[client] Keep NetBird traffic out of third-party fwmark rules (#7314)

client/iface/wgproxy/rawsocket/rawsocket_privileged_test.go

@@ -0,0 +1,77 @@
+//go:build linux && !android && privileged
+
+package rawsocket
+
+import (
+	"net"
+	"syscall"
+	"testing"
+
+	"golang.org/x/sys/unix"
+
+	nbnet "github.com/netbirdio/netbird/client/net"
+)
+
+// The sender sockets must stay unmarked: a NAT rule matching on fwmark that
+// rewrites the source of an injected packet makes WireGuard adopt the rewritten
+// address as the peer endpoint.
+func TestSenderRawSocketsCarryNoFwmark(t *testing.T) {
+	// the mark is only ever applied when advanced routing is available, so
+	// without it the assertion below would hold for the wrong reason
+	nbnet.Init()
+	if !nbnet.AdvancedRouting() {
+		t.Skip("advanced routing unsupported, the sockets carry no mark either way")
+	}
+
+	tests := []struct {
+		name    string
+		prepare func() (net.PacketConn, error)
+		// the proxy treats the IPv6 socket as optional, so a host without IPv6
+		// is a reason to skip rather than to fail
+		optional bool
+	}{
+		{name: "IPv4", prepare: PrepareSenderRawSocketIPv4},
+		{name: "IPv6", prepare: PrepareSenderRawSocketIPv6, optional: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			conn, err := tc.prepare()
+			if err != nil {
+				if tc.optional {
+					t.Skipf("prepare raw socket: %v", err)
+				}
+				t.Fatalf("prepare raw socket: %v", err)
+			}
+			defer func() {
+				if err := conn.Close(); err != nil {
+					t.Logf("close raw socket: %v", err)
+				}
+			}()
+
+			syscallConn, ok := conn.(syscall.Conn)
+			if !ok {
+				t.Fatalf("raw socket %T does not expose a syscall conn", conn)
+			}
+			raw, err := syscallConn.SyscallConn()
+			if err != nil {
+				t.Fatalf("syscall conn: %v", err)
+			}
+
+			var mark int
+			var markErr error
+			if err := raw.Control(func(fd uintptr) {
+				mark, markErr = unix.GetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_MARK)
+			}); err != nil {
+				t.Fatalf("control: %v", err)
+			}
+			if markErr != nil {
+				t.Fatalf("get SO_MARK: %v", markErr)
+			}
+
+			if mark != 0 {
+				t.Errorf("SO_MARK = %#x, want 0", mark)
+			}
+		})
+	}
+}