[Tutor] ICMP checksum

[the taloner]

Hi list,

I was just playing around with one of the examples given in Python
cook-book. This examples show you how to send data using ICMP packets.
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/439224

While running the script I encountered a problem which was with the
statement

icmp.set_icmp_cksum(0)

The error encountered was

Traceback (most recent call last):
File "icmpdata.py", line 38, in ?
icmp.set_icmp_chksum(0)
AttributeError: ICMP instance has no attribute 'set_icmp_chksum'


So I did the next logical thing, commented the line in code (which appears
twice). Script ran smoothly and I captured the traffic using tcpdump.
When the script sends first 54 bytes everything is good. When I send a
longer data string, and the script is forced to split the data over multiple
packets I start fasing a problem with checksum.

17:36:36.198543 IP (tos 0x0, ttl 255, id 16951, offset 0, flags [none],
length: 82) 192.168.3.37 <http://192.168.3.37> >
192.168.3.36<http://192.168.3.36>:
icmp 62: echo reply seq 0
0x0000: 0002 3fb9 17fe 000c 299c e8a5 0800 4500 ..?.....).....E.
0x0010: 0052 4237 0000 ff01 f1d9 c0a8 0325 c0a8 .RB7.........%..
0x0020: 0324 0000 bab9 0001 0000 6161 6161 6161 .$........aaaaaa
0x0030: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa
0x0040: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa
0x0050: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa
17:36:37.198470 IP (tos 0x0, ttl 255, id 16952, offset 0, flags [none],
length: 82) 192.168.3.37 <http://192.168.3.37> >
192.168.3.36<http://192.168.3.36>:
icmp 62: echo reply seq 0 (wrong icmp cksum bab9 (->bab8)!)
0x0000: 0002 3fb9 17fe 000c 299c e8a5 0800 4500 ..?.....).....E.
0x0010: 0052 4238 0000 ff01 f1d8 c0a8 0325 c0a8 .RB8.........%..
0x0020: 0324 0000 bab9 0002 0000 6161 6161 6161 .$........aaaaaa
0x0030: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa
0x0040: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa
0x0050: 6161 6161 6161 6161 6161 6161 6161 6161 aaaaaaaaaaaaaaaa

Also as you can see that seq is not increasing as ideally it should have
been as directed by this line:

seq_id = seq_id + 1

Can someone give some light on these issues?

Thanks
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