Re: order of matching rules

[Christopher Kruegel <chris () infosys tuwien ac at>]

Chris Green <cmg () sourcefire com> writes:

> > The site http://www.infosys.tuwien.ac.at/snort-ng/ mentions that
> > "For some strange reason, Snort stops the detection process for a
> > packet after the first matching rule - maybe to improve performance"
> > while talking about snort-ng. Is this the way it works in
> > Snort-1.9.0 too?
>
> For Snort-1.9.x yes.
> For Snort-2.0, no.
>
> There was a first exit match strategy first.  The strange reason was
> once you got something you care about, why bother keeping going on and
> let the ruleset editors worry about rule ordering.

I think the fact that Snort 2.0 changed this behavior clearly indicates that a 
first exit strategy causes more problems that it solves. The massive number 
of alerts generated includes many that you do not care about - especially 
probing attacks. These are often filtered out automatically. You definitely 
do not want an attack that you care about being hidden behind a benign alert 
that gets discarded in an automatic way.  Therefore, reporting _all_ rules 
that match seems to be a good idea.

> If you're looking at snort-ng, look at the HEAD snort branch too.
> You'll be pleasantly suprised if you have the facilities to compare
> the two.

I wonder what that means exactly - could you be a bit more specific :)

christopher kruegel


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