tcpdump, rl, sis, fxp and multicast problems

Roman Le Houelleur roman at
Fri Jan 19 10:06:42 UTC 2007

that's very clever indeed ! Well, I was not quite sure it was
actually performing the TCO (not to say I feel stupid ...).

not very important but wouldn't it be better to set the checksum
to 0 instead of some arbitrary (?) and confusing value then ?

thank you anyway,

Kris Kennaway wrote:
> On Thu, Jan 18, 2007 at 03:34:46PM +0100, Roman Le Houelleur wrote:
>> Hi everyone,
>> It's been about a year or so that I upgrade my box to RELENG_6
>> from time to time. I got some suprises this week, I've seen a
>> few things that made me think of hardware problems but some are
>> definitly soft.
>> First and easy one, tcpdump -v does not show checksum error
>> for UDP whereas it does for TCP (same w/ tcpdump -vvv).
>> checksum : I have a double port fxp 82550 Pro/100 board which
>> makes plenty of checksum errors (UDP & TCP), I suspect this
>> is related to checksum offloading but am not sure. I strongly
>> believe this card was working fine a few days/weeks ago.
> And a priori it still is: by definition, checksum offload means that
> the OS does not compute the checksum for outgoing packets, so tcpdump
> doesn't see a valid checksum either.  Unless you have evidence (from
> e.g. observation on another host) that the checksums are not being
> computed correctly, this is not a bug.  It is, however, a FAQ ;-)
> Kris

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