Add relayServerIP field to signal for foreign-relay fallback dial

client/internal/engine.go

@@ -2384,6 +2384,8 @@ func convertToOfferAnswer(msg *sProto.Message) (*peer.OfferAnswer, error) {
 		}
 	}
 
+	relayIP := decodeRelayIP(msg.GetBody().GetRelayServerIP())
+
 	offerAnswer := peer.OfferAnswer{
 		IceCredentials: peer.IceCredentials{
 			UFrag: remoteCred.UFrag,
@@ -2394,7 +2396,23 @@ func convertToOfferAnswer(msg *sProto.Message) (*peer.OfferAnswer, error) {
 		RosenpassPubKey: rosenpassPubKey,
 		RosenpassAddr:   rosenpassAddr,
 		RelaySrvAddress: msg.GetBody().GetRelayServerAddress(),
+		RelaySrvIP:      relayIP,
 		SessionID:       sessionID,
 	}
 	return &offerAnswer, nil
 }
+
+// decodeRelayIP decodes the proto relayServerIP bytes (4 or 16) into a
+// netip.Addr. Returns the zero value for empty input and logs a warning
+// for malformed payloads.
+func decodeRelayIP(b []byte) netip.Addr {
+	if len(b) == 0 {
+		return netip.Addr{}
+	}
+	ip, ok := netip.AddrFromSlice(b)
+	if !ok {
+		log.Warnf("invalid relayServerIP in signal message (%d bytes), ignoring", len(b))
+		return netip.Addr{}
+	}
+	return ip.Unmap()
+}

client/internal/peer/handshaker.go

@@ -3,6 +3,7 @@ package peer
 import (
 	"context"
 	"errors"
+	"net/netip"
 	"sync"
 	"sync/atomic"
 
@@ -40,6 +41,10 @@ type OfferAnswer struct {
 
 	// relay server address
 	RelaySrvAddress string
+	// RelaySrvIP is the IP the remote peer is connected to on its
+	// relay server. Used as a fallback dial target if DNS for RelaySrvAddress
+	// fails. Zero value if the peer did not advertise an IP.
+	RelaySrvIP netip.Addr
 	// SessionID is the unique identifier of the session, used to discard old messages
 	SessionID *ICESessionID
 }
@@ -219,6 +224,7 @@ func (h *Handshaker) buildOfferAnswer() OfferAnswer {
 
 	if addr, err := h.relay.RelayInstanceAddress(); err == nil {
 		answer.RelaySrvAddress = addr
+		answer.RelaySrvIP = h.relay.RelayInstanceIP()
 	}
 
 	return answer

client/internal/peer/signaler.go

@@ -54,19 +54,19 @@ func (s *Signaler) signalOfferAnswer(offerAnswer OfferAnswer, remoteKey string,
 			log.Warnf("failed to get session ID bytes: %v", err)
 		}
 	}
-	msg, err := signal.MarshalCredential(
-		s.wgPrivateKey,
-		offerAnswer.WgListenPort,
-		remoteKey,
-		&signal.Credential{
+	msg, err := signal.MarshalCredential(s.wgPrivateKey, remoteKey, signal.CredentialPayload{
+		Type:         bodyType,
+		WgListenPort: offerAnswer.WgListenPort,
+		Credential: &signal.Credential{
 			UFrag: offerAnswer.IceCredentials.UFrag,
 			Pwd:   offerAnswer.IceCredentials.Pwd,
 		},
-		bodyType,
-		offerAnswer.RosenpassPubKey,
-		offerAnswer.RosenpassAddr,
-		offerAnswer.RelaySrvAddress,
-		sessionIDBytes)
+		RosenpassPubKey: offerAnswer.RosenpassPubKey,
+		RosenpassAddr:   offerAnswer.RosenpassAddr,
+		RelaySrvAddress: offerAnswer.RelaySrvAddress,
+		RelaySrvIP:      offerAnswer.RelaySrvIP,
+		SessionID:       sessionIDBytes,
+	})
 	if err != nil {
 		return err
 	}

client/internal/peer/worker_relay.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"net"
+	"net/netip"
 	"sync"
 	"sync/atomic"
 
@@ -60,8 +61,12 @@ func (w *WorkerRelay) OnNewOffer(remoteOfferAnswer *OfferAnswer) {
 	}
 
 	srv := w.preferredRelayServer(currentRelayAddress, remoteOfferAnswer.RelaySrvAddress)
+	var fallbackIP netip.Addr
+	if srv == remoteOfferAnswer.RelaySrvAddress {
+		fallbackIP = remoteOfferAnswer.RelaySrvIP
+	}
 
-	relayedConn, err := w.relayManager.OpenConn(w.peerCtx, srv, w.config.Key)
+	relayedConn, err := w.relayManager.OpenConn(w.peerCtx, srv, w.config.Key, fallbackIP)
 	if err != nil {
 		if errors.Is(err, relayClient.ErrConnAlreadyExists) {
 			w.log.Debugf("handled offer by reusing existing relay connection")
@@ -94,6 +99,10 @@ func (w *WorkerRelay) RelayInstanceAddress() (string, error) {
 	return w.relayManager.RelayInstanceAddress()
 }
 
+func (w *WorkerRelay) RelayInstanceIP() netip.Addr {
+	return w.relayManager.RelayInstanceIP()
+}
+
 func (w *WorkerRelay) IsRelayConnectionSupportedWithPeer() bool {
 	return w.relaySupportedOnRemotePeer.Load() && w.RelayIsSupportedLocally()
 }

relay/test/benchmark_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/rand"
 	"fmt"
 	"net"
+	"net/netip"
 	"os"
 	"sync"
 	"testing"
@@ -101,7 +102,7 @@ func transfer(t *testing.T, testData []byte, peerPairs int) {
 
 	clientsSender := make([]*client.Client, peerPairs)
 	for i := 0; i < cap(clientsSender); i++ {
-		c := client.NewClient(serverConnURL, hmacTokenStore, "sender-"+fmt.Sprint(i), iface.DefaultMTU)
+		c := client.NewClient(serverConnURL, netip.Addr{}, hmacTokenStore, "sender-"+fmt.Sprint(i), iface.DefaultMTU)
 		err := c.Connect(ctx)
 		if err != nil {
 			t.Fatalf("failed to connect to server: %s", err)
@@ -111,7 +112,7 @@ func transfer(t *testing.T, testData []byte, peerPairs int) {
 
 	clientsReceiver := make([]*client.Client, peerPairs)
 	for i := 0; i < cap(clientsReceiver); i++ {
-		c := client.NewClient(serverConnURL, hmacTokenStore, "receiver-"+fmt.Sprint(i), iface.DefaultMTU)
+		c := client.NewClient(serverConnURL, netip.Addr{}, hmacTokenStore, "receiver-"+fmt.Sprint(i), iface.DefaultMTU)
 		err := c.Connect(ctx)
 		if err != nil {
 			t.Fatalf("failed to connect to server: %s", err)

relay/testec2/relay.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"net/netip"
 	"sync"
 	"time"
 
