[client] Scope the iOS file drop listener to the tunnel interface

The receiver's host listener is a socket of the Network Extension, so the
SYN-ACK of an accepted connection followed the extension's own-traffic
bypass onto the physical interface and the sender never got an answer.
Bind the listeners to the tunnel interface index the same way the dial
already is; accepted sockets inherit the scope. Other platforms pass a
nil control and keep the current behavior.

client/internal/engine_filedrop.go

@@ -43,7 +43,7 @@ func (e *Engine) startFileDrop() {
 	resolver := filedropResolver{status: e.statusRecorder}
 
 	netstackNet := e.wgInterface.GetNet()
-	if err := e.fileDrop.StartReceiver(e.ctx, addr, netstackNet, resolver); err != nil {
+	if err := e.fileDrop.StartReceiver(e.ctx, addr, netstackNet, resolver, fileDropListenControl(e.wgInterface)); err != nil {
 		log.Errorf("failed to start file drop receiver: %v", err)
 		return
 	}

client/internal/engine_filedrop_dial.go

@@ -12,3 +12,7 @@ func fileDropOSDial(WGIface) filedrop.DialFunc {
 	dialer := &net.Dialer{}
 	return dialer.DialContext
 }
+
+func fileDropListenControl(WGIface) filedrop.ListenControl {
+	return nil
+}

client/internal/engine_filedrop_dial_ios.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net"
 	"net/netip"
+	"strings"
 	"syscall"
 
 	"golang.org/x/sys/unix"
@@ -14,6 +15,31 @@ import (
 	"github.com/netbirdio/netbird/client/internal/filedrop"
 )
 
+// fileDropListenControl scopes the receiver's listeners to the tunnel
+// interface, so replies on accepted connections leave through the tunnel
+// instead of following the Network Extension's own-traffic bypass.
+func fileDropListenControl(wgIface WGIface) filedrop.ListenControl {
+	return func(network, _ string, c syscall.RawConn) error {
+		osIface, err := net.InterfaceByName(wgIface.Name())
+		if err != nil {
+			return fmt.Errorf("lookup interface %q: %w", wgIface.Name(), err)
+		}
+
+		proto, opt := unix.IPPROTO_IP, unix.IP_BOUND_IF
+		if strings.HasSuffix(network, "6") {
+			proto, opt = unix.IPPROTO_IPV6, unix.IPV6_BOUND_IF
+		}
+
+		var operr error
+		if err := c.Control(func(s uintptr) {
+			operr = unix.SetsockoptInt(int(s), proto, opt, osIface.Index)
+		}); err != nil {
+			return err
+		}
+		return operr
+	}
+}
+
 // fileDropOSDial scopes the dial to the tunnel interface, since a Network
 // Extension's own unscoped sockets bypass its tunnel and leave on the
 // physical interface.

client/internal/filedrop/manager.go

@@ -147,8 +147,9 @@ func (m *Manager) SetDestinationDir(dir string) error {
 	return m.policy.SetDestinationDir(dir)
 }
 
-// StartReceiver binds the receiving server on addr.
-func (m *Manager) StartReceiver(ctx context.Context, addr netip.AddrPort, netstackNet *netstack.Net, resolver PeerResolver) error {
+// StartReceiver binds the receiving server on addr. A non-nil control is
+// applied to host listeners before bind.
+func (m *Manager) StartReceiver(ctx context.Context, addr netip.AddrPort, netstackNet *netstack.Net, resolver PeerResolver, control ListenControl) error {
 	m.mu.Lock()
 	if m.server != nil {
 		m.mu.Unlock()
@@ -170,6 +171,9 @@ func (m *Manager) StartReceiver(ctx context.Context, addr netip.AddrPort, netsta
 	if netstackNet != nil {
 		server.SetNetstackNet(netstackNet)
 	}
+	if control != nil {
+		server.SetListenControl(control)
+	}
 
 	if err := server.Start(ctx, addr); err != nil {
 		return fmt.Errorf("start receiver: %w", err)

client/internal/filedrop/server.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/netip"
 	"sync"
+	"syscall"
 	"time"
 
 	log "github.com/sirupsen/logrus"
@@ -48,6 +49,10 @@ type ServerConfig struct {
 	SpoolMaxAge time.Duration
 }
 
+// ListenControl is a raw-socket hook applied to host listeners before bind,
+// mirroring net.ListenConfig.Control.
+type ListenControl func(network, address string, c syscall.RawConn) error
+
 // Server serves the receiver over HTTP on the overlay address and owns the
 // listener and janitor lifecycle; the protocol logic itself lives in receiver.
 type Server struct {
@@ -56,6 +61,7 @@ type Server struct {
 	listener       net.Listener
 	extraListeners []net.Listener
 	netstackNet    *netstack.Net
+	listenControl  ListenControl
 
 	recv      *receiver
 	boundPort uint16
@@ -90,6 +96,13 @@ func NewServer(cfg ServerConfig) (*Server, error) {
 	return &Server{recv: newReceiver(cfg, spool, maxAge)}, nil
 }
 
+// SetListenControl installs a raw-socket hook applied to host listeners.
+func (s *Server) SetListenControl(control ListenControl) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.listenControl = control
+}
+
 // SetNetstackNet routes listeners through the gVisor netstack instead of host sockets.
 func (s *Server) SetNetstackNet(n *netstack.Net) {
 	s.mu.Lock()
@@ -244,7 +257,7 @@ func (s *Server) createListener(ctx context.Context, addr netip.AddrPort) (net.L
 		return ln, fmt.Sprintf("netstack %s", addr), nil
 	}
 
-	var lc net.ListenConfig
+	lc := net.ListenConfig{Control: s.listenControl}
 	ln, err := lc.Listen(ctx, "tcp", net.TCPAddrFromAddrPort(addr).String())
 	if err != nil {
 		return nil, "", fmt.Errorf("listen: %w", err)