multiport: fix review findings in lane probing and recv_error

- Don't probe lanes on a tunnel idle in both directions, and don't count lane
  probes or their acks as tunnel traffic. A lane's own keepalive was keeping
  idle tunnels from ever being dropped by tunnels.drop_inactive.
- Accept a recv_error from an up lane's target. The peer answers lane traffic
  from the lane socket it arrived on, and the spoof guard only accepted the
  base remote, so recv_errors for lane traffic were ignored.
- Keep the probe generation across a lane reset, so an ack for a probe sent
  before a roam can't promote the lane on the new path.
- Bring handshake.proto in line with payload.go: field 3 is reserved and
  TxLanes is field 4.

connection_manager.go

@@ -197,26 +197,18 @@ func (cm *connectionManager) doTrafficCheck(localIndex uint32, p, nb, out []byte
 	}
 
 	cm.resetRelayTrafficCheck(hostinfo)
-	cm.maintainLanes(localIndex, decision, hostinfo, now, nb, out)
+	cm.maintainLanes(decision, hostinfo, now, nb, out)
 }
 
 // maintainLanes piggybacks multiport lane probing on the per-tunnel traffic
-// tick. This tick is the right place for it precisely because lanes are
-// demand-driven: a tunnel only lands here when it has traffic, which is the
-// same condition that raises lane demand.
-//
-// makeTrafficDecision returns a nil hostinfo on some keep-alive paths, so
-// re-resolve the index in that case.
-func (cm *connectionManager) maintainLanes(localIndex uint32, decision trafficDecision, hostinfo *HostInfo, now time.Time, nb, out []byte) {
-	if decision == deleteTunnel || decision == closeTunnel {
+// tick. makeTrafficDecision returns a nil hostinfo for a tunnel that is idle in
+// both directions, and that tunnel is skipped: probing it would cost a probe per
+// lane per keepalive for a tunnel nothing is using. Its lanes are probed again
+// on the first tick after traffic resumes.
+func (cm *connectionManager) maintainLanes(decision trafficDecision, hostinfo *HostInfo, now time.Time, nb, out []byte) {
+	if hostinfo == nil || decision == deleteTunnel || decision == closeTunnel {
 		return
 	}
-	if hostinfo == nil {
-		hostinfo = cm.hostMap.QueryIndex(localIndex)
-		if hostinfo == nil {
-			return
-		}
-	}
 	cm.intf.probeLanes(hostinfo, now, nb, out)
 }
 

design/multiport.md

@@ -203,8 +203,11 @@ everything it needs in one atomic read and there is no window where one is set
 and the other is not.
 
 Probing is driven by the connection manager's per-tunnel traffic tick
-(`maintainLanes` -> `probeLanes`), which only fires for a tunnel with traffic --
-the same condition that makes a lane worth having. Timers:
+(`maintainLanes` -> `probeLanes`), which skips a tunnel that is idle in both
+directions -- a lane is only worth having while there is traffic to put on it.
+Probes and their acks are not counted as tunnel traffic, so a lane's own
+keepalive cannot keep an otherwise idle tunnel from being dropped by
+`tunnels.drop_inactive`. Timers:
 
 | timer | value |
 |---|---|
@@ -445,9 +448,10 @@ and lane probe *acks* all use the base session and a socket on the base port
 would see the tunnel's address move and roam-thrash.
 
 `recv_error` is the one deliberate exception: it replies from the socket the
-offending packet arrived on, because a lane peer's spoof guard compares our
-source address against that lane's remote and would discard a reply from the
-base port.
+offending packet arrived on, so the reply to lane traffic travels the reverse of
+the lane's own 4-tuple, the only one the peer's NAT and stateful firewalls are
+known to pass. The receiving side's spoof guard accepts a `recv_error` from the
+base remote or from the target of any of its up lanes (`isLaneRemote`).
 
