Rebind for MacOS (#1816)

Co-authored-by: Jack Doan <me@jackdoan.com>

connection_manager_test.go

@@ -25,6 +25,7 @@ func newTestLighthouse() *LightHouse {
 	lighthouses := []netip.Addr{}
 	staticList := map[netip.Addr]struct{}{}
 
+	lh.localAddrsFn = func(*LocalAllowList) []netip.Addr { return nil }
 	lh.lighthouses.Store(&lighthouses)
 	lh.staticList.Store(&staticList)
 

control.go

@@ -53,6 +53,7 @@ type Control struct {
 	statsStart             func()
 	dnsStart               func()
 	lighthouseStart        func()
+	networkChangeStart     func(rebind func())
 	connectionManagerStart func(context.Context)
 }
 
@@ -104,6 +105,9 @@ func (c *Control) Start() error {
 	if c.dnsStart != nil {
 		go c.dnsStart()
 	}
+	if c.networkChangeStart != nil {
+		go c.networkChangeStart(c.RebindUDPServer)
+	}
 	if c.connectionManagerStart != nil {
 		go c.connectionManagerStart(c.ctx)
 	}
@@ -198,7 +202,11 @@ func (c *Control) RebindUDPServer() {
 		return
 	}
 
-	_ = c.f.outside.Rebind()
+	// A failure here means we are likely still pinned to the interface we came up on, so the rest of this is
+	// unlikely to help. Say so instead of silently carrying on as if we rebound.
+	if err := c.f.outside.Rebind(); err != nil {
+		c.l.Error("Failed to rebind udp socket", "error", err)
+	}
 
 	// Trigger a lighthouse update, useful for mobile clients that should have an update interval of 0
 	c.f.lightHouse.SendUpdate()

control_tester.go

@@ -108,7 +108,19 @@ func (c *Control) GetVpnAddrs() []netip.Addr {
 }
 
 func (c *Control) GetUDPAddr() netip.AddrPort {
-	return c.f.outside.(*udp.TesterConn).Addr
+	return c.f.outside.(*udp.TesterConn).GetAddr()
+}
+
+// SetUDPAddr moves this node to a new underlay address, standing in for a laptop waking up on a different
+// network. Register the new address with the router as well or nothing will route back.
+func (c *Control) SetUDPAddr(addr netip.AddrPort) {
+	c.f.outside.(*udp.TesterConn).SetAddr(addr)
+}
+
+// SetLocalAddrsFn replaces underlay address discovery so a test can advertise its simulated address instead of
+// whatever this machine's NICs happen to be. Call it before Start, SendUpdate reads it from the update worker.
+func (c *Control) SetLocalAddrsFn(fn func(*LocalAllowList) []netip.Addr) {
+	c.f.lightHouse.localAddrsFn = fn
 }
 
