[Snort-users] Rule Selection

Alex Butcher, ISC/ISYS Alex.Butcher at ...11254...
Fri Feb 11 03:22:38 EST 2005

--On 10 February 2005 23:15 -0800 Rudi Starcevic <tech at ...12014...> wrote:

>  >>  Depends on what your're looking for. I run some snort sensors "wide
> open" in order to monitor and profile all the attacks
>  >> that are occuring. In other cases, only selected rules are
> enabled.Miner, Jonathan W (CSC) (US SSA) wrote:
> Well I am very interested to know all attacks that may be ocurring but
> network performance is our main concern.
> This box is a commercial web app that stream digital media so it must
> have the best network speed it can.

I suspect you're either deploying snort in an unusual way, or you're 
misunderstanding what it does.

Snort will not have any impact on the performance of the streaming, unless:

a) you're running it on the same machine as the stream server(s). Don't do 

b) you're using your switch's SPAN port, and it can't keep up. Buy a better 
switch, or use a passive tap.

c) you're using snort in inline mode.

> Let say only port 80 is open.
> Which of the two would run faster
> a) Smort with all rules loaded
> b) Smort with only port 80 rules loaded.
> I tend to think it makes no difference. If port 80 is not being used
> snort will not apply those rules.
> Am I correct?

AFAIK, it depends on the traffic that snort sees. If you have 1000 port 80 
rules and 10 port 23 rules (say), then snort will perform (almost) 
identically in both these scenarios:

1) if you have all 1010 rules enabled, but there's no port 80 traffic
2) if you have only the port 23 rules enabled

If some port 80 traffic starts appearing, then the 1000 port 80 rules 
enabled in scenario 1 will be checked and snort will need more resources, 
or ignore ("drop") packets.

> Cheers
> Rudi

Alex Butcher: Security & Integrity, Personal Computer Systems Group
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