netdev - Re: SYN attack, with FIN flag set

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Date:	Sat, 03 Dec 2011 11:07:10 +0200
From:	Denys Fedoryshchenko <denys@...p.net.lb>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, <netdev@...r.kernel.org>
Subject: Re: SYN attack, with FIN flag set

 On Sat, 03 Dec 2011 13:03:54 +0400, Michael Tokarev wrote:
> On 03.12.2011 12:53, Eric Dumazet wrote:
> []
>> TCP stack first tries to lookup a socket, given the tuple found in
>> incoming packet.
>>
>> This is where your machine is hit : we find the listener socket and 
>> lock
>> it.
>>
>> Then, once socket was found and locked, state machine handle various
>> possible states.
>>
>> In your case, you want to bypass the lookup, and eventually bypass 
>> the
>> IP route lookup as well (to keep IP route cache small)
>>
>> iptables -t raw -I PREROUTING -p tcp --tcp-flags SYN,FIN SYN,FIN -j 
>> DROP
>
> Maybe it makes some sence to add a basic "sanity" check rule
> before the socket lookup?
>
> The question here is why SYN+FIN results in worse behavour than
> SYN alone - in the default setup, without iptables rules?  As
> far as I understand, "regular" SYN attack is handled just fine,
> but SYN+FIN attack makes the machine to "choke", and it is not
> obvious how to fix it -- naive --syn iptables rule does not help.
>
> The price for the sanity check appears to be small since there's
> already a check for RST.
>
> Just asking, not suggesting anything... ;)
>
> Thanks,
>
> /mjt
 No,no, as i understand it is just threating SYN+FIN as plain SYN.
 I think if it incurr additional expenses (verification), no need maybe 
 to fix it, it is job of iptables.
 But is FIN to listening socket - legitimate? Shouldn't it be dropped?

 ---
 System administrator
 Denys Fedoryshchenko
 Virtual ISP S.A.L.
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