Reinject captured first packet on lazy peer activation and keep peer alive across endpoint swap

client/iface/wgproxy/bind/proxy.go

@@ -136,6 +136,15 @@ func (p *ProxyBind) CloseConn() error {
 	return p.close()
 }
 
+// InjectPacket writes b to the remote peer over the underlying transport.
+func (p *ProxyBind) InjectPacket(b []byte) error {
+	if p.remoteConn == nil {
+		return fmt.Errorf("proxy not started")
+	}
+	_, err := p.remoteConn.Write(b)
+	return err
+}
+
 func (p *ProxyBind) close() error {
 	if p.remoteConn == nil {
 		return nil

client/iface/wgproxy/ebpf/wrapper.go

@@ -219,6 +219,15 @@ func (p *ProxyWrapper) RedirectAs(endpoint *net.UDPAddr) {
 	p.pausedCond.L.Unlock()
 }
 
+// InjectPacket writes b to the remote peer over the underlying transport.
+func (p *ProxyWrapper) InjectPacket(b []byte) error {
+	if p.remoteConn == nil {
+		return fmt.Errorf("proxy not started")
+	}
+	_, err := p.remoteConn.Write(b)
+	return err
+}
+
 // CloseConn close the remoteConn and automatically remove the conn instance from the map
 func (p *ProxyWrapper) CloseConn() error {
 	if p.cancel == nil {

client/iface/wgproxy/proxy.go

@@ -18,4 +18,8 @@ type Proxy interface {
 	RedirectAs(endpoint *net.UDPAddr)
 	CloseConn() error
 	SetDisconnectListener(disconnected func())
+
+	// InjectPacket writes a raw packet directly to the remote peer over the underlying transport,
+	// bypassing WireGuard. Used to replay the captured lazyconn handshake initiation.
+	InjectPacket(b []byte) error
 }

client/iface/wgproxy/udp/proxy.go

@@ -147,6 +147,15 @@ func (p *WGUDPProxy) RedirectAs(endpoint *net.UDPAddr) {
 	p.sendPkg = p.srcFakerConn.SendPkg
 }
 
+// InjectPacket writes b to the remote peer over the underlying transport.
+func (p *WGUDPProxy) InjectPacket(b []byte) error {
+	if p.remoteConn == nil {
+		return fmt.Errorf("proxy not started")
+	}
+	_, err := p.remoteConn.Write(b)
+	return err
+}
+
 // CloseConn close the localConn
 func (p *WGUDPProxy) CloseConn() error {
 	if p.cancel == nil {

client/internal/engine_test.go

@@ -224,6 +224,10 @@ func (m *MockWGIface) LastActivities() map[string]monotime.Time {
 	return nil
 }
 
+func (m *MockWGIface) MTU() uint16 {
+	return 1280
+}
+
 func (m *MockWGIface) SetPresharedKey(peerKey string, psk wgtypes.Key, updateOnly bool) error {
 	return nil
 }

client/internal/iface_common.go

@@ -44,4 +44,5 @@ type wgIfaceBase interface {
 	FullStats() (*configurer.Stats, error)
 	LastActivities() map[string]monotime.Time
 	SetPresharedKey(peerKey string, psk wgtypes.Key, updateOnly bool) error
+	MTU() uint16
 }

client/internal/lazyconn/activity/lazy_conn.go

@@ -4,15 +4,21 @@ import (
 	"context"
 	"io"
 	"net"
+	"sync"
 	"time"
 )
 
 // lazyConn detects activity when WireGuard attempts to send packets.
-// It does not deliver packets, only signals that activity occurred.
+// It does not deliver packets, only signals that activity occurred. The first packet WG-go writes
+// is captured so the lazyconn manager can reinject it through the real bind endpoint, avoiding the
+// handshake-retry wait when the real connection comes up.
 type lazyConn struct {
 	activityCh chan struct{}
 	ctx        context.Context
 	cancel     context.CancelFunc
+
+	mu             sync.Mutex
+	capturedPacket []byte
 }
 
 // newLazyConn creates a new lazyConn for activity detection.
@@ -31,12 +37,19 @@ func (c *lazyConn) Read(_ []byte) (n int, err error) {
 	return 0, io.EOF
 }
 
