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From Stas Bekman <>
Subject Re: Strategy against 'trivial' DOS attacks?
Date Thu, 08 May 2003 00:51:54 GMT
Gerd Knops wrote:
> Hi,
> Lately one of our servers has been subjected to a very trivial but 
> effective DOS attack: The attacker would simply open sessions (aka 
> telnet <server> 80) and not send any data. By default an apache child 
> would sit for 300 seconds and effectively be blocked. Just a handful of 
> those, and all available apache instances are blocked. The attacker 
> doesn't even need a high bandwidth pipe to do this. Even if the timeout 
> is reduced, it still doesn't need much to block the server.
> So how does one defend against this? Is there a (simple) mod-perl way of 
> detecting timed out sessions, then blocking the involved IP? It needs to 
> be simple and not require external hardware, as I have to replicate it 
> over several dozen (non-clustered) servers.
> Any ideas?

Since they send nothing at all, you can't possibly provide a mod_perl 
solution, since mod_perl is not going to be invoked at all.

Try lowering the timeout:
though read the warning in that section.

I think there are many non-Apache tools exist to prevent this kind of attacks, 
because it's a generic attack of opening the connection and doing nothing.

Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker     mod_perl Guide --->

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