#!/usr/local/bin/python3
# check wether path mtu to dst is as expected

import os
from addr import *
from scapy.all import *

# usage: ping_mtu src dst size icmp-size

srcaddr=sys.argv[1]
dstaddr=sys.argv[2]
size=int(sys.argv[3])
expect=int(sys.argv[4])
eid=os.getpid() & 0xffff
hdr=IP(flags="DF", src=srcaddr, dst=dstaddr)/ICMP(type='echo-request', id=eid)
payload="a" * (size - len(bytes(hdr)))
ip=hdr/payload
iplen=IP(bytes(ip)).len
eth=Ether(src=SRC_MAC, dst=PF_MAC)/ip
a=srp1(eth, iface=SRC_IF, timeout=2)

if a is None:
	print("no packet sniffed")
	exit(2)
if a and a.payload.payload.type==3 and a.payload.payload.code==4:
	mtu=a.payload.payload.nexthopmtu
	print("mtu=%d" % (mtu))
	if mtu != expect:
		print("MTU!=%d" % (expect))
		exit(1)
	iip=a.payload.payload.payload
	iiplen=iip.len
	if iiplen != iplen:
		print("inner IP len %d!=%d" % (iiplen, iplen))
		exit(1)
	isrc=iip.src
	if isrc != srcaddr:
		print("inner IP src %d!=%d" % (isrc, srcaddr))
		exit(1)
	idst=iip.dst
	if idst != dstaddr:
		print("inner IP dst %d!=%d" % (idst, dstaddr))
		exit(1)
	exit(0)
print("MTU=UNKNOWN")
exit(2)