From: Patrick McHardy <kaber@trash•net>
To: Maxime Bizon <mbizon@freebox•fr>
Cc: Eric Dumazet <edumazet@google•com>,
davem@davemloft•net, netdev <netdev@vger•kernel.org>
Subject: Re: problem forwarding IP fragments with DF bit set (caused by ipv4: fix path MTU discovery with connection tracking)
Date: Tue, 29 Apr 2014 15:33:27 +0100 [thread overview]
Message-ID: <20140429143324.GB12781@macbook.localnet> (raw)
In-Reply-To: <1398703056.12635.41.camel@sakura.staff.proxad.net>
On Mon, Apr 28, 2014 at 06:37:36PM +0200, Maxime Bizon wrote:
>
> Hello Patrick & Eric,
>
> After upgrading a router with a kernel that has patch 5f2d04f1f9 (ipv4:
> fix path MTU discovery with connection tracking), some packets are not
> forwarded anymore.
>
> (note: kernel is 3.11.10, and conntrack is enabled)
>
> Offending packets are IP fragments with DF bit set, MTU is the same on
> both interfaces involved in forwarding. All received fragments are
> (obviously) below MTU. The resulting packet after re-assembly is however
> above MTU.
>
> conntrack causes the packets to be re-assembled, but since the resulting
> skb now has IP_DF set, it fails the (DF + MTU) test in ip_forward.c and
> causes ICMP frag_needed to be sent.
That is the correct behaviour.
> Without the patch, the packet was (re-)fragmented in the output path
> (but as the patch says, breaking PMTUD because original fragment size is
> not considered).
Yes. The sending host should receive the ICMP frag needed and adjust it's
size accordingly.
next prev parent reply other threads:[~2014-04-29 14:33 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-28 16:37 problem forwarding IP fragments with DF bit set (caused by ipv4: fix path MTU discovery with connection tracking) Maxime Bizon
2014-04-28 17:59 ` Maxime Bizon
2014-04-29 14:35 ` Patrick McHardy
2014-04-29 14:33 ` Patrick McHardy [this message]
2014-04-29 14:42 ` Maxime Bizon
2014-04-29 14:45 ` Patrick McHardy
2014-04-29 15:23 ` Maxime Bizon
2014-04-29 15:37 ` Eric Dumazet
2014-04-29 20:13 ` Florian Westphal
2014-04-30 14:34 ` Maxime Bizon
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