sniffarp.py

#!/usr/bin/env python

# Written by Antoine 'NaPs' Millet
#
#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#                    Version 2, December 2004
#
# Copyright (C) 2004 Sam Hocevar
#  14 rue de Plaisance, 75014 Paris, France
# Everyone is permitted to copy and distribute verbatim or modified
# copies of this license document, and changing it is allowed as long
# as the name is changed.
#
#            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
#   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
#
#  0. You just DO WHAT THE FUCK YOU WANT TO.
#

import struct
from socket import *

# Configuration :

protocol = 0x0806 # ARP Protocol
if_name = 'eth0' # Network interface to bind to socket

# End of configuration

# Funcs :

def mac2string(macaddr):
	return ':'.join(['%02X' % (ord(c)) for c in macaddr])

def ip2string(ipaddr):
	return '.'.join([str(ord(c)) for c in ipaddr])

def val2int(val):
	return int(''.join(['%02d'%ord(c) for c in val]), 16)

# End of funcs

if __name__ == '__main__':
	s = socket(AF_PACKET, SOCK_RAW, SOCK_RAW) # This is a raw socket, 
											  # script must be executed
											  # as root !

	s.bind((if_name, SOCK_RAW))

	#mac_addr = s.getsockname()[4] # getting mac address

	while True: # Main while
		frame = s.recv(1048) # Getting last frame in netbuff
		
		# Ethernet frame :
		# +------------+------------+----+----------+
		# |     6B     |     6B     | 2B | ...1498B |
		# +------------+------------+----+----------+
		# |  @ dest    |    @ src   |type|  data... |
		# +------------+------------+----+----------+
		# 
		# So, frame[:14] are ethernet headers, and frame[14:] are data
		
		# ARP packet :
		# +----+----+--+--+----+------------------+
		# | 2B | 2B |1B|1B| 2B |    dep of *1*2   |
		# +----+----+--+--+----+------------------+
		# |HWty|PTty|*1|*2| op |       Addrs      |
		# +----+----+--+--+----+------------------+
		# *1 : Hardware address length
		# *2 : Protocol address length
		#
		# So, frame[14:22] are arp headers, and frame[22:x] addresses, 
		# x depending of hw and proto addresses sizes.

		# Decoding :
		
		# Getting "type" field in ethernet headers
		proto_type = val2int(struct.unpack('!2s', frame[12:14])[0])
		
		# Filter frames which are not arp frames
		if proto_type != protocol:
			continue
		
		arp_headers = frame[14:22]
		arp_headers_values = struct.unpack('!2s2s1s1s2s', arp_headers)
		
		hw_type, pt_type, hw_size, pt_size, operation = \
								[val2int(v) for v in arp_headers_values]
		
		total_addresses_byte = hw_size * 2 + pt_size * 2
		arp_addrs = frame[22:22 + total_addresses_byte]
		
		src_hw, src_pt, dst_hw, dst_pt = struct.unpack(
				'!%ss%ss%ss%ss' % (hw_size, pt_size, hw_size, pt_size), 
				arp_addrs)
		
		if operation == 1: # ARP Request
			print '[%s / %s] Who is %s ?' % (
				mac2string(src_hw), 
				ip2string(src_pt),
				ip2string(dst_pt)
				)
		elif operation == 2: # ARP Reply
			print '[%s / %s] I\'m %s' % (
				mac2string(src_hw), 
				ip2string(src_pt),
				ip2string(src_pt)
				)