@@ -71,7 +72,7 @@ func prepareConnsSender(serverConnURL string, peerPairs int) []net.Conn {
 	ctx := context.Background()
 	clientsSender := make([]*client.Client, peerPairs)
 	for i := 0; i < cap(clientsSender); i++ {
-		c := client.NewClient(serverConnURL, hmacTokenStore, "sender-"+fmt.Sprint(i), iface.DefaultMTU)
+		c := client.NewClient(serverConnURL, netip.Addr{}, hmacTokenStore, "sender-"+fmt.Sprint(i), iface.DefaultMTU)
 		if err := c.Connect(ctx); err != nil {
 			log.Fatalf("failed to connect to server: %s", err)
 		}
@@ -157,7 +158,7 @@ func runReader(conn net.Conn) time.Duration {
 func prepareConnsReceiver(serverConnURL string, peerPairs int) []net.Conn {
 	clientsReceiver := make([]*client.Client, peerPairs)
 	for i := 0; i < cap(clientsReceiver); i++ {
-		c := client.NewClient(serverConnURL, hmacTokenStore, "receiver-"+fmt.Sprint(i), iface.DefaultMTU)
+		c := client.NewClient(serverConnURL, netip.Addr{}, hmacTokenStore, "receiver-"+fmt.Sprint(i), iface.DefaultMTU)
 		err := c.Connect(context.Background())
 		if err != nil {
 			log.Fatalf("failed to connect to server: %s", err)

shared/relay/client/client.go

@@ -2,8 +2,12 @@ package client
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net"
+	"net/netip"
+	"net/url"
+	"strings"
 	"sync"
 	"time"
 
@@ -146,6 +150,7 @@ func (cc *connContainer) close() {
 type Client struct {
 	log            *log.Entry
 	connectionURL  string
+	fallbackIP     netip.Addr
 	authTokenStore *auth.TokenStore
 	hashedID       messages.PeerID
 
@@ -170,13 +175,16 @@ type Client struct {
 }
 
 // NewClient creates a new client for the relay server. The client is not connected to the server until the Connect
-func NewClient(serverURL string, authTokenStore *auth.TokenStore, peerID string, mtu uint16) *Client {
+// is called. fallbackIP, when valid, is used as a dial-time fallback if the FQDN-based dial fails. TLS
+// verification still uses the FQDN from serverURL via SNI.
+func NewClient(serverURL string, fallbackIP netip.Addr, authTokenStore *auth.TokenStore, peerID string, mtu uint16) *Client {
 	hashedID := messages.HashID(peerID)
 	relayLog := log.WithFields(log.Fields{"relay": serverURL})
 
 	c := &Client{
 		log:            relayLog,
 		connectionURL:  serverURL,
+		fallbackIP:     fallbackIP,
 		authTokenStore: authTokenStore,
 		hashedID:       hashedID,
 		mtu:            mtu,
@@ -304,6 +312,41 @@ func (c *Client) ServerInstanceURL() (string, error) {
 	return c.instanceURL.String(), nil
 }
 
+// ConnectedIP returns the IP address of the live relay-server connection,
+// extracted from the underlying socket's RemoteAddr. Zero value if not
+// connected or if the address is not an IP literal.
+func (c *Client) ConnectedIP() netip.Addr {
+	c.mu.Lock()
+	conn := c.relayConn
+	c.mu.Unlock()
+	if conn == nil {
+		return netip.Addr{}
+	}
+	addr := conn.RemoteAddr()
+	if addr == nil {
+		return netip.Addr{}
+	}
+	return extractIPLiteral(addr.String())
+}
+
+// extractIPLiteral returns the IP from address forms produced by the relay
+// dialers (URL or host:port). Zero value if the host is not an IP.
+func extractIPLiteral(s string) netip.Addr {
+	if u, err := url.Parse(s); err == nil && u.Host != "" {
+		s = u.Host
+	}
+	host, _, err := net.SplitHostPort(s)
+	if err != nil {
+		host = s
+	}
+	host = strings.Trim(host, "[]")
+	ip, err := netip.ParseAddr(host)
+	if err != nil {
+		return netip.Addr{}
+	}
+	return ip.Unmap()
+}
+
 // SetOnDisconnectListener sets a function that will be called when the connection to the relay server is closed.
 func (c *Client) SetOnDisconnectListener(fn func(string)) {
 	c.listenerMutex.Lock()
@@ -335,7 +378,11 @@ func (c *Client) connect(ctx context.Context) (*RelayAddr, error) {
 	rd := dialer.NewRaceDial(c.log, dialer.DefaultConnectionTimeout, c.connectionURL, dialers...)
 	conn, err := rd.Dial(ctx)
 	if err != nil {
-		return nil, err
+		fallbackConn, fbErr := c.dialFallback(ctx, dialers)
+		if fbErr != nil {
+			return nil, fmt.Errorf("primary dial: %w; fallback dial: %w", err, fbErr)
+		}
+		conn = fallbackConn
 	}
 	c.relayConn = conn
 
@@ -351,6 +398,58 @@ func (c *Client) connect(ctx context.Context) (*RelayAddr, error) {
 	return instanceURL, nil
 }
 
+// dialFallback retries the dial against c.fallbackIP, preserving the
+// original FQDN as the TLS ServerName for SNI. Returns an error if no
+// fallback IP is configured or if the substituted URL is malformed.
+func (c *Client) dialFallback(ctx context.Context, dialers []dialer.DialeFn) (net.Conn, error) {
+	if !c.fallbackIP.IsValid() || c.fallbackIP.IsUnspecified() {
+		return nil, errors.New("no usable fallback IP configured")
+	}
+
+	fallbackURL, serverName, err := substituteHost(c.connectionURL, c.fallbackIP)
+	if err != nil {
+		return nil, fmt.Errorf("substitute host: %w", err)
+	}
+
+	c.log.Infof("primary dial failed, retrying via fallback IP %s (SNI=%s)", c.fallbackIP, serverName)
+
+	rd := dialer.NewRaceDial(c.log, dialer.DefaultConnectionTimeout, fallbackURL, dialers...).
+		WithServerName(serverName)
+	return rd.Dial(ctx)
+}
+
+// substituteHost replaces the host portion of a rel/rels URL with ip,
+// preserving the scheme and port. Returns the rewritten URL and the
+// original host to use as the TLS ServerName, or empty if the original
+// host is itself an IP literal (SNI requires a DNS name).
+func substituteHost(serverURL string, ip netip.Addr) (string, string, error) {
+	u, err := url.Parse(serverURL)
+	if err != nil {
+		return "", "", fmt.Errorf("parse %q: %w", serverURL, err)
+	}
+	if u.Scheme == "" || u.Host == "" {
+		return "", "", fmt.Errorf("invalid relay URL %q", serverURL)
+	}
+	if !ip.IsValid() {
+		return "", "", errors.New("invalid fallback IP")
+	}
+	origHost := u.Hostname()
+	if _, err := netip.ParseAddr(origHost); err == nil {
+		origHost = ""
+	}
+	ip = ip.Unmap()
+	newHost := ip.String()
+	if ip.Is6() {
+		newHost = "[" + newHost + "]"
+	}
+	if port := u.Port(); port != "" {
+		u.Host = newHost + ":" + port
+	} else {
+		u.Host = newHost
+	}
+	return u.String(), origHost, nil
+}
+
 func (c *Client) handShake(ctx context.Context) (*RelayAddr, error) {
 	msg, err := messages.MarshalAuthMsg(c.hashedID, c.authTokenStore.TokenBinary())
 	if err != nil {

