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#!/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)
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