[Snort-users] Performance and rule tuning

Jefferson, Shawn Shawn.Jefferson at ...14448...
Wed Dec 3 14:57:25 EST 2008

I've been running the mmaped pcap module with snort on both my sensors for the last two days, and noticed quite an improvement, however I was still getting dropped packets.  I commented out all the Emerging Threats rules and this eliminated any dropped packets with over 100 MB/s of sustained traffic (at least that is what snort stats is showing me.)  Also, I noticed CPU usage went down considerably as well.  I guess I don't have enough horse power to run these rules.

Speaking of the stats though... I noticed that with each increase in the performance of my snort sensor, I'm recording more MBit/second.  Now it's up to around 150 Mb/s.  Is this number an accurate measure of what's on the wire, or does it depend somewhat on the performance of your sensor?

One more question about rule tuning:

I am getting some false positives from the ftp pre-processor.  How do I suppress these without disabling the pre-processor altogether?


-----Original Message-----
From: Jefferson, Shawn [mailto:Shawn.Jefferson at ...14448...]
Sent: December 02, 2008 11:40 AM
To: Snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] Performance and rule tuning

Thanks for your help everyone, I think I have this working.

The log was daemon.log not messages, and it wasn't using PCAP_FRAMES.  I did the following:

Apt-get remove libpcap0.8
Rebuilt snort
Used "export PCAP_FRAMES=32768" (I was confused as to use export or not... export seems to be required.)

Now it says "Using PCAP_FRAMES=32768" in daemon.log.

Now I'll do this on my main snort sensor and see if there is any performance improvement.

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