 func (c *Control) KillPendingTunnel(vpnIp netip.Addr) bool {

e2e/rebind_test.go

@@ -0,0 +1,225 @@
+//go:build e2e_testing
+// +build e2e_testing
+
+package e2e
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/slackhq/nebula"
+	"github.com/slackhq/nebula/cert"
+	"github.com/slackhq/nebula/cert_test"
+	"github.com/slackhq/nebula/e2e/router"
+	"github.com/slackhq/nebula/header"
+	"github.com/slackhq/nebula/udp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// reportedAddrs is what the lighthouse would hand a peer asking where vpnAddr is.
+func reportedAddrs(t *testing.T, lh *nebula.Control, vpnAddr netip.Addr) []netip.AddrPort {
+	t.Helper()
+	cm := lh.QueryLighthouse(vpnAddr)
+	if cm == nil {
+		return nil
+	}
+	var out []netip.AddrPort
+	for _, c := range *cm {
+		out = append(out, c.Reported...)
+		out = append(out, c.Learned...)
+	}
+	return out
+}
+
+// waitForLighthouseMsg routes until a lighthouse message lands on lh, or gives up. Reports whether one arrived.
+func waitForLighthouseMsg(t *testing.T, r *router.R, lh *nebula.Control, wait time.Duration) bool {
+	t.Helper()
+	h := &header.H{}
+	return r.RouteForAllExitFuncOrTimeout(wait, func(p *udp.Packet, c *nebula.Control) router.ExitType {
+		if c != lh {
+			return router.KeepRouting
+		}
+		// Punches are a single byte and never parse, they are just not what we are after
+		if err := h.Parse(p.Data); err != nil {
+			return router.KeepRouting
+		}
+		if h.Type == header.LightHouse {
+			return router.RouteAndExit
+		}
+		return router.KeepRouting
+	})
+}
+
+// A laptop that changes networks has to tell the lighthouse promptly, otherwise the lighthouse keeps handing peers
+// the old address and their punches land nowhere. On a long lighthouse interval the only thing that closes that
+// window is the rebind, which on darwin the network change monitor drives. The e2e build compiles the monitor out,
+// so we call RebindUDPServer directly, which is the same thing the monitor does.
+func TestRebindSendsLighthouseUpdate(t *testing.T) {
+	t.Parallel()
+	ca, _, caKey, _ := cert_test.NewTestCaCert(cert.Version2, cert.Curve_CURVE25519, time.Now(), time.Now().Add(10*time.Minute), nil, nil, []string{})
+
+	lhControl, lhVpnIpNet, lhUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "lh", "10.128.0.1/24", m{
+		"lighthouse": m{"am_lighthouse": true},
+	})
+
+	// 600s interval, so nothing scheduled can send an update during this test. A rebind is the only thing that can.
+	myControl, _, _, _ := newSimpleServer(cert.Version2, ca, caKey, "me", "10.128.0.2/24", m{
+		"lighthouse": m{
+			"hosts":    []any{lhVpnIpNet[0].Addr().String()},
+			"interval": 600,
+		},
+		"static_host_map": m{
+			lhVpnIpNet[0].Addr().String(): []any{lhUdpAddr.String()},
+		},
+	})
+
+	r := router.NewR(t, lhControl, myControl)
+	defer r.RenderFlow()
+
+	lhControl.Start()
+	myControl.Start()
+
+	// Let the startup registration finish, then clear everything it left behind
+	require.True(t, waitForLighthouseMsg(t, r, lhControl, time.Second*5), "expected an initial registration")
+	r.RouteFor(time.Millisecond * 400)
+
+	// Nothing should be talking to the lighthouse on its own now
+	require.False(t, waitForLighthouseMsg(t, r, lhControl, time.Millisecond*200),
+		"nothing should reach the lighthouse before the rebind")
+
+	myControl.RebindUDPServer()
+
+	assert.True(t, waitForLighthouseMsg(t, r, lhControl, time.Second*5),
+		"a rebind should push an update to the lighthouse rather than waiting out the interval")
+
+	lhControl.Stop()
+	myControl.Stop()
+}
+
+// The other half of a rebind: every live tunnel requeries the lighthouse on its next send. That query is what makes
+// the lighthouse tell the peer to punch toward our new address, which is the part that actually revives a tunnel
+// whose remote NAT state died while we were on a different network.
+func TestRebindRequeriesPeersOnNextSend(t *testing.T) {
+	t.Parallel()
+	ca, _, caKey, _ := cert_test.NewTestCaCert(cert.Version2, cert.Curve_CURVE25519, time.Now(), time.Now().Add(10*time.Minute), nil, nil, []string{})
+
+	lhControl, lhVpnIpNet, lhUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "lh", "10.128.0.1/24", m{
+		"lighthouse": m{"am_lighthouse": true},
+	})
+
+	lhCfg := m{
+		"lighthouse": m{
+			"hosts":    []any{lhVpnIpNet[0].Addr().String()},
+			"interval": 600,
+			// Without this the peers advertise this machine's real addresses and then try to punch at them,
+			// which the router has no route for.
+			"local_allow_list": m{
+				"10.0.0.0/24": true,
+				"::/0":        false,
