warn on uselessly low MTU

cmd/nebula-service/main.go

@@ -53,7 +53,12 @@ func main() {
 	l := logging.NewLogger(os.Stdout)
 
 	if *serviceFlag != "" {
-		if err := doService(configPath, configTest, Build, serviceFlag); err != nil {
+		if *configTest {
+			fmt.Println("-test is not supported with -service, run the config test without -service")
+			os.Exit(1)
+		}
+
+		if err := doService(configPath, Build, serviceFlag); err != nil {
 			l.Error("Service command failed", "error", err)
 			os.Exit(1)
 		}
@@ -93,15 +98,14 @@ func main() {
 	}
 
 	if !*configTest {
-		wait, err := ctrl.Start()
-		if err != nil {
+		if err := ctrl.Start(); err != nil {
 			util.LogWithContextIfNeeded("Error while running", err, l)
 			os.Exit(1)
 		}
 
 		go ctrl.ShutdownBlock()
 
-		if err := wait(); err != nil {
+		if err := ctrl.Wait(); err != nil {
 			l.Error("Nebula stopped due to fatal error", "error", err)
 			os.Exit(2)
 		}

cmd/nebula-service/service.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"log"
+	"os"
 
 	"github.com/kardianos/service"
 	"github.com/slackhq/nebula"
@@ -14,7 +15,6 @@ var logger service.Logger
 
 type program struct {
 	configPath *string
-	configTest *bool
 	build      string
 	control    *nebula.Control
 }
@@ -40,22 +40,41 @@ func (p *program) Start(s service.Service) error {
 		}
 	})
 
-	p.control, err = nebula.Main(c, *p.configTest, Build, l, nil)
+	p.control, err = nebula.Main(c, false, Build, l, nil)
 	if err != nil {
 		return err
 	}
 
-	p.control.Start()
+	if err := p.control.Start(); err != nil {
+		return err
+	}
+
+	// Nebula can stop itself on a fatal packet reader error, make sure to log it if it happens.
+	go func() {
+		if err := p.control.Wait(); err != nil {
+			logger.Error(fmt.Sprintf("Nebula stopped due to fatal error: %v", err))
+			os.Exit(2)
+		}
+	}()
+
 	return nil
 }
 
 func (p *program) Stop(s service.Service) error {
 	logger.Info("Nebula service stopping.")
+	if p.control == nil {
+		return nil
+	}
+
 	p.control.Stop()
+
+	// block until nebula has fully drained before reporting stopped.
+	// error logging is handled by Start.
+	_ = p.control.Wait()
 	return nil
 }
 
-func doService(configPath *string, configTest *bool, build string, serviceFlag *string) error {
+func doService(configPath *string, build string, serviceFlag *string) error {
 	if *configPath == "" {
 		p, err := config.DefaultPath()
 		if err != nil {
@@ -73,7 +92,6 @@ func doService(configPath *string, configTest *bool, build string, serviceFlag *
 
 	prg := &program{
 		configPath: configPath,
-		configTest: configTest,
 		build:      build,
 	}
 
@@ -105,8 +123,9 @@ func doService(configPath *string, configTest *bool, build string, serviceFlag *
 	switch *serviceFlag {
 	case "run":
 		if err := s.Run(); err != nil {
-			// Route any errors to the system logger
+			// Route any errors to the system logger and report the failure
 			logger.Error(err)
+			return err
 		}
 	default:
 		if err := service.Control(s, *serviceFlag); err != nil {

cmd/nebula/main.go

@@ -84,8 +84,7 @@ func main() {
 	}
 
 	if !*configTest {
-		wait, err := ctrl.Start()
-		if err != nil {
+		if err := ctrl.Start(); err != nil {
 			util.LogWithContextIfNeeded("Error while running", err, l)
 			os.Exit(1)
 		}
@@ -93,7 +92,7 @@ func main() {
 		go ctrl.ShutdownBlock()
 		notifyReady(l)
 
-		if err := wait(); err != nil {
+		if err := ctrl.Wait(); err != nil {
 			l.Error("Nebula stopped due to fatal error", "error", err)
 			os.Exit(2)
 		}

control.go

@@ -69,29 +69,29 @@ type ControlHostInfo struct {
 }
 
 // Start actually runs nebula, this is a nonblocking call.
-// The returned function blocks until nebula has fully stopped and returns the
-// first fatal reader error (if any). A nil error means nebula shut down
-// gracefully; a non-nil error means a reader hit an unexpected failure that
-// triggered the shutdown.
-func (c *Control) Start() (func() error, error) {
+// Use Wait to block until nebula has fully stopped and to learn whether a fatal reader error caused the shutdown.
+func (c *Control) Start() error {
 	c.stateLock.Lock()
 	defer c.stateLock.Unlock()
 	switch c.state {
 	case StateReady:
 		//yay!
 	case StateStopped, StateStopping:
-		return nil, ErrAlreadyStopped
+		return ErrAlreadyStopped
 	case StateStarted:
-		return nil, ErrAlreadyStarted
+		return ErrAlreadyStarted
 	default:
-		return nil, ErrUnknownState
+		return ErrUnknownState
 	}
 
 	// Activate the interface
 	err := c.f.activate()
 	if err != nil {
+		// Cancel before Close so a caller returning from Wait always observes a dead Context
+		c.cancel()
+		_ = c.f.Close()
 		c.state = StateStopped
-		return nil, err
+		return err
 	}
 
 	// Call all the delayed funcs that waited patiently for the interface to be created.
@@ -114,13 +114,9 @@ func (c *Control) Start() (func() error, error) {
 	c.f.triggerShutdown = c.Stop
 
 	// Start reading packets.
-	out, err := c.f.run()
-	if err != nil {
-		c.state = StateStopped
-		return nil, err
-	}
+	c.f.run()
 	c.state = StateStarted
-	return out, nil
+	return nil
 }
 
 func (c *Control) State() RunState {
@@ -133,10 +129,26 @@ func (c *Control) Context() context.Context {
 	return c.ctx
 }
 
-// Stop is a non-blocking call that signals nebula to close all tunnels and shut down
+// Stop tears nebula down, closing all tunnels and releasing everything it holds.
+// Use Wait to block until the shutdown has completed.
+// A Control that has been stopped cannot be started again, Start will return ErrAlreadyStopped.
 func (c *Control) Stop() {
 	c.stateLock.Lock()
-	if c.state != StateStarted {
+	switch c.state {
+	case StateStarted:
+		// Fall through to the full teardown below
+
+	case StateReady:
+		// Never started
+		c.cancel()
+		c.state = StateStopped
+		if err := c.f.Close(); err != nil {
+			c.l.Error("Close interface failed", "error", err)
+		}
+		c.stateLock.Unlock()
+		return
+
+	default:
 		c.stateLock.Unlock()
 		// We are stopping or stopped already
 		return
@@ -145,19 +157,26 @@ func (c *Control) Stop() {
 	c.state = StateStopping
 	c.stateLock.Unlock()
 
-	// Stop the handshakeManager (and other services), to prevent new tunnels from
-	// being created while we're shutting them all down.
+	// Closing tunnels can be slow with a large hostmap, don't hold the lock for it
 	c.cancel()
-
 	c.CloseAllTunnels(false)
+
+	c.stateLock.Lock()
+	c.state = StateStopped
 	if err := c.f.Close(); err != nil {
 		c.l.Error("Close interface failed", "error", err)
 	}
-	c.stateLock.Lock()
-	c.state = StateStopped
 	c.stateLock.Unlock()
 }
 
+// Wait blocks until nebula has fully stopped, either via Stop or an internal fatal error,
+// and returns the first fatal packet reader error if there was one.
+// It is safe to call from multiple goroutines and at any point in the lifecycle,
+// but a Wait on a Control that is never started and never stopped will block forever.
+func (c *Control) Wait() error {
+	return c.f.wait()
+}
+
 // ShutdownBlock will listen for and block on term and interrupt signals, calling Control.Stop() once signalled
 func (c *Control) ShutdownBlock() {
 	sigChan := make(chan os.Signal, 1)
@@ -170,8 +189,15 @@ func (c *Control) ShutdownBlock() {
 	c.Stop()
 }
 
-// RebindUDPServer asks the UDP listener to rebind it's listener. Mainly used on mobile clients when interfaces change
+// RebindUDPServer asks the UDP listener to rebind it's listener. Mainly used on mobile clients when interfaces change.
 func (c *Control) RebindUDPServer() {
+	c.stateLock.Lock()
+	defer c.stateLock.Unlock()
+
+	if c.state != StateStarted {
+		return
+	}
+
 	_ = c.f.outside.Rebind()
 
 	// Trigger a lighthouse update, useful for mobile clients that should have an update interval of 0

