On Thu, 2004-10-21 at 09:39 -0700, Guy Harris wrote:
> Radek Vokal wrote:
>
> > anyone experienced this: When I'm doing ping and capture UDP packets
> > (I'm usind UDP encapsulation) with ethereal there's incorrect udp-
> > checksum.
>
> I.e., Ethereal reports that the UDP checksum is incorrect?
See the attached file. It shows request which has wrong checksum and
respons which seems to be ok.
>
> > I've tested this on different machines and it appears only on
> > FC2 with different NIC.
>
> (I assume "FC2" means Fedora Core 2.)
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.
>
> Does it appear on all NICs? If so, do those NICs support offloading UDP
> checksums, as well as TCP checksums? If they support offloading UDP
> checksums, this might be the same problem that's seen with NICs that
> support TCP checksum offloading:
>
> http://www.ethereal.com/faq#q5.14
>
> > My questio is: How is ethereal computing the
> > correct checksum?
>
> It uses code based on BSD's Internet checksum code.
>
> > Is the ethereal one computed using libpcap
>
> No.
>
> > or is it vice versa?
>
> No. It's independent of libpcap - which contains no checksum code,
> although recent versions of tcpdump do have checksum code and does the
> same sort of checking Ethereal does.
> Tcpdump might correctly handle packets with source-routing IP options; I
> don't think Ethereal would currently do so.
Ok
Radek
>
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--
Radek Vokál <rvokal@xxxxxxxxxx> -- Red Hat, Inc.
Linux 2.6.8-1.521smp #1 SMP Mon Aug 16 09:25:06 EDT 2004 i686 GNU/Linux
Attachment:
udp_checksum
Description: Binary data