 ### Wire compatibility
 

handshake/handshake.proto

@@ -32,12 +32,14 @@ message NebulaHandshakeDetails {
   LaneDetails ResponderLanes = 10;
 }
 
-// LaneDetails advertises a host's multiport lane capability. On a base
-// handshake LaneIndex is 0 and PortCount/BasePort describe the sender's
-// consecutively bound UDP ports. On a lane handshake the initiator sets
-// LaneIndex to its (nonzero) lane number.
+// LaneDetails advertises a host's multiport lane capability. PortCount/BasePort
+// describe the sender's consecutively bound UDP ports, and TxLanes is how many
+// lanes the sender may send on, so the peer knows how many lane sessions it must
+// be prepared to receive on.
 message LaneDetails {
   uint32 PortCount = 1;
   uint32 BasePort = 2;
-  uint32 LaneIndex = 3;
+  // was a per-lane handshake index
+  reserved 3;
+  uint32 TxLanes = 4;
 }

inside.go

@@ -611,9 +611,9 @@ func (f *Interface) sendNoMetrics(t header.MessageType, st header.MessageSubType
 	//l.WithField("trace", string(debug.Stack())).Error("out Header ", &Header{Version, t, st, 0, hostinfo.remoteIndexId, c}, p)
 	out = header.Encode(out, header.Version, t, st, hostinfo.remoteIndexId, c)
 	// A closing tunnel is torn down right after this, so skip the connection manager entirely: no point recording
-	// traffic or asking the lighthouse for a punch. Otherwise, if we rebound since this tunnel last sent, ask the
-	// lighthouse to get the far side punching at us again.
-	if t != header.CloseTunnel && f.connectionManager.Out(hostinfo) {
+	// traffic or asking the lighthouse for a punch. Lane probe acks are not traffic either. Otherwise, if we rebound
+	// since this tunnel last sent, ask the lighthouse to get the far side punching at us again.
+	if t != header.CloseTunnel && !isLaneProbe(t, st) && f.connectionManager.Out(hostinfo) {
 		f.lightHouse.QueryServer(hostinfo.vpnAddrs[0])
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
 			f.l.Debug("Lighthouse update triggered for punch due to rebind epoch",

lanes.go

@@ -236,7 +236,7 @@ func newLaneSet(r *handshake.Result, myLanes int, myAddr, peerAddr netip.Addr) *
 	// and the kernel remembers that for as long as the flow stays busy. A flow that
 	// starts while the lanes are still down therefore gets pinned to queue 0 on
 	// both hosts for its whole life. This costs one probe per lane on any tunnel
-	// with traffic; an idle tunnel is never ticked, so it still costs nothing.
+	// with traffic; an idle tunnel is never probed, so it still costs nothing.
 	for s := 1; s < txLanes; s++ {
 		ls.demand[s].Store(true)
 	}
@@ -545,8 +545,8 @@ func (ls *laneSet) noteAck(s int, gen uint8, now time.Time) (netip.AddrPort, boo
 // probeLanes runs one lane maintenance pass for a peer: it demotes lanes whose
 // probe went unanswered, re-proves lanes that have been up a while without one,
 // and probes down lanes the data plane asked for. Driven by the connection
-// manager's per-tunnel traffic tick, which only fires for a live tunnel — the
-// same condition that produces lane demand in the first place.
+// manager's per-tunnel traffic tick, which skips a tunnel that is idle in both
+// directions.
 func (f *Interface) probeLanes(hostinfo *HostInfo, now time.Time, nb, out []byte) {
 	ls := hostinfo.lanes
 	if ls == nil || ls.txLanes < 2 {
@@ -618,6 +618,10 @@ func (f *Interface) probeLanes(hostinfo *HostInfo, now time.Time, nb, out []byte
 // resetLocked takes every lane down and clears its probe state, without
 // counting it as a failure.
 //
+// The probe generation survives the reset. Starting it over would let a late ack
+// for a probe sent before the reset match the first probe sent after it, and
+// promote the lane to a target nothing has proven.
+//
 // Demand is deliberately left standing: it records that a routine has real
 // traffic for this peer, which a roam or a relay detour does not change. Keeping
 // it re-probes the lanes that were actually carrying data as soon as a path
@@ -625,10 +629,32 @@ func (f *Interface) probeLanes(hostinfo *HostInfo, now time.Time, nb, out []byte
 func (ls *laneSet) resetLocked() {
 	for s := 1; s < len(ls.probe); s++ {
 		ls.txAddr[s].Store(nil)
-		ls.probe[s] = laneProbeState{}
+		ls.probe[s] = laneProbeState{gen: ls.probe[s].gen}
 	}
 }
 
