[Snort-users] snort dropping 48%

Sheahan, Paul Paul.Sheahan at ...2218...
Fri May 7 08:52:13 EDT 2004

I just performed a test. I'm running Snort 2.0.5 on RH Linux 8.

If I startup snort with -N and the default rules enabled, then NO
directories are created. If I then go into snort.conf and add my custom
"my.rules", then run snort again with the SAME script, directories are
then created!

This looks like a bug in Snort as the rules shouldn't determine whether
Snort creates the log directories. Thoughts from anyone?

I'll play around with my custom rules to see if I can narrow the issue
down further.

-----Original Message-----
From: Josh Berry [mailto:josh.berry at ...10221...] 
Sent: Friday, May 07, 2004 9:19 AM
To: Sheahan, Paul
Cc: snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] snort dropping 48%

I have had no problems running snort on Gigabit once I tuned the system.

One of the biggest differences was disabling the log directory creation
with -N, and completely removing the -l switch.  Using -l with -N
make sense, isn't that like saying log, don't log?

I also found information on tuning the Linux filesystem, and tuning the
network cards.  It helps to use a NIC capable of using NAPI.  It sounds
though you might have issues with the NIC drivers.  If it is not the
drivers you could try using the MMAP version of libpcap at:

I also have a custom kernel that I stripped everything out of that
completely necessary and then added performance enhancing patches for
scheduler, the low-latency patches and pre-emptive patches.

If you are running on a dual processor system you could set the CPU
affinity for the Snort process.

> Sheahan, Paul wrote:
>>Also Snort STILL creates individual directories for
>>each address it encounters. So many directories get created in reaches
>>the Linux limit after a while and crashes Snort. I suppose Snort could
>>be so busy with this that it may be contributing to the packet loss?
> If you specify the -N switch it should not do any packet logging. I
> tested this with `snort -d -l ./ -N -c /usr/local/etc/snort.conf'. It
> generates the alert file , but not any packet logs, sounds like you
> might not be using the -N switch properly (or the -N switch needs to
> in a certain spot?). I could see how default packet logging could
> kill a server that runs on gigabit though.
> While this may contribute to it, it doesn't sound like the root of
> problem though as you've previously tried logging binary format.
> Sheahan, Paul wrote:
>> The content rules are the issue, but it is still a mystery why old
>> hardware and Snort version worked.
> The real difference here is a amount of traffic snort needs to
> Gigabit ethernet is a 10x faster than standard. Thats a lot of
> What we really need is a response from someone who effectively runs
> snort on a gigabit network. Can snort run "out of the box" on a
> network efficiently (given decent hardware of course) or does it need
> be tweaked to prevent major packet loss?
> As for your current situation Paul, would it be feasible to share the
> load between multiple sensors? Each sensor containing 100 of your
> rules? That might work to get every packet on the wire without having
> sacrifice some of snort's features for speed.
> Just an idea. :)
> sgt_b
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