-// Write signals activity detection when ICEBind routes packets to this endpoint.
+// Write signals activity detection when ICEBind routes packets to this endpoint. The first packet
+// is cloned and stored so it can be replayed on the real endpoint once ICE/relay comes up.
 func (c *lazyConn) Write(b []byte) (n int, err error) {
 	if c.ctx.Err() != nil {
 		return 0, io.EOF
 	}
 
+	c.mu.Lock()
+	if c.capturedPacket == nil && len(b) > 0 {
+		c.capturedPacket = append([]byte(nil), b...)
+	}
+	c.mu.Unlock()
+
 	select {
 	case c.activityCh <- struct{}{}:
 	default:
@@ -45,6 +58,13 @@ func (c *lazyConn) Write(b []byte) (n int, err error) {
 	return len(b), nil
 }
 
+// CapturedPacket returns a copy of the first packet written to this connection, or nil.
+func (c *lazyConn) CapturedPacket() []byte {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.capturedPacket
+}
+
 // ActivityChan returns the channel that signals when activity is detected.
 func (c *lazyConn) ActivityChan() <-chan struct{} {
 	return c.activityCh

client/internal/lazyconn/activity/listener_bind.go

@@ -118,16 +118,23 @@ func (d *BindListener) ReadPackets() {
 		d.peerCfg.Log.Infof("exit from activity listener")
 	}
 
-	d.peerCfg.Log.Debugf("removing lazy endpoint for peer %s", d.peerCfg.PublicKey)
-	if err := d.wgIface.RemovePeer(d.peerCfg.PublicKey); err != nil {
-		d.peerCfg.Log.Errorf("failed to remove endpoint: %s", err)
-	}
-
+	// Leave the peer in place. ConfigureWGEndpoint will UpdatePeer with the real endpoint;
+	// removing the peer here wipes wireguard-go's staged queue and drops the user packet that
+	// triggered activation.
 	_ = d.lazyConn.Close()
 	d.bind.RemoveEndpoint(d.fakeIP)
 	d.done.Done()
 }
 
+// CapturedPacket returns the first packet that triggered activity, or nil if none was captured.
+// Safe to call after ReadPackets returns.
+func (d *BindListener) CapturedPacket() []byte {
+	if d.lazyConn == nil {
+		return nil
+	}
+	return d.lazyConn.CapturedPacket()
+}
+
 // Close stops the listener and cleans up resources.
 func (d *BindListener) Close() {
 	d.peerCfg.Log.Infof("closing activity listener (LazyConn)")

client/internal/lazyconn/activity/listener_bind_test.go

@@ -68,6 +68,10 @@ func (m *MockWGIfaceBind) GetBind() device.EndpointManager {
 	return m.endpointMgr
 }
 
+func (m *MockWGIfaceBind) MTU() uint16 {
+	return 1280
+}
+
 func TestBindListener_Creation(t *testing.T) {
 	mockEndpointMgr := newMockEndpointManager()
 	mockIface := &MockWGIfaceBind{endpointMgr: mockEndpointMgr}
@@ -207,8 +211,9 @@ func TestManager_BindMode(t *testing.T) {
 	require.NoError(t, err)
 
 	select {
-	case peerConnID := <-mgr.OnActivityChan:
-		assert.Equal(t, cfg.PeerConnID, peerConnID, "Received peer connection ID should match")
+	case ev := <-mgr.OnActivityChan:
+		assert.Equal(t, cfg.PeerConnID, ev.PeerConnID, "Received peer connection ID should match")
+		assert.Equal(t, []byte{0x01, 0x02, 0x03}, ev.FirstPacket, "First packet should be captured for reinjection")
 	case <-time.After(2 * time.Second):
 		t.Fatal("timeout waiting for activity notification")
 	}
@@ -266,8 +271,8 @@ func TestManager_BindMode_MultiplePeers(t *testing.T) {
 	receivedPeers := make(map[peerid.ConnID]bool)
 	for i := 0; i < 2; i++ {
 		select {
-		case peerConnID := <-mgr.OnActivityChan:
-			receivedPeers[peerConnID] = true
+		case ev := <-mgr.OnActivityChan:
+			receivedPeers[ev.PeerConnID] = true
 		case <-time.After(2 * time.Second):
 			t.Fatal("timeout waiting for activity notifications")
 		}

client/internal/lazyconn/activity/listener_udp.go

@@ -8,6 +8,7 @@ import (
 
 	log "github.com/sirupsen/logrus"
 
