'Get an invalid file descriptor.
@return: An integer which is not a valid file descriptor at the time of
this call.  After any future system call which allocates a new file
descriptor, there is no guarantee the returned file descriptor will
still be invalid.'

def _invalidFileDescriptor(self):

    fd = self.system.open('/dev/net/tun', os.O_RDWR)
    self.system.close(fd)
    return fd
