tcpdump, rl, sis, fxp and multicast problems
kris at obsecurity.org
Thu Jan 18 23:51:29 UTC 2007
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 ;-)
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