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From Dirk-Willem van Gulik <>
Subject Re: Mitigating the Slowloris DoS attack
Date Mon, 22 Jun 2009 08:06:47 GMT
Guenter Knauf wrote:

> Hi Andreas,
> Andreas Krennmair schrieb:

>> For those who are still unaware of the Slowloris attack, it's a
>> denial-of-service attack that consumes Apache's resources by opening up
>> a great number of parallel connections and slowly sending partial
>> attack including a PoC tool was published here:
>> I thought for some time about the whole issue, and then I developed a
>> proof-of-concept patch for Apache 2.2.11 (currently only touches the
>> prefork MPM), which you can download here:

> wouldnt limiting the number of simultanous connections from one IP
> already help? F.e. something like:

Keep in mind that, if this attack turns into a real issue, it is likely 
to be through a vector like botnets. It is pretty common* to see lots of 
bits behind a single (corporate) NAT gateway.

You would not nessesarily want to penalize an entire interanet for their 
lack of security that way. That is not our job :).

Also - these things are only a problem when the server is resource tight 
- and even then - it could be modified to just invest little at that 
point -- either by having a different accept mechanism -or- by detecting 
sluggishness and then hading the connection back to something more 
async/single-threaded which deals with all slow connections - freeing up 
the 'full' worker for real work.


*: e.g. see the conflicker stats.

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