add support for disabling relay via environment variable

* [relay] reduce QUIC initial packet size to 1280 (IPv6 min MTU)

* adjust QUIC initial packet size to 1232 based on RFC 9000 §14

client/iface/iface_test.go

@@ -589,6 +589,101 @@ func Test_ConnectPeers(t *testing.T) {
 
 }
 
+func Test_UserSpaceAddAllowedIPs(t *testing.T) {
+	ifaceName := fmt.Sprintf("utun%d", WgIntNumber+5)
+	wgIP := "10.99.99.21/30"
+	wgPort := 33105
+
+	newNet, err := stdnet.NewNet(context.Background(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	opts := WGIFaceOpts{
+		IFaceName:    ifaceName,
+		Address:      wgIP,
+		WGPort:       wgPort,
+		WGPrivKey:    key,
+		MTU:          DefaultMTU,
+		TransportNet: newNet,
+	}
+
+	iface, err := NewWGIFace(opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = iface.Create()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		if err := iface.Close(); err != nil {
+			t.Error(err)
+		}
+	}()
+
+	_, err = iface.Up()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	keepAlive := 15 * time.Second
+	initialAllowedIP := netip.MustParsePrefix("10.99.99.22/32")
+	endpoint, err := net.ResolveUDPAddr("udp", "127.0.0.1:9905")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Add peer with initial endpoint and first allowed IP
+	err = iface.UpdatePeer(peerPubKey, []netip.Prefix{initialAllowedIP}, keepAlive, endpoint, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Phase 1: generate 500 allowed IPs into a list
+	const extraIPs = 500
+	addedPrefixes := make([]netip.Prefix, 0, extraIPs)
+	for i := 0; i < extraIPs; i++ {
+		// Use 172.16.x.y/32 range: i encoded as two octets
+		prefix := netip.MustParsePrefix(fmt.Sprintf("172.16.%d.%d/32", i/256, i%256))
+		addedPrefixes = append(addedPrefixes, prefix)
+	}
+
+	// Phase 2: iterate over the list and add each allowed IP to the peer
+	phase2Start := time.Now()
+	for _, prefix := range addedPrefixes {
+		if addErr := iface.AddAllowedIP(peerPubKey, prefix); addErr != nil {
+			t.Fatalf("failed to add allowed IP %s: %v", prefix, addErr)
+		}
+	}
+	t.Logf("Phase 2 (add %d IPs to peer): %s", extraIPs, time.Since(phase2Start))
+
+	// Verify the peer has all 101 allowed IPs (1 initial + 100 added)
+	peer, err := getPeer(ifaceName, peerPubKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if peer.Endpoint.String() != endpoint.String() {
+		t.Fatalf("expected endpoint %s, got %s", endpoint, peer.Endpoint)
+	}
+
+	allExpected := append([]netip.Prefix{initialAllowedIP}, addedPrefixes...)
+	if len(peer.AllowedIPs) != len(allExpected) {
+		t.Fatalf("expected %d allowed IPs, got %d", len(allExpected), len(peer.AllowedIPs))
+	}
+
+	allowedIPSet := make(map[string]struct{}, len(peer.AllowedIPs))
+	for _, aip := range peer.AllowedIPs {
+		allowedIPSet[aip.String()] = struct{}{}
+	}
+	for _, expected := range allExpected {
+		if _, found := allowedIPSet[expected.String()]; !found {
+			t.Errorf("expected allowed IP %s not found in peer config", expected)
+		}
+	}
+}
+
 func getPeer(ifaceName, peerPubKey string) (wgtypes.Peer, error) {
 	wg, err := wgctrl.New()
 	if err != nil {

client/internal/connect.go

@@ -290,6 +290,10 @@ func (c *ConnectClient) run(mobileDependency MobileDependency, runningChan chan
 			return wrapErr(err)
 		}
 
+		if relayClient.IsDisableRelay() {
+			relayURLs = []string{}
+		}
+
 		relayManager := relayClient.NewManager(engineCtx, relayURLs, myPrivateKey.PublicKey().String(), engineConfig.MTU)
 		c.statusRecorder.SetRelayMgr(relayManager)
 		if len(relayURLs) > 0 {
@@ -310,6 +314,8 @@ func (c *ConnectClient) run(mobileDependency MobileDependency, runningChan chan
 		c.engineMutex.Lock()
 		engine := NewEngine(engineCtx, cancel, signalClient, mgmClient, relayManager, engineConfig, mobileDependency, c.statusRecorder, checks, stateManager)
 		engine.SetSyncResponsePersistence(c.persistSyncResponse)
+		engine.SetReadyChan(runningChan)
+		runningChan = nil
 		c.engine = engine
 		c.engineMutex.Unlock()
 
@@ -330,11 +336,6 @@ func (c *ConnectClient) run(mobileDependency MobileDependency, runningChan chan
 		log.Infof("Netbird engine started, the IP is: %s", peerConfig.GetAddress())
 		state.Set(StatusConnected)
 
-		if runningChan != nil {
-			close(runningChan)
-			runningChan = nil
-		}
-
 		<-engineCtx.Done()
 
 		c.engineMutex.Lock()

client/internal/engine.go

@@ -28,8 +28,8 @@ import (
 	"github.com/netbirdio/netbird/client/firewall"
 	firewallManager "github.com/netbirdio/netbird/client/firewall/manager"
 	"github.com/netbirdio/netbird/client/iface"
-	nbnetstack "github.com/netbirdio/netbird/client/iface/netstack"
 	"github.com/netbirdio/netbird/client/iface/device"
+	nbnetstack "github.com/netbirdio/netbird/client/iface/netstack"
 	"github.com/netbirdio/netbird/client/iface/udpmux"
 	"github.com/netbirdio/netbird/client/internal/acl"
 	"github.com/netbirdio/netbird/client/internal/debug"
@@ -217,6 +217,10 @@ type Engine struct {
 	// WireGuard interface monitor
 	wgIfaceMonitor *WGIfaceMonitor
 
