1#!/usr/local/bin/python3
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=b"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(bytes(p.payload)).chksum
17print("ipcksum=%#04x" % (ipcksum))
18udpcksum=IP(bytes(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