shared/relay/client/client_fallback_test.go

@@ -0,0 +1,280 @@
+package client
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/netip"
+	"testing"
+	"time"
+
+	"go.opentelemetry.io/otel"
+
+	"github.com/netbirdio/netbird/client/iface"
+	"github.com/netbirdio/netbird/relay/server"
+	"github.com/netbirdio/netbird/shared/relay/auth/allow"
+)
+
+// TestClient_FallbackIPRecoversFromUnresolvableFQDN verifies that when the
+// primary FQDN-based dial fails (unresolvable .invalid host), Connect
+// recovers via the fallback IP and SNI still uses the FQDN.
+func TestClient_FallbackIPRecoversFromUnresolvableFQDN(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	listenAddr, port := freeAddr(t)
+	srvCfg := server.Config{
+		Meter:          otel.Meter(""),
+		ExposedAddress: fmt.Sprintf("rel://test-unresolvable-host.invalid:%d", port),
+		TLSSupport:     false,
+		AuthValidator:  &allow.Auth{},
+	}
+	srv, err := server.NewServer(srvCfg)
+	if err != nil {
+		t.Fatalf("create server: %s", err)
+	}
+
+	errChan := make(chan error, 1)
+	go func() {
+		if err := srv.Listen(server.ListenerConfig{Address: listenAddr}); err != nil {
+			errChan <- err
+		}
+	}()
+	t.Cleanup(func() {
+		if err := srv.Shutdown(context.Background()); err != nil {
+			t.Errorf("shutdown server: %s", err)
+		}
+	})
+	if err := waitForServerToStart(errChan); err != nil {
+		t.Fatalf("server failed to start: %s", err)
+	}
+
+	t.Run("no fallback IP, primary fails", func(t *testing.T) {
+		c := NewClient(srvCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice-nofallback", iface.DefaultMTU)
+		err := c.Connect(ctx)
+		if err == nil {
+			_ = c.Close()
+			t.Fatalf("expected connect to fail without fallback IP, got nil")
+		}
+	})
+
+	t.Run("fallback IP recovers", func(t *testing.T) {
+		c := NewClient(srvCfg.ExposedAddress, netip.MustParseAddr("127.0.0.1"), hmacTokenStore, "alice-fallback", iface.DefaultMTU)
+		if err := c.Connect(ctx); err != nil {
+			t.Fatalf("connect with fallback IP: %s", err)
+		}
+		t.Cleanup(func() { _ = c.Close() })
+
+		if !c.Ready() {
+			t.Fatalf("client not ready after connect")
+		}
+		if got := c.ConnectedIP(); got.String() != "127.0.0.1" {
+			t.Fatalf("ConnectedIP = %q, want 127.0.0.1", got)
+		}
+	})
+}
+
+// TestClient_ConnectedIPAfterFQDNDial verifies ConnectedIP returns the
+// resolved IP after a successful FQDN-based dial. The underlying socket's
+// RemoteAddr must be exposed through the dialer wrappers; if it returns
+// the dial-time URL instead, ConnectedIP returns empty and the fallback
+// IP we advertise to peers is empty too.
+func TestClient_ConnectedIPAfterFQDNDial(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	defer cancel()
+
+	listenAddr, port := freeAddr(t)
+	srvCfg := server.Config{
+		Meter:          otel.Meter(""),
+		ExposedAddress: fmt.Sprintf("rel://localhost:%d", port),
+		TLSSupport:     false,
+		AuthValidator:  &allow.Auth{},
+	}
+	srv, err := server.NewServer(srvCfg)
+	if err != nil {
+		t.Fatalf("create server: %s", err)
+	}
+	errChan := make(chan error, 1)
+	go func() {
+		if err := srv.Listen(server.ListenerConfig{Address: listenAddr}); err != nil {
+			errChan <- err
+		}
+	}()
+	t.Cleanup(func() { _ = srv.Shutdown(context.Background()) })
+	if err := waitForServerToStart(errChan); err != nil {
+		t.Fatalf("server failed to start: %s", err)
+	}
+
+	c := NewClient(srvCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice-fqdn", iface.DefaultMTU)
+	if err := c.Connect(ctx); err != nil {
+		t.Fatalf("connect: %s", err)
+	}
+	t.Cleanup(func() { _ = c.Close() })
+
+	got := c.ConnectedIP().String()
+	if got != "127.0.0.1" && got != "::1" {
+		t.Fatalf("ConnectedIP after FQDN dial = %q, want 127.0.0.1 or ::1", got)
+	}
+}
+
+func TestSubstituteHost(t *testing.T) {
+	tests := []struct {
+		name           string
+		serverURL      string
+		ip             string
+		wantURL        string
+		wantServerName string
+		wantErr        bool
+	}{
+		{
+			name:           "rels with port",
+			serverURL:      "rels://relay.netbird.io:443",
+			ip:             "10.0.0.5",
+			wantURL:        "rels://10.0.0.5:443",
+			wantServerName: "relay.netbird.io",
+		},
+		{
+			name:           "rel with port",
+			serverURL:      "rel://relay.example.com:80",
+			ip:             "192.0.2.1",
+			wantURL:        "rel://192.0.2.1:80",
+			wantServerName: "relay.example.com",
+		},
+		{
+			name:           "ipv6 fallback bracketed",
+			serverURL:      "rels://relay.example.com:443",
+			ip:             "2001:db8::1",
+			wantURL:        "rels://[2001:db8::1]:443",
+			wantServerName: "relay.example.com",
+		},
+		{
+			name:           "no port",
+			serverURL:      "rels://relay.example.com",
+			ip:             "10.0.0.5",
+			wantURL:        "rels://10.0.0.5",
+			wantServerName: "relay.example.com",
+		},
+		{
+			name:           "ipv6 server with port returns empty SNI",
+			serverURL:      "rels://[2001:db8::5]:443",
+			ip:             "10.0.0.5",
+			wantURL:        "rels://10.0.0.5:443",
+			wantServerName: "",
+		},
+		{
+			name:           "ipv4 server with port returns empty SNI",
+			serverURL:      "rels://10.0.0.5:443",
+			ip:             "10.0.0.6",
+			wantURL:        "rels://10.0.0.6:443",
+			wantServerName: "",
+		},
+		{
+			name:           "ipv6 fallback no port",
+			serverURL:      "rels://relay.example.com",
+			ip:             "2001:db8::1",
+			wantURL:        "rels://[2001:db8::1]",
+			wantServerName: "relay.example.com",
+		},
+		{
+			name:      "missing scheme",
+			serverURL: "relay.example.com:443",
+			ip:        "10.0.0.5",
+			wantErr:   true,
+		},
+		{
+			name:      "empty",
+			serverURL: "",
+			ip:        "10.0.0.5",
+			wantErr:   true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var ip netip.Addr
+			if tt.ip != "" {
+				ip = netip.MustParseAddr(tt.ip)
+			}
+			gotURL, gotName, err := substituteHost(tt.serverURL, ip)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected error, got nil")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+			if gotURL != tt.wantURL {
+				t.Errorf("URL = %q, want %q", gotURL, tt.wantURL)
+			}
+			if gotName != tt.wantServerName {
+				t.Errorf("ServerName = %q, want %q", gotName, tt.wantServerName)
+			}
+		})
+	}
+}
+
+func TestClient_ConnectedIPEmptyWhenNotConnected(t *testing.T) {
+	c := NewClient("rel://example.invalid:80", netip.Addr{}, hmacTokenStore, "x", iface.DefaultMTU)
+	if got := c.ConnectedIP(); got.IsValid() {
+		t.Fatalf("ConnectedIP on disconnected client = %q, want zero", got)
+	}
+}
+
+// staticAddr is a net.Addr that returns a fixed string. Used to verify
+// ConnectedIP parses RemoteAddr correctly.
+type staticAddr struct{ s string }
+
+func (a staticAddr) Network() string { return "tcp" }
+func (a staticAddr) String() string  { return a.s }
+
+type stubConn struct {
+	net.Conn
+	remote net.Addr
+}
+
+func (s stubConn) RemoteAddr() net.Addr { return s.remote }
+
+func TestClient_ConnectedIPParsesRemoteAddr(t *testing.T) {
+	tests := []struct {
+		name string
+		s    string
+		want string
+	}{
+		{"hostport ipv4", "127.0.0.1:50301", "127.0.0.1"},
+		{"hostport ipv6 bracketed", "[::1]:50301", "::1"},
+		{"url with ipv4", "rel://127.0.0.1:50301", "127.0.0.1"},
+		{"url with ipv6", "rels://[2001:db8::1]:443", "2001:db8::1"},
+		{"fqdn url returns empty", "rel://relay.example.com:50301", ""},
+		{"fqdn hostport returns empty", "relay.example.com:50301", ""},
+		{"plain ipv4 no port", "10.0.0.1", "10.0.0.1"},
+		{"empty", "", ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := &Client{relayConn: stubConn{remote: staticAddr{s: tt.s}}}
+			got := c.ConnectedIP()
+			var gotStr string
+			if got.IsValid() {
+				gotStr = got.String()
+			}
+			if gotStr != tt.want {
+				t.Errorf("ConnectedIP(%q) = %q, want %q", tt.s, gotStr, tt.want)
+			}
+		})
+	}
+}
+
+// freeAddr returns a 127.0.0.1 address with an OS-assigned port. The
+// listener is closed before returning, so the port is briefly free for
+// the caller to bind. Avoids hardcoded ports that can collide.
+func freeAddr(t *testing.T) (string, int) {
+	t.Helper()
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("get free port: %s", err)
+	}
+	addr := l.Addr().(*net.TCPAddr)
+	_ = l.Close()
+	return addr.String(), addr.Port
+}

