netdev - Re: [RFC PATCH 0/2] Faster/parallel SYN handling to mitigate SYN floods

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Message-ID: <1338417630.7747.156.camel@localhost>
Date:	Thu, 31 May 2012 00:40:30 +0200
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	christoph.paasch@...ouvain.be
Cc:	netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Martin Topholm <mph@...h.dk>, Florian Westphal <fw@...len.de>,
	opurdila@...acom.com,
	Hans Schillstrom <hans.schillstrom@...csson.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC PATCH 0/2] Faster/parallel SYN handling to mitigate SYN
 floods

On Wed, 2012-05-30 at 10:53 +0200, Christoph Paasch wrote:
> On 05/30/2012 10:44 AM, Jesper Dangaard Brouer wrote:
> >> > 
> >> > Then the receiver will receive two SYN/ACK's for the same SYN with
> >> > different sequence-numbers. As the "SYN cookie SYN-ACK" will arrive
> >> > second, it will be discarded and seq-numbers from the first one will be
> >> > taken on the client-side.
> > I thought that the retransmitted SYN packet, were caused by the SYN-ACK
> > didn't reach the client?
> 
> Or, if the SYN/ACK got somehow delayed in the network and the
> SYN-retransmission timer on the client-side fires before the SYN/ACK
> reaches the client.

That seems like a very unlikely situation, which we perhaps should
neglect as we are under SYN attack.

I will test the attack vector, if we instead of dropping the reqsk, fall
back into the slow locked path.


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