Re: PCRE and cross packet matching

["vpiserchia () gmail com" <vpiserchia () gmail com>]

Hello Patrick, Tony

Thank you for your exhaustive answers.

Before all I have to say that the first experiments were made with the
PFRING DAQ
module and I can confirm that in this case the PCRE signature is not
able to match a cross packet content.

The way I launched snort is like this:

snort --pid-path /root/snort/ -D \
        -c /etc/snort/snort.conf -i eth1,eth2 \
        -l /root/snort/ -A fast \
        --daq-dir /opt/daq/lib/daq --daq pfring --daq-mode passive


Today I made more tests and changed a bit the setup of my experiments,
that is now I use the "standard" PCAP DAQ module
in this way:

snort --pid-path /root/snort/ -D \
        -c /etc/snort/snort.conf -i eth1,eth2 \
        -l /root/snort/ -A fast \
        --daq-dir /opt/daq/lib/daq --daq pcap --daq-mode passive


with this setup the PCRE signature now works well and alert as expected

So my new question for the list is:
has anyone already experienced this behaviour with the pfring daq module
and pcre signatures?

best regards
vito



On 08/03/2012 05:58 PM, Tony Robinson wrote:
> Just to further explain Patrick's message,
>
> While it isn't explicitly spelled out, Patrick is more or less
> referring to frag3 and stream 5. If you utilize ip defragmentation,
> and stream reassembly, we have an entire TCP stream that the rule can
> work against. If frag3/s5 are not being used to defragment/reassemble
> packets and TCP segments, you will only have individual packets to
> work with.
>
> A good, general rule of thumb for using PCRE in this instance is to
> have some sort of a content match prior to using PCRE so snort knows
> where in the packet or stream to use the PCRE engine to shred through
> the data from that point onward, so you don't run into the problem of
> snort giving up on a PCRE match.
>
> hope this helps,
>
> -Tony
>
> On Fri, Aug 3, 2012 at 9:53 AM, vpiserchia () gmail com
> <mailto:vpiserchia () gmail com> <vpiserchia () gmail com
> <mailto:vpiserchia () gmail com>> wrote:
>
>     Hello Snort Gurus
>
>     I have the following question for you:
>
>     does snort pcre signatures match cross-packets content?
>
>     I googled a bit and no other answers found about this topic, sry if aI
>     missed any
>
>     regards
>     vito piserchia
>
>
>
>     ------------------------------------------------------------------------------
>     Live Security Virtual Conference
>     Exclusive live event will cover all the ways today's security and
>     threat landscape has changed and how IT managers can respond.
>     Discussions
>     will include endpoint security, mobile security and the latest in
>     malware
>     threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/
>     _______________________________________________
>     Snort-users mailing list
>     Snort-users () lists sourceforge net
>     <mailto:Snort-users () lists sourceforge net>
>     Go to this URL to change user options or unsubscribe:
>     https://lists.sourceforge.net/lists/listinfo/snort-users
>     Snort-users list archive:
>     http://www.geocrawler.com/redir-sf.php3?list=snort-users
>
>     Please visit http://blog.snort.org to stay current on all the
>     latest Snort news!
>
>
>
>
> -- 
>
> Tony Robinson
> Security Consultant I
> SourceFIRE Professional Services Division

------------------------------------------------------------------------------
Live Security Virtual Conference
Exclusive live event will cover all the ways today's security and 
threat landscape has changed and how IT managers can respond. Discussions 
will include endpoint security, mobile security and the latest in malware 
threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/

_______________________________________________
Snort-users mailing list
Snort-users () lists sourceforge net
Go to this URL to change user options or unsubscribe:
https://lists.sourceforge.net/lists/listinfo/snort-users
Snort-users list archive:
http://www.geocrawler.com/redir-sf.php3?list=snort-users

Please visit http://blog.snort.org to stay current on all the latest Snort news!