shared/relay/client/client_test.go

@@ -3,6 +3,7 @@ package client
 import (
 	"context"
 	"net"
+	"net/netip"
 	"os"
 	"testing"
 	"time"
@@ -68,7 +69,7 @@ func TestClient(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 	t.Log("alice connecting to server")
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -76,7 +77,7 @@ func TestClient(t *testing.T) {
 	defer clientAlice.Close()
 
 	t.Log("placeholder connecting to server")
-	clientPlaceHolder := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "clientPlaceHolder", iface.DefaultMTU)
+	clientPlaceHolder := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "clientPlaceHolder", iface.DefaultMTU)
 	err = clientPlaceHolder.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -84,7 +85,7 @@ func TestClient(t *testing.T) {
 	defer clientPlaceHolder.Close()
 
 	t.Log("Bob connecting to server")
-	clientBob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "bob", iface.DefaultMTU)
+	clientBob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "bob", iface.DefaultMTU)
 	err = clientBob.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -144,7 +145,7 @@ func TestRegistration(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		_ = srv.Shutdown(ctx)
@@ -184,7 +185,7 @@ func TestRegistrationTimeout(t *testing.T) {
 		_ = fakeTCPListener.Close()
 	}(fakeTCPListener)
 
-	clientAlice := NewClient("127.0.0.1:50201", hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient("127.0.0.1:50201", netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err == nil {
 		t.Errorf("failed to connect to server: %s", err)
@@ -227,7 +228,7 @@ func TestEcho(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idAlice, iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idAlice, iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -239,7 +240,7 @@ func TestEcho(t *testing.T) {
 		}
 	}()
 
-	clientBob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idBob, iface.DefaultMTU)
+	clientBob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idBob, iface.DefaultMTU)
 	err = clientBob.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -319,7 +320,7 @@ func TestBindToUnavailabePeer(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Errorf("failed to connect to server: %s", err)
@@ -367,13 +368,13 @@ func TestBindReconnect(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
 	}
 
-	clientBob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "bob", iface.DefaultMTU)
+	clientBob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "bob", iface.DefaultMTU)
 	err = clientBob.Connect(ctx)
 	if err != nil {
 		t.Errorf("failed to connect to server: %s", err)
@@ -395,7 +396,7 @@ func TestBindReconnect(t *testing.T) {
 		t.Errorf("failed to close client: %s", err)
 	}
 
-	clientAlice = NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice = NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Errorf("failed to connect to server: %s", err)
@@ -470,13 +471,13 @@ func TestCloseConn(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	bob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "bob", iface.DefaultMTU)
+	bob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "bob", iface.DefaultMTU)
 	err = bob.Connect(ctx)
 	if err != nil {
 		t.Errorf("failed to connect to server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Errorf("failed to connect to server: %s", err)
@@ -534,13 +535,13 @@ func TestCloseRelayConn(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	bob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "bob", iface.DefaultMTU)
+	bob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "bob", iface.DefaultMTU)
 	err = bob.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, "alice", iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, "alice", iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -590,7 +591,7 @@ func TestCloseByServer(t *testing.T) {
 
 	idAlice := "alice"
 	log.Debugf("connect by alice")
-	relayClient := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idAlice, iface.DefaultMTU)
+	relayClient := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idAlice, iface.DefaultMTU)
 	if err = relayClient.Connect(ctx); err != nil {
 		log.Fatalf("failed to connect to server: %s", err)
 	}
@@ -648,7 +649,7 @@ func TestCloseByClient(t *testing.T) {
 
 	idAlice := "alice"
 	log.Debugf("connect by alice")
-	relayClient := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idAlice, iface.DefaultMTU)
+	relayClient := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idAlice, iface.DefaultMTU)
 	err = relayClient.Connect(ctx)
 	if err != nil {
 		log.Fatalf("failed to connect to server: %s", err)
@@ -701,7 +702,7 @@ func TestCloseNotDrainedChannel(t *testing.T) {
 		t.Fatalf("failed to start server: %s", err)
 	}
 
-	clientAlice := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idAlice, iface.DefaultMTU)
+	clientAlice := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idAlice, iface.DefaultMTU)
 	err = clientAlice.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)
@@ -713,7 +714,7 @@ func TestCloseNotDrainedChannel(t *testing.T) {
 		}
 	}()
 
