[Snort-users] Snort spikes to 100% CPU followed by network latency

Russ Combs (rucombs) rucombs at ...589...
Fri May 23 09:40:56 EDT 2014

From: Cody Brugh [cbrugh at ...11827...]
Sent: Friday, May 23, 2014 8:54 AM
To: Russ Combs (rucombs)
Cc: snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] Snort spikes to 100% CPU followed by network latency


We did not change anything before this started happening... I did upgrade snort to the latest version once this happened but it continues to spike even after upgrading.

This is starting to happen about every 2 or 3 days now... I've had a couple times where it fixes itself and snort CPU usage goes down.  Other times it goes on for a long period and I end up killing the snort process.

I already have the preprocessor sensitive_data commented out, however I do see this in my disablesid.conf (I used pulled pork to fetch VRT rules).


Should I adjust the PCRE stuff maybe?

* Can't comment on the above but the issue with pcre rule options that have /O is that they override the match limits that are configured and can therefore chew on a packet as long as needed.  You could check for such rules and comment them out.

Did you enable PPM?  That can both help limit your total maximum latency and catch the packets that trigger the problem.


On Fri, May 23, 2014 at 8:09 AM, Russ Combs (rucombs) <rucombs at ...589...<mailto:rucombs at ...589...>> wrote:

From: Cody Brugh [cbrugh at ...11827...<mailto:cbrugh at ...11827...>]
Sent: Thursday, May 22, 2014 8:13 PM
To: snort-users at lists.sourceforge.net<mailto:snort-users at ...3783...net>
Subject: [Snort-users] Snort spikes to 100% CPU followed by network latency


We have been running snort in-line for over a year now with no issues in terms of latency or CPU usage.  Recently (over the past month) snort will all of the sudden spike CPU usage up to 100% and network latency becomes real bad, 1000+ms.

I am really not sure where to start on figuring out what is causing this.  I am starting snort so it prints the alerts/drops on the console and don't see any specific rule that would be causing this.

Any advise on this issue?

* Did you change your Snort version or configuration around the time you started seeing the issue?  How frequently does this occur?  And when it happens does it resolve itself or do you restart or what?

You can turn on PPM (config ppm ...) and enable the PPM rules (gid 134).  That may catch the problem packet which you can log and examine for clues.

Without any clues I'd first check for SDF and PCRE.  If you have SDF (preprocessor sensitive_data) configured you can try commenting that out.  If you have any pcre/O rules (PCRE override) you can try commenting those out too.

Snort OS: CentOS, 64-bit

  o"  )~   Version GRE (Build 56)
   ''''    By Martin Roesch & The Snort Team: http://www.snort.org/snort/snort-team
           Copyright (C) 2014 Cisco and/or its affiliates. All rights reserved.
           Copyright (C) 1998-2013 Sourcefire, Inc., et al.
           Using libpcap version 1.0.0
           Using PCRE version: 7.8 2008-09-05
           Using ZLIB version: 1.2.3

DAQ version: 2.0.2


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