+	"github.com/netbirdio/netbird/client/iface/bufsize"
 	"github.com/netbirdio/netbird/client/internal/lazyconn"
 )
 
@@ -20,6 +21,8 @@ type UDPListener struct {
 	done     sync.Mutex
 
 	isClosed atomic.Bool
+
+	capturedPacket []byte
 }
 
 // NewUDPListener creates a listener that detects activity via UDP socket reads.
@@ -46,9 +49,13 @@ func NewUDPListener(wgIface WgInterface, cfg lazyconn.PeerConfig) (*UDPListener,
 }
 
 // ReadPackets blocks reading from the UDP socket until activity is detected or the listener is closed.
+// The first packet that triggers activity is captured so it can be reinjected through the real
+// transport once it is established. Without this, kernel WireGuard's handshake initiation would be
+// dropped and WG would only retry after REKEY_TIMEOUT.
 func (d *UDPListener) ReadPackets() {
+	buf := make([]byte, int(d.wgIface.MTU())+bufsize.WGBufferOverhead)
 	for {
-		n, remoteAddr, err := d.conn.ReadFromUDP(make([]byte, 1))
+		n, remoteAddr, err := d.conn.ReadFromUDP(buf)
 		if err != nil {
 			if d.isClosed.Load() {
 				d.peerCfg.Log.Infof("exit from activity listener")
@@ -62,20 +69,24 @@ func (d *UDPListener) ReadPackets() {
 			d.peerCfg.Log.Warnf("received %d bytes from %s, too short", n, remoteAddr)
 			continue
 		}
-		d.peerCfg.Log.Infof("activity detected")
+		d.capturedPacket = append([]byte(nil), buf[:n]...)
+		d.peerCfg.Log.Infof("activity detected, captured %d bytes for reinjection", n)
 		break
 	}
 
-	d.peerCfg.Log.Debugf("removing lazy endpoint: %s", d.endpoint.String())
-	if err := d.wgIface.RemovePeer(d.peerCfg.PublicKey); err != nil {
-		d.peerCfg.Log.Errorf("failed to remove endpoint: %s", err)
-	}
-
-	// Ignore close error as it may return "use of closed network connection" if already closed.
+	// Leave the peer in place. ConfigureWGEndpoint will UpdatePeer with the real endpoint;
+	// removing the peer here wipes kernel WG's staged queue and drops the user packet that
+	// triggered activation.
 	_ = d.conn.Close()
 	d.done.Unlock()
 }
 
+// CapturedPacket returns the first packet that triggered activity, or nil if none was captured.
+// Safe to call after ReadPackets returns.
+func (d *UDPListener) CapturedPacket() []byte {
+	return d.capturedPacket
+}
+
 // Close stops the listener and cleans up resources.
 func (d *UDPListener) Close() {
 	d.peerCfg.Log.Infof("closing activity listener: %s", d.conn.LocalAddr().String())

client/internal/lazyconn/activity/manager.go

@@ -19,6 +19,14 @@ import (
 type listener interface {
 	ReadPackets()
 	Close()
+	CapturedPacket() []byte
+}
+
+// Event reports activity on a managed peer. FirstPacket is the bytes that triggered activation,
+// captured for reinjection through the real transport.
+type Event struct {
+	PeerConnID  peerid.ConnID
+	FirstPacket []byte
 }
 
 type WgInterface interface {
@@ -26,10 +34,11 @@ type WgInterface interface {
 	UpdatePeer(peerKey string, allowedIps []netip.Prefix, keepAlive time.Duration, endpoint *net.UDPAddr, preSharedKey *wgtypes.Key) error
 	IsUserspaceBind() bool
 	Address() wgaddr.Address
+	MTU() uint16
 }
 
 type Manager struct {
-	OnActivityChan chan peerid.ConnID
+	OnActivityChan chan Event
 
 	wgIface WgInterface
 
@@ -41,7 +50,7 @@ type Manager struct {
 
 func NewManager(wgIface WgInterface) *Manager {
 	m := &Manager{
-		OnActivityChan: make(chan peerid.ConnID, 1),
+		OnActivityChan: make(chan Event, 1),
 		wgIface:        wgIface,
 		peers:          make(map[peerid.ConnID]listener),
 		done:           make(chan struct{}),
@@ -116,12 +125,12 @@ func (m *Manager) waitForTraffic(l listener, peerConnID peerid.ConnID) {
 	delete(m.peers, peerConnID)
 	m.mu.Unlock()
 