control_lifecycle_test.go

@@ -0,0 +1,292 @@
+package nebula
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/netip"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/gaissmai/bart"
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/routing"
+	"github.com/slackhq/nebula/test"
+	"github.com/slackhq/nebula/udp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type fakeDevice struct {
+	closeOnce sync.Once
+	closedCh  chan struct{}
+	closed    bool
+}
+
+func newFakeDevice() *fakeDevice {
+	return &fakeDevice{closedCh: make(chan struct{})}
+}
+
+// Read blocks until Close like a real tun with no traffic, then reports EOF
+// the same way a closed device does
+func (d *fakeDevice) Read(p []byte) (int, error) {
+	<-d.closedCh
+	return 0, io.EOF
+}
+
+func (d *fakeDevice) Write(p []byte) (int, error) { return len(p), nil }
+
+func (d *fakeDevice) Close() error {
+	d.closeOnce.Do(func() {
+		d.closed = true
+		close(d.closedCh)
+	})
+	return nil
+}
+
+func (d *fakeDevice) Activate() error                       { return nil }
+func (d *fakeDevice) Networks() []netip.Prefix              { return nil }
+func (d *fakeDevice) Name() string                          { return "fake" }
+func (d *fakeDevice) RoutesFor(netip.Addr) routing.Gateways { return nil }
+func (d *fakeDevice) SupportsMultiqueue() bool              { return false }
+func (d *fakeDevice) NewMultiQueueReader() (io.ReadWriteCloser, error) {
+	return nil, errors.New("unsupported")
+}
+
+// newReadyControl hand-builds the minimum Control that Main would have
+// produced right before Start, including the construction token NewInterface
+// takes so waiters block until Close releases the resources
+func newReadyControl(t *testing.T) (*Control, *fakeDevice, *fakeConn) {
+	l := test.NewLogger()
+	dev := newFakeDevice()
+	conn := &fakeConn{}
+	ctx, cancel := context.WithCancel(context.Background())
+
+	myVpnNet := netip.MustParsePrefix("10.128.0.1/16")
+	nt := new(bart.Lite)
+	nt.Insert(myVpnNet)
+	cs := &CertState{
+		myVpnNetworks:      []netip.Prefix{myVpnNet},
+		myVpnNetworksTable: nt,
+	}
+	lh, err := NewLightHouseFromConfig(ctx, l, config.NewC(l), cs, nil, nil)
+	require.NoError(t, err)
+
+	f := &Interface{
+		ctx:        ctx,
+		inside:     dev,
+		outside:    conn,
+		writers:    []udp.Conn{conn},
+		readers:    make([]io.ReadWriteCloser, 1),
+		routines:   1,
+		hostMap:    newHostMap(l),
+		lightHouse: lh,
+		l:          l,
+	}
+	f.wg.Add(1)
+
+	return &Control{
+		state:  StateReady,
+		f:      f,
+		l:      l,
+		ctx:    ctx,
+		cancel: cancel,
+	}, dev, conn
+}
+
+func TestControl_StopBeforeStart(t *testing.T) {
+	c, dev, conn := newReadyControl(t)
+
+	// A Stop on a never started control must release everything Main acquired
+	c.Stop()
+	assert.Equal(t, StateStopped, c.State())
+	assert.True(t, dev.closed, "the tun device should have been closed")
+	assert.True(t, conn.closed, "the udp socket should have been closed")
+	require.ErrorIs(t, c.ctx.Err(), context.Canceled, "the service context should have been cancelled")
+
+	// Wait must return promptly now that the resources are released
+	require.NoError(t, c.Wait())
+
+	// A stopped control can never be started
+	require.ErrorIs(t, c.Start(), ErrAlreadyStopped)
+
+	// A second Stop is a harmless no-op
+	c.Stop()
+	assert.Equal(t, StateStopped, c.State())
+	require.NoError(t, c.Wait())
+}
+
+func TestControl_WaitBlocksUntilStop(t *testing.T) {
+	c, _, _ := newReadyControl(t)
+
+	done := make(chan error, 1)
+	go func() { done <- c.Wait() }()
+
+	select {
+	case <-done:
+		t.Fatal("Wait returned before Stop")
+	case <-time.After(50 * time.Millisecond):
+	}
+
+	c.Stop()
+	select {
+	case err := <-done:
+		require.NoError(t, err)
+	case <-time.After(time.Second):
+		t.Fatal("Wait did not return after Stop")
+	}
+}
+
+type fakeConn struct {
+	closed  bool
+	rebinds int
+}
+
+func (c *fakeConn) Rebind() error                            { c.rebinds++; return nil }
+func (c *fakeConn) LocalAddr() (netip.AddrPort, error)       { return netip.AddrPort{}, nil }
+func (c *fakeConn) ListenOut(_ udp.EncReader) error          { return nil }
+func (c *fakeConn) WriteTo(_ []byte, _ netip.AddrPort) error { return nil }
+func (c *fakeConn) ReloadConfig(_ *config.C)                 {}
+func (c *fakeConn) SupportsMultipleReaders() bool            { return true }
+func (c *fakeConn) Close() error                             { c.closed = true; return nil }
+
+type multiqueueDevice struct {
+	*fakeDevice
+}
+
+func (d *multiqueueDevice) SupportsMultiqueue() bool { return true }
+
+func TestControl_StartMultiqueueFailureReleases(t *testing.T) {
+	dev := &multiqueueDevice{fakeDevice: newFakeDevice()}
+	conn := &fakeConn{}
+	ctx, cancel := context.WithCancel(context.Background())
+	f := &Interface{
+		ctx:      ctx,
+		inside:   dev,
+		outside:  conn,
+		writers:  []udp.Conn{conn},
+		readers:  make([]io.ReadWriteCloser, 2),
+		routines: 2,
+		l:        test.NewLogger(),
+	}
+	f.wg.Add(1)
+
+	c := &Control{
+		state:  StateReady,
+		f:      f,
+		l:      test.NewLogger(),
+		ctx:    ctx,
+		cancel: cancel,
+	}
+
+	// The second reader fails to open, everything must be released
+	require.Error(t, c.Start())
+	assert.Equal(t, StateStopped, c.State())
+	assert.True(t, dev.closed, "the tun device should have been closed")
+	assert.True(t, conn.closed, "the udp socket should have been closed")
+	require.ErrorIs(t, c.ctx.Err(), context.Canceled)
+
+	// And Wait must not hang on the construction token
+	require.NoError(t, c.Wait())
+}
+
+func TestInterface_CloseIsIdempotent(t *testing.T) {
+	dev := newFakeDevice()
+	f := &Interface{
+		inside: dev,
+		l:      test.NewLogger(),
+	}
+	f.wg.Add(1)
+
+	require.NoError(t, f.Close())
+	assert.True(t, dev.closed)
+
+	// A second Close must not double release the wg token or the device
+	require.NoError(t, f.Close())
+	require.NoError(t, f.wait())
+}
+
+func TestControl_FatalErrorReportsThroughWait(t *testing.T) {
+	c, dev, conn := newReadyControl(t)
+
+	// Mirror what Start wires up, without needing real packet readers
+	c.f.triggerShutdown = c.Stop
+	c.state = StateStarted
+
+	boom := errors.New("boom")
+	c.f.onFatal(boom)
+
+	require.ErrorIs(t, c.Wait(), boom)
+	assert.Equal(t, StateStopped, c.State())
+	assert.True(t, dev.closed)
+	assert.True(t, conn.closed)
+
+	// A second fatal error must not fire the shutdown again or replace the first
+	c.f.onFatal(errors.New("later"))
+	require.ErrorIs(t, c.Wait(), boom)
+
+	// Wait stays factual, a Stop after the death does not mask the error
+	c.Stop()
+	require.ErrorIs(t, c.Wait(), boom)
+}
+
+func TestControl_ConcurrentStopAndStart(t *testing.T) {
+	c, _, _ := newReadyControl(t)
+
+	var wg sync.WaitGroup
+	for i := 0; i < 2; i++ {
+		wg.Go(func() { c.Stop() })
+	}
+	wg.Go(func() { _ = c.Start() })
+	wg.Go(func() {
+		_ = c.Wait()
+		// A returned Wait must always observe the final state, no matter how
+		// the race resolved
+		assert.Equal(t, StateStopped, c.State())
+	})
+	wg.Wait()
+
+	// However the race resolves, the control must end fully stopped with no
+	// panic and Wait must observe the final state
+	require.NoError(t, c.Wait())
+	assert.Equal(t, StateStopped, c.State())
+	require.ErrorIs(t, c.Start(), ErrAlreadyStopped)
+}
+
+func TestControl_StartStopLifecycle(t *testing.T) {
+	c, dev, conn := newReadyControl(t)
+
+	require.NoError(t, c.Start())
+	assert.Equal(t, StateStarted, c.State())
+	require.ErrorIs(t, c.Start(), ErrAlreadyStarted)
+
+	// Stop must unpark the reader blocked in the device and release everything
+	c.Stop()
+	assert.Equal(t, StateStopped, c.State())
+	assert.True(t, dev.closed, "the tun device should have been closed")
+	assert.True(t, conn.closed, "the udp socket should have been closed")
+	require.ErrorIs(t, c.ctx.Err(), context.Canceled)
+
+	// The reader drained off a closed device, that is not a fatal error
+	require.NoError(t, c.Wait())
+	require.ErrorIs(t, c.Start(), ErrAlreadyStopped)
+}
+
+func TestControl_RebindIsGatedByState(t *testing.T) {
+	c, _, conn := newReadyControl(t)
+
+	// A rebind before Start reaches nothing, the interface is not up
+	c.RebindUDPServer()
+	assert.Equal(t, 0, conn.rebinds, "rebind before start must be a no-op")
+
+	require.NoError(t, c.Start())
+	c.RebindUDPServer()
+	assert.Equal(t, 1, conn.rebinds, "rebind while started must reach the conn")
+
+	// A rebind racing a completed stop must not touch the closed conn
+	c.Stop()
+	require.NoError(t, c.Wait())
+	c.RebindUDPServer()
+	assert.Equal(t, 1, conn.rebinds, "rebind after stop must be a no-op")
+}

control_tester.go

@@ -125,6 +125,14 @@ func (c *Control) GetHostmap() *HostMap {
 	return c.f.hostMap
 }
 
+// GetHostmapIndexCount returns the number of entries in the main hostmap Indexes table, holding
+// the hostmap read lock so tests can poll it while connection manager churns tunnels.
+func (c *Control) GetHostmapIndexCount() int {
+	c.f.hostMap.RLock()
+	defer c.f.hostMap.RUnlock()
+	return len(c.f.hostMap.Indexes)
+}
+
 func (c *Control) GetF() *Interface {
 	return c.f
 }

e2e/handshakes_test.go

@@ -405,7 +405,7 @@ func TestStage1Race(t *testing.T) {
 
 	r.Log("Spin until connection manager tears down a tunnel")
 
-	for len(myControl.GetHostmap().Indexes)+len(theirControl.GetHostmap().Indexes) > 2 {
+	for myControl.GetHostmapIndexCount()+theirControl.GetHostmapIndexCount() > 2 {
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 		t.Log("Connection manager hasn't ticked yet")
 		time.Sleep(time.Second)
@@ -453,9 +453,11 @@ func TestUncleanShutdownRaceLoser(t *testing.T) {
 
 	r.Log("Nuke my hostmap")
 	myHostmap := myControl.GetHostmap()
+	myHostmap.Lock()
 	myHostmap.Hosts = map[netip.Addr]*nebula.HostInfo{}
 	myHostmap.Indexes = map[uint32]*nebula.HostInfo{}
 	myHostmap.RemoteIndexes = map[uint32]*nebula.HostInfo{}
+	myHostmap.Unlock()
 
 	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("Hi from me again")))
 	p = r.RouteForAllUntilTxTun(theirControl)
@@ -465,10 +467,10 @@ func TestUncleanShutdownRaceLoser(t *testing.T) {
 	assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 
 	r.Log("Wait for the dead index to go away")
-	start := len(theirControl.GetHostmap().Indexes)
+	start := theirControl.GetHostmapIndexCount()
 	for {
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
-		if len(theirControl.GetHostmap().Indexes) < start {
+		if theirControl.GetHostmapIndexCount() < start {
 			break
 		}
 		time.Sleep(time.Second)
@@ -504,9 +506,11 @@ func TestUncleanShutdownRaceWinner(t *testing.T) {
 
 	r.Log("Nuke my hostmap")
 	theirHostmap := theirControl.GetHostmap()
+	theirHostmap.Lock()
 	theirHostmap.Hosts = map[netip.Addr]*nebula.HostInfo{}
 	theirHostmap.Indexes = map[uint32]*nebula.HostInfo{}
 	theirHostmap.RemoteIndexes = map[uint32]*nebula.HostInfo{}
+	theirHostmap.Unlock()
 
 	theirControl.InjectTunPacket(BuildTunUDPPacket(myVpnIpNet[0].Addr(), 80, theirVpnIpNet[0].Addr(), 80, []byte("Hi from them again")))
 	p = r.RouteForAllUntilTxTun(myControl)
@@ -517,10 +521,10 @@ func TestUncleanShutdownRaceWinner(t *testing.T) {
 	assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 
 	r.Log("Wait for the dead index to go away")
-	start := len(myControl.GetHostmap().Indexes)
+	start := myControl.GetHostmapIndexCount()
 	for {
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
-		if len(myControl.GetHostmap().Indexes) < start {
+		if myControl.GetHostmapIndexCount() < start {
 			break
 		}
 		time.Sleep(time.Second)
@@ -628,10 +632,10 @@ func TestReestablishRelays(t *testing.T) {
 	r.Log("Close the tunnel")
 	relayControl.CloseTunnel(theirVpnIpNet[0].Addr(), true)
 
-	start := len(myControl.GetHostmap().Indexes)
-	curIndexes := len(myControl.GetHostmap().Indexes)
+	start := myControl.GetHostmapIndexCount()
+	curIndexes := myControl.GetHostmapIndexCount()
 	for curIndexes >= start {
-		curIndexes = len(myControl.GetHostmap().Indexes)
+		curIndexes = myControl.GetHostmapIndexCount()
 		r.Logf("Wait for the dead index to go away:start=%v indexes, current=%v indexes", start, curIndexes)
 		myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("Hi from me should fail")))
 
