Snort mailing list archives
Re: Rule efficiency
From: Alex Kirk <akirk () sourcefire com>
Date: Fri, 23 Jul 2010 15:01:11 -0400
For what it's worth, the use of the fast_pattern keyword when there's a single content clause is actually unnecessary. The fast pattern matcher by default chooses the longest string available out of a rule, and if you've only got one string, well, it'll choose that every time. Good luck with your management quandary. On Fri, Jul 23, 2010 at 2:33 PM, Isherwood, Jeffrey - IS < Jeffrey.Isherwood () itt com> wrote:
Thanks for the reply Alex… For reasons that I can’t go into, I am not able to check the DNS queries (alas, that was my original thought as well). I have to check on the http_header part, I was told to watch for “Any Traffic” but often management hands down directives that include the words “any” or “all” when they could be more precise. If it turns out that they are only interested in web traffic then using adding “fast_pattern” and “http_header” look like they might help out thanks… If they turn around and insist on any/all traffic – I can’t see any other way to tighten those rules up, other than maybe using the “fast_pattern” unless the domain names are in the tcp packet headers and I am able to craft the rule to search more narrow… *From:* Alex Kirk [mailto:akirk () sourcefire com] *Sent:* Friday, July 23, 2010 1:57 PM *To:* Isherwood, Jeffrey - IS *Cc:* snort-users () lists sourceforge net *Subject:* Re: [Snort-users] Rule efficiency Multiple rules should be faster, due to the way Snort works. Snort's first step for any packet is to use the fast pattern matcher to find appropriate packets to operate on; the patterns used are based on the port used in the rule, and either the longest static string specified in a content clause, or the content clause specifically declared to be used by the "fast_pattern" keyword. If the fast pattern matcher finds something, the rest of the rule options are evaluated in order. For cases where you've got a really small pattern, you're going to get a lot more matches out of the fast pattern matcher, and thus force Snort to do more work. Since the fast pattern matcher is, well, fast (so much so that the dev team has called additional fast pattern checks "nearly free"), it makes clear sense to get it to do as much sorting as possible for you before you dig into the rules themselves. Meanwhile, let me give you some thoughts on these rules in particular. If you're looking for HTTP access, as I would guess based on your fictional names, you'll need to specify the http_header keyword to go along with those contents for Snort 2.8.6 and beyond - since hostnames appear in HTTP headers, and you need that keyword to make Snort look there. Additionally, you might consider switching these over to be rules that look for DNS queries to the domains in question (assuming you're confident this is not bot-generated traffic that's going off of an internal hosts file) - such rules are almost as easy to write, and there's *way* less UDP traffic to inspect than HTTP, which will help improve your overall performance pretty dramatically. ------------------------------ This e-mail and any files transmitted with it may be proprietary and are intended solely for the use of the individual or entity to whom they are addressed. If you have received this e-mail in error please notify the sender. Please note that any views or opinions presented in this e-mail are solely those of the author and do not necessarily represent those of ITT Corporation. The recipient should check this e-mail and any attachments for the presence of viruses. ITT accepts no liability for any damage caused by any virus transmitted by this e-mail.
-- Alex Kirk AEGIS Program Lead Sourcefire Vulnerability Research Team +1-410-423-1937 alex.kirk () sourcefire com
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Current thread:
- Rule efficiency Isherwood, Jeffrey - IS (Jul 23)
- Re: Rule efficiency Alex Kirk (Jul 23)
- Re: Rule efficiency Korodev (Jul 23)
- Re: Rule efficiency Isherwood, Jeffrey - IS (Jul 23)
- Re: Rule efficiency Alex Kirk (Jul 23)
- Re: Rule efficiency Joel Esler (Jul 23)
- Re: Rule efficiency Alex Kirk (Jul 23)
- Re: Rule efficiency waldo kitty (Jul 23)
- Re: Rule efficiency Isherwood, Jeffrey - IS (Jul 23)
- Re: Rule efficiency Alex Kirk (Jul 23)
- MP3's are evil... Searching for traffic based upon uploaded file type... Isherwood, Jeffrey - IS (Aug 05)
- Re: MP3's are evil... Searching for traffic based upon uploaded file type... Joel Esler (Aug 05)
- Re: MP3's are evil... Searching for traffic basedupon uploaded file type... Castle, Shane (Aug 05)
- Re: MP3's are evil... Searching for traffic based upon uploaded file type... Joel Esler (Aug 05)
- Re: MP3's are evil... Searching for traffic based upon uploaded file type... Isherwood, Jeffrey - IS (Aug 05)
- Re: Rule efficiency Alex Kirk (Jul 23)