Re: Non-Determinism in Snort detection engine

["Asad, Hafiz ul" <Hafiz-ul.Asad () city ac uk>]

Hi,

Thanks for your reply. The files are actually quite big, in hundereds of gigs, but I don't replay them on the network. 
I am using this ,


snort --pcap-file=/path_to_pcap_file.txt  -c  snort.conf -l /var/log/snort


However, the path to the list of pcap files, is on another machine having the pcap files. I am just mounting that drive 
through the "sshfs" to the machine where I am running snort. Could that be the reason?


Asad

________________________________
From: Felix Erlacher <felix.erlacher () uibk ac at>
Sent: Friday, July 7, 2017 12:44:17 PM
To: snort-users () lists snort org
Subject: Re: [Snort-users] Non-Determinism in Snort detection engine

Hi Asad,

I assume you also have the same rule files for different runs.
How do you feed the pcap data to you Snort instance?
Reading from a pcap file or replaying it over the network?
How big is your pcap dump?

Replaying over the network might lead to different packets being lost on
different runs and thus leading to different results.

greets

felix

On 07/07/17 13:37, Asad, Hafiz ul wrote:
> > > Snort team,
>
> > > I have recently observed that snort, having same rules (Pre-processor
> rules to be precise), have generated different
> > > number of alerts for the same pcap traffic when run twice. Is there
> any non-determinism in the snort engine or I might
> > > have done something wrong with the experiment?
>
>
>
> To be more precise, in the alerts data in the mysql database, different
> packets (same source IP, destination but different IP ID) of the same
> TCP session have been alerted by the same preprocessor rule, SID=
> 33,GID=119,msg:http_inspect: UNESCAPED SPACE IN HTTP URI . This is after
> I run the experiment twice for the same pcap data.
>
>
> Asad
>
> ------------------------------------------------------------------------
> *From:* Asad, Hafiz ul <Hafiz-ul.Asad () city ac uk>
> *Sent:* Friday, July 7, 2017 12:11:15 PM
> *To:* Snort-users () lists snort org; snort-users () lists sourceforge net
> *Subject:* [Snort-users] Fw: Non-Determinism in Snort detection engine
>
>
>
>
>
> ------------------------------------------------------------------------
> *From:* Asad, Hafiz ul
> *Sent:* Thursday, July 6, 2017 5:50 PM
> *To:* snort-users () lists sourceforge net
> *Subject:* Non-Determinism in Snort detection engine
>
>
> Snort team,
>
>
> I have recently observed that snort, having same rules (Pre-processor
> rules to be precise), have generated different number of alerts for the
> same pcap traffic when run twice. Is there any non-determinism in the
> snort engine or I might have done something wrong with the experiment?
>
>
> regards
>
> Asad
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