@@ -819,18 +823,18 @@ func TestStage1RaceRelays2(t *testing.T) {
 
 	t.Log("Wait until we remove extra tunnels")
 	t.Logf("Waiting for hostinfos to be removed... myControl=%d theirControl=%d relayControl=%d",
-		len(myControl.GetHostmap().Indexes),
-		len(theirControl.GetHostmap().Indexes),
-		len(relayControl.GetHostmap().Indexes),
+		myControl.GetHostmapIndexCount(),
+		theirControl.GetHostmapIndexCount(),
+		relayControl.GetHostmapIndexCount(),
 	)
-	hostInfos := len(myControl.GetHostmap().Indexes) + len(theirControl.GetHostmap().Indexes) + len(relayControl.GetHostmap().Indexes)
+	hostInfos := myControl.GetHostmapIndexCount() + theirControl.GetHostmapIndexCount() + relayControl.GetHostmapIndexCount()
 	retries := 60
 	for hostInfos > 6 && retries > 0 {
-		hostInfos = len(myControl.GetHostmap().Indexes) + len(theirControl.GetHostmap().Indexes) + len(relayControl.GetHostmap().Indexes)
+		hostInfos = myControl.GetHostmapIndexCount() + theirControl.GetHostmapIndexCount() + relayControl.GetHostmapIndexCount()
 		t.Logf("Waiting for hostinfos to be removed... myControl=%d theirControl=%d relayControl=%d",
-			len(myControl.GetHostmap().Indexes),
-			len(theirControl.GetHostmap().Indexes),
-			len(relayControl.GetHostmap().Indexes),
+			myControl.GetHostmapIndexCount(),
+			theirControl.GetHostmapIndexCount(),
+			relayControl.GetHostmapIndexCount(),
 		)
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 		t.Log("Connection manager hasn't ticked yet")
@@ -924,24 +928,24 @@ func TestRehandshakingRelays(t *testing.T) {
 	assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 	r.RenderHostmaps("working hostmaps", myControl, relayControl, theirControl)
 	// We should have two hostinfos on all sides
-	for len(myControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for myControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(myControl.GetHostmap().Indexes))
+	for myControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for myControl hostinfos (%v != 2) to get cleaned up from lack of use...", myControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
 		time.Sleep(time.Second)
 	}
 	t.Logf("myControl hostinfos got cleaned up!")
-	for len(theirControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for theirControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(theirControl.GetHostmap().Indexes))
+	for theirControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for theirControl hostinfos (%v != 2) to get cleaned up from lack of use...", theirControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
 		time.Sleep(time.Second)
 	}
 	t.Logf("theirControl hostinfos got cleaned up!")
-	for len(relayControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for relayControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(relayControl.GetHostmap().Indexes))
+	for relayControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for relayControl hostinfos (%v != 2) to get cleaned up from lack of use...", relayControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
@@ -1029,24 +1033,24 @@ func TestRehandshakingRelaysPrimary(t *testing.T) {
 	assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 	r.RenderHostmaps("working hostmaps", myControl, relayControl, theirControl)
 	// We should have two hostinfos on all sides
-	for len(myControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for myControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(myControl.GetHostmap().Indexes))
+	for myControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for myControl hostinfos (%v != 2) to get cleaned up from lack of use...", myControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
 		time.Sleep(time.Second)
 	}
 	t.Logf("myControl hostinfos got cleaned up!")
-	for len(theirControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for theirControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(theirControl.GetHostmap().Indexes))
+	for theirControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for theirControl hostinfos (%v != 2) to get cleaned up from lack of use...", theirControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
 		time.Sleep(time.Second)
 	}
 	t.Logf("theirControl hostinfos got cleaned up!")
-	for len(relayControl.GetHostmap().Indexes) != 2 {
-		t.Logf("Waiting for relayControl hostinfos (%v != 2) to get cleaned up from lack of use...", len(relayControl.GetHostmap().Indexes))
+	for relayControl.GetHostmapIndexCount() != 2 {
+		t.Logf("Waiting for relayControl hostinfos (%v != 2) to get cleaned up from lack of use...", relayControl.GetHostmapIndexCount())
 		r.Log("Assert the relay tunnel still works")
 		assertTunnel(t, theirVpnIpNet[0].Addr(), myVpnIpNet[0].Addr(), theirControl, myControl, r)
 		r.Log("yupitdoes")
@@ -1123,7 +1127,7 @@ func TestRehandshaking(t *testing.T) {
 	theirConfig.ReloadConfigString(string(rc))
 
 	r.Log("Spin until there is only 1 tunnel")
-	for len(myControl.GetHostmap().Indexes)+len(theirControl.GetHostmap().Indexes) > 2 {
+	for myControl.GetHostmapIndexCount()+theirControl.GetHostmapIndexCount() > 2 {
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 		t.Log("Connection manager hasn't ticked yet")
 		time.Sleep(time.Second)
@@ -1223,7 +1227,7 @@ func TestRehandshakingLoser(t *testing.T) {
 	myConfig.ReloadConfigString(string(rc))
 
 	r.Log("Spin until there is only 1 tunnel")
-	for len(myControl.GetHostmap().Indexes)+len(theirControl.GetHostmap().Indexes) > 2 {
+	for myControl.GetHostmapIndexCount()+theirControl.GetHostmapIndexCount() > 2 {
 		assertTunnel(t, myVpnIpNet[0].Addr(), theirVpnIpNet[0].Addr(), myControl, theirControl, r)
 		t.Log("Connection manager hasn't ticked yet")
 		time.Sleep(time.Second)

e2e/tunnels_test.go

@@ -43,8 +43,8 @@ func TestDropInactiveTunnels(t *testing.T) {
 	r.Log("Go inactive and wait for the tunnels to get dropped")
 	waitStart := time.Now()
 	for {
-		myIndexes := len(myControl.GetHostmap().Indexes)
-		theirIndexes := len(theirControl.GetHostmap().Indexes)
+		myIndexes := myControl.GetHostmapIndexCount()
+		theirIndexes := theirControl.GetHostmapIndexCount()
 		if myIndexes == 0 && theirIndexes == 0 {
 			break
 		}
@@ -493,8 +493,8 @@ func TestCloseTunnelAuthenticated(t *testing.T) {
 
 	waitStart := time.Now()
 	for {
-		myIndexes := len(myControl.GetHostmap().Indexes)
-		theirIndexes := len(theirControl.GetHostmap().Indexes)
+		myIndexes := myControl.GetHostmapIndexCount()
+		theirIndexes := theirControl.GetHostmapIndexCount()
 		if myIndexes == 0 && theirIndexes == 0 {
 			break
 		}
@@ -548,8 +548,8 @@ func TestCloseTunnelAuthenticated(t *testing.T) {
 	r.Log("Injected bogus close tunnel. Let's see!")
 	waitStart = time.Now()
 	for {
-		myIndexes := len(myControl.GetHostmap().Indexes)
-		theirIndexes := len(theirControl.GetHostmap().Indexes)
+		myIndexes := myControl.GetHostmapIndexCount()
+		theirIndexes := theirControl.GetHostmapIndexCount()
 		if myIndexes == 0 {
 			t.Fatal("myIndexes should not be 0")
 		}

examples/config.yml

@@ -110,6 +110,15 @@ lighthouse:
     #- "1.1.1.1:4242"
     #- "1.2.3.4:0" # port will be replaced with the real listening port
 
+  # Locally discovered addresses are checked against the MTU of the link they were found on. If the link cannot fit
+  # a full-size packet from the nebula tun device (`tun.mtu` plus encapsulation overhead, which is larger for relayed
+  # traffic) without fragmenting, a warning is logged.
+  # When omit_low_mtu_addrs is true, addresses whose links cannot fit normal nebula traffic are dropped from
+  # lighthouse reports entirely.
+  # Addresses that can fit normal nebula traffic but not relayed traffic are always still advertised.
+  # This does not apply to addresses listed in advertise_addrs.
+  #omit_low_mtu_addrs: false
+
   # EXPERIMENTAL: This option may change or disappear in the future.
   # This setting allows us to "guess" what the remote might be for a host
   # while we wait for the lighthouse response.

handshake_manager.go

@@ -430,14 +430,11 @@ func (hm *HandshakeManager) CheckAndComplete(hostinfo *HostInfo, handshakePacket
 	// Check if we already have a tunnel with this vpn ip
 	existingHostInfo, found := hm.mainHostMap.Hosts[hostinfo.vpnAddrs[0]]
 	if found && existingHostInfo != nil {
-		testHostInfo := existingHostInfo
-		for testHostInfo != nil {
-			// Is it just a delayed handshake packet?
+		// Is it just a delayed handshake packet? Check every hostinfo we hold for this address.
+		for _, testHostInfo := range hm.mainHostMap.unlockedGetHostList(hostinfo.vpnAddrs[0]) {
 			if bytes.Equal(hostinfo.HandshakePacket[handshakePacket], testHostInfo.HandshakePacket[handshakePacket]) {
 				return testHostInfo, ErrAlreadySeen
 			}
-
-			testHostInfo = testHostInfo.next
 		}
 
 		// Is this a newer handshake?

hostmap.go

@@ -56,11 +56,20 @@ type Relay struct {
 }
 
 type HostMap struct {
-	sync.RWMutex    //Because we concurrently read and write to our maps
-	Indexes         map[uint32]*HostInfo
-	Relays          map[uint32]*HostInfo // Maps a Relay IDX to a Relay HostInfo object
-	RemoteIndexes   map[uint32]*HostInfo
+	sync.RWMutex  //Because we concurrently read and write to our maps
+	Indexes       map[uint32]*HostInfo
+	Relays        map[uint32]*HostInfo // Maps a Relay IDX to a Relay HostInfo object
+	RemoteIndexes map[uint32]*HostInfo
+	// Hosts maps a vpn address to its primary hostinfo, one entry per address we hold a tunnel
+	// for. moreHosts only has an entry while an address is held by 2 or more hostinfos and stores
+	// the full most-recent-first list; moreHosts[a][0] is always the same hostinfo as Hosts[a].
+	// Each address gets its own independent list, so a hostinfo owning multiple addresses can
+	// never corrupt another address's ordering the way the old shared next/prev chain could.
+	// Entries in moreHosts are only ever written by unlockedSetHostsForAddr; Hosts is written
+	// directly only in the single-hostinfo fast paths where moreHosts is known to have no entry,
+	// and unlockedDeleteHostInfo swaps either map for a fresh one when it fully drains.
 	Hosts           map[netip.Addr]*HostInfo
+	moreHosts       map[netip.Addr][]*HostInfo
 	preferredRanges atomic.Pointer[[]netip.Prefix]
 	l               *slog.Logger
 }
@@ -266,10 +275,6 @@ type HostInfo struct {
 	lastRoam       time.Time
 	lastRoamRemote netip.AddrPort
 
-	// Used to track other hostinfos for this vpn ip since only 1 can be primary
-	// Synchronised via hostmap lock and not the hostinfo lock.
-	next, prev *HostInfo
-
 	//TODO: in, out, and others might benefit from being an atomic.Int32. We could collapse connectionManager pendingDeletion, relayUsed, and in/out into this 1 thing
 	in, out, pendingDeletion atomic.Bool
 
@@ -334,6 +339,7 @@ func newHostMap(l *slog.Logger) *HostMap {
 		Relays:        map[uint32]*HostInfo{},
 		RemoteIndexes: map[uint32]*HostInfo{},
 		Hosts:         map[netip.Addr]*HostInfo{},
+		moreHosts:     map[netip.Addr][]*HostInfo{},
 		l:             l,
 	}
 }
@@ -382,13 +388,55 @@ func (hm *HostMap) EmitStats() {
 	metrics.GetOrRegisterGauge("hostmap.main.relayIndexes", nil).Update(int64(relaysLen))
 }
 
