netdev - Re: [PATCH 1/4] netfilter: xt_connlimit: fix daddr connlimit in SNAT scenario

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Message-ID: <AANLkTi=QgVBVAPcTmb_La=VJOLq5LmGPqMfw7bbCuByP@mail.gmail.com>
Date:	Mon, 14 Mar 2011 20:42:29 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] netfilter: xt_connlimit: fix daddr connlimit in SNAT scenario

On Mon, Mar 14, 2011 at 8:26 PM, Jan Engelhardt <jengelh@...ozas.de> wrote:
> On Monday 2011-03-14 07:50, Changli Gao wrote:
>
>>We use the reply tuples when limiting the connections by the destination
>>addresses, however, in SNAT scenario, the final reply tuples won't be
>>ready until SNAT is done in POSTROUING or INPUT chain
>
> If I am not mistaken: if you do daddr counting, SNAT is irrelevant.
> Consider ruleset
>  -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to 1.2.3.4:80
>  -t nat -A PREROUTING -p tcp --dport 443 -j DNAT --to 1.2.3.5:443
>
> The tuple will first be (as per conntrack -L):
>  src=home dst=router src=router dst=home
> After DNAT:
>  src=home dst=router src=1.2.3.4 dst=home
>
> Thus looking at the src of the reply tuple seems correct — at least this
> is what was wanted, counting per stashed servers (=1 customer), not per
> globally visible address.
>

Yes, you are correct only when there is no SNAT rule. If there is an SNAT rule:

-t nat -A POSTROUTING -p tcp --dport 80 -j SNAT --to-source 192.168.0.1

the final tuples will be:
src = home dst = router src=1.2.3.4 dst=192.168.0.1

However, the tuple saved by connlimit is src=1.2.3.4 dst=home, so this
conn will be removed later as there isn't any conntrack, which has
this tuple in any direction.

You can't prevent a user from doing such a configuration, although you
might think it is stupid to do that.

Thanks for your review.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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