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]
9uport=os.getpid() & 0xffff
10# inetd ignores UDP packets from privileged port or nfs
11if uport < 1024 or uport == 2049:
12	uport+=1024
13payload="a" * 1472
14p=(Ether(src=SRC_MAC, dst=PF_MAC)/IP(flags="DF", src=SRC_OUT, dst=dstaddr)/
15    UDP(sport=uport, dport=9)/payload)
16ipcksum=IP(str(p.payload)).chksum
17print "ipcksum=%#04x" % (ipcksum)
18udpcksum=IP(str(p.payload)).payload.chksum
19print "udpcksum=%#04x" % (udpcksum)
20a=srp1(p, iface=SRC_IF, timeout=2)
21if a and a.type == ETH_P_IP and \
22    a.payload.proto == 1 and \
23    icmptypes[a.payload.payload.type] == 'dest-unreach' and \
24    icmpcodes[a.payload.payload.type][a.payload.payload.code] == \
25    'fragmentation-needed':
26	outeripcksum=a.payload.chksum
27	print "outeripcksum=%#04x" % (outeripcksum)
28	outercksum=a.payload.payload.chksum
29	print "outercksum=%#04x" % (outercksum)
30	q=a.payload.payload.payload
31	inneripcksum=q.chksum
32	print "inneripcksum=%#04x" % (inneripcksum)
33	if q.proto == 17:
34		innercksum=q.payload.chksum
35		print "innercksum=%#04x" % (innercksum)
36		if innercksum == udpcksum:
37			exit(0)
38		print "INNERCKSUM!=UDPCKSUM"
39		exit(1)
40	print "NO INNER UDP REQUEST"
41	exit(2)
42print "NO FRAGMENTATION NEEDED"
43exit(2)
44