-	clientBob := NewClient(serverCfg.ExposedAddress, hmacTokenStore, idBob, iface.DefaultMTU)
+	clientBob := NewClient(serverCfg.ExposedAddress, netip.Addr{}, hmacTokenStore, idBob, iface.DefaultMTU)
 	err = clientBob.Connect(ctx)
 	if err != nil {
 		t.Fatalf("failed to connect to server: %s", err)

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

@@ -23,7 +23,7 @@ func (d Dialer) Protocol() string {
 	return Network
 }
 
-func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
+func (d Dialer) Dial(ctx context.Context, address, serverName string) (net.Conn, error) {
 	quicURL, err := prepareURL(address)
 	if err != nil {
 		return nil, err
@@ -32,11 +32,14 @@ func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
 	// Get the base TLS config
 	tlsClientConfig := quictls.ClientQUICTLSConfig()
 
-	// Set ServerName to hostname if not an IP address
-	host, _, splitErr := net.SplitHostPort(quicURL)
-	if splitErr == nil && net.ParseIP(host) == nil {
-		// It's a hostname, not an IP - modify directly
-		tlsClientConfig.ServerName = host
+	switch {
+	case serverName != "" && net.ParseIP(serverName) == nil:
+		tlsClientConfig.ServerName = serverName
+	case serverName == "":
+		host, _, splitErr := net.SplitHostPort(quicURL)
+		if splitErr == nil && net.ParseIP(host) == nil {
+			tlsClientConfig.ServerName = host
+		}
 	}
 
 	quicConfig := &quic.Config{

shared/relay/client/dialer/race_dialer.go

@@ -14,7 +14,9 @@ const (
 )
 
 type DialeFn interface {
-	Dial(ctx context.Context, address string) (net.Conn, error)
+	// Dial connects to address. serverName, when non-empty, overrides the TLS
+	// ServerName used for SNI/cert validation. Empty means derive from address.
+	Dial(ctx context.Context, address, serverName string) (net.Conn, error)
 	Protocol() string
 }
 
@@ -27,6 +29,7 @@ type dialResult struct {
 type RaceDial struct {
 	log               *log.Entry
 	serverURL         string
+	serverName        string
 	dialerFns         []DialeFn
 	connectionTimeout time.Duration
 }
@@ -40,6 +43,16 @@ func NewRaceDial(log *log.Entry, connectionTimeout time.Duration, serverURL stri
 	}
 }
 
+// WithServerName sets a TLS SNI/cert validation override. Used when serverURL
+// contains an IP literal but the cert is issued for a different hostname.
+//
+// Mutates the receiver and is not safe for concurrent reconfiguration; a
+// RaceDial is intended to be constructed per dial and discarded.
+func (r *RaceDial) WithServerName(serverName string) *RaceDial {
+	r.serverName = serverName
+	return r
+}
+
 func (r *RaceDial) Dial(ctx context.Context) (net.Conn, error) {
 	connChan := make(chan dialResult, len(r.dialerFns))
 	winnerConn := make(chan net.Conn, 1)
@@ -64,7 +77,7 @@ func (r *RaceDial) dial(dfn DialeFn, abortCtx context.Context, connChan chan dia
 	defer cancel()
 
 	r.log.Infof("dialing Relay server via %s", dfn.Protocol())
-	conn, err := dfn.Dial(ctx, r.serverURL)
+	conn, err := dfn.Dial(ctx, r.serverURL, r.serverName)
 	connChan <- dialResult{Conn: conn, Protocol: dfn.Protocol(), Err: err}
 }
 

shared/relay/client/dialer/race_dialer_test.go

@@ -28,7 +28,7 @@ type MockDialer struct {
 	protocolStr string
 }
 
-func (m *MockDialer) Dial(ctx context.Context, address string) (net.Conn, error) {
+func (m *MockDialer) Dial(ctx context.Context, address, _ string) (net.Conn, error) {
 	return m.dialFunc(ctx, address)
 }
 

shared/relay/client/dialer/ws/conn.go

@@ -12,14 +12,24 @@ import (
 type Conn struct {
 	ctx context.Context
 	*websocket.Conn
-	remoteAddr WebsocketAddr
+	remoteAddr net.Addr
 }
 
-func NewConn(wsConn *websocket.Conn, serverAddress string) net.Conn {
+// NewConn builds a relay ws.Conn. underlying is the raw TCP/TLS conn captured
+// from the http transport's DialContext; when set, RemoteAddr returns its
+// peer address (an IP literal). When nil (e.g. wasm), RemoteAddr falls back
+// to the dial-time URL.
+func NewConn(wsConn *websocket.Conn, serverAddress string, underlying net.Conn) net.Conn {
+	var addr net.Addr = WebsocketAddr{serverAddress}
+	if underlying != nil {
+		if ra := underlying.RemoteAddr(); ra != nil {
+			addr = ra
+		}
+	}
 	return &Conn{
 		ctx:        context.Background(),
 		Conn:       wsConn,
-		remoteAddr: WebsocketAddr{serverAddress},
+		remoteAddr: addr,
 	}
 }
 

shared/relay/client/dialer/ws/dialopts_generic.go

@@ -2,10 +2,14 @@
 
 package ws
 
-import "github.com/coder/websocket"
+import (
+	"net"
 
-func createDialOptions() *websocket.DialOptions {
+	"github.com/coder/websocket"
+)
+
+func createDialOptions(serverName string, underlyingOut *net.Conn) *websocket.DialOptions {
 	return &websocket.DialOptions{
-		HTTPClient: httpClientNbDialer(),
+		HTTPClient: httpClientNbDialer(serverName, underlyingOut),
 	}
 }

shared/relay/client/dialer/ws/dialopts_js.go

@@ -2,9 +2,13 @@
 
 package ws
 
-import "github.com/coder/websocket"
+import (
+	"net"
 
-func createDialOptions() *websocket.DialOptions {
-	// WASM version doesn't support HTTPClient
+	"github.com/coder/websocket"
+)
+
+func createDialOptions(_ string, _ *net.Conn) *websocket.DialOptions {
+	// WASM version doesn't support HTTPClient or custom TLS config.
 	return &websocket.DialOptions{}
 }

shared/relay/client/dialer/ws/ws.go

@@ -26,13 +26,14 @@ func (d Dialer) Protocol() string {
 	return "WS"
 }
 
-func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
+func (d Dialer) Dial(ctx context.Context, address, serverName string) (net.Conn, error) {
 	wsURL, err := prepareURL(address)
 	if err != nil {
 		return nil, err
 	}
 
-	opts := createDialOptions()
+	var underlying net.Conn
+	opts := createDialOptions(serverName, &underlying)
 
 	parsedURL, err := url.Parse(wsURL)
 	if err != nil {
@@ -52,7 +53,7 @@ func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
 		_ = resp.Body.Close()
 	}
 
-	conn := NewConn(wsConn, address)
+	conn := NewConn(wsConn, address, underlying)
 	return conn, nil
 }
 
@@ -64,7 +65,10 @@ func prepareURL(address string) (string, error) {
 	return strings.Replace(address, "rel", "ws", 1), nil
 }
 
