wgengine: send disco key via TSMP on first contact

When we have not yet communicated with a peer, send a
TSMPDiscoAdvertisement to let the peer know of our disco key. This is in
most cases redundant, but will allow us to set up direct connections
when the client cannot access control.

Some parts taken from: #18073

Updates #12639

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -75,12 +75,13 @@ type endpoint struct {
 	// mu protects all following fields.
 	mu syncs.Mutex // Lock ordering: Conn.mu, then endpoint.mu
 
-	heartBeatTimer            *time.Timer    // nil when idle
-	lastSendExt               mono.Time      // last time there were outgoing packets sent to this peer from an external trigger (e.g. wireguard-go or disco pingCLI)
-	lastSendAny               mono.Time      // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
-	lastFullPing              mono.Time      // last time we pinged all disco or wireguard only endpoints
-	lastUDPRelayPathDiscovery mono.Time      // last time we ran UDP relay path discovery
-	derpAddr                  netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)
+	heartBeatTimer             *time.Timer // nil when idle
+	lastSendExt                mono.Time   // last time there were outgoing packets sent to this peer from an external trigger (e.g. wireguard-go or disco pingCLI)
+	lastSendAny                mono.Time   // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
+	lastFullPing               mono.Time   // last time we pinged all disco or wireguard only endpoints
+	lastUDPRelayPathDiscovery  mono.Time   // last time we ran UDP relay path discovery
+	lastTSMPDiscoAdvertisement mono.Time
+	derpAddr                   netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)
 
 	bestAddr           addrQuality // best non-DERP path; zero if none; mutate via setBestAddrLocked()
 	bestAddrAt         mono.Time   // time best address re-confirmed

wgengine/magicsock/magicsock.go

@@ -178,9 +178,10 @@ type Conn struct {
 
 	// A publisher for synchronization points to ensure correct ordering of
 	// config changes between magicsock and wireguard.
-	syncPub               *eventbus.Publisher[syncPoint]
-	allocRelayEndpointPub *eventbus.Publisher[UDPRelayAllocReq]
-	portUpdatePub         *eventbus.Publisher[router.PortUpdate]
+	syncPub                  *eventbus.Publisher[syncPoint]
+	allocRelayEndpointPub    *eventbus.Publisher[UDPRelayAllocReq]
+	portUpdatePub            *eventbus.Publisher[router.PortUpdate]
+	tsmpDiscoKeyAvailablePub *eventbus.Publisher[NewDiscoKeyAvailable]
 
 	// pconn4 and pconn6 are the underlying UDP sockets used to
 	// send/receive packets for wireguard and other magicsock
@@ -691,6 +692,7 @@ func NewConn(opts Options) (*Conn, error) {
 	c.syncPub = eventbus.Publish[syncPoint](ec)
 	c.allocRelayEndpointPub = eventbus.Publish[UDPRelayAllocReq](ec)
 	c.portUpdatePub = eventbus.Publish[router.PortUpdate](ec)
+	c.tsmpDiscoKeyAvailablePub = eventbus.Publish[NewDiscoKeyAvailable](ec)
 	eventbus.SubscribeFunc(ec, c.onPortMapChanged)
 	eventbus.SubscribeFunc(ec, c.onFilterUpdate)
 	eventbus.SubscribeFunc(ec, c.onNodeViewsUpdate)
@@ -2239,6 +2241,7 @@ func (c *Conn) handleDiscoMessage(msg []byte, src epAddr, shouldBeRelayHandshake
 		if debugDisco() {
 			c.logf("magicsock: disco: failed to open naclbox from %v (wrong rcpt?) via %s", sender, via)
 		}
+
 		metricRecvDiscoBadKey.Add(1)
 		return
 	}
@@ -2646,6 +2649,8 @@ func (c *Conn) enqueueCallMeMaybe(derpAddr netip.AddrPort, de *endpoint) {
 		return
 	}
 
+	c.maybeSendTSMPDiscoAdvert(de)
+
 	eps := make([]netip.AddrPort, 0, len(c.lastEndpoints))
 	for _, ep := range c.lastEndpoints {
 		eps = append(eps, ep.Addr)
@@ -4306,3 +4311,16 @@ func (c *Conn) HandleDiscoKeyAdvertisement(node tailcfg.NodeView, update packet.
 	c.logf("magicsock: updated disco key for peer %v to %v", nodeKey.ShortString(), discoKey.ShortString())
 	metricTSMPDiscoKeyAdvertisementApplied.Add(1)
 }
+
+type NewDiscoKeyAvailable struct {
+	EpAddr netip.Addr
+}
+
+func (c *Conn) maybeSendTSMPDiscoAdvert(de *endpoint) {
+	if de.lastTSMPDiscoAdvertisement == 0 {
+		de.lastTSMPDiscoAdvertisement = mono.Now()
+		c.tsmpDiscoKeyAvailablePub.Publish(NewDiscoKeyAvailable{
+			EpAddr: de.nodeAddr,
+		})
+	}
+}

wgengine/userspace.go

@@ -54,6 +54,7 @@ import (
 	"tailscale.com/util/execqueue"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/set"
+	"tailscale.com/util/singleflight"
 	"tailscale.com/util/testenv"
 	"tailscale.com/util/usermetric"
 	"tailscale.com/version"
@@ -568,6 +569,14 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 		}
 		e.magicConn.HandleDiscoKeyAdvertisement(peer.Node, pkt)
 	})
+	var tsmpRequestGroup singleflight.Group[netip.Addr, struct{}]
+	eventbus.SubscribeFunc(ec, func(req magicsock.NewDiscoKeyAvailable) {
+		go tsmpRequestGroup.Do(req.EpAddr, func() (struct{}, error) {
+			e.sendTSMPDiscoAdvertisement(req.EpAddr)
+			e.logf("wgengine: sending TSMP disco key advertisement to %v", req.EpAddr)
+			return struct{}{}, nil
+		})
+	})
 	e.eventClient = ec
 	e.logf("Engine created.")
 	return e, nil