[Snort-users] Inline snort negative impact on network

Y M snort at ...15979...
Thu Nov 13 13:07:06 EST 2014

Date: Thu, 13 Nov 2014 09:46:24 -0800
Subject: Re: [Snort-users] Inline snort negative impact on network
From: charles.heselton at ...11827...
To: snort at ...15979...
CC: snort-users at lists.sourceforge.net

On Wed, Nov 12, 2014 at 10:59 PM, Y M <snort at ...15979...> wrote:

I would say tuning; NIC (gro, lro, etc), kernel (networking stack), and Snort itself (number of rules/processors, etc). Since you are already on Snort, why not using daq 2.0.4? And there is the "unknown/unexpected" hardware behavior. If all the tuning does not improve things, see if you can test with different NICs if possible.

I've done some sysctl tuning, but it hasn't seemed to make much of a difference.  ifconfig shows that there are 5 (out of 600K+) dropped RX packets on only 1 of the 2 bridged interfaces.  All of the other error-indicating counters are 0.  Again, system resources remain low when the system is inline.  So I don't know that performance is really an issue. 
Using daq 2.0.2 because that's what's avilable in Gentoo's software repository.  If/when 2.0.4 becomes available, I'll upgrade and see if it makes a difference.
I suspect that snort is dropping random packets, but have no way to confirm.
Thanks for the response YM, Still hoping for some useful advice from the community.

# I see. Have you also disabled lro, gro, and the rest of the gang? They have been the most part of the issue when setting up Snort inline. And while you are at the NIC level, you may also want to adjust RX/TX buffers.
Another thing that I would tune in specific is the http_inspect preprocessor, and then move to the remaining configurations, like disabling unwanted preprocessors and rules..Hope this can helps.

Date: Wed, 12 Nov 2014 20:31:31 -0800
From: charles.heselton at ...11827...
To: snort-users at lists.sourceforge.net
Subject: [Snort-users] Inline snort negative impact on network

I'm attempting to install/configure a standalone, inline snort box.  When I have the sensor inline, with snort running, the traffic seems to be flowing properly; snort is alerting, as expected.  
However, browsing the web, and downloads, becomes significantly impacted.  speedtest.net fails to load.  wget downloads files at ~6Kbps, when it should be closer to 6Mbps.
The question is why?
Hardware:  Intel Celeron 4 core, 8GB RAM, 64GB SSD, dual Gigabit (Realtek) NICs onboard, USB3.0->Gigabit dongle NIC (for admin).
Software:  Gentoo x86_64 linux; kernel 3.16.5; snort 2.7.0; daq 2.0.2.
When snort is running, and traffic is passing, both gkrellm and top show almost 0 CPU activity.  This is on a relatively low traffic, home network, so I wouldn't expect the system to be loaded.  The admin interface shows more activity than the 2 bridged interfaces.
What gives?  Any advice appreciated.

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