netdev - Re: [PATCH net-next] ipv6: ip6_fragment() should check CHECKSUM

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Message-Id: <20120518.235409.1621981320285066566.davem@davemloft.net>
Date:	Fri, 18 May 2012 23:54:09 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, tore@....no
Subject: Re: [PATCH net-next] ipv6: ip6_fragment() should check
 CHECKSUM_PARTIAL

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 18 May 2012 23:02:15 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Quoting Tore Anderson from :
> 
> If the allfrag feature has been set on a host route (due to an ICMPv6
> Packet Too Big received indicating a MTU of less than 1280),
> TCP SYN/ACK packets to that destination appears to get an incorrect
> TCP checksum. This in turn means they are thrown away as invalid.
> 
> In the case of an IPv4 client behind a link with a MTU of less than
> 1260, accessing an IPv6 server through a stateless translator,
> this means that the client can only download a single large file
> from the server, because once it is in the server's routing cache
> with the allfrag feature set, new TCP connections can no longer
> be established.
> 
> </endquote>
> 
> It appears ip6_fragment() doesn't handle CHECKSUM_PARTIAL properly.
> 
> As network drivers are not prepared to fetch correct transport header, a
> safe fix is to call skb_checksum_help() before fragmenting packet.
> 
> Reported-by: Tore Anderson <tore@....no>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Tested-by: Tore Anderson <tore@....no>

Applied, thanks.
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