code: func setSocketMark(fd, mark int) error {
  	if err := syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_MARK, mark); err != nil {
  		return os.NewSyscallError("failed to set mark", err)
  	}
  	return nil
  }
docs: // setSocketMark sets packet marking on the given socket.