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Date: Tue, 16 Oct 2007 14:06:07 +0200
From: Jelte Jansen <jelte@NLnetLabs.nl>
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Subject: Re: re(4) checksum offload problem
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Pyun YongHyeon wrote:
>  > i'm seeing bad IP/UDP checksums on packets on a realtek gigabit network
>  > card. Turning offloading off (ifconfig -txcsum) seems to make the
>  > problem disappear.
>  > 
> Would you give me more information for your network configuration?
> Which host use re(4) driver? Which host was used to capture the
> traffic?
> 
> If you see bad checksum messages on hosts with Tx checksum enabled
> driver it's quite normal to see bad checksummed frames. Hardware
> will insert computed checksum into the frame after bpf listeners
> see the frame.
> 

Ah, right, they both run re(4) drivers, so what i thought was bad
checksums from the other host were actually not-filled-in-yet checksums
for response packets to those packets (...).

I get it now, when you don't offload they are computed before the
filter, so in that case you don't get these 'errors'. But with
offloading it's not done until the packets are sent on the wire.

Annoying, that :)

But thanks, and sorry for the noise.

Jelte

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