+	// readyChan is closed when the first sync message is received from management
+	readyChan     chan struct{}
+	readyChanOnce sync.Once
+
 	// shutdownWg tracks all long-running goroutines to ensure clean shutdown
 	shutdownWg sync.WaitGroup
 
@@ -275,6 +279,10 @@ func NewEngine(
 	return engine
 }
 
+func (e *Engine) SetReadyChan(ch chan struct{}) {
+	e.readyChan = ch
+}
+
 func (e *Engine) Stop() error {
 	if e == nil {
 		// this seems to be a very odd case but there was the possibility if the netbird down command comes before the engine is fully started
@@ -834,6 +842,13 @@ func (e *Engine) handleSync(update *mgmProto.SyncResponse) error {
 	defer func() {
 		log.Infof("sync finished in %s", time.Since(started))
 	}()
+
+	e.readyChanOnce.Do(func() {
+		if e.readyChan != nil {
+			close(e.readyChan)
+		}
+	})
+
 	e.syncMsgMux.Lock()
 	defer e.syncMsgMux.Unlock()
 
@@ -880,9 +895,11 @@ func (e *Engine) handleSync(update *mgmProto.SyncResponse) error {
 		// todo update signal
 	}
 
+	uCheckTime := time.Now()
 	if err := e.updateChecksIfNew(update.Checks); err != nil {
 		return err
 	}
+	log.Infof("update check finished in %s", time.Since(uCheckTime))
 
 	nm := update.GetNetworkMap()
 	if nm == nil {
@@ -925,7 +942,9 @@ func (e *Engine) handleRelayUpdate(update *mgmProto.RelayConfig) error {
 			return fmt.Errorf("update relay token: %w", err)
 		}
 
-		e.relayManager.UpdateServerURLs(update.Urls)
+		if !relayClient.IsDisableRelay() {
+			e.relayManager.UpdateServerURLs(update.Urls)
+		}
 
 		// Just in case the agent started with an MGM server where the relay was disabled but was later enabled.
 		// We can ignore all errors because the guard will manage the reconnection retries.

client/internal/peer/ice/agent.go

@@ -12,6 +12,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	"github.com/netbirdio/netbird/client/internal/stdnet"
+	relayClient "github.com/netbirdio/netbird/shared/relay/client"
 )
 
 const (
@@ -125,6 +126,10 @@ func GenerateICECredentials() (string, string, error) {
 }
 
 func CandidateTypes() []ice.CandidateType {
+	if relayClient.IsDisableRelay() {
+		return []ice.CandidateType{ice.CandidateTypeHost, ice.CandidateTypeServerReflexive, ice.CandidateTypeRelay}
+	}
+
 	if hasICEForceRelayConn() {
 		return []ice.CandidateType{ice.CandidateTypeRelay}
 	}

relay/server/listener/quic/listener.go

@@ -11,6 +11,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	"github.com/netbirdio/netbird/relay/protocol"
+	nbRelay "github.com/netbirdio/netbird/shared/relay"
 )
 
 const Proto protocol.Protocol = "quic"
@@ -27,7 +28,7 @@ type Listener struct {
 func (l *Listener) Listen(acceptFn func(conn net.Conn)) error {
 	quicCfg := &quic.Config{
 		EnableDatagrams:   true,
-		InitialPacketSize: 1452,
+		InitialPacketSize: nbRelay.QUICInitialPacketSize,
 	}
 	listener, err := quic.ListenAddr(l.Address, l.TLSConfig, quicCfg)
 	if err != nil {

shared/relay/client/dialer/quic/quic.go

@@ -12,6 +12,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	nbnet "github.com/netbirdio/netbird/client/net"
+	nbRelay "github.com/netbirdio/netbird/shared/relay"
 	quictls "github.com/netbirdio/netbird/shared/relay/tls"
 )
 
@@ -42,7 +43,7 @@ func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
 		KeepAlivePeriod:   30 * time.Second,
 		MaxIdleTimeout:    4 * time.Minute,
 		EnableDatagrams:   true,
-		InitialPacketSize: 1452,
+		InitialPacketSize: nbRelay.QUICInitialPacketSize,
 	}
 
 	udpConn, err := nbnet.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 0})

shared/relay/client/env.go

@@ -0,0 +1,15 @@
+package client
+
+import (
+	"os"
+	"strconv"
+)
+
+const (
+	envKeyNBDebugDisableRelay = "NB_DEBUG_DISABLE_RELAY"
+)
+
+func IsDisableRelay() bool {
+	v, _ := strconv.ParseBool(os.Getenv(envKeyNBDebugDisableRelay))
+	return v
+}

shared/relay/constants.go

@@ -3,4 +3,9 @@ package relay
 const (
 	// WebSocketURLPath is the path for the websocket relay connection
 	WebSocketURLPath = "/relay"
+
+	// QUICInitialPacketSize is the conservative initial QUIC packet size (bytes)
+	// for unknown-path PMTU, per RFC 9000 §14: 1280 (IPv6 min MTU) − 40 (IPv6
+	// header) − 8 (UDP header) = 1232. DPLPMTUD may probe larger sizes later.
+	QUICInitialPacketSize = 1232
 )