Open file corresponding to the TUN device.

def open(self):
        ''' Open file corresponding to the TUN device. '''
        self.fd = open('/dev/net/tun', 'rb+', buffering=0)
        tun_flags = IFF_TAP | IFF_NO_PI | IFF_PERSIST
        ifr = struct.pack('16sH', self.name, tun_flags)
        fcntl.ioctl(self.fd, TUNSETIFF, ifr)
        fcntl.ioctl(self.fd, TUNSETOWNER, os.getuid())
        self.ifflags = self.ifflags | IFF_RUNNING