+			},
+		},
+		"static_host_map": m{
+			lhVpnIpNet[0].Addr().String(): []any{lhUdpAddr.String()},
+		},
+	}
+
+	myControl, myVpnIpNet, myUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "me", "10.128.0.2/24", lhCfg)
+	theirControl, theirVpnIpNet, theirUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "them", "10.128.0.3/24", lhCfg)
+
+	r := router.NewR(t, lhControl, myControl, theirControl)
+	defer r.RenderFlow()
+
+	lhControl.Start()
+	myControl.Start()
+	theirControl.Start()
+	r.RouteFor(time.Millisecond * 500)
+
+	// Point the peers at each other directly, this test is about the rebind and not about lighthouse discovery
+	myControl.InjectLightHouseAddr(theirVpnIpNet[0].Addr(), theirUdpAddr)
+	theirControl.InjectLightHouseAddr(myVpnIpNet[0].Addr(), myUdpAddr)
+
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("initial")))
+	r.RouteFor(time.Second)
+	require.NotNil(t, myControl.GetHostInfoByVpnAddr(theirVpnIpNet[0].Addr(), false), "expected a tunnel to them")
+	r.RouteFor(time.Millisecond * 300)
+
+	// Assert on what the peer sees rather than on lighthouse traffic. A query for them makes the lighthouse send
+	// them a punch notification, which is the whole point. Our own update to the lighthouse sends them nothing,
+	// so this cannot be satisfied by the update the rebind itself pushes.
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("quiet")))
+	require.False(t, waitForLighthouseMsg(t, r, theirControl, time.Millisecond*300),
+		"an ordinary send should not requery the lighthouse")
+
+	myControl.RebindUDPServer()
+	r.RouteFor(time.Millisecond * 300) // let the update the rebind itself sends pass by
+
+	myControl.InjectTunPacket(BuildTunUDPPacket(theirVpnIpNet[0].Addr(), 80, myVpnIpNet[0].Addr(), 80, []byte("after rebind")))
+	assert.True(t, waitForLighthouseMsg(t, r, theirControl, time.Second*5),
+		"the first send after a rebind should requery the lighthouse, which then tells the peer to punch at us")
+
+	lhControl.Stop()
+	myControl.Stop()
+	theirControl.Stop()
+}
+
+// The scenario this whole thing exists for: a laptop sleeps at the office and wakes up at home on a new address.
+// Until it tells the lighthouse, the lighthouse keeps handing peers the office address, so their punches land
+// nowhere and the tunnel stays dead. On a long interval the rebind is the only thing that closes that window.
+func TestRebindAdvertisesNewAddressAfterMove(t *testing.T) {
+	t.Parallel()
+	ca, _, caKey, _ := cert_test.NewTestCaCert(cert.Version2, cert.Curve_CURVE25519, time.Now(), time.Now().Add(10*time.Minute), nil, nil, []string{})
+
+	lhControl, lhVpnIpNet, lhUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "lh", "10.128.0.1/24", m{
+		"lighthouse": m{"am_lighthouse": true},
+	})
+
+	myControl, myVpnIpNet, myUdpAddr, _ := newSimpleServer(cert.Version2, ca, caKey, "me", "10.128.0.2/24", m{
+		"lighthouse": m{
+			"hosts":    []any{lhVpnIpNet[0].Addr().String()},
+			"interval": 600,
+		},
+		"static_host_map": m{
+			lhVpnIpNet[0].Addr().String(): []any{lhUdpAddr.String()},
+		},
+	})
+
+	// Advertise wherever we currently are rather than this machine's real NICs, read fresh each time so a move
+	// is picked up.
+	myControl.SetLocalAddrsFn(func(*nebula.LocalAllowList) []netip.Addr {
+		return []netip.Addr{myControl.GetUDPAddr().Addr()}
+	})
+
+	r := router.NewR(t, lhControl, myControl)
+	defer r.RenderFlow()
+
+	lhControl.Start()
+	myControl.Start()
+
+	require.True(t, waitForLighthouseMsg(t, r, lhControl, time.Second*5), "expected an initial registration")
+	r.RouteFor(time.Millisecond * 400)
+
+	require.Contains(t, reportedAddrs(t, lhControl, myVpnIpNet[0].Addr()), myUdpAddr,
+		"the lighthouse should know the address we started on")
+
+	// Wake up somewhere else
+	newAddr := netip.MustParseAddrPort("10.0.0.99:4242")
+	myControl.SetUDPAddr(newAddr)
+	r.AddRoute(newAddr.Addr(), newAddr.Port(), myControl)
+
+	// Nothing has told the lighthouse, and with interval 600 nothing scheduled will
+	r.RouteFor(time.Millisecond * 400)
+	require.NotContains(t, reportedAddrs(t, lhControl, myVpnIpNet[0].Addr()), newAddr,
+		"the lighthouse should still be handing out the old address before the rebind")
+
+	myControl.RebindUDPServer()
+	require.True(t, waitForLighthouseMsg(t, r, lhControl, time.Second*5), "expected an update after the rebind")
+	r.RouteFor(time.Millisecond * 400)
+
+	assert.Contains(t, reportedAddrs(t, lhControl, myVpnIpNet[0].Addr()), newAddr,
+		"after the rebind the lighthouse should hand peers our new address")
+
+	lhControl.Stop()
+	myControl.Stop()
+}

