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Re: Snort 2.9, RHEL 5 and afpacket DAQ
From: Michael Altizer <maltizer () sourcefire com>
Date: Wed, 20 Oct 2010 15:14:14 -0400

 On 10/20/2010 02:59 PM, Rich Graves wrote:
On Wed, Oct 20, 2010 at 1:12 PM, Jeff Kell wrote:

    I had rebuilt snort 2.8.6 with libpcap 1.1.1 and  had some worse
    performance than before, but then there was a discussion on one of
    the snort lists regarding sids 4676 and 4677 in the oracle-rules
    being a pcre "hog".

    Disabling those two sids dropped my average CPU over half...

Wow. Mine dropped over 2/3.

sid 4676 is limited to POSTs, so if you have a requirement to detect ancient oracle attacks, keep that one and drop just 4677.

The problem of the maximum 49MB buffer on RHEL5 64-bit remains (does not affect Ubuntu 64-bit or RHEL5 32-bit; I'd expect it to effect CentOS and other rebuilds as well), but since I'm no longer regularly filling the buffer, my 2.9.0 installation is now stable enough that I can start looking at the new rule options, and hope the buffer issue gets addressed in 2.9.1.

I've replicated the issue on a 64-bit CentOS 5.5 VM. It's going to take some investigation from the kernel side of af_packet to figure out the issue since it appears to be limited to 64-bit CentOS/RHEL as you indicated. Unfortunately, they really don't make building a custom kernel with their source easy, but I'm getting there...

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