Skip to content

Instantly share code, notes, and snippets.

Created September 18, 2010 04:49
Show Gist options
  • Star 63 You must be signed in to star a gist
  • Fork 18 You must be signed in to fork a gist
  • Save glacjay/585369 to your computer and use it in GitHub Desktop.
Save glacjay/585369 to your computer and use it in GitHub Desktop.
Reading/writing Linux's TUN/TAP device using Python.
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
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 pointopoint up',
while True:
# Read an IP packet been sent to this TUN device.
packet = list(, 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))
Copy link

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).

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment