#! /usr/bin/env python
#
# pmtu.py client_ip server_ip rouer_ip mtu
#
# (c) 2009 by packetlevel.ch 
#
# version 0.3 / 8.Nov.2009
#
#
# based on 
# ICMP message Type 3 code 4 (destination unreachable, don't fragment (DF) bit sent and fragmentation required)
#
# import requert parts
#
from scapy.all import *
from sys import argv 
from sys import exit
#
# command line options
#
if len(argv) !=5:
   print"Usage:\n\tpmtu.py Client_IP Server_IP Router_IP MTU"
   print"\tExample pmtu.py 192.168.0.12 172.16.1.1 10.1.1.1 1000"
   sys.exit(0)

ip_client = argv[1]
ip_server = argv[2]
ip_router = argv[3]
mtu = int(argv[4])
#
# build answer packet
#
ip = IP()
icmp = ICMP()
#
# IP Packet sent to client
#
ip.dst = ip_client
ip.src = ip_router 
#
# icmp type 3 + code 4 
#
icmp.type = 3
icmp.code = 4
icmp.unused = mtu
#
# build original packet for ICMP ping request
#
ip_orig = IP()
ip_orig.src = ip_client
ip_orig.dst = ip_server
icmp_orig = ICMP()
icmp_orig.type = 8 
icmp_orig.code = 0 
#
# send the packet
#
send (ip/icmp/ip_orig/icmp_orig)