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

From: Patrick McHardy <[email-masked]>
Date: Mon, 14 Mar 2011 19:32:11 +0100

On 14.03.2011 13:42, Changli Gao wrote:
> On Mon, Mar 14, 2011 at 8:26 PM, Jan Engelhardt <[email-masked]> 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.

Jan, please let me know whether you want me to apply these patches.
Thanks.