[Snort-devel] Mis-Matching traffic with PCRE Rules

Joel Esler jesler at ...402...
Fri Mar 8 09:28:33 EST 2013

On Mar 8, 2013, at 5:43 AM, waseem sarwar <waseemsarwar103 at ...445...> wrote:

> I have a pcre based rule as follow in my rules file,
> alert udp any any -> any 53 (msg:"MALWARE domain capodeicapi.eu"; pcre:"m/capodeicapi.eu/i"; classtype:trojan-activity; sid:5000968;)

I'm not sure what you are doing with the "m" in your pcre there..  but if you are trying to match on a domain name look up, that rule won't work.  The "." in a domain name is actually a number.  And it would be faster and better to do a content match there.  content:"capodeicapi|02|eu"; or something like that.

> The issue I am facing is that this rule also matches for the domain http://capo.eu which it should not match. I am also facing similar problem with more pcre rules such that they match sub string based url of actual rules . I am using snort version 2.9.1.

First thing I am going to ask you to do is upgrade your version of Snort.  We are on now, support for 2.9.1 ended about a year ago.  In addition to that, I also need to know what version of pcre you have installed on the box.

Joel Esler
Senior Research Engineer, VRT
OpenSource Community Manager
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