Let us sniff the packets using the sniff() function of Scapy and define a callback function, modify_packet_header(), which adds an extra header of certain packets.

Listing 8.3 gives the code for adding an extra header in HTTP packets, as follows:

#!/usr/bin/env python 
# Python Network Programming Cookbook, Second Edition -- Chapter - 8 
# This program is optimized for Python 2.7.12 and Python 3.5.2. 
# It may run on any other version with/without modifications. 
 
 
from scapy.all import * 
 
def modify_packet_header(pkt): 
    """ Parse the header and add an extra header""" 
    if pkt.haslayer(TCP) and pkt.getlayer(TCP).dport == 80
     and pkt.haslayer(Raw): 
        hdr = pkt[TCP].payload.__dict__         
        extra_item = {'Extra Header' : ' extra value'} 
        hdr.update(extra_item)      
        send_hdr = '
'.join(hdr) 
        pkt[TCP].payload = send_hdr 
         
        pkt.show() 
         
        del pkt[IP].chksum 
        send(pkt) 
 
if __name__ == '__main__': 
    # start sniffing 
    sniff(filter="tcp and ( port 80 )", prn=modify_packet_header) 
 

If you run this script, it will show a captured packet; print the modified version of it and send it to the network, as shown in the following output. This can be verified by other packet capturing tools such as tcpdump or wireshark:

$ python 8_3_add_extra_http_header_in_sniffed_packet.py 
    
###[ Ethernet ]###
  dst       = 52:54:00:12:35:02
  src       = 08:00:27:95:0d:1a
  type      = 0x800
###[ IP ]###
     version   = 4L
     ihl       = 5L
     tos       = 0x0
     len       = 525
     id        = 13419
     flags     = DF
     frag      = 0L
     ttl       = 64
     proto     = tcp
     chksum    = 0x171
     src       = 10.0.2.15
     dst       = 82.94.164.162
     options   
###[ TCP ]###
        sport     = 49273
        dport     = www
        seq       = 107715690
        ack       = 216121024
        dataofs   = 5L
        reserved  = 0L
        flags     = PA
        window    = 6432
        chksum    = 0x50f
        urgptr    = 0
        options   = []
###[ Raw ]###
           load      = 'Extra Header
sent_time
fields
aliastypes
post_transforms
underlayer
fieldtype
time
initialized
overloaded_fields
packetfields
payload
default_fields'
.
Sent 1 packets.