-// DeleteHostInfo will fully unlink the hostinfo and return true if it was the final hostinfo for this vpn ip
+// unlockedSetHostsForAddr stores the per-address hostinfo list (list[0] is the primary). An empty
+// list removes the address. This is the one place Hosts and moreHosts are written together, keep
+// it that way. Callers must hold the write lock.
+func (hm *HostMap) unlockedSetHostsForAddr(addr netip.Addr, list []*HostInfo) {
+	if len(list) == 0 {
+		delete(hm.Hosts, addr)
+		delete(hm.moreHosts, addr)
+		return
+	}
+	hm.Hosts[addr] = list[0]
+	if len(list) > 1 {
+		hm.moreHosts[addr] = list
+	} else {
+		delete(hm.moreHosts, addr)
+	}
+}
+
+// unlockedGetHostList returns every hostinfo holding addr, primary first, or nil if we have no
+// tunnel for addr. The common single-hostinfo case builds a fresh one element list, so keep this
+// off the packet hot path; the primary is a direct Hosts read. Callers must hold the lock (read
+// or write).
+func (hm *HostMap) unlockedGetHostList(addr netip.Addr) []*HostInfo {
+	if list, ok := hm.moreHosts[addr]; ok {
+		return list
+	}
+	if h, ok := hm.Hosts[addr]; ok {
+		return []*HostInfo{h}
+	}
+	return nil
+}
+
+// removeHostInfo returns list with hi removed (order preserved), or list unchanged if hi is
+// absent. It deletes in place: every mutator holds the hostmap write lock and no reader ever
+// retains a slice across a mutation (readers iterate under RLock), so there is no snapshot to
+// invalidate.
+func removeHostInfo(list []*HostInfo, hi *HostInfo) []*HostInfo {
+	idx := slices.Index(list, hi)
+	if idx < 0 {
+		return list
+	}
+	return slices.Delete(list, idx, idx+1)
+}
+
+// DeleteHostInfo will fully unlink the hostinfo and return true if no other hostinfo still holds
+// any of its vpn addrs, meaning we no longer have a tunnel to the peer
 func (hm *HostMap) DeleteHostInfo(hostinfo *HostInfo) bool {
 	// Delete the host itself, ensuring it's not modified anymore
 	hm.Lock()
-	// If we have a previous or next hostinfo then we are not the last one for this vpn ip
-	final := (hostinfo.next == nil && hostinfo.prev == nil)
-	hm.unlockedDeleteHostInfo(hostinfo)
+	final := hm.unlockedDeleteHostInfo(hostinfo)
 	hm.Unlock()
 
 	return final
@@ -400,71 +448,66 @@ func (hm *HostMap) MakePrimary(hostinfo *HostInfo) {
 	hm.unlockedMakePrimary(hostinfo)
 }
 
-func (hm *HostMap) unlockedMakePrimary(hostinfo *HostInfo) {
-	// Get the current primary, if it exists
-	oldHostinfo := hm.Hosts[hostinfo.vpnAddrs[0]]
-
-	// Every address in the hostinfo gets elevated to primary
-	for _, vpnAddr := range hostinfo.vpnAddrs {
-		//NOTE: It is possible that we leave a dangling hostinfo here but connection manager works on
-		// indexes so it should be fine.
-		hm.Hosts[vpnAddr] = hostinfo
+// unlockedMakePrimary reports whether hostinfo is (now) the primary for each of its addresses,
+// false only when it is no longer in the hostmap at all.
+func (hm *HostMap) unlockedMakePrimary(hostinfo *HostInfo) bool {
+	// A hostinfo that is no longer in the hostmap must not be re-inserted here. Callers can race
+	// tunnel teardown, deciding to promote under the read lock and only taking the write lock
+	// after a delete fully unlinked the hostinfo (connection manager swapPrimary, AddRelay). Every
+	// live hostinfo is registered in Indexes by unlockedAddHostInfo, so this is a membership test.
+	if hm.Indexes[hostinfo.localIndexId] != hostinfo {
+		return false
 	}
 
-	// If we are already primary then we won't bother re-linking
-	if oldHostinfo == hostinfo {
-		return
-	}
-
-	// Unlink this hostinfo
-	if hostinfo.prev != nil {
-		hostinfo.prev.next = hostinfo.next
-	}
-	if hostinfo.next != nil {
-		hostinfo.next.prev = hostinfo.prev
-	}
-
-	// If there wasn't a previous primary then clear out any links
-	if oldHostinfo == nil {
-		hostinfo.next = nil
-		hostinfo.prev = nil
-		return
-	}
-
-	// Relink the hostinfo as primary
-	hostinfo.next = oldHostinfo
-	oldHostinfo.prev = hostinfo
-	hostinfo.prev = nil
-}
-
-func (hm *HostMap) unlockedDeleteHostInfo(hostinfo *HostInfo) {
-	isLastHostinfo := hostinfo.next == nil && hostinfo.prev == nil
-
+	// Move hostinfo to the front (primary) of each of its address lists. The lists are
+	// independent per address, so this can never leave a dangling entry the way promoting
+	// against a single shared chain could.
 	for _, addr := range hostinfo.vpnAddrs {
-		if hm.Hosts[addr] != hostinfo {
+		if hm.Hosts[addr] == hostinfo {
+			// Already primary for this address, the list is already in the right order
 			continue
 		}
-		if hostinfo.next != nil {
-			// Promote the next hostinfo in the shared chain to primary for this address
-			hm.Hosts[addr] = hostinfo.next
-		} else {
-			delete(hm.Hosts, addr)
+		list := removeHostInfo(hm.unlockedGetHostList(addr), hostinfo)
+		list = append([]*HostInfo{hostinfo}, list...)
+		hm.unlockedSetHostsForAddr(addr, list)
+	}
+	return true
+}
+
+// unlockedDeleteHostInfo removes hostinfo from every one of its address lists and from the index
+// maps. It returns true if this was the last hostinfo for all of its addresses (we no longer have
+// any tunnel to the peer), which the caller uses to decide whether to clear learned lighthouse
+// state and disestablish relays.
+func (hm *HostMap) unlockedDeleteHostInfo(hostinfo *HostInfo) bool {
+	// Remove this hostinfo from each of its address lists. The lists are independent, so a
+	// sibling is never promoted to an address it does not own and no other list is touched.
+	final := true
+	for _, addr := range hostinfo.vpnAddrs {
+		if list, ok := hm.moreHosts[addr]; ok {
+			list = removeHostInfo(list, hostinfo)
+			hm.unlockedSetHostsForAddr(addr, list)
+			if len(list) > 0 {
+				final = false
+			}
+		} else if existing, ok := hm.Hosts[addr]; ok {
+			if existing == hostinfo {
+				// Common case, the only hostinfo for this address. moreHosts has no entry to clean up.
+				delete(hm.Hosts, addr)
+			} else {
+				// We don't hold this address but another hostinfo does, we still have a tunnel to the peer
+				final = false
+			}
 		}
 	}
+
+	// Go maps never shrink their buckets, replace fully drained maps so a node that churned
+	// through a large peer count gives the memory back. Same idiom as the index maps below.
 	if len(hm.Hosts) == 0 {
 		hm.Hosts = map[netip.Addr]*HostInfo{}
 	}
-
-	// Splice this hostinfo out of the shared chain exactly once
-	if hostinfo.prev != nil {
-		hostinfo.prev.next = hostinfo.next
+	if len(hm.moreHosts) == 0 {
+		hm.moreHosts = map[netip.Addr][]*HostInfo{}
 	}
-	if hostinfo.next != nil {
-		hostinfo.next.prev = hostinfo.prev
-	}
-
-	hostinfo.next = nil
-	hostinfo.prev = nil
 
 	// The remote index uses index ids outside our control so lets make sure we are only removing
 	// the remote index pointer here if it points to the hostinfo we are deleting
@@ -488,7 +531,7 @@ func (hm *HostMap) unlockedDeleteHostInfo(hostinfo *HostInfo) {
 		)
 	}
 
-	if isLastHostinfo {
+	if final {
 		// I have lost connectivity to my peers. My relay tunnel is likely broken. Mark the next
 		// hops as 'Requested' so that new relay tunnels are created in the future.
 		hm.unlockedDisestablishVpnAddrRelayFor(hostinfo)
@@ -497,6 +540,8 @@ func (hm *HostMap) unlockedDeleteHostInfo(hostinfo *HostInfo) {
 	for _, localRelayIdx := range hostinfo.relayState.CopyRelayForIdxs() {
 		delete(hm.Relays, localRelayIdx)
 	}
+
+	return final
 }
 
 func (hm *HostMap) QueryIndex(index uint32) *HostInfo {
@@ -540,19 +585,30 @@ func (hm *HostMap) QueryVpnAddrsRelayFor(targetIps []netip.Addr, relayHostIp net
 	hm.RLock()
 	defer hm.RUnlock()
 
+	// This runs per relayed packet, so check the primary with a single map probe and only consult
+	// moreHosts when the primary can't relay for us.
 	h, ok := hm.Hosts[relayHostIp]
 	if !ok {
 		return nil, nil, errors.New("unable to find host")
 	}
 
-	for h != nil {
-		for _, targetIp := range targetIps {
-			r, ok := h.relayState.QueryRelayForByIp(targetIp)
-			if ok && r.State == Established {
-				return h, r, nil
+	for _, targetIp := range targetIps {
+		r, ok := h.relayState.QueryRelayForByIp(targetIp)
+		if ok && r.State == Established {
+			return h, r, nil
+		}
+	}
+
+	if list, ok := hm.moreHosts[relayHostIp]; ok {
+		// list[0] is the primary we already checked
+		for _, h := range list[1:] {
+			for _, targetIp := range targetIps {
+				r, ok := h.relayState.QueryRelayForByIp(targetIp)
+				if ok && r.State == Established {
+					return h, r, nil
+				}
 			}
 		}
-		h = h.next
 	}
 
 	return nil, nil, errors.New("unable to find host with relay")
@@ -560,20 +616,14 @@ func (hm *HostMap) QueryVpnAddrsRelayFor(targetIps []netip.Addr, relayHostIp net
 
 func (hm *HostMap) unlockedDisestablishVpnAddrRelayFor(hi *HostInfo) {
 	for _, relayHostIp := range hi.relayState.CopyRelayIps() {
-		if h, ok := hm.Hosts[relayHostIp]; ok {
-			for h != nil {
-				h.relayState.UpdateRelayForByIpState(hi.vpnAddrs[0], Disestablished)
-				h = h.next
-			}
+		for _, h := range hm.unlockedGetHostList(relayHostIp) {
+			h.relayState.UpdateRelayForByIpState(hi.vpnAddrs[0], Disestablished)
 		}
 	}
 	for _, rs := range hi.relayState.CopyAllRelayFor() {
 		if rs.Type == ForwardingType {
-			if h, ok := hm.Hosts[rs.PeerAddr]; ok {
-				for h != nil {
-					h.relayState.UpdateRelayForByIpState(hi.vpnAddrs[0], Disestablished)
-					h = h.next
-				}
+			for _, h := range hm.unlockedGetHostList(rs.PeerAddr) {
+				h.relayState.UpdateRelayForByIpState(hi.vpnAddrs[0], Disestablished)
 			}
 		}
 	}
@@ -623,22 +673,27 @@ func (hm *HostMap) unlockedAddHostInfo(hostinfo *HostInfo, f *Interface) {
 }
 
 func (hm *HostMap) unlockedInnerAddHostInfo(vpnAddr netip.Addr, hostinfo *HostInfo, f *Interface) {
-	existing := hm.Hosts[vpnAddr]
-	hm.Hosts[vpnAddr] = hostinfo
-
-	if existing != nil && existing != hostinfo {
-		hostinfo.next = existing
-		existing.prev = hostinfo
+	existing, ok := hm.Hosts[vpnAddr]
+	if !ok {
+		// Common case, the first hostinfo for this address. moreHosts stays empty.
+		hm.Hosts[vpnAddr] = hostinfo
+		return
 	}
 
-	i := 1
-	check := hostinfo
-	for check != nil {
-		if i > MaxHostInfosPerVpnIp {
-			hm.unlockedDeleteHostInfo(check)
-		}
-		check = check.next
-		i++
+	// The new hostinfo becomes the primary for this address. Remove any stale copy of it first so
+	// we never hold a duplicate, then prepend.
+	list, ok := hm.moreHosts[vpnAddr]
+	if !ok {
+		list = []*HostInfo{existing}
+	}
+	list = removeHostInfo(list, hostinfo)
+	list = append([]*HostInfo{hostinfo}, list...)
+	hm.unlockedSetHostsForAddr(vpnAddr, list)
+
+	// Enforce the per-address cap by fully retiring the oldest hostinfo once we exceed it.
+	// Deleting it removes it from all of its addresses and the index maps, matching prior behavior.
+	if len(list) > MaxHostInfosPerVpnIp {
+		hm.unlockedDeleteHostInfo(list[len(list)-1])
 	}
 }
 
