Ethereal-dev: Re: [Ethereal-dev] udp checksum incorrect

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Fri, 22 Oct 2004 01:01:05 -0700
Radek Vokal wrote:


See the attached file. It shows request which has wrong checksum and
respons which seems to be ok.


So what's the IP address of the machine running Ethereal?
If it's 192.168.1.13, I'll bet the problem is that the network interface is doing UDP checksum offloading. 


Yes, I'm on Fedora Core 2 (sorry for the abbreviation, I'm too used to
it) NICs I've tested this on are intels and starfire. As I realized,
they both have zero copy issue and as I was told, the checksum of
packets I send will only be calculated when they go over the wire. Maybe
that's the problem, but I saw this happen only with TCP and ICMP.
I'd be a bit surprised if an interface offloads ICMP checksums but not UDP checksums - there's probably at least as much UDP traffic as ICMP traffic on a typical network, especially if you're doing NFS-over-UDP or Windows Media.