[Snort-users] snort dropping 48%

Josh Berry josh.berry at ...10221...
Fri May 7 06:28:04 EDT 2004


The only way that I could get the directories to stop being created was to
remove the -l /path/to/logfiles and put in -N (this gave me a big
performance boost).  I also use the -z switch and have disabled the frag
preprocessor because of its overhead.  I disable the ttcp alerts, ip
options alerts, and tcp options alerts in the snort.conf file.

>
> Thanks again, your tips were very helpful. You are right; I disabled a
> lot of stuff just for testing purposes. I plan to put everything back in
> once I figure out the packet loss issue. Couple of things I've done:
>
> 1) I just upgraded to the new libpcap released yesterday and rebooted
> for fun
> 2) Moved -N to the end of my startup script.
>
> Still 49% packet loss using only one rule file with about 400
> content-type rules. 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?
>
> Funny how this rules file and startup script worked perfectly on Snort
> 1.9 on 100mb Ethernet and a low end server, and I was using all the
> other default rules too. Odd. I've always loved Snort but now it has
> become completely useless.
>
> Note that I don't have much packet loss at all when I take out my
> content rules and put in the default rule files. The content rules are
> the issue, but it is still a mystery why old hardware and Snort version
> worked.
>
> Thanks for the help.
> Paul
>
> -----Original Message-----
> From: sgt_b [mailto:sgt_b at ...11733...]
> Sent: Thursday, May 06, 2004 4:15 PM
> To: Sheahan, Paul
> Cc: snort-users at lists.sourceforge.net
> Subject: Re: [Snort-users] snort dropping 48%
>
> Well looks like you've got snort all tuned up for speed! By utilizing
> the -N switch you're not doing any logging at all so the -b and -L
> switches are confusing. If the -N switch comes before the -b and -l
> switches then snort WILL log packets. If the -N switch comes AFTER the
> -b and -l switches snort will NOT logs packets. Just thought I'd clear
> that up.
> Also the -k none switch exposes you to some NIDS evasion techniques. An
> attacker could inject seemingly valid packets with bad checksums. With
> -k none, snort will see bad packets as part of a valid stream while the
> remote system you're protecting will drop these packets. This could lead
>
> to snort becoming "desynchronized", and thus miss packets or streams it
> should be alerting on.
>
> Now, all that being said, I'm sure you turned these on due to the packet
>
> loss issue you're having. From the looks of things, you really shouldn't
>
> be seeing many dropped packets. That's an opinion coming from someone
> who has never used snort on a gigabit network mind you. ;).
>
> Keep in mind that even if you do get packet loss down to a minimum, are
> the sacrifices you're making worth it? By not implementing checksum
> verification, and by not utilizing the stream4 preprocessor you're
> exposing your IDS to some of the most basic NIDS evasion techniques.
> Without packet logging, and only using "fast" alert methods, you may get
>
> very limited information from your IDS in the event of an alert.
>
> As stated previously I have really no experience implementing snort on a
>
> gigabit network, so take what I say with a grain of salt. It may have
> something to do with all the content rules...I'm really not sure
> (disabling them for testing would help verify if this is the issue).
> Even though this reply doesn't help solve the problem, maybe it helps a
> little.
>
> sgt_b
> Sheahan, Paul wrote:
>
>>Thanks for the feedback. Yes, I use -b in my startup script. I have
>>tried many different options in the script, or in the config file. Here
>>is what I normally run to start Snort:
>>
>>/usr/local/bin/snort -A fast -c /etc/snort/custom.conf -i eth2 -l
>>/var/log/custom -k none -o -N -b -L traces
>>
>>Used to work fine with my custom content rules until I switched to
>>Gigabit and a higher end server.
>>
>>Thanks!
>>Paul
>>
>>P.S. My bare-bones snort config is below in my original message as
> well.
>>
>>
>>-----Original Message-----
>>From: sgt_b [mailto:sgt_b at ...11733...]
