on amd64 tcp4 cksums are bad (FYI)

Ruslan Ermilov ru at freebsd.org
Fri Aug 20 12:54:24 PDT 2004

On Fri, Aug 20, 2004 at 03:41:26PM -0400, Andrew Gallatin wrote:
> David W. Hankins writes:
>  > 
>  > This is as observed via tcpdump on [client], which is what is producing
>  > the bad checksums.  Obviously it doesn't cause a problem since no one
>  > listens to TCP checksums, but it's interesting.  I only noticed it
>  > because I was tcpdump'ing for completely unrelated reasons, and it caught
>  > my eye.
> <...>
>  > Client machine is amd64, running 64-bit mode 5-current fresh as of
>  > yesterday.  Network interface is e1000, so fxp.  Server is also freebsd
> e1000 is actually em.  
> You're almost certainly using a driver which offloads transmit
> checksums.  (both fxp and em do) Since BPF sniffs the packet before it
> leaves the host, the checksum has not yet been calculated, so it looks
> bad.
> See this thread:
> http://lists.FreeBSD.org/pipermail/freebsd-net/2004-August/004703.html
This should be added to FAQ.

Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer
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