open utun device in OS X

utun.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# open utun device in OS X
# Refs:
# - https://github.com/python/cpython/blob/master/Modules/socketmodule.c
# - https://opensource.apple.com/source/xnu/xnu-4570.31.3/bsd/sys/kern_control.h.auto.html
# - https://opensource.apple.com/source/xnu/xnu-344.49/bsd/sys/sys_domain.h.auto.html
# - https://opensource.apple.com/source/xnu/xnu-2782.20.48/bsd/net/if_utun.h.auto.html
#

import fcntl
import socket
import struct

MAX_KCTL_NAME = 96
UTUN_CONTROL_NAME = 'com.apple.net.utun_control'.encode()
CTLIOCGINFO = 3227799043

PF_SYSTEM = getattr(socket, 'PF_SYSTEM') if hasattr(socket, 'PF_SYSTEM') else 32
SYSPROTO_CONTROL = getattr(socket, 'SYSPROTO_CONTROL') if hasattr(socket, 'SYSPROTO_CONTROL') else 2

FMT = '<I{}s'.format(MAX_KCTL_NAME)


# raise OSError: [Errno 16] Resource busy
def utun(idx):
    sock = socket.socket(PF_SYSTEM, socket.SOCK_DGRAM, SYSPROTO_CONTROL)
    arg = struct.pack(FMT, 0, UTUN_CONTROL_NAME)
    ctl_info = fcntl.ioctl(sock, CTLIOCGINFO, arg)
    ctl_id, ctl_name = struct.unpack(FMT, ctl_info)
    sock.connect((ctl_id, idx + 1))
    return sock