@@ -814,9 +869,17 @@ func (i *HostInfo) logger(l *slog.Logger) *slog.Logger {
 
 // Utility functions
 
-func localAddrs(l *slog.Logger, allowList *LocalAllowList) []netip.Addr {
+// localAddr is a locally discovered address candidate for lighthouse
+// advertisement, along with details about the link it was found on.
+type localAddr struct {
+	addr    netip.Addr
+	ifName  string
+	linkMTU int // MTU reported for the link, or <= 0 if unknown
+}
+
+func localAddrs(l *slog.Logger, allowList *LocalAllowList) []localAddr {
 	//FIXME: This function is pretty garbage
-	var finalAddrs []netip.Addr
+	var finalAddrs []localAddr
 	ifaces, _ := net.Interfaces()
 	for _, i := range ifaces {
 		allow := allowList.AllowName(i.Name)
@@ -861,7 +924,7 @@ func localAddrs(l *slog.Logger, allowList *LocalAllowList) []netip.Addr {
 					continue
 				}
 
-				finalAddrs = append(finalAddrs, addr)
+				finalAddrs = append(finalAddrs, localAddr{addr: addr, ifName: i.Name, linkMTU: i.MTU})
 			}
 		}
 	}

hostmap_test.go

@@ -2,6 +2,7 @@ package nebula
 
 import (
 	"net/netip"
+	"slices"
 	"testing"
 
 	"github.com/slackhq/nebula/config"
@@ -10,78 +11,84 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// chainIds returns the localIndexIds of the hostinfos holding addr, primary (index 0) first. It
+// also validates the Hosts/moreHosts sync contract on every call so a mutation that broke it
+// fails fast.
+func chainIds(t *testing.T, hm *HostMap, addr netip.Addr) []uint32 {
+	t.Helper()
+	assertHostMapInvariants(t, hm)
+	list := hm.unlockedGetHostList(addr)
+	ids := make([]uint32, len(list))
+	for i, h := range list {
+		ids[i] = h.localIndexId
+	}
+	return ids
+}
+
+// assertHostMapInvariants checks the Hosts/moreHosts contract: moreHosts only holds addresses
+// with 2 or more hostinfos, its first entry is always the primary in Hosts, lists never hold
+// duplicates, every hostinfo in a list owns the address and is registered in Indexes, and every
+// indexed hostinfo is reachable through each of its addresses.
+func assertHostMapInvariants(t *testing.T, hm *HostMap) {
+	t.Helper()
+	for addr, list := range hm.moreHosts {
+		require.GreaterOrEqualf(t, len(list), 2, "moreHosts[%s] must hold at least 2 hostinfos", addr)
+		require.Samef(t, hm.Hosts[addr], list[0], "moreHosts[%s][0] must match the primary in Hosts", addr)
+		seen := map[*HostInfo]bool{}
+		for _, h := range list {
+			require.NotNilf(t, h, "moreHosts[%s] must never hold a nil hostinfo", addr)
+			require.Falsef(t, seen[h], "moreHosts[%s] holds hostinfo %d twice", addr, h.localIndexId)
+			seen[h] = true
+			require.Samef(t, hm.Indexes[h.localIndexId], h, "moreHosts[%s] member %d is not registered in Indexes", addr, h.localIndexId)
+			require.Truef(t, slices.Contains(h.vpnAddrs, addr), "moreHosts[%s] member %d does not own the address", addr, h.localIndexId)
+		}
+	}
+	for addr, h := range hm.Hosts {
+		require.NotNilf(t, h, "Hosts[%s] must never be nil", addr)
+		require.Samef(t, hm.Indexes[h.localIndexId], h, "Hosts[%s] primary %d is not registered in Indexes", addr, h.localIndexId)
+		require.Truef(t, slices.Contains(h.vpnAddrs, addr), "Hosts[%s] primary (index %d) does not own the address", addr, h.localIndexId)
+	}
+	for idx, h := range hm.Indexes {
+		require.Equalf(t, idx, h.localIndexId, "Indexes[%d] holds hostinfo with localIndexId %d", idx, h.localIndexId)
+		for _, va := range h.vpnAddrs {
+			require.Truef(t, slices.Contains(hm.unlockedGetHostList(va), h), "indexed hostinfo %d is missing from the list for %s", idx, va)
+		}
+	}
+}
+
 func TestHostMap_MakePrimary(t *testing.T) {
 	l := test.NewLogger()
 	hm := newHostMap(l)
 
 	f := &Interface{}
+	a := netip.MustParseAddr("0.0.0.1")
 
-	h1 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 1}
-	h2 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 2}
-	h3 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 3}
-	h4 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 4}
+	h1 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 1}
+	h2 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 2}
+	h3 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 3}
+	h4 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 4}
 
 	hm.unlockedAddHostInfo(h4, f)
 	hm.unlockedAddHostInfo(h3, f)
 	hm.unlockedAddHostInfo(h2, f)
 	hm.unlockedAddHostInfo(h1, f)
 
-	// Make sure we go h1 -> h2 -> h3 -> h4
-	prim := hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h1.localIndexId, prim.localIndexId)
-	assert.Equal(t, h2.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h1.localIndexId, h2.prev.localIndexId)
-	assert.Equal(t, h3.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h2.localIndexId, h3.prev.localIndexId)
-	assert.Equal(t, h4.localIndexId, h3.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h4.prev.localIndexId)
-	assert.Nil(t, h4.next)
+	// Most-recently-added is primary: h1, h2, h3, h4
+	assert.Equal(t, []uint32{1, 2, 3, 4}, chainIds(t, hm, a))
+	assert.Equal(t, h1, hm.QueryVpnAddr(a))
 
-	// Swap h3/middle to primary
+	// Swap the middle to primary: h3, h1, h2, h4
 	hm.MakePrimary(h3)
+	assert.Equal(t, []uint32{3, 1, 2, 4}, chainIds(t, hm, a))
+	assert.Equal(t, h3, hm.QueryVpnAddr(a))
 
-	// Make sure we go h3 -> h1 -> h2 -> h4
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h3.localIndexId, prim.localIndexId)
-	assert.Equal(t, h1.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h2.localIndexId, h1.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h1.prev.localIndexId)
-	assert.Equal(t, h4.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h1.localIndexId, h2.prev.localIndexId)
-	assert.Equal(t, h2.localIndexId, h4.prev.localIndexId)
-	assert.Nil(t, h4.next)
-
-	// Swap h4/tail to primary
+	// Swap the tail to primary: h4, h3, h1, h2
 	hm.MakePrimary(h4)
+	assert.Equal(t, []uint32{4, 3, 1, 2}, chainIds(t, hm, a))
 
-	// Make sure we go h4 -> h3 -> h1 -> h2
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h4.localIndexId, prim.localIndexId)
-	assert.Equal(t, h3.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h1.localIndexId, h3.next.localIndexId)
-	assert.Equal(t, h4.localIndexId, h3.prev.localIndexId)
-	assert.Equal(t, h2.localIndexId, h1.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h1.prev.localIndexId)
-	assert.Equal(t, h1.localIndexId, h2.prev.localIndexId)
-	assert.Nil(t, h2.next)
-
-	// Swap h4 again should be no-op
+	// Swapping the current primary again is a no-op
 	hm.MakePrimary(h4)
-
-	// Make sure we go h4 -> h3 -> h1 -> h2
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h4.localIndexId, prim.localIndexId)
-	assert.Equal(t, h3.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h1.localIndexId, h3.next.localIndexId)
-	assert.Equal(t, h4.localIndexId, h3.prev.localIndexId)
-	assert.Equal(t, h2.localIndexId, h1.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h1.prev.localIndexId)
-	assert.Equal(t, h1.localIndexId, h2.prev.localIndexId)
-	assert.Nil(t, h2.next)
+	assert.Equal(t, []uint32{4, 3, 1, 2}, chainIds(t, hm, a))
 }
 
 func TestHostMap_DeleteHostInfo(t *testing.T) {
@@ -89,13 +96,14 @@ func TestHostMap_DeleteHostInfo(t *testing.T) {
 	hm := newHostMap(l)
 
 	f := &Interface{}
+	a := netip.MustParseAddr("0.0.0.1")
 
-	h1 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 1}
-	h2 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 2}
-	h3 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 3}
-	h4 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 4}
-	h5 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 5}
-	h6 := &HostInfo{vpnAddrs: []netip.Addr{netip.MustParseAddr("0.0.0.1")}, localIndexId: 6}
+	h1 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 1}
+	h2 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 2}
+	h3 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 3}
+	h4 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 4}
+	h5 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 5}
+	h6 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 6}
 
 	hm.unlockedAddHostInfo(h6, f)
 	hm.unlockedAddHostInfo(h5, f)
@@ -104,94 +112,110 @@ func TestHostMap_DeleteHostInfo(t *testing.T) {
 	hm.unlockedAddHostInfo(h2, f)
 	hm.unlockedAddHostInfo(h1, f)
 
-	// h6 should be deleted
-	assert.Nil(t, h6.next)
-	assert.Nil(t, h6.prev)
-	h := hm.QueryIndex(h6.localIndexId)
-	assert.Nil(t, h)
+	// h6 is evicted by the MaxHostInfosPerVpnIp cap; the rest are newest-first.
+	assert.Nil(t, hm.QueryIndex(h6.localIndexId))
+	assert.Equal(t, []uint32{1, 2, 3, 4, 5}, chainIds(t, hm, a))
 
-	// Make sure we go h1 -> h2 -> h3 -> h4 -> h5
-	prim := hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h1.localIndexId, prim.localIndexId)
-	assert.Equal(t, h2.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h1.localIndexId, h2.prev.localIndexId)
-	assert.Equal(t, h3.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h2.localIndexId, h3.prev.localIndexId)
-	assert.Equal(t, h4.localIndexId, h3.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h4.prev.localIndexId)
-	assert.Equal(t, h5.localIndexId, h4.next.localIndexId)
-	assert.Equal(t, h4.localIndexId, h5.prev.localIndexId)
-	assert.Nil(t, h5.next)
+	// Delete primary; not final since siblings remain.
+	assert.False(t, hm.DeleteHostInfo(h1))
+	assert.Equal(t, []uint32{2, 3, 4, 5}, chainIds(t, hm, a))
 
-	// Delete primary
-	hm.DeleteHostInfo(h1)
-	assert.Nil(t, h1.prev)
-	assert.Nil(t, h1.next)
+	// Deleting the same hostinfo again must not report final while siblings remain and must not
+	// disturb the list. The old chain code got this wrong: the first delete nil'd next/prev, so a
+	// second delete looked final and wiped lighthouse state out from under the live sibling.
+	assert.False(t, hm.DeleteHostInfo(h1))
+	assert.Equal(t, []uint32{2, 3, 4, 5}, chainIds(t, hm, a))
 
-	// Make sure we go h2 -> h3 -> h4 -> h5
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h2.localIndexId, prim.localIndexId)
-	assert.Equal(t, h3.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h3.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h2.localIndexId, h3.prev.localIndexId)
-	assert.Equal(t, h4.localIndexId, h3.next.localIndexId)
-	assert.Equal(t, h3.localIndexId, h4.prev.localIndexId)
-	assert.Equal(t, h5.localIndexId, h4.next.localIndexId)
-	assert.Equal(t, h4.localIndexId, h5.prev.localIndexId)
-	assert.Nil(t, h5.next)
+	// Delete a middle node.
+	assert.False(t, hm.DeleteHostInfo(h3))
+	assert.Equal(t, []uint32{2, 4, 5}, chainIds(t, hm, a))
 
