Python packet sniffer using ctypes crashes when copying socket buffer -

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i'm trying capture first 20 bytes (full packet minus options) of ip packet, populate struct of ctype members, , print information want screen (protocol, source , destination address). ip class follows:

import socket import os import struct ctypes import *  # host listen on host = "192.168.0.187" # our ip header class ip(structure):         _fields_ = [             ("ihl", c_ubyte, 4),             ("version", c_ubyte, 4),             ("tos", c_ubyte),             ("len", c_ushort),             ("id", c_ushort),             ("offset", c_ushort),             ("ttl", c_ubyte),             ("protocol_num", c_ubyte),             ("sum", c_ushort),             ("src", c_ulong),             ("dst", c_ulong)         ]     def __new__(self, socket_buffer=none):         return self.from_buffer_copy(socket_buffer)     def __init__(self, socket_buffer=none):         # map protocol constants names         self.protocol_map = {1:"icmp", 6:"tcp", 17:"udp"}         # human readable ip addresses         self.src_address = socket.inet_ntoa(struct.pack("<l",self.src))         self.dst_address = socket.inet_ntoa(struct.pack("<l",self.dst))         # human readable protocol         try:             self.protocol = self.protocol_map[self.protocol_num]         except:             self.protocol = str(self.protocol_num)

now, create socket, bind host, , loop packets:

# create socket , bind public interface (os dependent) if os.name == "nt":     socket_protocol = socket.ipproto_ip else:     socket_protocol = socket.ipproto_icmp  # create raw socket sniffer = socket.socket(socket.af_inet, socket.sock_raw, socket_protocol) sniffer.bind((host, 0)) # include header information sniffer.setsockopt(socket.ipproto_ip, socket.ip_hdrincl, 1)  if os.name == "nt":     sniffer.ioctl(socket.sio_rcvall, socket.rcvall_on)  try:     while true:         # read in packet         raw_buffer = sniffer.recvfrom(65565)[0]         # create ip header first 20 bytes of buffer         ip_header = ip(raw_buffer[0:20])         # print out protocol detected , hosts         print "protocol: %s %s -> %s" % (ip_header.protocol, ip_header.src_¬                                 address, ip_header.dst_address) # handle ctrl-c except keyboardinterrupt:     # if we're using windows, turn off promiscuous mode     if os.name == "nt":         sniffer.ioctl(socket.sio_rcvall, socket.rcvall_off)

when run using c_ulong data type "src" , "dst" _fields_ struct members, following error (running in 1 terminal while pinging nostarch.com in another):

enter image description here

i postulated perhaps size of c_ulong larger byte, throwing off requirement first 20 bytes (i'm new python). changed c_ulong c_ushort , ran again:

enter image description here

actual ping path:

enter image description here

so, while script ran without error, it's cutting off src , dst addresses.

why asking @ least 32 bytes when i'm telling want first 20?

(i'm in kali64 vbox vm, running on win7 host, using python 2.7)

any appreciated.

the size of ip should verified, i.e.

print(sizeof(ip))

should return 20 bytes. since ctypes.u_long 8 in case of 64bit linux, size 32 bytes (4 bytes due padding, 8 bytes due integer sizes). either use ctypes.u_int or explicit sizes follows:

from ctypes import *  class ip(structure):     _fields_ = [ ("version", c_uint8, 4),                  ("ihl", c_uint8, 4),                  ("tos", c_uint8),                  ("len", c_uint16),                  ("id", c_uint16),                  ("offset", c_uint16),                  ("ttl", c_uint8),                  ("protocol_num", c_uint8),                  ("sum", c_uint16),                  ("src", c_uint32),                  ("dst", c_uint32) ]  print(sizeof(ip))

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