Simple packet manipulation with fqrouter's fork of python-netfilterqueue (https://github.com/fqrouter/python-netfilterqueue)

nfqueue

#! /usr/bin/env python2.7

from scapy.all import *
from netfilterqueue import NetfilterQueue

def modify(packet):
  pkt = IP(packet.get_payload()) #converts the raw packet to a scapy compatible string

  #modify the packet all you want here

  packet.set_payload(str(pkt)) #set the packet content to our modified version

  packet.accept() #accept the packet 



nfqueue = NetfilterQueue()
#1 is the iptabels rule queue number, modify is the callback function
nfqueue.bind(1, modify) 
try:
  print "[*] waiting for data"
  nfqueue.run()
except KeyboardInterrupt:
  pass