-	m.notify(peerConnID)
+	m.notify(Event{PeerConnID: peerConnID, FirstPacket: l.CapturedPacket()})
 }
 
-func (m *Manager) notify(peerConnID peerid.ConnID) {
+func (m *Manager) notify(ev Event) {
 	select {
 	case <-m.done:
-	case m.OnActivityChan <- peerConnID:
+	case m.OnActivityChan <- ev:
 	}
 }

client/internal/lazyconn/activity/manager_test.go

@@ -44,6 +44,10 @@ func (m MocWGIface) Address() wgaddr.Address {
 	}
 }
 
+func (m MocWGIface) MTU() uint16 {
+	return 1280
+}
+
 // GetPeerListener is a test helper to access listeners
 func (m *Manager) GetPeerListener(peerConnID peerid.ConnID) (listener, bool) {
 	m.mu.Lock()
@@ -86,9 +90,9 @@ func TestManager_MonitorPeerActivity(t *testing.T) {
 	}
 
 	select {
-	case peerConnID := <-mgr.OnActivityChan:
-		if peerConnID != peerCfg1.PeerConnID {
-			t.Fatalf("unexpected peerConnID: %v", peerConnID)
+	case ev := <-mgr.OnActivityChan:
+		if ev.PeerConnID != peerCfg1.PeerConnID {
+			t.Fatalf("unexpected peerConnID: %v", ev.PeerConnID)
 		}
 	case <-time.After(1 * time.Second):
 	}

client/internal/lazyconn/manager/manager.go

@@ -130,8 +130,8 @@ func (m *Manager) Start(ctx context.Context) {
 		select {
 		case <-ctx.Done():
 			return
-		case peerConnID := <-m.activityManager.OnActivityChan:
-			m.onPeerActivity(peerConnID)
+		case ev := <-m.activityManager.OnActivityChan:
+			m.onPeerActivity(ev)
 		case peerIDs := <-m.inactivityManager.InactivePeersChan():
 			m.onPeerInactivityTimedOut(peerIDs)
 		}
@@ -513,13 +513,13 @@ func (m *Manager) checkHaGroupActivity(haGroup route.HAUniqueID, peerID string,
 	return false
 }
 
-func (m *Manager) onPeerActivity(peerConnID peerid.ConnID) {
+func (m *Manager) onPeerActivity(ev activity.Event) {
 	m.managedPeersMu.Lock()
 	defer m.managedPeersMu.Unlock()
 
-	mp, ok := m.managedPeersByConnID[peerConnID]
+	mp, ok := m.managedPeersByConnID[ev.PeerConnID]
 	if !ok {
-		log.Errorf("peer not found by conn id: %v", peerConnID)
+		log.Errorf("peer not found by conn id: %v", ev.PeerConnID)
 		return
 	}
 
@@ -536,7 +536,7 @@ func (m *Manager) onPeerActivity(peerConnID peerid.ConnID) {
 
 	m.activateHAGroupPeers(mp.peerCfg)
 
