[Snort-users] snort.conf "detection engine"

Jason Wallace jason.r.wallace at ...11827...
Wed Mar 31 09:01:14 EDT 2010

"memory usage for ac (fastest and the biggest memory hog) could be
hundreds of megs or even over a gig for big (gigabit-ish) links..."

You don't need a lot of traffic with ac to use a lot of memory. On one
segment I'm using ac on we see between 10-20Mb/s and my snort process
is using ~2300MB of memory.


On Tue, Mar 30, 2010 at 6:39 PM, Mike Lococo <mikelococo at ...11827...> wrote:
>> basicly low->high mem and low->high performance or combinations there of.
>> What would be considered 'low' or for that matter 'high', with current
>> multi-core systems, is this setting still valid/useful? Or should it
>> just be left to default? for that matter what is default, as I don't see
>> that mentioned.
> It's pretty load dependent.  You can tell what you're running by
> watching the snort startup output and looking for "Search Info Summary".
>  I believe that ac-bnfa is default in the current stable snort,
> although I don't think that has always been the case.
> I don't have a link handy, but when I researched this a few months ago I
> believe I found a posting from a SourceFire employee suggesting that the
> difference in performance between the best and worst algorithms were on
> the order of 10%, but that the memory usage for ac (fastest and the
> biggest memory hog) could be hundreds of megs or even over a gig for big
> (gigabit-ish) links... which is much worse than similarly fast
> lower-memory alternatives like ac-bnfa.
> I'm currently using ac-bnfa with a 300-400megabit link, and memory usage
> is roughly 1.5G for a snort process, with a little over 2/3rds of that
> going to stream and frag preprocessors.  I decided that the likely
> single-digit performance gains going from ac-bnfa to ac were not worth
> the time to test and extra memory overhead to me.
> Thanks,
> Mike Lococo
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