+// isLaneProbe reports whether t/st is a lane probe or its ack. Probing is upkeep
+// the lanes do for themselves, not traffic: counting it would let a lane's own
+// keepalive keep an idle tunnel from ever being dropped.
+func isLaneProbe(t header.MessageType, st header.MessageSubType) bool {
+	return t == header.Test && (st == header.LaneProbe || st == header.LaneProbeAck)
+}
+
+// isLaneRemote reports whether addr is the target of one of our up lanes. The
+// peer answers a lane packet from the socket it arrived on, so a recv_error for
+// lane traffic comes from that lane's target rather than the base remote.
+func (ls *laneSet) isLaneRemote(addr netip.AddrPort) bool {
+	if ls == nil {
+		return false
+	}
+	for s := 1; s < ls.txLanes; s++ {
+		if a := ls.txAddr[s].Load(); a != nil && *a == addr {
+			return true
+		}
+	}
+	return false
+}
+
 // sendLaneProbe sends a probe on lane s to addr from a socket on lane s's port.
 // The probe is an ordinary Test packet encrypted with the lane's session, so an
 // ack proves the whole lane: our source port reached the peer, its reply reached

lanes_test.go

@@ -593,6 +593,14 @@ func TestLaneProbeRoamResets(t *testing.T) {
 	assert.Equal(t, netip.MustParseAddr("198.51.100.7"), ls.peerAddr)
 	ls.mu.Unlock()
 
+	// A late ack for the probe sent before the roam must not promote the lane
+	// on the new path, so the generation has to carry across the reset.
+	ls.demand[1].Store(true)
+	ifce.probeLanes(hi, now, nb, out)
+	_, promoted = ls.noteAck(1, gen, now)
+	assert.False(t, promoted, "an ack from before the roam promoted the lane")
+	assert.Nil(t, ls.txAddr[1].Load())
+
 	// Relayed (no direct remote) means no lanes at all.
 	ls.demand[1].Store(true)
 	ifce.probeLanes(hi, now, nb, out)
@@ -606,6 +614,71 @@ func TestLaneProbeRoamResets(t *testing.T) {
 	assert.Nil(t, ls.txAddr[1].Load(), "lane survived losing the direct path")
 }
 
