[Snort-users] Performance and rule tuning

Nathaniel Richmond nate+snort at ...14258...
Tue Dec 2 13:30:03 EST 2008

Replies inline.


Jefferson, Shawn wrote:
> Hi,
> I have a couple of questions about performance and rule tuning.
> Performance:
> I'm seeing quite a bit of dropped packets on one of my sensors.
> Traffic is about 30-60 Mb/s.  From the reading I've done, it seems
> like the first thing is to make sure your variables are set in
> snort.conf, and probably the next is to move to mmaped pcap.  I've
> attempted to do both of these, however, I was wondering if snort is
> actually using the mmapped pcap or not. Is there any way to tell?
> I did the following:
> - apt-get remove libpcap-dev

Maybe you also need to remove the libpcap package.

> - built the mmapped pcap
> - rebuilt snort
> - put PCAP_FRAMES=32768 in my script file that starts snort

You should then get "Using PCAP_FRAMES=32768" in /var/log/messages
when you start Snort. The normal message without Phil Wood's libpcap
is "Not Using PCAP_FRAMES".

Phil Wood also has an example in the README on his site showing how
to test PCAP_FRAMES after building tcpdump with his libpcap.

> There aren't many "how-to" articles out there for doing this, and I
> hope I did everything right.
> Rule Tuning:
> Is the optimal way of tuning out false positives using suppress
> rules in threshold.conf ?  I am using oinkmaster to download new
> rules each day, so I'm assuming that commenting out rules won't
> work.

Use disablesid or enablesid in oinkmaster.conf to either comment out
rules that are enabled by default or enable rules that are commented
by default. If you want to disable a rule without running oinkmaster
again then you can manually comment the rule. You should still add
the disablesid line in your oinkmaster.conf or it will get
re-enabled the next time you run Oinkmaster.

> Thanks!
> Shawn
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