Using raw socket in Python

Question

I have this simple client code which is sending a message to a server and then echoes the server response.

import socket, sys, time


HOST = 'localhost'   
PORT = 11000             
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
msg = raw_input()
s.send(msg)
data = s.recv(len(msg))
s.close()
print 'Received: ', data

I wish to use raw socket here in order to be able to get the IP_ID field out of the server's response packet, but I have no idea how.

No matter how much I read about it on the internet I did not find a good example of how to do it.

(Just to clarify, I don't really need to send a raw packet to the server, I can use the normal SOCK_STREAM to send the message, but I need to use raw socket to get the response' IP_ID.)

Can anyone just edit my code and show me, in the simpliest way, how to do it? All I managed to find out is that I need to replace socket.SOCK_STREAM with socket.SOCK_RAW, but im still far away.

Thanks alot

Answer 1

Reading from a raw socket and parsing out the IP_ID is trivial:

response, addr = s.recvfrom(65535)
response_id = struct.unpack('!H', response[4:6])
print response_id

The hard part is getting someone to send you a packet in the first place. I'm pretty sure you can't use the same socket in raw mode and stream mode at the same time, so you're going to need to replace that connect and send with a bunch of much more complicated code that constructs and sends the appropriate TCP packets to initiate the connection. There are libraries like scapy that will do all that hard stuff for you, but if you want to do it manually, you just need to read RFC 791 and RFC 793 carefully, do all the tedious stuff (making sure you get all the endianness right), and you're on your way.

On *BSD (including OS X), the kernel will fill in the IP length, TCP length, and, best of all, the TCP checksum. It gets much more painful if you have to handle those yourself. (If this doesn't work on your platform, you probably do… either that, or I screwed up something else that OS X fixes for me automagically and your platform doesn't.)

import socket
import struct

def make_ip(proto, srcip, dstip, ident=54321):
    saddr = socket.inet_aton(srcip)
    daddr = socket.inet_aton(dstip)
    ihl_ver = (4 << 4) | 5
    return struct.pack('!BBHHHBBH4s4s' , 
                       ihl_ver, 0, 0, ident, 0, 255, proto, 0, saddr, daddr)

def make_tcp(srcport, dstport, payload, seq=123, ackseq=0,
             fin=False, syn=True, rst=False, psh=False, ack=False, urg=False,
             window=5840):
    offset_res = (5 << 4) | 0
    flags = (fin | (syn << 1) | (rst << 2) | 
             (psh <<3) | (ack << 4) | (urg << 5))
    return struct.pack('!HHLLBBHHH', 
                       srcport, dstport, seq, ackseq, offset_res, 
                       flags, window, 0, 0)

srcip = dstip = '127.0.0.1'
srcport, dstport = 11001, 11000
payload = '[TESTING]\n'

ip = make_ip(socket.IPPROTO_TCP, srcip, dstip)
tcp = make_tcp(srcport, dstport, payload)
packet = ip + tcp + payload

s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)
s.sendto(packet, (dstip, 0))
response, addr = s.recvfrom(65535)
response_id = struct.unpack('!H', response[4:6])
print response_id

Each time I run this, I get a nak (if no one's listening on port 11000) or ack packet with a randomized IP_ID, just as you'd expect.