Follow HTTP Stream (with decompression)

| dpkt | gzip | http | pcap | python

I was using Wireshark to capture an exchange of HTTP packets, however, some of the HTTP responses were using “content-encoding: gzip”, which meant I couldn’t view them decompressed in the “Follow TCP Stream”. Wireshark does decompress them in Packet Details view, but it is hard to follow the full stream like this.

The solution was to write some Python which made use of the dpkt library. My code naively reassembles the TCP flow and then assumes traffic on port 80 is HTTP. Therefore there is much room for improvement, but here is the code anyway.

#!/usr/bin/env python
# Turns a pcap file with http gzip compressed data into plain text, making it
# easier to follow.

import dpkt

def tcp_flags(flags):
	ret = ''
	if flags & dpkt.tcp.TH_FIN:
		ret = ret + 'F'
	if flags & dpkt.tcp.TH_SYN:
		ret = ret + 'S'
	if flags & dpkt.tcp.TH_RST:
		ret = ret + 'R'
	if flags & dpkt.tcp.TH_PUSH:
		ret = ret + 'P'
	if flags & dpkt.tcp.TH_ACK:
		ret = ret + 'A'
	if flags & dpkt.tcp.TH_URG:
		ret = ret + 'U'
	if flags & dpkt.tcp.TH_ECE:
		ret = ret + 'E'
	if flags & dpkt.tcp.TH_CWR:
		ret = ret + 'C'

	return ret

def parse_http_stream(stream):
	while len(stream) > 0:
		if stream[:4] == 'HTTP':
			http = dpkt.http.Response(stream)
			print http.status
		else:
			http = dpkt.http.Request(stream)
			print http.method, http.uri
		stream = stream[len(http):]

def parse_pcap_file(filename):
	# Open the pcap file
	f = open('market.pcap', 'rb')
	pcap = dpkt.pcap.Reader(f)
	
	# I need to reassmble the TCP flows before decoding the HTTP
	conn = dict() # Connections with current buffer
	for ts, buf in pcap:
		eth = dpkt.ethernet.Ethernet(buf)
		if eth.type != dpkt.ethernet.ETH_TYPE_IP:
			continue
	
		ip = eth.data
		if ip.p != dpkt.ip.IP_PROTO_TCP:
			continue
	
		tcp = ip.data
	
		tupl = (ip.src, ip.dst, tcp.sport, tcp.dport)
		#print tupl, tcp_flags(tcp.flags)
	
		# Ensure these are in order! TODO change to a defaultdict
		if tupl in conn:
			conn[ tupl ] = conn[ tupl ] + tcp.data
		else:
			conn[ tupl ] = tcp.data
	
		# TODO Check if it is a FIN, if so end the connection
	
		# Try and parse what we have
		try:
			stream = conn[ tupl ]
			if stream[:4] == 'HTTP':
				http = dpkt.http.Response(stream)
				#print http.status
			else:
				http = dpkt.http.Request(stream)
				#print http.method, http.uri
	
			print http
			print

			# If we reached this part an exception hasn't been thrown
			stream = stream[len(http):]
			if len(stream) == 0:
				del conn[ tupl ]
			else:
				conn[ tupl ] = stream
		except dpkt.UnpackError:
			pass

	f.close()

if __name__ == '__main__':
	import sys
	if len(sys.argv) <= 1:
		print "%s <pcap filename>" % sys.argv[0]
		sys.exit(2)

	parse_pcap_file(sys.argv[1])

Please note, I had to make a couple of changes to the dpkt library, which I have submitted back for review. Those changes can be found in the following patches 1 2 3. I will update this code if/when the patches get accepted.

comments powered by Disqus