[Tutor] ICMP checksum

[Kent Johnson]

the taloner wrote:
> 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'

This recipe uses a package called impacket which is available from
http://oss.coresecurity.com/projects/impacket.html

Looking at the implementation of ICMP in the latest release of ImpactPacket.py, the correct spelling of the method is 'set_icmp_cksum'

Kent

> 
> 
> 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.

That shouldn't be too surprising if you commented out the line that sets the 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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