1#!/usr/local/bin/python3
2# check ip and icmp checksum in returned icmp packet
3
4import os
5from addr import *
6from scapy.all import *
7
8dstaddr=sys.argv[1]
9eid=os.getpid() & 0xffff
10payload=b"a" * 1472
11p=(Ether(src=SRC_MAC, dst=PF_MAC)/IP(flags="DF", src=SRC_OUT, dst=dstaddr)/
12    ICMP(type='echo-request', id=eid)/payload)
13ipcksum=IP(bytes(p.payload)).chksum
14print("ipcksum=%#04x" % (ipcksum))
15echocksum=IP(bytes(p.payload)).payload.chksum
16print("echocksum=%#04x" % (echocksum))
17a=srp1(p, iface=SRC_IF, timeout=2)
18if a and a.type == ETH_P_IP 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 == 1 and \
31	    icmptypes[q.payload.type] == 'echo-request':
32		innercksum=q.payload.chksum
33		print("innercksum=%#04x" % (innercksum))
34		if innercksum == echocksum:
35			exit(0)
36		print("INNERCKSUM!=ECHOCKSUM")
37		exit(1)
38	print("NO INNER ECHO REQUEST")
39	exit(2)
40print("NO FRAGMENTATION NEEDED")
41exit(2)
42