How to recalculate IP checksum with scapy? [duplicate]

Question

Possible Duplicate:
How to calculate a packet checksum without sending it?

I've spoofed a source IP and MAC address in a captured packet, but now I need to recalculate the checksum so that it checks out once its been received (after being injected into the network of course). I didn't really want to implement the checksum myself, and I was thinking that scapy could do this for me. I read that the show2() function should recalculate the checksum, but I can't seem to get it to work.

So, how can I use scapy to recalculate (and replace) the checksum for a captured + spoofed packet?

Thanks!

Dyche

Also, if there is another easy to use Python library that will work nicely in this situation - I'm all ears (eyes). – Dyche

Answer 1

As shown here, you have to delete the .chksum attribute before calling the show2() method from scapy

Borage

I selected this answer because the key was that I had to del the chksum field. Previously I was manually rebuilding the packet with null bytes in the checksum field. Doing that and then calling show2() did not recalculate the checksum. Deleting the field first with del provides the correct behaviour. – Dyche

Answer 2

Let's say for argument's sake that we're processing an IP header and want to recalculate the checksum after the next hop:

>>> iph = IP(import_hexcap())
0000 4500 0064 000f 0000 fe01 3726 c0a8 0108
0010 c0a8 030b
>>> iph.ttl = iph.ttl - 1
>>> del iph.chksum
>>> iph.show2()
###[ IP ]###
version= 4L
ihl= 5L
tos= 0x0
len= 100
id= 15
flags= 
frag= 0L
ttl= 253
proto= icmp
chksum= 0x3826
src= 192.168.1.8
dst= 192.168.3.11
options= 

The .chksum field has your answer.

Pereyra