+// A peer answers lane traffic it can't place with a recv_error from the lane
+// socket it arrived on, so the spoof guard must accept one from an up lane's
+// target and still refuse one from anywhere else.
+func TestHandleRecvErrorFromLane(t *testing.T) {
+	hostMap := newHostMap(test.NewLogger())
+	ifce := newLaneTestInterface(hostMap)
+
+	initR, _ := runTestHandshake(t)
+	ls := newTestLaneSet(t, initR, 4, 4, 4242, 4)
+	hi := newTestLaneHostInfo(t, initR, ls)
+	hostMap.unlockedAddHostInfo(hi, ifce)
+
+	now := time.Now()
+	ifce.probeLanes(hi, now, make([]byte, 12), make([]byte, mtu))
+	ls.mu.Lock()
+	gen := ls.probe[1].gen
+	ls.mu.Unlock()
+	laneAddr, promoted := ls.noteAck(1, gen, now)
+	require.True(t, promoted)
+	require.NotEqual(t, hi.GetRemote(), laneAddr)
+
+	h := &header.H{Type: header.RecvError, RemoteIndex: hi.remoteIndexId}
+
+	ifce.handleRecvError(netip.MustParseAddrPort("192.0.2.1:9999"), h)
+	require.NotNil(t, hostMap.QueryReverseIndex(hi.remoteIndexId), "a recv_error from an unknown port closed the tunnel")
+
+	ifce.handleRecvError(laneAddr, h)
+	assert.Nil(t, hostMap.QueryReverseIndex(hi.remoteIndexId), "a recv_error from an up lane's target was ignored")
+}
+
+// An idle tunnel is not probed, so its lanes' keepalives can't keep it alive.
+// Probing resumes on the first tick that sees traffic.
+func TestIdleTunnelSkipsLaneProbes(t *testing.T) {
+	hostMap := newHostMap(test.NewLogger())
+	ifce := newLaneTestInterface(hostMap)
+
+	initR, _ := runTestHandshake(t)
+	ls := newTestLaneSet(t, initR, 4, 4, 4242, 4)
+	hi := newTestLaneHostInfo(t, initR, ls)
+	hostMap.unlockedAddHostInfo(hi, ifce)
+
+	// An empty pool fails every cert, which with pki.disconnect_invalid off
+	// leaves the tunnel up.
+	ifce.pki.caPool.Store(cert.NewCAPool())
+
+	cm := ifce.connectionManager
+	p := []byte("")
+	nb := make([]byte, 12)
+	out := make([]byte, mtu)
+	probed := func() bool {
+		ls.mu.Lock()
+		defer ls.mu.Unlock()
+		return !ls.probe[1].sentAt.IsZero()
+	}
+
+	// Adding the tunnel records outbound traffic; drop it so the tick sees idle.
+	hi.takeTraffic()
+	cm.doTrafficCheck(hi.localIndexId, p, nb, out, time.Now())
+	assert.False(t, probed(), "an idle tunnel was probed")
+
+	cm.In(hi)
+	cm.doTrafficCheck(hi.localIndexId, p, nb, out, time.Now())
+	assert.True(t, probed(), "a tunnel with traffic was not probed")
+}
+
 // A lane probe is answered on the base tunnel, echoing the header's lane, so a
 // peer cannot get us to vouch for a lane it never probed.
 func TestHandleLaneProbe(t *testing.T) {
@@ -629,6 +702,7 @@ func TestHandleLaneProbe(t *testing.T) {
 	assert.Equal(t, header.Test, h.Type)
 	assert.Equal(t, header.LaneProbeAck, h.Subtype)
 	assert.Equal(t, uint8(0), h.Lane(), "the ack is base-tunnel traffic")
+	assert.False(t, hi.sentSinceCheck(), "a probe ack must not count as tunnel traffic")
 
 	pt, err := laneSessionFor(t, newTestLaneSet(t, initR, 4, 4, 4242, 4), 1).dKey.DecryptDanger(
 		nil, sent.bufs[0][:header.Len], sent.bufs[0][header.Len:], h.MessageCounter, make([]byte, 12))

outside.go

@@ -186,7 +186,9 @@ func (f *Interface) readOutsidePackets(via ViaSender, packet []byte, rxc *rxCont
 	if lane == 0 {
 		f.handleHostRoaming(hostinfo, via)
 	}
-	f.connectionManager.In(hostinfo)
+	if !isLaneProbe(h.Type, h.Subtype) {
+		f.connectionManager.In(hostinfo)
+	}
 
 	switch h.Type {
 	case header.Message:
@@ -552,9 +554,10 @@ func (f *Interface) maybeSendRecvError(endpoint netip.AddrPort, index uint32, q
 	}
 }
 
-// sendRecvError replies from the socket the offending packet arrived on (q).
-// A lane peer's spoof guard compares our source addr against the lane's
-// remote, so a reply from the base port would be discarded.
+// sendRecvError replies from the socket the offending packet arrived on (q), so
+// the reply to lane traffic rides the reverse of the lane's own 4-tuple: the
+// only one the peer's NAT and stateful firewalls are known to let through. The
+// peer's handleRecvError accepts it from that lane's target.
 func (f *Interface) sendRecvError(endpoint netip.AddrPort, index uint32, q int) {
 	f.messageMetrics.Tx(header.RecvError, 0, 1)
 
@@ -591,7 +594,7 @@ func (f *Interface) handleRecvError(addr netip.AddrPort, h *header.H) {
 	}
 
 	hr := hostinfo.GetRemote()
-	if hr.IsValid() && hr != addr {
+	if hr.IsValid() && hr != addr && !hostinfo.lanes.isLaneRemote(addr) {
 		f.l.Info("Someone spoofing recv_errors?",
 			"addr", addr,
 			"hostinfoRemote", hr,