netdev - Re: SYN attack, with FIN flag set

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Date:	Sat, 03 Dec 2011 09:53:21 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Denys Fedoryshchenko <denys@...p.net.lb>
Cc:	netdev@...r.kernel.org
Subject: Re: SYN attack, with FIN flag set

Le samedi 03 décembre 2011 à 10:18 +0200, Denys Fedoryshchenko a écrit :
> On Sat, 03 Dec 2011 08:55:02 +0100, Eric Dumazet wrote:
> > Le samedi 03 décembre 2011 à 08:27 +0100, Eric Dumazet a écrit :
> >
> >> I believe netfilter tcp conntrack considers SYN|FIN as INVALID
> >>
> >
> > Or if you cannot afford conntracking, just do
> >
> > iptables -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
> 
>  Sure i did,thanks, but i just was curious, why connection with such 
>  flags are threated as SYN.
> 

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




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