Snort mailing list archives
PCRE and the Snort-specific modifiers
From: Joshua.Kinard () us-cert gov
Date: Fri, 13 Aug 2010 20:05:34 -0400
Hi all, I've got a couple of questions regarding the 'pcre' rule option and its modifier stack. With a 'content' rule option, we cannot mix options like 'http_header' and 'http_raw_header', otherwise, Snort throws an error. Does the same apply to using the various Snort-specific URI modifiers in 'pcre'? I.e., should using 'U' (like http_uri) forbid 'I' (like http_raw_uri)? Ditto for 'H' & 'D', and 'C' & 'K'. How about using the 'B' (like rawbytes) modifier? Again with 'content', we cannot use 'rawbytes' and any http_* modifier. Should the use of 'B' forbid the use of 'U', 'I', 'P', 'H', 'D', 'M' , 'C', 'K', 'S', & 'Y'? Currently, Snort doesn't enforce any parser check against a rule using such a combination. I've only been testing the parser (and looking at the code for the parser for some of these options), so I haven't tried validating this against any actual traffic as of yet. And how is the 'O' modifier used? Its description in the manual simply states that it overrides pcre_match_limit....but with what? Does it simply instruct Snort to just ignore the aforementioned limit, or does this modifier take an argument somehow? I haven't studied the source well enough to figure out what this specific modifier is doing. Also, the manual needs to mention the exclusivity of the 'R' (relative) and URI modifiers. An error condition is thrown when the parser detects them used together, but I see no mention in the manual of this check. The manual does, however, mention not to use 'R' and 'B' together (why?), but the parser does not actually enforce this (which is inconsistent). Thanks!, --J ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Snort-devel mailing list Snort-devel () lists sourceforge net https://lists.sourceforge.net/lists/listinfo/snort-devel
Current thread:
- PCRE and the Snort-specific modifiers Joshua . Kinard (Aug 13)
- Re: PCRE and the Snort-specific modifiers Matthew Watchinski (Aug 14)