[Snort-users] snort dropping 48%

Sheahan, Paul Paul.Sheahan at ...2218...
Fri May 7 10:03:06 EDT 2004


Found the problem with directories being created. I took my custom rules
file and narrowed it down to one of each type of rule. I ran Snort with
-N and it still created directories for each source IP. Then I finally
narrowed it down to one rule, then finally one keyword.

I can reproduce the problem every time now. If I have the "session:
printable" keyword in ANY rule, Snort creates directories for each
source IP in the logging directory, regardless of -N or other options I
use to start Snort. Bug in Snort I guess!

Now I will try removing this keyword from all of my custom rules and
once I am sure these directories are no longer being created, I will
check to see if packets dropped is reduced. The Snort manual mentions
this keyword will slow Snort considerably, so we'll see what
happens......

Thanks,
Paul


-----Original Message-----
From: snort-users-admin at lists.sourceforge.net
[mailto:snort-users-admin at lists.sourceforge.net] On Behalf Of Sheahan,
Paul
Sent: Friday, May 07, 2004 11:52 AM
To: snort-users at lists.sourceforge.net
Cc: Josh Berry
Subject: RE: [Snort-users] snort dropping 48%


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
doesn't
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
as
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:
http://public.lanl.gov/cpw/.

I also have a custom kernel that I stripped everything out of that
wasn't
completely necessary and then added performance enhancing patches for
the
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
just
> 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
be
> in a certain spot?). I could see how default packet logging could
easily
> kill a server that runs on gigabit though.
> While this may contribute to it, it doesn't sound like the root of
your
> 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
analyze.
> Gigabit ethernet is a 10x faster than standard. Thats a lot of
packets!
> 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
gigabit
> network efficiently (given decent hardware of course) or does it need
to
> 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
custom
> rules? That might work to get every packet on the wire without having
to
> sacrifice some of snort's features for speed.
> Just an idea. :)
>
> sgt_b
>
>
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