>>Sent: Thursday, May 06, 2004 3:20 PM
>>To: Sheahan, Paul
>>Cc: snort-users at lists.sourceforge.net
>>Subject: Re: [Snort-users] snort dropping 48%
>>
>>Hi Paul,
>>
>>I'm sure you've already tried this, but I want to make sure I cover all
>
>>bases. :)
>>How are you logging? If its to the console (-v), I can easily see near
>>50% of packets being dropped on an gigabit network. Have you tried
> using
>>
>>-b? It logs files in binary, and is much faster. I'd recommend you try
>>that. If you've already tried the various logging methods, but got the
>>same results, let us know so we can try and troubleshoot this issue. It
>
>>would also be helpful if you show us how you're running snort (all the
>>flags).
>>
>>sgt_b
>>
>>Sheahan, Paul wrote:
>>
>>
>>
>>>I still don't have an answer either. 49% of packets being dropped is
>>>absolutely ridiculous.
>>>
>>>I recently ran TOP to check memory while Snort was running my
>>>content-based rules and noticed that even though I had 1 gig of ram in
>>>my server, there was almost no free memory. So I upgraded to 4 gig of
>>>RAM figuring Snort just needed more RAM, but the same problem is still
>>>occurring, 49% of packets are still being dropped.
>>>
>>>Should I take a look at libpcap? I understand there are multiple
>>>versions. What version should I be running?
>>>
>>>Thanks
>>>
>>>
>>>-----Original Message-----
>>>From: snort user [mailto:snortuser at ...125...]
>>>Sent: Wednesday, May 05, 2004 1:42 PM
>>>To: Sheahan, Paul
>>>Subject: RE: [Snort-users] snort dropping 48%
>>>
>>>Im actually getting the same problem on a Debian machine. When the
>>>traffic
>>>exceeds 100Mb/s snort really starts dropping packets fast. If I remove
>
>>>content based rules then dropped apckets significantly drop. I never
>>>
>>>
>>saw
>>
>>
>>>a
>>>reply other than it could be a RedHat problem so I was wondering if
>>>anyone
>>>else had any ideas since I am not on RedHat.
>>>
>>>
>>>
>>>
>>>
>>>
>>>>From: "Sheahan, Paul" <Paul.Sheahan at ...2218...>
>>>>To: <snort-users at lists.sourceforge.net>
>>>>Subject: [Snort-users] snort dropping 48%
>>>>Date: Wed, 28 Apr 2004 13:46:55 -0400
>>>>
>>>>Can anyone give me a tip in this situation?
>>>>
>>>>
>>>>
>>>>I used to have a Snort 1.9 sensor running on RHLinux7 on a 100mb
>>>>Ethernet network. On that sensor I ran the most of the default rules
>>>>plus my own custom rule file, which contained a lot of content-based
>>>>rules. It handled it no problem and didn't drop any packets.
>>>>
>>>>
>>>>
>>>>Now I've upgraded to a big beefy server, gig Ethernet, RH Linux 8.0
>>>>
>>>>
>>and
>>
>>
>>>>Snort 2.0.5 using the same Snort config as above. Traffic levels are
>>>>
>>>>
>>>>
>>>>
>>>the
>>>
>>>
>>>
>>>
>>>>same. Now I noticed it was dropping half of the traffic! My custom
>>>>content rules are extremely important to me, so I performed a test. I
>>>>created this bare bones snort.conf which basically disables all
>>>>
>>>>
>>>>
>>>>
>>>standard
>>>
>>>
>>>
>>>
>>>>rules and extra preprocessors:
>>>>
>>>>
>>>>
>>>>var HOME_NET [10.10.0.0/16]
>>>>
>>>>var EXTERNAL_NET !$HOME_NET
>>>>
>>>>preprocessor frag2
>>>>
>>>>preprocessor http_decode: 80 unicode iis_alt_unicode double_encode
>>>>iis_flip_slash full_whitespace
>>>>
>>>>include classification.config
>>>>
>>>>include reference.config
>>>>
>>>>include /etc/snort/my.rules
>>>>
>>>>include /etc/snort/pass.rules
>>>>
>>>>
>>>>
>>>>Then I started Snort and let it capture traffic for a while. I
> stopped
>>>>Snort and it is STILL dropping 48% of the traffic! My "my.rules" file
>>>>contains a few hundred content-based rules. What gives? Can Snort no
>>>>longer handle content-based rules? Or am I missing something here?
>>>>
>>>>
>>>>
>>>>Thanks,
>>>>
>>>>Paul
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>_________________________________________________________________
>>>Mother's Day is May 9. Make it special with great ideas from the
>>>Mother's
>>>Day Guide! http://special.msn.com/network/04mothersday.armx
>>>
>>>
>>>
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>>>
>>
>>
>>
>>
>>
>>
>
>
>
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