on amd64 tcp4 cksums are bad (FYI)
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
> See this thread:
This should be added to FAQ.
ru at FreeBSD.org
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