Re: snort & regular expressions

[Martin Roesch <roesch () sourcefire com>]

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Hi Sevil,

We use a two-stage process in the Snort detection engine these days.   
In it's standard configuration all the rules that are loaded in at  
runtime have their longest pattern matching option (content/ 
uricontent) loaded into a fast set-wise pattern matching engine.   
(Set-wise pattern matchers match all patterns in the set  
simultaneously.)  Once the engine is up and running, traffic is run  
thru the set-wise pattern matcher to pre-qualify rules that *may*  
fire.  These rules are chained together and tested after the  
prequalification stage, greatly reducing the number of rules that  
have to be analyzed for any given data set.  For the sake of building  
the prequalification set-wise matching data, the PCRE rule options  
are ignored and only tested when the full rules themselves are tested  
after prequalification.

There are three basic pattern matching algorithms that we use in  
Snort today, Wu-Manber, Aho-Corasick and Boyer-Moore.  PCRE uses its  
own DFA/NFA mechanisms behind the scenes.

Hope that helps!

     -Marty

On Jan 25, 2006, at 2:01 PM, Sevil SEN wrote:

> Hello,
> I know that Snort uses efficient multiple-string algorithms. If the  
> set of strings contain regular expressions, which algorithm is used  
> in Snort?
> thanks..
>
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- --
Martin Roesch - Founder/CTO, Sourcefire Inc. - +1-410-290-1616
Sourcefire - Security for the Real World - http://www.sourcefire.com
Snort: Open Source Network IDS - http://www.snort.org




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