-func httpClientNbDialer() *http.Client {
+// httpClientNbDialer builds the http client used by the websocket library.
+// underlyingOut, when non-nil, is populated with the raw conn from the
+// transport's DialContext so the caller can read its RemoteAddr.
+func httpClientNbDialer(serverName string, underlyingOut *net.Conn) *http.Client {
 	customDialer := nbnet.NewDialer()
 
 	certPool, err := x509.SystemCertPool()
@@ -75,10 +79,15 @@ func httpClientNbDialer() *http.Client {
 
 	customTransport := &http.Transport{
 		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
-			return customDialer.DialContext(ctx, network, addr)
+			c, err := customDialer.DialContext(ctx, network, addr)
+			if err == nil && underlyingOut != nil {
+				*underlyingOut = c
+			}
+			return c, err
 		},
 		TLSClientConfig: &tls.Config{
-			RootCAs: certPool,
+			RootCAs:    certPool,
+			ServerName: serverName,
 		},
 	}
 

shared/relay/client/manager.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"net/netip"
 	"reflect"
 	"sync"
 	"time"
@@ -117,7 +118,10 @@ func (m *Manager) Serve() error {
 // OpenConn opens a connection to the given peer key. If the peer is on the same relay server, the connection will be
 // established via the relay server. If the peer is on a different relay server, the manager will establish a new
 // connection to the relay server. It returns back with a net.Conn what represent the remote peer connection.
-func (m *Manager) OpenConn(ctx context.Context, serverAddress, peerKey string) (net.Conn, error) {
+//
+// fallbackIP, when valid and serverAddress is foreign, is used as a dial-time fallback if the FQDN-based
+// dial fails. Ignored for the local home-server path. TLS verification still uses the FQDN via SNI.
+func (m *Manager) OpenConn(ctx context.Context, serverAddress, peerKey string, fallbackIP netip.Addr) (net.Conn, error) {
 	m.relayClientMu.RLock()
 	defer m.relayClientMu.RUnlock()
 
@@ -138,7 +142,7 @@ func (m *Manager) OpenConn(ctx context.Context, serverAddress, peerKey string) (
 		netConn, err = m.relayClient.OpenConn(ctx, peerKey)
 	} else {
 		log.Debugf("open peer connection via foreign server: %s", serverAddress)
-		netConn, err = m.openConnVia(ctx, serverAddress, peerKey)
+		netConn, err = m.openConnVia(ctx, serverAddress, peerKey, fallbackIP)
 	}
 	if err != nil {
 		return nil, err
@@ -202,6 +206,19 @@ func (m *Manager) RelayInstanceAddress() (string, error) {
 	return m.relayClient.ServerInstanceURL()
 }
 
+// RelayInstanceIP returns the IP address of the live home relay connection.
+// Zero value if not connected. Sent alongside RelayInstanceAddress so remote
+// peers can dial directly without their own DNS lookup.
+func (m *Manager) RelayInstanceIP() netip.Addr {
+	m.relayClientMu.RLock()
+	defer m.relayClientMu.RUnlock()
+
+	if m.relayClient == nil {
+		return netip.Addr{}
+	}
+	return m.relayClient.ConnectedIP()
+}
+
 // ServerURLs returns the addresses of the relay servers.
 func (m *Manager) ServerURLs() []string {
 	return m.serverPicker.ServerURLs.Load().([]string)
@@ -223,7 +240,7 @@ func (m *Manager) UpdateToken(token *relayAuth.Token) error {
 	return m.tokenStore.UpdateToken(token)
 }
 
-func (m *Manager) openConnVia(ctx context.Context, serverAddress, peerKey string) (net.Conn, error) {
+func (m *Manager) openConnVia(ctx context.Context, serverAddress, peerKey string, fallbackIP netip.Addr) (net.Conn, error) {
 	// check if already has a connection to the desired relay server
 	m.relayClientsMutex.RLock()
 	rt, ok := m.relayClients[serverAddress]
@@ -258,7 +275,7 @@ func (m *Manager) openConnVia(ctx context.Context, serverAddress, peerKey string
 	m.relayClients[serverAddress] = rt
 	m.relayClientsMutex.Unlock()
 
-	relayClient := NewClient(serverAddress, m.tokenStore, m.peerID, m.mtu)
+	relayClient := NewClient(serverAddress, fallbackIP, m.tokenStore, m.peerID, m.mtu)
 	err := relayClient.Connect(m.ctx)
 	if err != nil {
 		rt.err = err

shared/relay/client/manager_fallback_test.go

@@ -0,0 +1,146 @@
+package client
+
+import (
+	"context"
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/netbirdio/netbird/client/iface"
+	"github.com/netbirdio/netbird/relay/server"
+)
+
+// TestManager_ForeignRelayFallbackIP exercises the foreign-relay path
+// end-to-end through Manager.OpenConn. Alice and Bob register on different
+// relay servers; Alice dials Bob's foreign relay using an unresolvable
+// FQDN. Without a fallback IP the dial fails; with Bob's advertised IP it
+// recovers and a payload round-trips between the peers.
+func TestManager_ForeignRelayFallbackIP(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
+	defer cancel()
+
+	// Alice's home relay
+	homeCfg := server.ListenerConfig{Address: "127.0.0.1:52401"}
+	homeSrv, err := server.NewServer(newManagerTestServerConfig(homeCfg.Address))
+	if err != nil {
+		t.Fatalf("create home server: %s", err)
+	}
+	homeErr := make(chan error, 1)
+	go func() {
+		if err := homeSrv.Listen(homeCfg); err != nil {
+			homeErr <- err
+		}
+	}()
+	t.Cleanup(func() { _ = homeSrv.Shutdown(context.Background()) })
+	if err := waitForServerToStart(homeErr); err != nil {
+		t.Fatalf("home server: %s", err)
+	}
+
+	// Bob's foreign relay
+	foreignCfg := server.ListenerConfig{Address: "127.0.0.1:52402"}
+	foreignSrv, err := server.NewServer(newManagerTestServerConfig(foreignCfg.Address))
+	if err != nil {
+		t.Fatalf("create foreign server: %s", err)
+	}
+	foreignErr := make(chan error, 1)
+	go func() {
+		if err := foreignSrv.Listen(foreignCfg); err != nil {
+			foreignErr <- err
+		}
+	}()
+	t.Cleanup(func() { _ = foreignSrv.Shutdown(context.Background()) })
+	if err := waitForServerToStart(foreignErr); err != nil {
+		t.Fatalf("foreign server: %s", err)
+	}
+
+	mCtx, mCancel := context.WithCancel(ctx)
+	t.Cleanup(mCancel)
+
+	mgrAlice := NewManager(mCtx, toURL(homeCfg), "alice", iface.DefaultMTU)
+	if err := mgrAlice.Serve(); err != nil {
+		t.Fatalf("alice manager serve: %s", err)
+	}
+
+	mgrBob := NewManager(mCtx, toURL(foreignCfg), "bob", iface.DefaultMTU)
+	if err := mgrBob.Serve(); err != nil {
+		t.Fatalf("bob manager serve: %s", err)
+	}
+
+	// Bob's real relay URL (what mgrBob.RelayInstanceAddress returns).
+	bobRealAddr, err := mgrBob.RelayInstanceAddress()
+	if err != nil {
+		t.Fatalf("bob relay address: %s", err)
+	}
+	// What Bob's RelayInstanceIP() reports — this is the field that
+	// would ride along in signal as relayServerIP.
+	bobAdvertisedIP := mgrBob.RelayInstanceIP()
+	if !bobAdvertisedIP.IsValid() {
+		t.Fatalf("expected valid RelayInstanceIP for bob, got zero")
+	}
+
+	// .invalid is reserved (RFC 2606), so DNS resolution always fails.
+	const brokenFQDN = "rel://relay-bob-instance.invalid:52402"
+	if brokenFQDN == bobRealAddr {
+		t.Fatalf("broken FQDN must differ from bob's real address (%s)", bobRealAddr)
+	}
+
+	t.Run("no fallback IP, dial fails", func(t *testing.T) {
+		dialCtx, dialCancel := context.WithTimeout(ctx, 5*time.Second)
+		defer dialCancel()
+		_, err := mgrAlice.OpenConn(dialCtx, brokenFQDN, "bob", netip.Addr{})
+		if err == nil {
+			t.Fatalf("expected OpenConn to fail without fallback IP, got success")
+		}
+	})
+
+	t.Run("fallback IP recovers", func(t *testing.T) {
+		// Bob waits for Alice's incoming peer connection on his side.
+		bobSideCh := make(chan error, 1)
+		go func() {
+			conn, err := mgrBob.OpenConn(ctx, bobRealAddr, "alice", netip.Addr{})
+			if err != nil {
+				bobSideCh <- err
+				return
+			}
+			buf := make([]byte, 1024)
+			n, err := conn.Read(buf)
+			if err != nil {
+				bobSideCh <- err
+				return
+			}
+			if _, err := conn.Write(buf[:n]); err != nil {
+				bobSideCh <- err
+				return
+			}
+			bobSideCh <- nil
+		}()
+
+		aliceConn, err := mgrAlice.OpenConn(ctx, brokenFQDN, "bob", bobAdvertisedIP)
+		if err != nil {
+			t.Fatalf("alice OpenConn with fallback IP: %s", err)
+		}
+		t.Cleanup(func() { _ = aliceConn.Close() })
+
+		payload := []byte("alice-to-bob")
+		if _, err := aliceConn.Write(payload); err != nil {
+			t.Fatalf("alice write: %s", err)
+		}
+
+		buf := make([]byte, len(payload))
+		if _, err := aliceConn.Read(buf); err != nil {
+			t.Fatalf("alice read echo: %s", err)
+		}
+		if string(buf) != string(payload) {
+			t.Fatalf("echo mismatch: got %q want %q", buf, payload)
+		}
+
+		select {
+		case err := <-bobSideCh:
+			if err != nil {
+				t.Fatalf("bob side: %s", err)
+			}
+		case <-time.After(5 * time.Second):
+			t.Fatalf("timed out waiting for bob side")
+		}
+	})
+}