e2e/router/router.go

@@ -114,6 +114,28 @@ type packet struct {
 	packet *udp.Packet
 	tun    bool // a packet pulled off a tun device
 	rx     bool // the packet was received by a udp device
+
+	// h is the nebula header, parsed once when the packet is recorded. parseErr says why there isn't one, which
+	// the flow log reports rather than hiding. Punchy sends a single byte, so an unparseable packet is normal.
+	h        header.H
+	parseErr error
+}
+
+// fromAddr and toAddr are the addresses this packet actually travelled between. Reading them off the control
+// instead would misreport the whole history once a test moves a node. Tun packets are synthesized without
+// addresses, so they fall back to the control.
+func (p *packet) fromAddr() netip.AddrPort {
+	if p.tun || !p.packet.From.IsValid() {
+		return p.from.GetUDPAddr()
+	}
+	return p.packet.From
+}
+
+func (p *packet) toAddr() netip.AddrPort {
+	if p.tun || !p.packet.To.IsValid() {
+		return p.to.GetUDPAddr()
+	}
+	return p.packet.To
 }
 
 func (p *packet) WasReceived() {
@@ -249,7 +271,7 @@ func (r *R) renderFlow() {
 			continue
 		}
 
-		addr := e.packet.from.GetUDPAddr()
+		addr := e.packet.fromAddr()
 		if _, ok := participants[addr]; ok {
 			continue
 		}
@@ -268,7 +290,6 @@ func (r *R) renderFlow() {
 	}
 
 	// Print packets
-	h := &header.H{}
 	for _, e := range r.flow {
 		if e.packet == nil {
 			//fmt.Fprintf(f, "    note over %s: %s\n", strings.Join(participantsVals, ", "), e.note)
@@ -280,21 +301,22 @@ func (r *R) renderFlow() {
 			fmt.Fprintln(f, r.formatUdpPacket(p))
 
 		} else {
-			if err := h.Parse(p.packet.Data); err != nil {
-				panic(err)
-			}
-
 			line := "--x"
 			if p.rx {
 				line = "->>"
 			}
 
-			fmt.Fprintf(f,
-				"    %s%s%s: %s(%s), index %v, counter: %v\n",
-				normalizeName(p.from.GetUDPAddr().String()),
+			detail := fmt.Sprintf("%s(%s), index %v, counter: %v",
+				p.h.TypeName(), p.h.SubTypeName(), p.h.RemoteIndex, p.h.MessageCounter)
+			if p.parseErr != nil {
+				detail = fmt.Sprintf("unparsed, %v (%d bytes)", p.parseErr, len(p.packet.Data))
+			}
+
+			fmt.Fprintf(f, "    %s%s%s: %s\n",
+				normalizeName(p.fromAddr().String()),
 				line,
-				normalizeName(p.to.GetUDPAddr().String()),
-				h.TypeName(), h.SubTypeName(), h.RemoteIndex, h.MessageCounter,
+				normalizeName(p.toAddr().String()),
+				detail,
 			)
 		}
 	}
@@ -408,21 +430,24 @@ func (r *R) unlockedInjectFlow(from, to *nebula.Control, p *udp.Packet, tun bool
 
 	r.renderHostmaps(fmt.Sprintf("Packet %v", len(r.flow)))
 
-	if len(r.ignoreFlows) > 0 {
 	var h header.H
-		err := h.Parse(p.Data)
-		if err != nil {
-			panic(err)
+	var parseErr error
+	if !tun {
+		parseErr = h.Parse(p.Data)
 	}
 
+	// Decide before copying, the copy comes from a freelist and an ignored packet would never be released
 	for _, i := range r.ignoreFlows {
-			if !tun {
-				if i.messageType == h.Type && i.subType == h.Subtype {
+		if tun {
+			if i.tun.HasValue && i.tun.IsTrue {
 				return nil
 			}
-			} else if i.tun.HasValue && i.tun.IsTrue {
-				return nil
+			continue
 		}
+
+		// A packet we could not parse has no type to match against, so no rule can ignore it
+		if parseErr == nil && i.messageType == h.Type && i.subType == h.Subtype {
+			return nil
 		}
 	}
 
