1#!/usr/local/bin/python2.7
2# check ip and udp checksum in returned icmp packet
3
4import os
5from addr import *
6from scapy.all import *
7
8dstaddr=sys.argv[1]
9pid=os.getpid()
10payload="a" * 1452
11p=(Ether(src=SRC_MAC, dst=PF_MAC)/IP(flags="DF", src=SRC_OUT, dst=dstaddr)/
12    UDP(sport=pid,dport=9)/payload)
13ipcksum=IP(str(p.payload)).chksum
14print "ipcksum=%#04x" % (ipcksum)
15udpcksum=IP(str(p.payload)).payload.chksum
16print "udpcksum=%#04x" % (udpcksum)
17a=srp1(p, iface=SRC_IF, timeout=2)
18if a and a.type == scapy.layers.dot11.ETHER_TYPES.IPv4 and \
19    a.payload.proto == 1 and \
20    icmptypes[a.payload.payload.type] == 'dest-unreach' and \
21    icmpcodes[a.payload.payload.type][a.payload.payload.code] == \
22    'fragmentation-needed':
23	outeripcksum=a.payload.chksum
24	print "outeripcksum=%#04x" % (outeripcksum)
25	outercksum=a.payload.payload.chksum
26	print "outercksum=%#04x" % (outercksum)
27	q=a.payload.payload.payload
28	inneripcksum=q.chksum
29	print "inneripcksum=%#04x" % (inneripcksum)
30	if q.proto == 17:
31		innercksum=q.payload.chksum
32		print "innercksum=%#04x" % (innercksum)
33		if innercksum == udpcksum:
34			exit(0)
35		print "INNERCKSUM!=UDPCKSUM"
36		exit(1)
37	print "NO INNER UDP REQUEST"
38	exit(2)
39print "NO FRAGMENTATION NEEDED"
40exit(2)
41