Re: [Bleeding-sigs] Re: Assessing Your Malware Exposure with Snort

You could put the entire variable in an included file. That'd probably 
be easier.

I've heard a number of discussions on how large variable aren't as 
efficient as smaller ones though. That'd be the limiter I bet.

Matt

Wes Young wrote:
> I wonder, is it possible to define a variable with an 'include' statement?
> 
> var $BAD_DNS = include bad_domains.rules
> 
> having the snort alert dynamically generate the content, or make the 
> content a var?... i haven't had a to check it out to see if it will even 
> work in snort, but maybe a conceptual idea that might work in teh future 
> for things like this?
> 
> then you can add/remove domains in a rule-type file instead?
> 
> 
> On Mar 10, 2005, at 4:21 PM, Matt Jonkman wrote:
> 
>     Thanks for the clarification Brian.
> 
>     I had started to combine these rules into multi-condition pcre.
>     Making one rule out of 30 or 40 original single domain rules. And I
>     can't remember where the heck I put them now or I'd link to them for
>     you.
> 
>     So my question is, will this be more efficient in a single comdin
>     match per rule, or multiple?
> 
>     Matt
> 
>     Brian wrote:
> 
>             I have written a few thousand Snort rules that are intended
>             to detect successful HTTP communication with hosts known to
>             be evil. They look for domain names in the Host string so
>             they are not subject to evasion by changing IP addresses.
> 
>             If you would like to give them a try you can grab them from
>             http://www.kgb.to/malware.html .
> 
>         Interesting.
>         The meat of one of your rules is:
>         flow:established;
>         pcre:"/(Host\:)\s[a-zA-Z0-9.-]+(\.whenyousearch.com\r\n)/";
>         Your rules are going to be slow. Here are the few issues I see so
>         far:
>         * Your regular expression is matching when you don't need to,
>         causing
>         pcre to do un-needed work. * By not using content, you are not
>         making use of the multi-pattern match engine in Snort. * Since
>         all of the rules I looked at are looking for traffic going
>         towards the web server, you should limit your flow to
>         "to_server" as well.
>         My tweaked "for speed" meat of the rule version is:
>         flow:established,to_server; content:"whenyousearch.com"; nocase;
>         pcre:"/^Host\x3a\s*[a-z0-0\.-]+\.whenyousearch.com/smi";
>         Brian Caswell
>         _______________________________________________
>         Intrusions mailing list
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> 
>     -- 
>     --------------------------------------------
>     Matthew Jonkman, CISSP
>     Senior Security Engineer
>     Infotex
>     765-429-0398 Direct Anytime
>     765-448-6847 Office
>     866-679-5177 24x7 NOC
>     my.infotex.com
>     www.offsitefilter.com
>     www.bleedingsnort.com
>     --------------------------------------------
> 
> 
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> 
> -- 
> Wes
> 
> I wish I could come up with some witty sigs
> 
> 
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-- 
--------------------------------------------
Matthew Jonkman, CISSP
Senior Security Engineer
Infotex
765-429-0398 Direct Anytime
765-448-6847 Office
866-679-5177 24x7 NOC
my.infotex.com
www.offsitefilter.com
www.bleedingsnort.com
--------------------------------------------


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