@@ -431,6 +456,8 @@ func (r *R) unlockedInjectFlow(from, to *nebula.Control, p *udp.Packet, tun bool
 		to:       to,
 		packet:   p.Copy(),
 		tun:      tun,
+		h:        h,
+		parseErr: parseErr,
 	}
 
 	r.flow = append(r.flow, flowEntry{packet: fp})
@@ -690,6 +717,81 @@ func (r *R) RouteUntilAfterMsgType(sender *nebula.Control, msgType header.Messag
 	})
 }
 
+// RouteFor routes everything that shows up for the given duration and then returns. Use it to let a test settle
+// deterministically rather than sleeping and hoping: a single FlushAll races a completing handshake, which queues
+// more packets right behind it.
+func (r *R) RouteFor(d time.Duration) {
+	r.RouteForAllExitFuncOrTimeout(d, func(*udp.Packet, *nebula.Control) ExitType {
+		return KeepRouting
+	})
+}
+
+// RouteForAllExitFuncOrTimeout is RouteForAllExitFunc with a deadline, reporting whether whatDo asked to exit
+// before time ran out. The unbounded version blocks forever on a quiet network, so this is what a test needs to
+// assert that something does NOT happen, or to route for a fixed settling period.
+func (r *R) RouteForAllExitFuncOrTimeout(timeout time.Duration, whatDo ExitFunc) bool {
+	sc := make([]reflect.SelectCase, 0, len(r.controls)+1)
+	cm := make([]*nebula.Control, 0, len(r.controls))
+
+	for _, c := range r.controls {
+		sc = append(sc, reflect.SelectCase{
+			Dir:  reflect.SelectRecv,
+			Chan: reflect.ValueOf(c.GetUDPTxChan()),
+			Send: reflect.Value{},
+		})
+		cm = append(cm, c)
+	}
+
+	timer := time.NewTimer(timeout)
+	defer timer.Stop()
+	sc = append(sc, reflect.SelectCase{
+		Dir:  reflect.SelectRecv,
+		Chan: reflect.ValueOf(timer.C),
+		Send: reflect.Value{},
+	})
+
+	for {
+		x, rx, _ := reflect.Select(sc)
+		if x == len(cm) {
+			return false
+		}
+
+		r.Lock()
+		p := rx.Interface().(*udp.Packet)
+		receiver := r.getControl(cm[x].GetUDPAddr(), p.To, p)
+		if receiver == nil {
+			r.Unlock()
+			panic("Can't RouteForAllExitFuncOrTimeout for host: " + p.To.String())
+		}
+
+		e := whatDo(p, receiver)
+		switch e {
+		case ExitNow:
+			r.Unlock()
+			p.Release()
+			return true
+
+		case RouteAndExit:
+			fp := r.unlockedInjectFlow(cm[x], receiver, p, false)
+			receiver.InjectUDPPacket(p)
+			fp.WasReceived()
+			r.Unlock()
+			p.Release()
+			return true
+
+		case KeepRouting:
+			fp := r.unlockedInjectFlow(cm[x], receiver, p, false)
+			receiver.InjectUDPPacket(p)
+			fp.WasReceived()
+
+		default:
+			panic(fmt.Sprintf("Unknown exitFunc return: %v", e))
+		}
+		r.Unlock()
+		p.Release()
+	}
+}
+
 func (r *R) RouteForAllUntilAfterMsgTypeTo(receiver *nebula.Control, msgType header.MessageType, subType header.MessageSubType) {
 	h := &header.H{}
 	r.RouteForAllExitFunc(func(p *udp.Packet, r *nebula.Control) ExitType {

examples/config.yml

@@ -146,6 +146,14 @@ listen:
   # Default true; set to false to leave WDF in charge of inbound decisions on the listener port. Not reloadable.
   #windows_bypass_wdf: true
 
+  # On macOS only
+  # macOS scopes the udp socket to the interface it was created on, so moving between networks (wifi to wired,
+  # office to home) leaves Nebula sending out an interface that no longer has a route. When true, Nebula watches
+  # the routing socket and rebinds the listener once the change settles.
+  # iOS does not use this, the host app drives the same rebind itself.
+  # Default true. Not reloadable.
+  #rebind_on_network_change: true
+
   # By default, Nebula replies to packets it has no tunnel for with a "recv_error" packet. This packet helps speed up reconnection
   # in the case that Nebula on either side did not shut down cleanly. This response can be abused as a way to discover if Nebula is running
   # on a host though. This option lets you configure if you want to send "recv_error" packets always, never, or only to private network remotes.

lighthouse.go

@@ -36,6 +36,10 @@ type LightHouse struct {
 	myVpnNetworksTable *bart.Lite
 	punchy             *Punchy
 
+	// localAddrsFn enumerates the underlay addresses we advertise. It is a field so tests can supply simulated
+	// addresses rather than whatever this machine's NICs happen to be. Set it before Start.
+	localAddrsFn func(*LocalAllowList) []netip.Addr
+
 	// Local cache of answers from light houses
 	// map of vpn addr to answers
 	addrMap map[netip.Addr]*RemoteList
@@ -107,6 +111,10 @@ func NewLightHouseFromConfig(ctx context.Context, l *slog.Logger, c *config.C, c
 		queryChan:          make(chan netip.Addr, c.GetUint32("handshakes.query_buffer", 64)),
 		l:                  l,
 	}
+	h.localAddrsFn = func(al *LocalAllowList) []netip.Addr {
+		return localAddrs(h.l, al)
+	}
+
 	lighthouses := make([]netip.Addr, 0)
 	h.lighthouses.Store(&lighthouses)
 	staticList := make(map[netip.Addr]struct{})
@@ -918,7 +926,7 @@ func (lh *LightHouse) SendUpdate() {
 	}
 
 	lal := lh.GetLocalAllowList()
-	for _, e := range localAddrs(lh.l, lal) {
+	for _, e := range lh.localAddrsFn(lal) {
 		if lh.myVpnNetworksTable.Contains(e) {
 			continue
 		}

main.go

@@ -268,6 +268,8 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 
 	attachCommands(l, c, ssh, ifce)
 