-	// Delete in the middle
-	hm.DeleteHostInfo(h3)
-	assert.Nil(t, h3.prev)
-	assert.Nil(t, h3.next)
+	// Delete the tail.
+	assert.False(t, hm.DeleteHostInfo(h5))
+	assert.Equal(t, []uint32{2, 4}, chainIds(t, hm, a))
 
-	// Make sure we go h2 -> h4 -> h5
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h2.localIndexId, prim.localIndexId)
-	assert.Equal(t, h4.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h4.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h2.localIndexId, h4.prev.localIndexId)
-	assert.Equal(t, h5.localIndexId, h4.next.localIndexId)
-	assert.Equal(t, h4.localIndexId, h5.prev.localIndexId)
-	assert.Nil(t, h5.next)
+	// Delete the head; h4 remains and becomes primary.
+	assert.False(t, hm.DeleteHostInfo(h2))
+	assert.Equal(t, []uint32{4}, chainIds(t, hm, a))
+	assert.Equal(t, h4, hm.QueryVpnAddr(a))
 
-	// Delete the tail
-	hm.DeleteHostInfo(h5)
-	assert.Nil(t, h5.prev)
-	assert.Nil(t, h5.next)
+	// Delete the only remaining item; final is true and the address is gone.
+	assert.True(t, hm.DeleteHostInfo(h4))
+	assert.Empty(t, chainIds(t, hm, a))
+	assert.Nil(t, hm.QueryVpnAddr(a))
 
-	// Make sure we go h2 -> h4
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h2.localIndexId, prim.localIndexId)
-	assert.Equal(t, h4.localIndexId, prim.next.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Equal(t, h4.localIndexId, h2.next.localIndexId)
-	assert.Equal(t, h2.localIndexId, h4.prev.localIndexId)
-	assert.Nil(t, h4.next)
+	// Deleting an already-gone hostinfo is still final; nothing holds the address anymore.
+	assert.True(t, hm.DeleteHostInfo(h4))
+	assert.Empty(t, chainIds(t, hm, a))
+}
 
-	// Delete the head
-	hm.DeleteHostInfo(h2)
-	assert.Nil(t, h2.prev)
-	assert.Nil(t, h2.next)
+// TestHostMap_MakePrimary_DeletedHostInfo covers promoting a hostinfo that lost a race with
+// tunnel teardown: swapPrimary and AddRelay decide to promote while holding a stale pointer and
+// only take the write lock after a delete fully unlinked the hostinfo. MakePrimary must be a
+// no-op, not a resurrection that installs an unmanaged primary.
+func TestHostMap_MakePrimary_DeletedHostInfo(t *testing.T) {
+	l := test.NewLogger()
+	hm := newHostMap(l)
+	f := &Interface{}
+	a := netip.MustParseAddr("0.0.0.1")
 
-	// Make sure we only have h4
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Equal(t, h4.localIndexId, prim.localIndexId)
-	assert.Nil(t, prim.prev)
-	assert.Nil(t, prim.next)
-	assert.Nil(t, h4.next)
+	h1 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 1}
+	h2 := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 2}
+	hm.unlockedAddHostInfo(h1, f)
+	hm.unlockedAddHostInfo(h2, f)
 
-	// Delete the only item
-	hm.DeleteHostInfo(h4)
-	assert.Nil(t, h4.prev)
-	assert.Nil(t, h4.next)
+	// h1 is fully deleted while another goroutine still holds a pointer to it.
+	assert.False(t, hm.DeleteHostInfo(h1))
+	assert.Equal(t, []uint32{2}, chainIds(t, hm, a))
 
-	// Make sure we have nil
-	prim = hm.QueryVpnAddr(netip.MustParseAddr("0.0.0.1"))
-	assert.Nil(t, prim)
+	// The stale promote must not bring it back.
+	hm.MakePrimary(h1)
+	assert.Equal(t, []uint32{2}, chainIds(t, hm, a))
+	assert.Equal(t, h2, hm.QueryVpnAddr(a))
+	assert.Nil(t, hm.QueryIndex(h1.localIndexId))
+}
+
+// TestHostMap_QueryVpnAddrsRelayFor_NonPrimary makes sure a relay established on an older
+// hostinfo is still found after a newer tunnel without relay state takes primary for the same
+// address. The lookup checks the primary first and falls back to the rest of the list.
+func TestHostMap_QueryVpnAddrsRelayFor_NonPrimary(t *testing.T) {
+	l := test.NewLogger()
+	hm := newHostMap(l)
+	f := &Interface{}
+	relayAddr := netip.MustParseAddr("0.0.0.9")
+	target := netip.MustParseAddr("0.0.0.1")
+
+	older := &HostInfo{
+		vpnAddrs:     []netip.Addr{relayAddr},
+		localIndexId: 1,
+		relayState: RelayState{
+			relayForByAddr: map[netip.Addr]*Relay{},
+			relayForByIdx:  map[uint32]*Relay{},
+		},
+	}
+	older.relayState.InsertRelay(target, 100, &Relay{Type: ForwardingType, State: Established, LocalIndex: 100, PeerAddr: target})
+	hm.unlockedAddHostInfo(older, f)
+
+	// The relay is found on the primary.
+	h, r, err := hm.QueryVpnAddrsRelayFor([]netip.Addr{target}, relayAddr)
+	require.NoError(t, err)
+	assert.Equal(t, older, h)
+	assert.Equal(t, uint32(100), r.LocalIndex)
+
+	// A re-handshake with no relay state takes primary; the established relay on the older
+	// hostinfo must still be found through the fallback.
+	newer := &HostInfo{vpnAddrs: []netip.Addr{relayAddr}, localIndexId: 2}
+	hm.unlockedAddHostInfo(newer, f)
+	assert.Equal(t, []uint32{2, 1}, chainIds(t, hm, relayAddr))
+
+	h, r, err = hm.QueryVpnAddrsRelayFor([]netip.Addr{target}, relayAddr)
+	require.NoError(t, err)
+	assert.Equal(t, older, h)
+	assert.Equal(t, uint32(100), r.LocalIndex)
+
+	// No hostinfo at all is a plain miss.
+	_, _, err = hm.QueryVpnAddrsRelayFor([]netip.Addr{target}, netip.MustParseAddr("0.0.0.42"))
+	require.Error(t, err)
 }
 
 // TestHostMap_DeleteHostInfo_MultipleVpnAddrs exercises the case where a hostinfo carries more than one
@@ -216,32 +240,82 @@ func TestHostMap_DeleteHostInfo_MultipleVpnAddrs(t *testing.T) {
 	hm.unlockedAddHostInfo(other, f)
 	hm.unlockedAddHostInfo(head, f)
 
-	// head is primary for both addresses, other is next in the shared chain
-	assert.Equal(t, head.localIndexId, hm.QueryVpnAddr(a).localIndexId)
-	assert.Equal(t, head.localIndexId, hm.QueryVpnAddr(b).localIndexId)
-	assert.Equal(t, other.localIndexId, head.next.localIndexId)
-	assert.Equal(t, head.localIndexId, other.prev.localIndexId)
+	// head is primary for both addresses, other is next in each address's list.
+	assert.Equal(t, head, hm.QueryVpnAddr(a))
+	assert.Equal(t, head, hm.QueryVpnAddr(b))
+	assert.Equal(t, []uint32{2, 1}, chainIds(t, hm, a))
+	assert.Equal(t, []uint32{2, 1}, chainIds(t, hm, b))
 
 	// Delete the head. other is still live, so it must become primary for BOTH addresses.
-	hm.DeleteHostInfo(head)
+	assert.False(t, hm.DeleteHostInfo(head))
+	assert.Equal(t, other, hm.QueryVpnAddr(a))
+	assert.Equal(t, other, hm.QueryVpnAddr(b))
+	assert.Equal(t, []uint32{1}, chainIds(t, hm, a))
+	assert.Equal(t, []uint32{1}, chainIds(t, hm, b))
 
-	// Pre-fix: QueryVpnAddr(b) came back nil here because the second address was deleted rather than
-	// promoted, leaving other unreachable at b.
-	require.NotNil(t, hm.QueryVpnAddr(a))
-	require.NotNil(t, hm.QueryVpnAddr(b))
-	assert.Equal(t, other.localIndexId, hm.QueryVpnAddr(a).localIndexId)
-	assert.Equal(t, other.localIndexId, hm.QueryVpnAddr(b).localIndexId)
-
-	// other is now the only hostinfo in the chain
-	assert.Nil(t, other.prev)
-	assert.Nil(t, other.next)
-
-	// head is fully detached
-	assert.Nil(t, head.prev)
-	assert.Nil(t, head.next)
+	// head is fully removed from the index map.
 	assert.Nil(t, hm.QueryIndex(head.localIndexId))
 }
 
+// TestHostMap_DeleteHostInfo_DivergentVpnAddrs covers chained hostinfos for the same peer whose
+// vpnAddrs sets differ (a re-handshake cert added a second address). Deleting the superset node
+// must not promote a sibling to an address it does not own.
+func TestHostMap_DeleteHostInfo_DivergentVpnAddrs(t *testing.T) {
+	l := test.NewLogger()
+	hm := newHostMap(l)
+	f := &Interface{}
+	a := netip.MustParseAddr("0.0.0.1")
+	b := netip.MustParseAddr("0.0.0.2")
+
+	// sub owns only a; super (a newer handshake) owns a and b.
+	sub := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 1}
+	super := &HostInfo{vpnAddrs: []netip.Addr{a, b}, localIndexId: 2}
+	hm.unlockedAddHostInfo(sub, f)
+	hm.unlockedAddHostInfo(super, f)
+
+	assert.Equal(t, []uint32{2, 1}, chainIds(t, hm, a))
+	assert.Equal(t, []uint32{2}, chainIds(t, hm, b))
+
+	// Delete super: a promotes to sub (which owns it); b has no remaining owner and must be
+	// removed, not dangled at sub (which does not own b).
+	assert.False(t, hm.DeleteHostInfo(super))
+	assert.Equal(t, []uint32{1}, chainIds(t, hm, a))
+	assert.Empty(t, chainIds(t, hm, b))
+	assert.Equal(t, sub, hm.QueryVpnAddr(a))
+	assert.Nil(t, hm.QueryVpnAddr(b))
+	assert.Nil(t, hm.QueryIndex(super.localIndexId))
+
+	// Deleting sub cleans up fully.
+	assert.True(t, hm.DeleteHostInfo(sub))
+	assert.Nil(t, hm.QueryVpnAddr(a))
+	assertHostMapInvariants(t, hm)
+}
+
+// TestHostMap_AddDivergentOverlap covers a new hostinfo claiming addresses currently owned by two
+// DIFFERENT hostinfos. The old single shared next/prev chain overwrote a pointer and orphaned one
+// of them (in Indexes but unreachable via its address); independent per-address lists cannot.
+func TestHostMap_AddDivergentOverlap(t *testing.T) {
+	l := test.NewLogger()
+	hm := newHostMap(l)
+	f := &Interface{}
+	a := netip.MustParseAddr("0.0.0.1")
+	b := netip.MustParseAddr("0.0.0.2")
+
+	hiA := &HostInfo{vpnAddrs: []netip.Addr{a}, localIndexId: 1}
+	hiP := &HostInfo{vpnAddrs: []netip.Addr{b}, localIndexId: 2}
+	hm.unlockedAddHostInfo(hiA, f)
+	hm.unlockedAddHostInfo(hiP, f)
+
+	hiB := &HostInfo{vpnAddrs: []netip.Addr{a, b}, localIndexId: 3}
+	hm.unlockedAddHostInfo(hiB, f)
+
+	assert.Equal(t, []uint32{3, 1}, chainIds(t, hm, a))
+	assert.Equal(t, []uint32{3, 2}, chainIds(t, hm, b))
+	// hiA is still reachable via its address (not orphaned) and still indexed.
+	assert.Contains(t, chainIds(t, hm, a), hiA.localIndexId)
+	assert.NotNil(t, hm.QueryIndex(hiA.localIndexId))
+}
+
 // TestHostMap_MaxHostInfosPerVpnIp_MultipleVpnAddrs verifies the MaxHostInfosPerVpnIp overflow prune
 // (unlockedInnerAddHostInfo calls unlockedDeleteHostInfo on the oldest node once the chain is too long)
 // still behaves when hostinfos carry more than one vpnAddr. The pruned node is always the tail, so it is
@@ -267,32 +341,14 @@ func TestHostMap_MaxHostInfosPerVpnIp_MultipleVpnAddrs(t *testing.T) {
 
 	oldest := hostinfos[len(hostinfos)-1]
 
