One of the most frequent topics that comes up when I'm out speaking to customers, or when anyone from the VRT is discussing Snort on a mailing list, IRC channel, etc., is performance. Everyone wants to know how to make their rules faster - and many people are willing to go to extreme lengths to get the speed they want, devling into arcane corners of PCRE, looking for obscure Snort rule options, the works.
This wouldn't be notable, except for the fact that the people I've seen work the hardest at extreme performance tweaks in their rules are missing one of the most simple, fundamental things you can do to keep your Snort/Sourcefire boxes humming along: steering clear of IP-only rules. You know, rules that look like this:
alert ip 220.127.116.11 any -> any any (msg:"Evil stuff from IP 18.104.22.168"; classtype:bad-unknown;)
Ther problem with rules like that is that, without a static content match to supply to the fast pattern matcher, these rules will be evaluated by the main Snort engine on every packet that crosses the IDS. While each individual entry into the engine will be very brief - there's almost nothing to do - the process of entering the engine itself entails a non-zero amount of work, setting up structures, initializing memory, etc. If you're running hundreds, or even thousands, of IP-only rules (as we've observed at more than one production site in the field), the work necessary to enter the engine across potentially hundreds of millions of packets per second stacks up to the point that you end up chewing up all available system resources.
I understand why people were doing this in the past. There are plenty of groups out there who get or compile lists of known malicious IP addresses, and want to capture all activity related to those IPs; since many of those same groups have an IDS but not a full packet capture tool, they fell back to using Snort as that tool. Other groups simply wanted to ensure that their systems never visited these known malicious IPs, regardless of logging. Both groups of users spent years clamoring for a way to blacklist known-bad IP addresses in Snort; as far back as 2009, Marty Roesch was writing experimental code that provided that functionality. For many years, production systems had no way of dealing with intelligence that only provided IP addresses.
That all changed, though, with the release of Snort 2.9.1 in August of last year, however. That release included our IP reputation preprocessor, which checks IP addresses against a blacklist and/or whitelist at the time packet headers are decoded. Since this decoding is obviously already done across all packets crossing the engine, and since checking the obtained IP address against a properly formatted list in memory is exceptionally fast, this preprocessor allows you to have a list of tens of thousands or more IP addresses that you can check against with minimal performance overhead. (Note for Sourcefire customers: this feature is currently being beta tested on Sourcefire hardware.)
Many users seem to be unaware of this feature, despite it having been widely announced on the Snort Blog; others who are aware of it have been reticent to make use of it, for reasons I don't completely understand. In cases where we have seen the switch made, however, we've seen systems that were dropping up to 50% of their packets suddenly begin running smoothly again, processing everything that crossed them with ease.
So if you've got a sensor that's not up to speed, so to speak, do yourself a favor, and start with something easy - eliminating your IP-only rules in favor of the IP reputation preprocessor. You may be amazed at the difference it makes.