Re: Synfloods - SNAT slow down

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----- Original Message -----
From: "David Cannings" <lists@xxxxxxxxx>
To: <netfilter@xxxxxxxxxxxxxxxxxxx>
Sent: Friday, April 23, 2004 12:31 PM
Subject: Re: Synfloods - SNAT slow down

> On Friday 23 April 2004 02:23, krv wrote:
> > We have a Linux gateway (2.4.22) which does NAT for all local hosts.
> > Where there is ICMP or SYN floods to be forwarded, the gateway starts
> > slowing down an there will be serious drop in packets being forwarded.
> You could try using the limit match in your FORWARD chain, with --limit
> and --limit-burst to limit the number of ICMP or packets with only the
> SYN flag set per second.  Your gateway would still have to process the
> packets, at least as far as deciding to drop them, but would not have to
> forward them on so you might see an improvement in performance.
> If you do examine this route, be careful you don't quench good ICMP
> packets as there is no retransmission in ICMP and you'll never know if
> certain wanted messages didn't get through.  For example, host X is
> sending you an ICMP flood so netfilter starts to drop ICMP packets, but
> host Y tries to send you a host unreachable message.
> Also don't forget that even if you decide not to forward the packets they
> are still there "on the wire", thus you will not see any improvement with
> external speeds.
> David
I have two thousand hosts and two thousand forward rules :(

Even if I completely block a attacking host, the gateway is getting bogged
The gateway would be processing atleast 30Mbps at peak loads.


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