Reading/writing Linux's TUN/TAP device using Python.

tun-ping-linux.py

import fcntl
import os
import struct
import subprocess


# Some constants used to ioctl the device file. I got them by a simple C
# program.
TUNSETIFF = 0x400454ca
TUNSETOWNER = TUNSETIFF + 2
IFF_TUN = 0x0001
IFF_TAP = 0x0002
IFF_NO_PI = 0x1000

# Open TUN device file.
tun = open('/dev/net/tun', 'r+b')
# Tall it we want a TUN device named tun0.
ifr = struct.pack('16sH', 'tun0', IFF_TUN | IFF_NO_PI)
fcntl.ioctl(tun, TUNSETIFF, ifr)
# Optionally, we want it be accessed by the normal user.
fcntl.ioctl(tun, TUNSETOWNER, 1000)

# Bring it up and assign addresses.
subprocess.check_call('ifconfig tun0 192.168.7.1 pointopoint 192.168.7.2 up',
        shell=True)

while True:
    # Read an IP packet been sent to this TUN device.
    packet = list(os.read(tun.fileno(), 2048))

    # Modify it to an ICMP Echo Reply packet.
    #
    # Note that I have not checked content of the packet, but treat all packets
    # been sent to our TUN device as an ICMP Echo Request.

    # Swap source and destination address.
    packet[12:16], packet[16:20] = packet[16:20], packet[12:16]

    # Under Linux, the code below is not necessary to make the TUN device to
    # work. I don't know why yet, but if you run tcpdump, you can see the
    # difference.
    if True:
        # Change ICMP type code to Echo Reply (0).
        packet[20] = chr(0)
        # Clear original ICMP Checksum field.
        packet[22:24] = chr(0), chr(0)
        # Calculate new checksum.
        checksum = 0
        # for every 16-bit of the ICMP payload:
        for i in range(20, len(packet), 2):
            half_word = (ord(packet[i]) << 8) + ord(packet[i+1])
            checksum += half_word
        # Get one's complement of the checksum.
        checksum = ~(checksum + 4) & 0xffff
        # Put the new checksum back into the packet.
        packet[22] = chr(checksum >> 8)
        packet[23] = chr(checksum & ((1 << 8) -1))

    # Write the reply packet into TUN device.
    os.write(tun.fileno(), ''.join(packet))

Thanks for code, could you tell me how to create device which survive reboot and how to remove that's devices?

Have you tried this with python3? I get an error on the open call:

tun = open('/dev/net/tun', 'r+b')
Traceback (most recent call last):
File "", line 1, in
io.UnsupportedOperation: File or stream is not seekable.

You'll need to make a few changes for it to work with Python3. Disable buffering on the open call, and use an array instead of a list for the packet data.

Something like this works for me: https://gist.github.com/shawnlower/928cc78614ba726aabab1a299c810f2e/revisions

and see https://bugs.python.org/issue20074

Can you provide the code to get the ioctl constants ? I don't know how to find them on my FreeNAS device... Thanks !

Can you provide the code to get the ioctl constants ? I don't know how to find them on my FreeNAS device... Thanks !

The constants are defined in /linux/if_tun.h

i tried on python 3 but getting this error on ioctl call
fcntl.ioctl(tun, TUNSETIFF, ifr)
PermissionError: [Errno 1] Operation not permitted

i tried on python 3 but getting this error on ioctl call
fcntl.ioctl(tun, TUNSETIFF, ifr)
PermissionError: [Errno 1] Operation not permitted

You need to be root to do this kind of operations (touching system files).