We can take all the useful command-line arguments such as network interface name, protocol name, source IP, source port, destination IP, destination port, and optional TCP flags.

We can use the Scapy library to create a custom TCP or UDP packet and send it over the network. As with the previous recipes, this recipe requires admin privilege to run.

Listing 8.5 gives the code for customizing the IP address of a packet, 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. 
 
 
import argparse 
import sys 
import re 
from random import randint 
 
from scapy.all import IP,TCP,UDP,conf,send 
 
 
def send_packet(protocol=None, src_ip=None, src_port=None, flags=None, dst_ip=None, dst_port=None, iface=None): 
    """Modify and send an IP packet.""" 
    if protocol == 'tcp': 
        packet = IP(src=src_ip, dst=dst_ip)/TCP(flags=flags,
                    sport=src_port, dport=dst_port) 
    elif protocol == 'udp': 
        if flags: raise Exception(" Flags are not supported for udp") 
        packet = IP(src=src_ip, dst=dst_ip)/UDP(sport=src_port,
                     dport=dst_port) 
    else: 
        raise Exception("Unknown protocol %s" % protocol) 
 
    send(packet, iface=iface) 
    
 
if __name__ == '__main__': 
    # setup commandline arguments 
    parser = argparse.ArgumentParser(description='Packet Modifier') 
    parser.add_argument('--iface', action="store",
                        dest="iface", default='eth0') 
    parser.add_argument('--protocol', action="store",
                        dest="protocol", default='tcp') 
    parser.add_argument('--src-ip', action="store",
                        dest="src_ip", default='1.1.1.1') 
    parser.add_argument('--src-port', action="store",
                       dest="src_port", default=randint(0, 65535)) 
    parser.add_argument('--dst-ip', action="store",
                       dest="dst_ip", default='192.168.1.51') 
    parser.add_argument('--dst-port', action="store",
                       dest="dst_port", default=randint(0, 65535)) 
    parser.add_argument('--flags', action="store",
                       dest="flags", default=None) 
    # parse arguments 
    given_args = parser.parse_args() 
    iface, protocol, src_ip,  src_port, dst_ip, dst_port,
    flags =  given_args.iface, given_args.protocol,
    given_args.src_ip, 
      given_args.src_port, given_args.dst_ip,
      given_args.dst_port, given_args.flags 
    send_packet(protocol, src_ip, src_port, flags,
      dst_ip, dst_port, iface) 

In order to run this script, enter the following commands:

$ sudo tcpdump src 192.168.1.66
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
^C18:37:34.309992 IP 192.168.1.66.60698 > 192.168.1.51.666: Flags [S], seq 0, win 8192, length 0
    
1 packets captured
1 packets received by filter
0 packets dropped by kernel
    
$ python 18_5_modify_ip_in_a_packet.py 
WARNING: No route found for IPv6 destination :: (no default route?)
.
Sent 1 packets.