+		})
+	}
+}