shared/relay/client/manager_test.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"net/netip"
 	"testing"
 	"time"
 
@@ -104,11 +105,11 @@ func TestForeignConn(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to get relay address: %s", err)
 	}
-	connAliceToBob, err := clientAlice.OpenConn(ctx, bobsSrvAddr, "bob")
+	connAliceToBob, err := clientAlice.OpenConn(ctx, bobsSrvAddr, "bob", netip.Addr{})
 	if err != nil {
 		t.Fatalf("failed to bind channel: %s", err)
 	}
-	connBobToAlice, err := clientBob.OpenConn(ctx, bobsSrvAddr, "alice")
+	connBobToAlice, err := clientBob.OpenConn(ctx, bobsSrvAddr, "alice", netip.Addr{})
 	if err != nil {
 		t.Fatalf("failed to bind channel: %s", err)
 	}
@@ -208,7 +209,7 @@ func TestForeginConnClose(t *testing.T) {
 	if err != nil {
 		t.Fatalf("failed to serve manager: %s", err)
 	}
-	conn, err := mgr.OpenConn(ctx, toURL(srvCfg2)[0], "bob")
+	conn, err := mgr.OpenConn(ctx, toURL(srvCfg2)[0], "bob", netip.Addr{})
 	if err != nil {
 		t.Fatalf("failed to bind channel: %s", err)
 	}
@@ -300,7 +301,7 @@ func TestForeignAutoClose(t *testing.T) {
 	}
 
 	t.Log("open connection to another peer")
-	if _, err = mgr.OpenConn(ctx, foreignServerURL, "anotherpeer"); err == nil {
+	if _, err = mgr.OpenConn(ctx, foreignServerURL, "anotherpeer", netip.Addr{}); err == nil {
 		t.Fatalf("should have failed to open connection to another peer")
 	}
 
@@ -369,7 +370,7 @@ func TestAutoReconnect(t *testing.T) {
 	if err != nil {
 		t.Errorf("failed to get relay address: %s", err)
 	}
-	conn, err := clientAlice.OpenConn(ctx, ra, "bob")
+	conn, err := clientAlice.OpenConn(ctx, ra, "bob", netip.Addr{})
 	if err != nil {
 		t.Errorf("failed to bind channel: %s", err)
 	}
@@ -387,7 +388,7 @@ func TestAutoReconnect(t *testing.T) {
 	time.Sleep(reconnectingTimeout + 1*time.Second)
 
 	log.Infof("reopent the connection")
-	_, err = clientAlice.OpenConn(ctx, ra, "bob")
+	_, err = clientAlice.OpenConn(ctx, ra, "bob", netip.Addr{})
 	if err != nil {
 		t.Errorf("failed to open channel: %s", err)
 	}
@@ -434,7 +435,7 @@ func TestNotifierDoubleAdd(t *testing.T) {
 		t.Fatalf("failed to serve manager: %s", err)
 	}
 
-	conn1, err := clientAlice.OpenConn(ctx, clientAlice.ServerURLs()[0], "bob")
+	conn1, err := clientAlice.OpenConn(ctx, clientAlice.ServerURLs()[0], "bob", netip.Addr{})
 	if err != nil {
 		t.Fatalf("failed to bind channel: %s", err)
 	}

shared/relay/client/picker.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net/netip"
 	"sync/atomic"
 	"time"
 
@@ -69,7 +70,7 @@ func (sp *ServerPicker) PickServer(parentCtx context.Context) (*Client, error) {
 
 func (sp *ServerPicker) startConnection(ctx context.Context, resultChan chan connResult, url string) {
 	log.Infof("try to connecting to relay server: %s", url)
-	relayClient := NewClient(url, sp.TokenStore, sp.PeerID, sp.MTU)
+	relayClient := NewClient(url, netip.Addr{}, sp.TokenStore, sp.PeerID, sp.MTU)
 	err := relayClient.Connect(ctx)
 	resultChan <- connResult{
 		RelayClient: relayClient,

shared/signal/client/client.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"net/netip"
 	"strings"
 
 	"github.com/netbirdio/netbird/shared/signal/proto"
@@ -14,17 +15,17 @@ import (
 
 // A set of tools to exchange connection details (Wireguard endpoints) with the remote peer.
 