+	networkChanges := udp.NewNetworkChangeMonitor(ctx, l, c)
+
 	return &Control{
 		state:                  StateReady,
 		f:                      ifce,
@@ -278,6 +280,7 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		statsStart:             stats.Start,
 		dnsStart:               ds.Start,
 		lighthouseStart:        lightHouse.StartUpdateWorker,
+		networkChangeStart:     networkChanges.Start,
 		connectionManagerStart: connManager.Start,
 	}, nil
 }

udp/netchange.go

@@ -0,0 +1,61 @@
+package udp
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/slackhq/nebula/config"
+)
+
+// NetworkChangeMonitor rebinds the udp listener when the local network moves out from under it.
+//
+// Detection lives here in the udp package, next to the socket it concerns and the platform matrix that already knows
+// which sockets go stale. What to do about a change — updating the lighthouse, requerying tunnels — is not the udp
+// package's business, so Start takes the reaction as a plain function. Passing it at Start rather than holding it
+// keeps this package from referencing whatever owns the rebind.
+//
+// On platforms whose sockets do not go stale, watchNetworkChanges hands back a nil channel and Start returns.
+type NetworkChangeMonitor struct {
+	l       *slog.Logger
+	ctx     context.Context
+	enabled bool
+}
+
+// NewNetworkChangeMonitor builds a monitor for local network changes. The returned monitor is always usable: Start
+// is safe to call unconditionally, it no-ops when disabled or on a platform that does not need it.
+func NewNetworkChangeMonitor(ctx context.Context, l *slog.Logger, c *config.C) *NetworkChangeMonitor {
+	return &NetworkChangeMonitor{
+		l:       l,
+		ctx:     ctx,
+		enabled: c.GetBool("listen.rebind_on_network_change", true),
+	}
+}
+
+// Start watches for network changes until the context is cancelled, calling rebind once per settled change. It
+// blocks, so callers run it in a goroutine, and it no-ops when disabled, unsupported, or with nothing to rebind.
+func (m *NetworkChangeMonitor) Start(rebind func()) {
+	if !m.enabled || rebind == nil || m.ctx.Err() != nil {
+		return
+	}
+
+	changes, err := watchNetworkChanges(m.ctx, m.l)
+	if err != nil {
+		// Not fatal. Everything else still works, we just won't notice a network change on our own.
+		m.l.Error("Failed to watch for network changes, will not rebind the udp listener when the network moves",
+			"error", err,
+		)
+		return
+	}
+
+	if changes == nil {
+		// This platform's sockets don't go stale, so there is nothing to watch for.
+		return
+	}
+
+	m.l.Info("Watching for network changes to rebind the udp listener")
+
+	for range changes {
+		m.l.Info("Local network changed, rebinding the udp listener")
+		rebind()
+	}
+}

udp/netchange_darwin.go

@@ -0,0 +1,164 @@
+//go:build darwin && !ios && !e2e_testing
+// +build darwin,!ios,!e2e_testing
+
+package udp
+
+import (
+	"context"
+	"encoding/binary"
+	"errors"
+	"log/slog"
+	"os"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+const (
+	// netChangeSettleWindow is how long we keep swallowing routing messages after the first interesting one. A
+	// single network change is never a single message, it is a burst: the link drops, addresses go away, new ones
+	// arrive, routes get rewritten. Reporting part way through that just means reporting again.
+	netChangeSettleWindow = time.Second
+
+	// netChangeReadBuffer is sized well past any rt_msghdr plus its addresses. A short read would be discarded by
+	// the kernel, so being generous here is how we avoid missing a message.
+	netChangeReadBuffer = 4096
+)
+
+// watchNetworkChanges reports when the local network moves out from under us, so the listener can be rebound.
+//
+// Darwin scopes a udp socket to whatever interface it came up on. Move between networks and we keep sending out an
+// interface that no longer has a route, which surfaces as an instant "no route to host" with no packet ever leaving
+// the box. Rebind clears that, but only if something notices the change and calls it. iOS has always been told by
+// the host app off NWPathMonitor. This is the equivalent for everything else that runs on darwin.
+//
+// The returned channel is buffered and coalescing: a send is dropped if one is already pending, since both mean the
+// same thing to a reader. It is closed when ctx is cancelled or the routing socket fails, so a caller can simply
+// range over it. Platforms whose sockets do not need rebinding return a nil channel and no error.
+func watchNetworkChanges(ctx context.Context, l *slog.Logger) (<-chan struct{}, error) {
+	sock, err := openRouteSocket()
+	if err != nil {
+		return nil, err
+	}
+
+	changes := make(chan struct{}, 1)
+
+	go func() {
+		defer close(changes)
+		defer func() { _ = sock.Close() }()
+
+		// Closing the socket is what unblocks the read in watchRouteSocket, so this turns cancellation into a
+		// close. It is scoped to this call so it cannot outlive the watch it belongs to.
+		done := make(chan struct{})
+		defer close(done)
+		go func() {
+			select {
+			case <-ctx.Done():
+				_ = sock.Close()
+			case <-done:
+			}
+		}()
+
+		watchRouteSocket(l, sock, changes)
+	}()
+
+	return changes, nil
+}
+
+// watchRouteSocket blocks reading the routing socket, reporting once per settled burst of changes. It returns when
+// the socket is closed, which is how cancellation gets us out of here.
+func watchRouteSocket(l *slog.Logger, sock *os.File, changes chan<- struct{}) {
+	buf := make([]byte, netChangeReadBuffer)
+
+	for {
+		n, err := sock.Read(buf)
+		if err != nil {
+			logRouteSocketError(l, err)
+			return
+		}
+
+		if !isNetworkChange(buf[:n]) {
+			continue
+		}
+
+		// Swallow the rest of the burst. The deadline is absolute and not extended by what arrives, so this always
+		// ends after the settle window no matter how chatty the socket is. Changes that land after the window
+		// simply produce another report, which is the correct outcome anyway.
+		deadline := time.Now().Add(netChangeSettleWindow)
+		for {
+			if err = sock.SetReadDeadline(deadline); err != nil {
+				logRouteSocketError(l, err)
+				return
+			}
+
+			if _, err = sock.Read(buf); err != nil {
+				if os.IsTimeout(err) {
+					break
+				}
+				logRouteSocketError(l, err)
+				return
+			}
+		}
+
+		if err = sock.SetReadDeadline(time.Time{}); err != nil {
+			logRouteSocketError(l, err)
+			return
+		}
+
+		select {
+		case changes <- struct{}{}:
+		default:
+			// One already pending, and a second "the network moved" tells the reader nothing new.
+		}
+	}
+}
+
+// logRouteSocketError reports a routing socket failure unless it is just us shutting the socket down.
+func logRouteSocketError(l *slog.Logger, err error) {
+	if errors.Is(err, os.ErrClosed) {
+		return
+	}
+
+	l.Error("Error reading the routing socket, will no longer notice local network changes", "error", err)
+}
+
+// openRouteSocket returns the routing socket as a non blocking os.File. Going through os.File puts reads on the go
+// poller, which buys us both a working read deadline and a Close that unblocks a read in progress.
+func openRouteSocket() (*os.File, error) {
+	fd, err := unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW, unix.AF_UNSPEC)
+	if err != nil {
+		return nil, err
+	}
+
+	if err = unix.SetNonblock(fd, true); err != nil {
+		_ = unix.Close(fd)
+		return nil, err
+	}
+
+	return os.NewFile(uintptr(fd), "route"), nil
+}
+
+// isNetworkChange reports whether a routing message means our local addressing may have moved out from under us.
+//
+// We read the header instead of parsing the message because the type is the only part we need, and a full parse can
+// fail on shapes we don't care about, which would turn "a message I can't parse" into "a change I missed".
+// rt_msghdr, if_msghdr and ifa_msghdr all begin with the same three fields, so this is the same for every type.
+func isNetworkChange(msg []byte) bool {
+	if len(msg) < 4 {
+		return false
+	}
+
+	// u_short msglen, u_char version, u_char type
+	if int(binary.NativeEndian.Uint16(msg[0:2])) > len(msg) || msg[2] != unix.RTM_VERSION {
+		return false
+	}
+
+	switch msg[3] {
+	case unix.RTM_NEWADDR, unix.RTM_DELADDR, unix.RTM_IFINFO:
+		// An address arrived or left, or a link changed state. Anything else on this socket is either a route
+		// churning underneath us, which a rebind doesn't help with, or unrelated traffic.
+		return true
+	default:
+		return false
+	}
+}