-	m.peerStore.PeerConnOpen(m.engineCtx, mp.peerCfg.PublicKey)
+	m.peerStore.PeerConnOpenWithFirstPacket(m.engineCtx, mp.peerCfg.PublicKey, ev.FirstPacket)
 }
 
 func (m *Manager) onPeerInactivityTimedOut(peerIDs map[string]struct{}) {

client/internal/lazyconn/wgiface.go

@@ -17,4 +17,5 @@ type WGIface interface {
 	IsUserspaceBind() bool
 	Address() wgaddr.Address
 	LastActivities() map[string]monotime.Time
+	MTU() uint16
 }

client/internal/peer/conn.go

@@ -136,6 +136,51 @@ type Conn struct {
 	// Connection stage timestamps for metrics
 	metricsRecorder MetricsRecorder
 	metricsStages   *MetricsStages
+
+	// pendingFirstPacket is the lazyconn-captured handshake init, replayed once the real
+	// transport is up.
+	pendingFirstPacket []byte
+}
+
+// SetPendingFirstPacket stashes a packet to be replayed once the connection is established.
+func (conn *Conn) SetPendingFirstPacket(b []byte) {
+	conn.mu.Lock()
+	conn.pendingFirstPacket = append([]byte(nil), b...)
+	conn.mu.Unlock()
+}
+
+// takePendingFirstPacket returns and clears the stashed first packet. Caller must hold conn.mu.
+func (conn *Conn) takePendingFirstPacket() []byte {
+	pkt := conn.pendingFirstPacket
+	conn.pendingFirstPacket = nil
+	return pkt
+}
+
+// injectPendingFirstPacket replays the captured handshake through the proxy if present, else
+// directly through the ICE conn. Caller must hold conn.mu.
+func (conn *Conn) injectPendingFirstPacket(proxy wgproxy.Proxy, directConn net.Conn) {
+	pkt := conn.takePendingFirstPacket()
+	if len(pkt) == 0 {
+		return
+	}
+
+	switch {
+	case proxy != nil:
+		if err := proxy.InjectPacket(pkt); err != nil {
+			conn.Log.Debugf("failed to reinject captured first packet via proxy: %v", err)
+			return
+		}
+	case directConn != nil:
+		if _, err := directConn.Write(pkt); err != nil {
+			conn.Log.Debugf("failed to reinject captured first packet via direct conn: %v", err)
+			return
+		}
+	default:
+		conn.Log.Debugf("no transport available to reinject captured first packet")
+		return
+	}
+
+	conn.Log.Debugf("reinjected captured first packet (%d bytes)", len(pkt))
 }
 
 // NewConn creates a new not opened Conn to the remote peer.
@@ -423,6 +468,8 @@ func (conn *Conn) onICEConnectionIsReady(priority conntype.ConnPriority, iceConn
 		conn.wgProxyRelay.RedirectAs(ep)
 	}
 
+	conn.injectPendingFirstPacket(wgProxy, iceConnInfo.RemoteConn)
+
 	conn.currentConnPriority = priority
 	conn.statusICE.SetConnected()
 	conn.updateIceState(iceConnInfo, updateTime)
@@ -546,6 +593,8 @@ func (conn *Conn) onRelayConnectionIsReady(rci RelayConnInfo) {
 
 	wgConfigWorkaround()
 
+	conn.injectPendingFirstPacket(wgProxy, nil)
+
 	conn.rosenpassRemoteKey = rci.rosenpassPubKey
 	conn.currentConnPriority = conntype.Relay
 	conn.statusRelay.SetConnected()

client/internal/peerstore/store.go

@@ -81,6 +81,12 @@ func (s *Store) PeerConn(pubKey string) (*peer.Conn, bool) {
 }
 
 func (s *Store) PeerConnOpen(ctx context.Context, pubKey string) {
+	s.PeerConnOpenWithFirstPacket(ctx, pubKey, nil)
+}
+
+// PeerConnOpenWithFirstPacket opens the peer connection and stashes a first packet to be
+// reinjected once the real transport is established.
+func (s *Store) PeerConnOpenWithFirstPacket(ctx context.Context, pubKey string, firstPacket []byte) {
 	s.peerConnsMu.RLock()
 	defer s.peerConnsMu.RUnlock()
 
@@ -88,11 +94,13 @@ func (s *Store) PeerConnOpen(ctx context.Context, pubKey string) {
 	if !ok {
 		return
 	}
+	if len(firstPacket) > 0 {
+		p.SetPendingFirstPacket(firstPacket)
+	}
 	// this can be blocked because of the connect open limiter semaphore
 	if err := p.Open(ctx); err != nil {
 		p.Log.Errorf("failed to open peer connection: %v", err)
 	}
-
 }
 
 func (s *Store) PeerConnIdle(pubKey string) {