-// Status is the status of the client
-type Status string
-
-const StreamConnected Status = "Connected"
-const StreamDisconnected Status = "Disconnected"
-
 const (
+	StreamConnected    Status = "Connected"
+	StreamDisconnected Status = "Disconnected"
+
 	// DirectCheck indicates support to direct mode checks
 	DirectCheck uint32 = 1
 )
 
+// Status is the status of the client
+type Status string
+
 type Client interface {
 	io.Closer
 	StreamConnected() bool
@@ -38,6 +39,24 @@ type Client interface {
 	SetOnReconnectedListener(func())
 }
 
+// Credential is an instance of a GrpcClient's Credential
+type Credential struct {
+	UFrag string
+	Pwd   string
+}
+
+// CredentialPayload bundles the fields of a signal Body for MarshalCredential.
+type CredentialPayload struct {
+	Type            proto.Body_Type
+	WgListenPort    int
+	Credential      *Credential
+	RosenpassPubKey []byte
+	RosenpassAddr   string
+	RelaySrvAddress string
+	RelaySrvIP      netip.Addr
+	SessionID       []byte
+}
+
 // UnMarshalCredential parses the credentials from the message and returns a Credential instance
 func UnMarshalCredential(msg *proto.Message) (*Credential, error) {
 
@@ -52,27 +71,26 @@ func UnMarshalCredential(msg *proto.Message) (*Credential, error) {
 }
 
 // MarshalCredential marshal a Credential instance and returns a Message object
-func MarshalCredential(myKey wgtypes.Key, myPort int, remoteKey string, credential *Credential, t proto.Body_Type, rosenpassPubKey []byte, rosenpassAddr string, relaySrvAddress string, sessionID []byte) (*proto.Message, error) {
+func MarshalCredential(myKey wgtypes.Key, remoteKey string, p CredentialPayload) (*proto.Message, error) {
+	var relayIPBytes []byte
+	if p.RelaySrvIP.IsValid() {
+		relayIPBytes = p.RelaySrvIP.Unmap().AsSlice()
+	}
 	return &proto.Message{
 		Key:       myKey.PublicKey().String(),
 		RemoteKey: remoteKey,
 		Body: &proto.Body{
-			Type:           t,
-			Payload:        fmt.Sprintf("%s:%s", credential.UFrag, credential.Pwd),
-			WgListenPort:   uint32(myPort),
+			Type:           p.Type,
+			Payload:        fmt.Sprintf("%s:%s", p.Credential.UFrag, p.Credential.Pwd),
+			WgListenPort:   uint32(p.WgListenPort),
 			NetBirdVersion: version.NetbirdVersion(),
 			RosenpassConfig: &proto.RosenpassConfig{
-				RosenpassPubKey:     rosenpassPubKey,
-				RosenpassServerAddr: rosenpassAddr,
+				RosenpassPubKey:     p.RosenpassPubKey,
+				RosenpassServerAddr: p.RosenpassAddr,
 			},
-			RelayServerAddress: relaySrvAddress,
-			SessionId:          sessionID,
+			RelayServerAddress: p.RelaySrvAddress,
+			RelayServerIP:      relayIPBytes,
+			SessionId:          p.SessionID,
 		},
 	}, nil
 }
-
-// Credential is an instance of a GrpcClient's Credential
-type Credential struct {
-	UFrag string
-	Pwd   string
-}

shared/signal/proto/signalexchange.pb.go

@@ -231,6 +231,11 @@ type Body struct {
 	// relayServerAddress is url of the relay server
 	RelayServerAddress string `protobuf:"bytes,8,opt,name=relayServerAddress,proto3" json:"relayServerAddress,omitempty"`
 	SessionId          []byte `protobuf:"bytes,10,opt,name=sessionId,proto3,oneof" json:"sessionId,omitempty"`
+	// relayServerIP is the IP the sender is connected to on its relay server,
+	// encoded as 4 bytes (IPv4) or 16 bytes (IPv6). Receivers may use it as a
+	// fallback dial target when DNS resolution of relayServerAddress fails.
+	// SNI/TLS verification still uses relayServerAddress.
+	RelayServerIP []byte `protobuf:"bytes,11,opt,name=relayServerIP,proto3" json:"relayServerIP,omitempty"`
 }
 
 func (x *Body) Reset() {
@@ -328,6 +333,13 @@ func (x *Body) GetSessionId() []byte {
 	return nil
 }
 
+func (x *Body) GetRelayServerIP() []byte {
+	if x != nil {
+		return x.RelayServerIP
+	}
+	return nil
+}
+
 // Mode indicates a connection mode
 type Mode struct {
 	state         protoimpl.MessageState
@@ -451,7 +463,7 @@ var file_signalexchange_proto_rawDesc = []byte{
 	0x52, 0x09, 0x72, 0x65, 0x6d, 0x6f, 0x74, 0x65, 0x4b, 0x65, 0x79, 0x12, 0x28, 0x0a, 0x04, 0x62,
 	0x6f, 0x64, 0x79, 0x18, 0x04, 0x20, 0x01, 0x28, 0x0b, 0x32, 0x14, 0x2e, 0x73, 0x69, 0x67, 0x6e,
 	0x61, 0x6c, 0x65, 0x78, 0x63, 0x68, 0x61, 0x6e, 0x67, 0x65, 0x2e, 0x42, 0x6f, 0x64, 0x79, 0x52,
-	0x04, 0x62, 0x6f, 0x64, 0x79, 0x22, 0xe4, 0x03, 0x0a, 0x04, 0x42, 0x6f, 0x64, 0x79, 0x12, 0x2d,
+	0x04, 0x62, 0x6f, 0x64, 0x79, 0x22, 0x90, 0x04, 0x0a, 0x04, 0x42, 0x6f, 0x64, 0x79, 0x12, 0x2d,
 	0x0a, 0x04, 0x74, 0x79, 0x70, 0x65, 0x18, 0x01, 0x20, 0x01, 0x28, 0x0e, 0x32, 0x19, 0x2e, 0x73,
 	0x69, 0x67, 0x6e, 0x61, 0x6c, 0x65, 0x78, 0x63, 0x68, 0x61, 0x6e, 0x67, 0x65, 0x2e, 0x42, 0x6f,
 	0x64, 0x79, 0x2e, 0x54, 0x79, 0x70, 0x65, 0x52, 0x04, 0x74, 0x79, 0x70, 0x65, 0x12, 0x18, 0x0a,
@@ -476,35 +488,38 @@ var file_signalexchange_proto_rawDesc = []byte{
 	0x28, 0x09, 0x52, 0x12, 0x72, 0x65, 0x6c, 0x61, 0x79, 0x53, 0x65, 0x72, 0x76, 0x65, 0x72, 0x41,
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 }
 
 var (

shared/signal/proto/signalexchange.proto

@@ -65,7 +65,15 @@ message Body {
   // relayServerAddress is url of the relay server
   string relayServerAddress = 8;
 
+  reserved 9;
+
   optional bytes sessionId = 10;
+
+  // relayServerIP is the IP the sender is connected to on its relay server,
+  // encoded as 4 bytes (IPv4) or 16 bytes (IPv6). Receivers may use it as a
+  // fallback dial target when DNS resolution of relayServerAddress fails.
+  // SNI/TLS verification still uses relayServerAddress.
+  bytes relayServerIP = 11;
 }
 
 // Mode indicates a connection mode