udp/netchange_darwin_test.go

@@ -0,0 +1,244 @@
+//go:build darwin && !ios && !e2e_testing
+// +build darwin,!ios,!e2e_testing
+
+package udp
+
+import (
+	"context"
+	"encoding/binary"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/test"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/goleak"
+	"golang.org/x/sys/unix"
+)
+
+// routeMsg builds the first four bytes of a routing message, which is all isNetworkChange reads.
+func routeMsg(msgType uint8, extra int) []byte {
+	msg := make([]byte, 4+extra)
+	binary.NativeEndian.PutUint16(msg[0:2], uint16(len(msg)))
+	msg[2] = unix.RTM_VERSION
+	msg[3] = msgType
+	return msg
+}
+
+func TestIsNetworkChange(t *testing.T) {
+	// The three that mean our addressing may have moved
+	assert.True(t, isNetworkChange(routeMsg(unix.RTM_NEWADDR, 0)))
+	assert.True(t, isNetworkChange(routeMsg(unix.RTM_DELADDR, 0)))
+	assert.True(t, isNetworkChange(routeMsg(unix.RTM_IFINFO, 0)))
+
+	// Route churn is not something a rebind helps with
+	assert.False(t, isNetworkChange(routeMsg(unix.RTM_ADD, 0)))
+	assert.False(t, isNetworkChange(routeMsg(unix.RTM_DELETE, 0)))
+	assert.False(t, isNetworkChange(routeMsg(unix.RTM_GET, 0)))
+
+	// Garbage must not be mistaken for a change
+	assert.False(t, isNetworkChange(nil), "empty")
+	assert.False(t, isNetworkChange([]byte{0, 0, 0}), "short header")
+
+	wrongVersion := routeMsg(unix.RTM_NEWADDR, 0)
+	wrongVersion[2] = unix.RTM_VERSION + 1
+	assert.False(t, isNetworkChange(wrongVersion), "wrong rtm_version")
+
+	lying := routeMsg(unix.RTM_NEWADDR, 0)
+	binary.NativeEndian.PutUint16(lying[0:2], 512)
+	assert.False(t, isNetworkChange(lying), "msglen longer than what we read")
+}
+
+// socketPair returns a connected pair of datagram sockets, the first wrapped the same way the routing socket is. It
+// stands in for the kernel so the watch loop can be driven with synthetic messages.
+func socketPair(t *testing.T) (*os.File, int) {
+	t.Helper()
+
+	fds, err := unix.Socketpair(unix.AF_UNIX, unix.SOCK_DGRAM, 0)
+	require.NoError(t, err)
+	require.NoError(t, unix.SetNonblock(fds[0], true))
+
+	f := os.NewFile(uintptr(fds[0]), "route")
+	t.Cleanup(func() {
+		_ = f.Close()
+		_ = unix.Close(fds[1])
+	})
+
+	return f, fds[1]
+}
+
+func TestWatchRouteSocketCoalescesABurst(t *testing.T) {
+	sock, kernel := socketPair(t)
+	changes := make(chan struct{}, 1)
+
+	done := make(chan struct{})
+	go func() {
+		watchRouteSocket(test.NewLogger(), sock, changes)
+		close(done)
+	}()
+
+	// One network change is a burst of messages. All of these land inside the settle window, so they must produce
+	// exactly one report rather than one apiece.
+	for range 5 {
+		_, err := unix.Write(kernel, routeMsg(unix.RTM_NEWADDR, 8))
+		require.NoError(t, err)
+	}
+	// Uninteresting messages in the middle of a burst must not add a report of their own either.
+	_, err := unix.Write(kernel, routeMsg(unix.RTM_ADD, 8))
+	require.NoError(t, err)
+
+	select {
+	case <-changes:
+	case <-time.After(netChangeSettleWindow * 4):
+		t.Fatal("a burst should have reported a change")
+	}
+
+	// Nothing more from that burst
+	select {
+	case <-changes:
+		t.Fatal("a burst should report exactly once")
+	case <-time.After(netChangeSettleWindow):
+	}
+
+	// A change after the window has closed is a separate event and gets its own report.
+	_, err = unix.Write(kernel, routeMsg(unix.RTM_IFINFO, 8))
+	require.NoError(t, err)
+	select {
+	case <-changes:
+	case <-time.After(netChangeSettleWindow * 4):
+		t.Fatal("a later change should report again")
+	}
+
+	// Closing the socket is how the real thing shuts down
+	require.NoError(t, sock.Close())
+	select {
+	case <-done:
+	case <-time.After(time.Second * 5):
+		t.Fatal("watchRouteSocket did not return after the socket was closed")
+	}
+}
+
+func TestWatchRouteSocketIgnoresUninterestingMessages(t *testing.T) {
+	sock, kernel := socketPair(t)
+	changes := make(chan struct{}, 1)
+
+	done := make(chan struct{})
+	go func() {
+		watchRouteSocket(test.NewLogger(), sock, changes)
+		close(done)
+	}()
+
+	for _, msgType := range []uint8{unix.RTM_ADD, unix.RTM_DELETE, unix.RTM_GET, unix.RTM_MISS} {
+		_, err := unix.Write(kernel, routeMsg(msgType, 8))
+		require.NoError(t, err)
+	}
+
+	select {
+	case <-changes:
+		t.Fatal("route churn alone must not report a change")
+	case <-time.After(netChangeSettleWindow * 2):
+	}
+
+	require.NoError(t, sock.Close())
+	select {
+	case <-done:
+	case <-time.After(time.Second * 5):
+		t.Fatal("watchRouteSocket did not return after the socket was closed")
+	}
+}
+
+// TestWatchRouteSocketDropsRatherThanBlocks covers the coalescing send. A reader that is busy rebinding must not
+// wedge the watcher, and a second pending "the network moved" tells it nothing new anyway.
+func TestWatchRouteSocketDropsRatherThanBlocks(t *testing.T) {
+	sock, kernel := socketPair(t)
+	changes := make(chan struct{}, 1)
+
+	done := make(chan struct{})
+	go func() {
+		watchRouteSocket(test.NewLogger(), sock, changes)
+		close(done)
+	}()
+
+	// Nobody is reading changes, so after the first report the buffer is full for the rest of this test
+	for range 3 {
+		_, err := unix.Write(kernel, routeMsg(unix.RTM_NEWADDR, 8))
+		require.NoError(t, err)
+		time.Sleep(netChangeSettleWindow + time.Millisecond*250)
+	}
+
+	// The watcher must still be alive and responsive to a close
+	require.NoError(t, sock.Close())
+	select {
+	case <-done:
+	case <-time.After(time.Second * 5):
+		t.Fatal("watchRouteSocket wedged on a full channel")
+	}
+
+	assert.Len(t, changes, 1, "the pending report should have coalesced, not queued")
+}
+
+// TestWatchNetworkChangesStopsWithContext covers the detection path against a real routing socket, including that
+// cancelling the context closes the channel so a ranging caller falls out of its loop.
+func TestWatchNetworkChangesStopsWithContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+
+	changes, err := watchNetworkChanges(ctx, test.NewLogger())
+	require.NoError(t, err)
+	require.NotNil(t, changes, "darwin should support watching")
+
+	drained := make(chan struct{})
+	go func() {
+		for range changes {
+		}
+		close(drained)
+	}()
+
+	cancel()
+	select {
+	case <-drained:
+	case <-time.After(time.Second * 5):
+		t.Fatal("cancelling the context should close the changes channel")
+	}
+}
+
+// TestNetworkChangeMonitorStopsWithContext drives the whole monitor against a real routing socket: Start must block
+// watching, and cancelling the context (which is all Control does on shutdown, it never stops the monitor directly)
+// must return it and clean up the watch goroutines.
+func TestNetworkChangeMonitorStopsWithContext(t *testing.T) {
+	// IgnoreCurrent because other tests in this package leave readers running; we only care about what this test
+	// leaks itself.
+	defer goleak.VerifyNone(t, goleak.IgnoreCurrent())
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	l := test.NewLogger()
+	c := config.NewC(l)
+	require.NoError(t, c.LoadString("listen:\n  rebind_on_network_change: true\n"))
+	m := NewNetworkChangeMonitor(ctx, l, c)
+
+	done := make(chan struct{})
+	go func() {
+		m.Start(func() {})
+		close(done)
+	}()
+
+	// Start should be sitting on the routing socket, not have fallen out. If it returned early it either failed to
+	// watch or no-op'd, both of which we want to catch.
+	select {
+	case <-done:
+		t.Fatal("Start returned instead of watching")
+	case <-time.After(time.Millisecond * 250):
+	}
+
+	cancel()
+	select {
+	case <-done:
+	case <-time.After(time.Second * 5):
+		t.Fatal("Start did not return after the context was cancelled")
+	}
+
+	// Starting again after the context is dead must not open anything.
+	m.Start(func() {})
+}

