netdev - Re: jme: UDP checksum error, and lots of them

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Message-ID: <20101203230250.GA3612@electric-eye.fr.zoreil.com>
Date:	Sat, 4 Dec 2010 00:02:50 +0100
From:	Francois Romieu <romieu@...zoreil.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	David Miller <davem@...emloft.net>, cooldavid@...ldavid.org,
	netdev@...r.kernel.org
Subject: Re: jme: UDP checksum error, and lots of them

Jan Engelhardt <jengelh@...ozas.de> :
> On Friday 2010-12-03 00:15, Francois Romieu wrote:
> >David Miller <davem@...emloft.net> :
> >[...]
> >> Something isn't right here.  The only thing that makes sense is if
> >> the tcpdump checksum validation is wrong for some reason.  Because
> >> only then could we give a reason for the UDP frames to not be
> >> dropped before vpnc can see them.
> >
> >Wild guess : 192.168... is the local address and tcpdump chokes on
> >an outgoing, yet-not-checksummed packet.
> 
> Then the checksums of all other packets I am emitting would also be 0000 
> or wrong, which isn't the case.

No. :o)

net/ipv4/udp.c::udp4_hwcsum_outgoing provides a packet data length
dependent partial checksum and it checksums fragments.

-- 
Ueimor
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