Snort and high-traffic lines

[Jens Krabbenhoeft <tschenz-snort-users () noris net>]

Hi all,

  I have been playing around with snort (1.9.0b6) quite a while now,
trying to get snort work with a big ruleset (~ 1200 rules, HOME_NET set
to one /19 and one /16, EXTERNAL_NET set to any) in a high-traffic
environment. 

My first problem seemed to be the libpcap which dropped about 0.5% of
the packets on a 25MBit-average line. After having installed the MMAP
enabled pcap from Phil Wood, everything is fine with the pcap (except
RAM usage ;)).

But after having solved the pcap-drops, snort began to drop packets with
the ruleset mentioned above and speeds over 30MBit (snort drops about
40% of the packets on a 65MBit-average line). To solve that problem I
used snort's binary logging (-b) and barnyard to log the incidents to
the database - but still drops with my ruleset.

Snort seems to keep up with about 65MBit traffic with a <300 rules
ruleset (and -b/barnyard) whereas snort logging to the DB generates
drops even with that small ruleset. 

My snort-box is a PIII-700 (running linux at the moment) with 256MB
RAM, 3c905B Ethernet-Card. As I planned to have a snort-box capable
of snorting about - let's say - 200-300MBit peak-traffic, I have a
question to all the people out there who successfully deployed snort in
a high-speed environment.

What would be the right hardware to snort that much traffic?
What would be the right OS?
How can I improve snort's performance in general (when changing the
ruleset and changing HOME/EXTERNAL_NET is not possible)?
How did you deploy snort successfully in a high-bandwidth environment?
Any tweaks for the OSes such as adjusting buffers, ...?

Thanks in advance,

        Jens


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