udp/netchange_generic.go

@@ -0,0 +1,22 @@
+//go:build !darwin || ios || e2e_testing
+// +build !darwin ios e2e_testing
+
+package udp
+
+import (
+	"context"
+	"log/slog"
+)
+
+// watchNetworkChanges is a no-op outside of darwin.
+//
+// Darwin is the platform that scopes a udp socket to the interface it came up on, so it is the platform whose socket
+// goes stale when the local network changes. Everywhere else Rebind has nothing to do, so there is nothing to watch
+// for. iOS is excluded on purpose even though it is darwin: the host app already drives the rebind off NWPathMonitor,
+// and two things racing to rebind the same socket is worse than one.
+//
+// A nil channel means "not supported here", which callers must treat as "do not start a watcher" rather than
+// selecting on it, since a receive from a nil channel blocks forever.
+func watchNetworkChanges(_ context.Context, _ *slog.Logger) (<-chan struct{}, error) {
+	return nil, nil
+}

udp/netchange_test.go

@@ -0,0 +1,39 @@
+package udp
+
+import (
+	"context"
+	"testing"
+
+	"github.com/slackhq/nebula/config"
+	"github.com/slackhq/nebula/test"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newMonitor(t *testing.T, ctx context.Context, cfg string) *NetworkChangeMonitor {
+	t.Helper()
+	l := test.NewLogger()
+	c := config.NewC(l)
+	require.NoError(t, c.LoadString(cfg))
+	return NewNetworkChangeMonitor(ctx, l, c)
+}
+
+func TestNetworkChangeMonitorDefaultsOn(t *testing.T) {
+	// Says nothing about rebinding, so this covers the default.
+	m := newMonitor(t, context.Background(), "listen:\n  host: 0.0.0.0\n")
+	assert.True(t, m.enabled, "should default to on")
+}
+
+func TestNetworkChangeMonitorDisabledIsANoOp(t *testing.T) {
+	m := newMonitor(t, context.Background(), "listen:\n  rebind_on_network_change: false\n")
+	require.False(t, m.enabled)
+
+	// Must return without opening a socket. If it watched anything this would block.
+	m.Start(func() {})
+}
+
+func TestNetworkChangeMonitorNilRebindIsANoOp(t *testing.T) {
+	// Nothing to rebind, so there is no point watching, on any platform.
+	m := newMonitor(t, context.Background(), "listen:\n  rebind_on_network_change: true\n")
+	m.Start(nil)
+}

udp/udp_darwin.go

@@ -187,6 +187,9 @@ func (u *StdConn) SupportsMultipleReaders() bool {
 	return false
 }
 
+// Rebind clears the interface the kernel scoped this socket to, so that sends are routed against the current
+// routing table instead of the interface we happened to be on when the socket was created. Darwin pins sockets
+// this way on its own, which is what strands us after the underlying network changes.
 func (u *StdConn) Rebind() error {
 	var err error
 	if u.isV4 {
@@ -195,9 +198,5 @@ func (u *StdConn) Rebind() error {
 		err = syscall.SetsockoptInt(int(u.sysFd), syscall.IPPROTO_IPV6, syscall.IPV6_BOUND_IF, 0)
 	}
 
-	if err != nil {
-		u.l.Error("Failed to rebind udp socket", "error", err)
-	}
-
-	return nil
+	return err
 }

udp/udp_tester.go

@@ -10,6 +10,7 @@ import (
 	"net/netip"
 	"os"
 	"sync"
+	"sync/atomic"
 
 	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/header"
@@ -64,7 +65,9 @@ func acquirePacket() *Packet {
 }
 
 type TesterConn struct {
-	Addr netip.AddrPort
+	// addr is read by nebula's own goroutines on every send and by the router's flow renderer, and a test can
+	// move it mid-run to simulate roaming, so it is atomic rather than a plain field.
+	addr atomic.Pointer[netip.AddrPort]
 
 	RxPackets chan *Packet // Packets to receive into nebula
 	TxPackets chan *Packet // Packets transmitted outside by nebula
@@ -82,13 +85,24 @@ type TesterConn struct {
 }
 
 func NewListener(l *slog.Logger, ip netip.Addr, port int, _ bool, _ int) (Conn, error) {
-	return &TesterConn{
-		Addr:      netip.AddrPortFrom(ip, uint16(port)),
+	c := &TesterConn{
 		RxPackets: make(chan *Packet, 10),
 		TxPackets: make(chan *Packet, 10),
 		done:      make(chan struct{}),
 		l:         l,
-	}, nil
+	}
+	c.SetAddr(netip.AddrPortFrom(ip, uint16(port)))
+	return c, nil
+}
+
+// GetAddr returns the underlay address this conn currently sends from.
+func (u *TesterConn) GetAddr() netip.AddrPort {
+	return *u.addr.Load()
+}
+
+// SetAddr moves this conn to a new underlay address, standing in for a host waking up on a different network.
+func (u *TesterConn) SetAddr(addr netip.AddrPort) {
+	u.addr.Store(&addr)
 }
 
 // Send will place a UdpPacket onto the receive queue for nebula to consume
@@ -147,7 +161,7 @@ func (u *TesterConn) WriteTo(b []byte, addr netip.AddrPort) error {
 		p.Data = p.Data[:len(b)]
 	}
 	copy(p.Data, b)
-	p.From = u.Addr
+	p.From = u.GetAddr()
 	p.To = addr
 	select {
 	case <-u.done:
@@ -178,7 +192,7 @@ func NewUDPStatsEmitter(_ []Conn) func() {
 }
 
 func (u *TesterConn) LocalAddr() (netip.AddrPort, error) {
-	return u.Addr, nil
+	return u.GetAddr(), nil
 }
 
 func (u *TesterConn) SupportsMultipleReaders() bool {