-	// The oldest hostinfo should have been pruned and fully detached
-	assert.Nil(t, oldest.next)
-	assert.Nil(t, oldest.prev)
+	// The oldest hostinfo was pruned from both lists and the index map.
 	assert.Nil(t, hm.QueryIndex(oldest.localIndexId))
 
-	// Both addresses resolve to the same head, and that head is one of the survivors (not the pruned one)
-	primA := hm.QueryVpnAddr(a)
-	primB := hm.QueryVpnAddr(b)
-	require.NotNil(t, primA)
-	require.NotNil(t, primB)
-	assert.Equal(t, primA.localIndexId, primB.localIndexId)
-	assert.NotEqual(t, oldest.localIndexId, primA.localIndexId)
-
-	// Walk the shared chain: exactly MaxHostInfosPerVpnIp survivors, no cycles, oldest absent
-	seen := map[uint32]struct{}{}
-	for h := primA; h != nil; h = h.next {
-		_, dup := seen[h.localIndexId]
-		require.False(t, dup, "cycle detected in hostinfo chain")
-		seen[h.localIndexId] = struct{}{}
-		if h.next != nil {
-			assert.Equal(t, h.localIndexId, h.next.prev.localIndexId, "prev pointer must mirror next")
-		}
-	}
-	assert.Len(t, seen, MaxHostInfosPerVpnIp)
-	_, prunedStillPresent := seen[oldest.localIndexId]
-	assert.False(t, prunedStillPresent)
+	// Both addresses hold exactly MaxHostInfosPerVpnIp survivors in the same order; oldest is absent.
+	require.Len(t, chainIds(t, hm, a), MaxHostInfosPerVpnIp)
+	assert.Equal(t, chainIds(t, hm, a), chainIds(t, hm, b), "both addresses must list the same survivors in the same order")
+	assert.NotContains(t, chainIds(t, hm, a), oldest.localIndexId)
+	assert.Equal(t, hm.QueryVpnAddr(a), hm.QueryVpnAddr(b))
 }
 
 func TestHostMap_reload(t *testing.T) {

interface.go

@@ -215,6 +215,9 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 
 	ifce.connectionManager.intf = ifce
 
+	// Held until Close so waiting on the interface blocks until the resources are actually released
+	ifce.wg.Add(1)
+
 	return ifce, nil
 }
 
@@ -258,17 +261,16 @@ func (f *Interface) activate() error {
 		f.readers[i] = reader
 	}
 
-	f.wg.Add(1) // for us to wait on Close() to return
+	// On error the caller owns the cleanup, Control.Start cancels the service context
+	// before releasing our resources so a waiter never observes a live context
 	if err = f.inside.Activate(); err != nil {
-		f.wg.Done()
-		f.inside.Close()
 		return err
 	}
 
 	return nil
 }
 
-func (f *Interface) run() (func() error, error) {
+func (f *Interface) run() {
 	// Launch n queues to read packets from udp
 	for i := 0; i < f.routines; i++ {
 		f.wg.Go(func() {
@@ -283,13 +285,14 @@ func (f *Interface) run() (func() error, error) {
 		})
 	}
 
-	return func() error {
-		f.wg.Wait()
-		if e := f.fatalErr.Load(); e != nil {
-			return *e
-		}
-		return nil
-	}, nil
+}
+
+func (f *Interface) wait() error {
+	f.wg.Wait()
+	if e := f.fatalErr.Load(); e != nil {
+		return *e
+	}
+	return nil
 }
 
 // onFatal stores the first fatal reader error, and calls triggerShutdown if it was the first one
@@ -322,7 +325,10 @@ func (f *Interface) listenOut(i int) {
 		f.readOutsidePackets(ViaSender{UdpAddr: fromUdpAddr}, plaintext[:0], payload, h, fwPacket, lhh, nb, i, ctCache.Get())
 	})
 
-	if err != nil && !f.closed.Load() {
+	// An error after teardown began is shutdown noise, the closed flag covers resources
+	// Close releases itself and the cancelled ctx covers ones torn down by their owners
+	// reacting to it, like the user device pipes
+	if err != nil && !f.closed.Load() && f.ctx.Err() == nil {
 		f.l.Error("Error while reading inbound packet, closing", "error", err)
 		f.onFatal(err)
 	}
@@ -341,7 +347,8 @@ func (f *Interface) listenIn(reader io.ReadWriteCloser, i int) {
 	for {
 		n, err := reader.Read(packet)
 		if err != nil {
-			if !f.closed.Load() {
+			// Same shutdown noise handling as listenOut
+			if !f.closed.Load() && f.ctx.Err() == nil {
 				f.l.Error("Error while reading outbound packet, closing", "error", err, "reader", i)
 				f.onFatal(err)
 			}
@@ -542,9 +549,15 @@ func (f *Interface) GetCertState() *CertState {
 	return f.pki.getCertState()
 }
 
+// Close releases the interface's resources: the udp sockets and the tun device.
+// It is idempotent and safe to call at any point in the lifecycle, including on an interface that never activated,
+// calls after the first return nil without doing anything.
 func (f *Interface) Close() error {
+	if !f.closed.CompareAndSwap(false, true) {
+		return nil
+	}
+
 	var errs []error
-	f.closed.Store(true)
 
 	// Release the udp readers
 	for i, u := range f.writers {
@@ -560,6 +573,8 @@ func (f *Interface) Close() error {
 	if closeErr != nil {
 		errs = append(errs, closeErr)
 	}
+
+	// Release the construction token so waiters know the resources are gone
 	f.wg.Done()
 	return errors.Join(errs...)
 }

lighthouse.go

@@ -19,8 +19,11 @@ import (
 	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/logging"
+	"github.com/slackhq/nebula/overlay"
 	"github.com/slackhq/nebula/udp"
 	"github.com/slackhq/nebula/util"
+	"golang.org/x/net/ipv4"
+	"golang.org/x/net/ipv6"
 )
 
 var ErrHostNotKnown = errors.New("host not known")
@@ -65,6 +68,18 @@ type LightHouse struct {
 
 	advertiseAddrs atomic.Pointer[[]netip.AddrPort]
 
+	// tunMTU mirrors tun.mtu so locally discovered addrs can be checked for
+	// links too small to carry a full-size nebula packet without fragmenting.
+	tunMTU atomic.Int64
+	// omitLowMTUAddrs drops such addrs from lighthouse updates (and demotes
+	// the associated warnings to debug logs) instead of advertising them.
+	omitLowMTUAddrs atomic.Bool
+	// mtuWarned tracks the last classification logged per local addr so a
+	// warning is only emitted when the classification changes, not on every
+	// periodic update.
+	mtuWarnLock sync.Mutex
+	mtuWarned   map[mtuWarnKey]linkMTUTier
+
 	// Addr's of relays that can be used by peers to access me
 	relaysForMe atomic.Pointer[[]netip.Addr]
 
@@ -105,6 +120,7 @@ func NewLightHouseFromConfig(ctx context.Context, l *slog.Logger, c *config.C, c
 		punchy:             p,
 		updateTrigger:      make(chan struct{}, 1),
 		queryChan:          make(chan netip.Addr, c.GetUint32("handshakes.query_buffer", 64)),
+		mtuWarned:          make(map[mtuWarnKey]linkMTUTier),
 		l:                  l,
 	}
 	lighthouses := make([]netip.Addr, 0)
@@ -216,6 +232,23 @@ func (lh *LightHouse) reload(c *config.C, initial bool) error {
 		}
 	}
 
+	if initial || c.HasChanged("tun.mtu") || c.HasChanged("lighthouse.omit_low_mtu_addrs") {
+		lh.tunMTU.Store(int64(c.GetInt("tun.mtu", overlay.DefaultMTU)))
+		lh.omitLowMTUAddrs.Store(c.GetBool("lighthouse.omit_low_mtu_addrs", false))
+
+		// Re-log any addrs whose links are still too small under the new values
+		lh.mtuWarnLock.Lock()
+		clear(lh.mtuWarned)
+		lh.mtuWarnLock.Unlock()
+
+		if !initial {
+			lh.l.Info("tun.mtu and/or lighthouse.omit_low_mtu_addrs has changed",
+				"tunMTU", lh.tunMTU.Load(),
+				"omitLowMTUAddrs", lh.omitLowMTUAddrs.Load(),
+			)
+		}
+	}
+
 	if initial || c.HasChanged("lighthouse.interval") {
 		lh.interval.Store(int64(c.GetInt("lighthouse.interval", 10)))
 
@@ -905,6 +938,108 @@ func (lh *LightHouse) TriggerUpdate() {
 	}
 }
 
+// linkMTUTier classifies how well a local addr's link MTU can carry
+// full-size nebula packets built from a tun packet of tun.mtu bytes.
+type linkMTUTier uint8
+
+// mtuWarnKey identifies a local addr for MTU warning dedup purposes. The
+// interface name is included because the same addr can exist on multiple
+// links with different MTUs.
+type mtuWarnKey struct {
+	ifName string
+	addr   netip.Addr
+}
+
+const (
+	// The link can carry both normal and relayed nebula traffic
+	linkMTUOk linkMTUTier = iota
+	// The link can carry normal nebula traffic, but relayed traffic (which
+	// adds a second layer of encapsulation) will not fit
+	linkMTUTooSmallForRelay
+	// Even normal nebula traffic will not fit
+	linkMTUTooSmall
+)
+
+const (
+	// Both AES-256-GCM and ChaCha20-Poly1305 append a 16 byte AEAD tag
+	cipherTagLen = 16
+	udpHeaderLen = 8
+)
+
+// requiredLinkMTU returns the minimum underlay link MTU that can carry a
+// full-size tun packet to an addr of the given family without fragmentation,
+// both directly and via a relay (which wraps the packet in a second nebula
+// header and AEAD tag).
+func requiredLinkMTU(tunMTU int, is4 bool) (direct, relayed int) {
+	ipHeaderLen := ipv6.HeaderLen
+	if is4 {
+		ipHeaderLen = ipv4.HeaderLen
+	}
+
+	direct = tunMTU + header.Len + cipherTagLen + udpHeaderLen + ipHeaderLen
+	relayed = direct + header.Len + cipherTagLen
+	return direct, relayed
+}
+
+// checkLocalLinkMTU classifies e's link MTU, logs when the classification
+// changes, and reports whether e should be advertised to lighthouses.
+func (lh *LightHouse) checkLocalLinkMTU(e localAddr) bool {
+	tunMTU := int(lh.tunMTU.Load())
+	omit := lh.omitLowMTUAddrs.Load()
+
+	tier := linkMTUOk
+	direct, relayed := requiredLinkMTU(tunMTU, e.addr.Is4())
+	if e.linkMTU > 0 { // links with an unknown MTU are advertised as-is
+		if e.linkMTU < direct {
+			tier = linkMTUTooSmall
+		} else if e.linkMTU < relayed {
+			tier = linkMTUTooSmallForRelay
+		}
+	}
+	advertise := tier != linkMTUTooSmall || !omit
+
+	key := mtuWarnKey{ifName: e.ifName, addr: e.addr}
+	lh.mtuWarnLock.Lock()
+	changed := lh.mtuWarned[key] != tier
+	if changed {
+		if tier == linkMTUOk {
+			delete(lh.mtuWarned, key)
+		} else {
+			lh.mtuWarned[key] = tier
+		}
+	}
+	lh.mtuWarnLock.Unlock()
+
+	if !changed || tier == linkMTUOk {
+		return advertise
+	}
+
+	level := slog.LevelWarn
+	if omit {
+		level = slog.LevelDebug
+	}
+
+	if lh.l.Enabled(context.Background(), level) {
+		msg := "Link MTU too small for nebula traffic, expect fragmentation or drops"
+		if !advertise {
+			msg = "Omitting addr with too-small link MTU from lighthouse report"
+		} else if tier == linkMTUTooSmallForRelay {
+			msg = "Link MTU too small for relayed nebula traffic"
+		}
+
+		lh.l.Log(context.Background(), level, msg,
+			"localAddr", e.addr,
+			"interface", e.ifName,
+			"linkMTU", e.linkMTU,
+			"requiredMTU", direct,
+			"requiredRelayMTU", relayed,
+			"tunMTU", tunMTU,
+		)
+	}
+
+	return advertise
+}
+
 func (lh *LightHouse) SendUpdate() {
 	var v4 []*V4AddrPort
 	var v6 []*V6AddrPort
@@ -919,15 +1054,19 @@ func (lh *LightHouse) SendUpdate() {
 
 	lal := lh.GetLocalAllowList()
 	for _, e := range localAddrs(lh.l, lal) {
-		if lh.myVpnNetworksTable.Contains(e) {
+		if lh.myVpnNetworksTable.Contains(e.addr) {
+			continue
+		}
+
+		if !lh.checkLocalLinkMTU(e) {
 			continue
 		}
 
