Python packet sniffer using ctypes crashes when copying socket buffer

Question

I'm trying to capture the first 20 bytes (full packet minus the options) of an IP packet, populate a struct of ctype members, and print the information I want to the screen (protocol, source and destination address). My IP class is as follows:

import socket
import os
import struct
from ctypes import *

# host to 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 to their 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, I create the socket, bind it to the host, and loop to get the packets:

# create socket and bind to 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 a packet
        raw_buffer = sniffer.recvfrom(65565)[0]
        # create an IP header from the first 20 bytes of the buffer
        ip_header = IP(raw_buffer[0:20])
        # print out the protocol that was detected and the 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 as data type for the "src" and "dst" _fields_ struct members, I get the following error (running in one terminal while pinging nostarch.com in another):

I postulated that perhaps the size of the c_ulong was larger than a byte, thus throwing off my requirement for the first 20 bytes (I'm very new to python). I then changed the c_ulong to c_ushort and ran it again:

Actual ping path:

So, while the script ran without error, it's cutting off the src and dst addresses.

Why is it asking for at least 32 bytes when I'm telling it I only want the first 20?

(I'm in Kali64 VBox VM, running on Win7 host, using Python 2.7)

Any help is appreciated.

Answer 1

The size of IP should be verified, i.e.

print(sizeof(IP))

should return 20 bytes. Since ctypes.u_long is 8 in case of 64bit linux, the size will be 32 bytes (4 bytes extra due to padding, 8 bytes due to integer sizes). Either use ctypes.u_int or explicit sizes as 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))