[Snort-users] Stupid Question

larosa, vjay larosa_vjay at ...3331...
Tue May 11 20:57:02 EDT 2004

kill -SIGUSR1 I believe. But personally I like (no LOVE) the perfstats
output. In a previous post I had talked about how to configure it to get
useful information. The file that is created will have tons of great info on
what snort is seeing. Watch out for frag timeouts and frag faults, they are
a serious performance killer. If you are seeing these increase your frag2
memory and frag2 timeout. I am now running a SourceFire NS3000 on a gig link
that is watching 300-500 MB/s with no packet loss. The only time I run into
trouble is when I introduce tons (200-300 MB/s) of fragmented NFS traffic on
top of the 300-500 MB/s of normal traffic. Then I suffer some bad packet
loss because we chew up all of the available memory allocated for IP
de-fragmentation. Anyway, give this a try and see what you find.

1) cp snort.conf /tmp/snort.conf
2) comment out all your rules and event generating pre-processors in the
3) Add the following line to your /tmp/snort.conf

preprocessor perfmonitor: time 10 console flow file
/tmp/now pktcnt 10000

4) Make the directory called /tmp/now.
5) ifconfig eth# up
6) Run snort (make sure that you are in the bash or bourne shell for this),

snort -i eth# -A none -N -c /tmp/snort.conf -l /tmp > /tmp/perf.txt 2>&1

7) Let that run for a while, then CTRL-C to stop it.
8) Take a look in the perf.txt file and see if you are losing packets, and
how many Mb per second you are seeing. If everything looks good, then try
slowly adding rules and preprocessors back in until packets start getting
lost. It may be something simple like IP fragmentation, you may need to
increase the memory allocated or the timeout values. Or maybe you just have
a lot of any any rules. 

Good luck!


-----Original Message-----
From: snort-users-admin at lists.sourceforge.net
[mailto:snort-users-admin at lists.sourceforge.net] On Behalf Of Bell, Josh
Sent: Tuesday, May 11, 2004 11:06 PM
To: snort-users at lists.sourceforge.net
Subject: [Snort-users] Stupid Question

When I run Snort manually (non-daemon mode), let it go for a while, then
stop it, I get a nice summary screen telling me (among other things) how
many packets are being dropped.  I periodically stop Snort and run it
manually for 10-15 minutes just to see this summary screen.  On a gigabit
link, the packet loss is usually around 1-3%, but I've seen it as high as

Is there any way to get this same information on the fly when Snort is
running in daemon mode?  Possibly even how much is being lost over time?
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