netdev - Re: [PATCH nf] conntrack: RFC5961 challenge ACK confuse conntrack LAST-ACK transition

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Date:	Fri, 15 May 2015 21:06:33 +0200
From:	Pablo Neira Ayuso <pablo@...filter.org>
To:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH nf] conntrack: RFC5961 challenge ACK confuse conntrack
 LAST-ACK transition

On Mon, May 11, 2015 at 12:02:17PM +0200, Jozsef Kadlecsik wrote:
> Hi,
> 
> On Thu, 7 May 2015, Jesper Dangaard Brouer wrote:
> 
> > In compliance with RFC5961, the network stack send challenge ACK in
> > response to spurious SYN packets, since commit 0c228e833c88 ("tcp:
> > Restore RFC5961-compliant behavior for SYN packets").
> > 
> > This pose a problem for netfilter conntrack in state LAST_ACK, because
> > this challenge ACK is (falsely) seen as ACKing last FIN, causing a
> > false state transition (into TIME_WAIT).
> > 
> > The challenge ACK is hard to distinguish from real last ACK.  Thus,
> > solution introduce a flag that tracks the potential for seeing a
> > challenge ACK, in case a SYN packet is let through and current state
> > is LAST_ACK.
> > 
> > When conntrack transition LAST_ACK to TIME_WAIT happens, this flag is
> > used for determining if we are expecting a challenge ACK.
> > 
> > Scapy based reproducer script avail here:
> >  https://github.com/netoptimizer/network-testing/blob/master/scapy/tcp_hacks_3WHS_LAST_ACK.py
> > 
> > Fixes: 0c228e833c88 ("tcp: Restore RFC5961-compliant behavior for SYN packets")
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> 
> The patch looks OK to me, thanks Jesper.
> 
> Acked-by: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>

Applied, thanks.
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