 		// Only add addrs that aren't my VPN/tun networks
-		if e.Is4() {
-			v4 = append(v4, netAddrToProtoV4AddrPort(e, uint16(lh.nebulaPort)))
+		if e.addr.Is4() {
+			v4 = append(v4, netAddrToProtoV4AddrPort(e.addr, uint16(lh.nebulaPort)))
 		} else {
-			v6 = append(v6, netAddrToProtoV6AddrPort(e, uint16(lh.nebulaPort)))
+			v6 = append(v6, netAddrToProtoV6AddrPort(e.addr, uint16(lh.nebulaPort)))
 		}
 	}
 

lighthouse_test.go

@@ -738,3 +738,86 @@ func TestLighthouse_DeletesWork(t *testing.T) {
 	out = lh.Query(testHost)
 	assert.Nil(t, out)
 }
+
+func Test_requiredLinkMTU(t *testing.T) {
+	// tun packet + nebula header (16) + AEAD tag (16) + udp (8) + ip header
+	direct, relayed := requiredLinkMTU(1300, true)
+	assert.Equal(t, 1360, direct)
+	assert.Equal(t, 1392, relayed)
+
+	direct, relayed = requiredLinkMTU(1300, false)
+	assert.Equal(t, 1380, direct)
+	assert.Equal(t, 1412, relayed)
+}
+
+func Test_checkLocalLinkMTU(t *testing.T) {
+	lh := &LightHouse{l: test.NewLogger(), mtuWarned: make(map[mtuWarnKey]linkMTUTier)}
+	lh.tunMTU.Store(1300)
+
+	v4 := netip.MustParseAddr("192.168.1.2")
+	v6 := netip.MustParseAddr("fd00::2")
+	mkAddr := func(a netip.Addr, mtu int) localAddr {
+		return localAddr{addr: a, ifName: "test0", linkMTU: mtu}
+	}
+
+	// Plenty of room, no state recorded
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 1500)))
+	assert.Empty(t, lh.mtuWarned)
+
+	// Unknown link MTU is not classified
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 0)))
+	assert.Empty(t, lh.mtuWarned)
+
+	// Too small for even normal traffic, still advertised by default
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 1359)))
+	assert.Equal(t, linkMTUTooSmall, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v4}])
+
+	// Fits normal traffic but not relayed traffic
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 1360)))
+	assert.Equal(t, linkMTUTooSmallForRelay, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v4}])
+
+	// Exactly enough for relayed traffic clears the state
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 1392)))
+	assert.Empty(t, lh.mtuWarned)
+
+	// v6 addrs need 20 more bytes of headroom
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v6, 1380)))
+	assert.Equal(t, linkMTUTooSmallForRelay, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v6}])
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v6, 1379)))
+	assert.Equal(t, linkMTUTooSmall, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v6}])
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v6, 1412)))
+	assert.Empty(t, lh.mtuWarned)
+
+	// With omit enabled, only addrs that can't fit normal traffic are dropped
+	lh.omitLowMTUAddrs.Store(true)
+	assert.False(t, lh.checkLocalLinkMTU(mkAddr(v4, 1359)))
+	assert.Equal(t, linkMTUTooSmall, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v4}])
+	assert.True(t, lh.checkLocalLinkMTU(mkAddr(v4, 1360)))
+	assert.Equal(t, linkMTUTooSmallForRelay, lh.mtuWarned[mtuWarnKey{ifName: "test0", addr: v4}])
+	assert.False(t, lh.checkLocalLinkMTU(mkAddr(v4, 1359)))
+}
+
+func Test_lighthouseMTUConfig(t *testing.T) {
+	l := test.NewLogger()
+	myVpnNet := netip.MustParsePrefix("10.128.0.1/16")
+	nt := new(bart.Lite)
+	nt.Insert(myVpnNet)
+	cs := &CertState{
+		myVpnNetworks:      []netip.Prefix{myVpnNet},
+		myVpnNetworksTable: nt,
+	}
+
+	c := config.NewC(l)
+	lh, err := NewLightHouseFromConfig(t.Context(), l, c, cs, nil, nil)
+	require.NoError(t, err)
+	assert.Equal(t, int64(1300), lh.tunMTU.Load())
+	assert.False(t, lh.omitLowMTUAddrs.Load())
+
+	c = config.NewC(l)
+	c.Settings["tun"] = map[string]any{"mtu": 8000}
+	c.Settings["lighthouse"] = map[string]any{"omit_low_mtu_addrs": true}
+	lh, err = NewLightHouseFromConfig(t.Context(), l, c, cs, nil, nil)
+	require.NoError(t, err)
+	assert.Equal(t, int64(8000), lh.tunMTU.Load())
+	assert.True(t, lh.omitLowMTUAddrs.Load())
+}

main.go

@@ -130,6 +130,17 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 	udpConns := make([]udp.Conn, routines)
 	port := c.GetInt("listen.port", 0)
 
+	// Callers get no handle to these until the Control is returned, release them on any error.
+	defer func() {
+		if reterr != nil {
+			for _, u := range udpConns {
+				if u != nil {
+					_ = u.Close()
+				}
+			}
+		}
+	}()
+
 	if !configTest {
 		rawListenHost := c.GetString("listen.host", "0.0.0.0")
 		var listenHost netip.Addr

overlay/tun_android.go

@@ -40,6 +40,7 @@ func newTunFromFd(c *config.C, l *slog.Logger, deviceFd int, vpnNetworks []netip
 
 	err := t.reload(c, true)
 	if err != nil {
+		_ = file.Close()
 		return nil, err
 	}
 

overlay/tun_freebsd.go

@@ -659,7 +659,6 @@ func addRoute(prefix netip.Prefix, gateway netroute.Addr) error {
 				return fmt.Errorf("failed to create route.RouteMessage for change: %w", err)
 			}
 			_, err = unix.Write(sock, data[:])
-			fmt.Println("DOING CHANGE")
 			return err
 		}
 		return fmt.Errorf("failed to write route.RouteMessage to socket: %w", err)

overlay/tun_ios.go

@@ -18,6 +18,7 @@ import (
 	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/routing"
 	"github.com/slackhq/nebula/util"
+	"golang.org/x/sys/unix"
 )
 
 type tun struct {
@@ -33,6 +34,12 @@ func newTun(_ *config.C, _ *slog.Logger, _ []netip.Prefix, _ bool) (*tun, error)
 }
 
 func newTunFromFd(c *config.C, l *slog.Logger, deviceFd int, vpnNetworks []netip.Prefix) (*tun, error) {
+	if err := unix.SetNonblock(deviceFd, true); err != nil {
+		// We own the fd from the moment it is handed to us, same as the reload error path below
+		_ = unix.Close(deviceFd)
+		return nil, fmt.Errorf("failed to set the tun fd to non-blocking mode: %w", err)
+	}
+
 	file := os.NewFile(uintptr(deviceFd), "/dev/tun")
 	t := &tun{
 		vpnNetworks:     vpnNetworks,
@@ -42,6 +49,7 @@ func newTunFromFd(c *config.C, l *slog.Logger, deviceFd int, vpnNetworks []netip
 
 	err := t.reload(c, true)
 	if err != nil {
+		_ = file.Close()
 		return nil, err
 	}
 

relay_manager.go

@@ -107,7 +107,10 @@ func (rm *relayManager) StartRelays(f *Interface, vpnIp netip.Addr, hh *Handshak
 			if relayHostInfo.GetRemote().IsValid() {
 				idx, err := AddRelay(rm.l, relayHostInfo, rm.hostmap, vpnIp, nil, TerminalType, Requested)
 				if err != nil {
+					// No local relay state was installed, so a CreateRelayRequest would hand the
+					// peer an index we could never resolve. Skip it.
 					hl.Info("Failed to add relay to hostmap", "relay", relay.String(), "error", err)
+					continue
 				}
 
 				m := NebulaControl{
@@ -237,7 +240,12 @@ func AddRelay(l *slog.Logger, relayHostInfo *HostInfo, hm *HostMap, vpnIp netip.
 			// Avoid standing up a relay that can't be used since only the primary hostinfo
 			// will be pointed to by the relay logic
 			//TODO: if there was an existing primary and it had relay state, should we merge?
-			hm.unlockedMakePrimary(relayHostInfo)
+			if !hm.unlockedMakePrimary(relayHostInfo) {
+				// The tunnel was torn down after the caller grabbed relayHostInfo. A relay standing
+				// on an unlinked hostinfo would never carry traffic, and its Relays entry could
+				// never be reclaimed since the delete-time cleanup has already run.
+				return 0, errors.New("relay hostinfo is no longer in the hostmap")
+			}
 
 			hm.Relays[index] = relayHostInfo
 			newRelay := Relay{

service/service.go

@@ -43,12 +43,25 @@ type Service struct {
 	}
 }
 
-func New(control *nebula.Control) (*Service, error) {
-	wait, err := control.Start()
+func New(control *nebula.Control) (_ *Service, reterr error) {
+	// Check this before Start so a failure doesn't leave a running nebula
+	device, ok := control.Device().(*overlay.UserDevice)
+	if !ok {
+		return nil, errors.New("must be using user device")
+	}
+
+	err := control.Start()
 	if err != nil {
 		return nil, err
 	}
 
+	// Anything that fails after a successful Start must tear nebula back down
+	defer func() {
+		if reterr != nil {
+			control.Stop()
+		}
+	}()
+
 	ctx := control.Context()
 	eg, ctx := errgroup.WithContext(ctx)
 	s := Service{
@@ -57,11 +70,6 @@ func New(control *nebula.Control) (*Service, error) {
 	}
 	s.mu.listeners = map[uint16]*tcpListener{}
 
-	device, ok := control.Device().(*overlay.UserDevice)
-	if !ok {
-		return nil, errors.New("must be using user device")
-	}
-
 	s.ipstack = stack.New(stack.Options{
 		NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol, ipv6.NewProtocol},
 		TransportProtocols: []stack.TransportProtocolFactory{tcp.NewProtocol, udp.NewProtocol, icmp.NewProtocol4, icmp.NewProtocol6},
@@ -147,7 +155,7 @@ func New(control *nebula.Control) (*Service, error) {
 	// Add the nebula wait function to the group so a fatal reader error
 	// propagates out through errgroup.Wait().
 	eg.Go(func() error {
-		return wait()
+		return control.Wait()
 	})
 
 	return &s, nil