XL driver checksum producing corrupted but checksum-correct packets

[Robert Watson]

On Sun, 25 Jan 2004, Mike Silbersack wrote:

> On Sat, 24 Jan 2004, Robert Watson wrote:
> 
> > To pick up the corrupted packet on the machine where the corruption is
> > occurring, you might want to try hooking up the UDP checksum drop case to
> > BPF_MTAP() for a special BPF device or rule, or have it spit them into a
> > raw socket (probably easier).
> 
> He said that the packet's checksum passes, but it is corrupt, so this
> won't work. 

I may have misread: my reading was that the if_xl card marks the packet as
having passed the checksum test, but if you let the OS do the checksum,
the checksum fails.  I.e., either the hardware checksumming is broken, or
the data is corrupted between when the hardware does the checksum, and it
reaches the OS buffer.  As such, Sam's patch works because it tells the OS
to ignore the checksum results from the hardware (although it doesn't
disable the checking of checksums), causing the OS to recalculate the
checksums and drop the packets rather than accepting them.  The goal of
the change I suggested would be to also do the checksums in the OS as
well, which allows you to detect the bad packets, but instead of dropping
them, funnel them aside for later analysis.   However, if I've misread,
sorry for the confusion!

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at fledge.watson.org      Senior Research Scientist, McAfee Research