linux-kernel - Re: [PATCH v3] netfilter: save the hash of the tuple in the original direction for latter use

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Date:	Fri, 20 Aug 2010 17:10:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] netfilter: save the hash of the tuple in the
 original direction for latter use

Le vendredi 20 août 2010 à 22:53 +0800, Changli Gao a écrit :
> Since we don't change the tuple in the original direction, we can save it
> in ct->tuplehash[IP_CT_DIR_REPLY].hnode.pprev for __nf_conntrack_confirm()
> use.
> 
> __hash_conntrack() is split into two steps: ____hash_conntrack() is used
> to get the raw hash, and __hash_bucket() is used to get the bucket id.
> 
> In SYN-flood case, early_drop() doesn't need to recompute the hash again.
> 
> Signed-off-by: Changli Gao <xiaosuo@...il.com>
> ---

Hmm... so to accept a few more SYN packets per second in SYNFLOOD
attack, we slow a bit normal operations ? (adding one test on each
packet going through conntrack)

If yes (I dont think we should, hackers are stronger than you anyway,
just face it)

v4:
	__read_mostly on nf_conntrack_rnd


What would happen if we let the initialization of nf_conntrack_rnd
only in the insertion case (like currently done) ? 
Only the first packet received on the machine/conntrack might be hashed
on a wrong slot. Is it a big deal ? If yes, maybe find a way to
recompute the hash in this case, instead of reusing 'wrong' one ?



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