[Snort-sigs] Matching the beginning or end of a (preprocessor) content buffer

Mike Cox mike.cox52 at ...2420...
Fri Nov 9 09:55:40 EST 2012


So I can probably do some tests when I get the time (thanks for the
responses BTW), but I'm somewhat concerned with the comment, "...it
would be against static pcaps which doesn't test performance.  (Some
people think that looping a pcap through a system a bunch of times
test performance..)"

Can you elaborate on this?

I understand that using the '-r' option to tell Snort to read a pcap
will not test performance of things like bandwidth, dropped packets,
etc.  However, in a case like this when you want to test *relative*
performance between rules, is Performance Profiling not accurate for
thing like avg_ticks, total_ticks, etc.?  Does the engine not load the
rules, build the matching data structures/logic, and process thing the
same way when the '-r' option is used?  Let me say again that I am
asking about relative performance numbers between rules, not absolute
numbers necessarily.

Thanks.

-Mike Cox

On Thu, Nov 8, 2012 at 2:59 PM, Joel Esler <jesler at ...435...> wrote:
> If you have the content as a pre qualifier, then the pcre should execute
> (especially something that small) really quickly.  It shouldn't be hard to
> perf test that, but if I did it, it would be against static pcaps which
> doesn't test performance.  (Some people think that looping a pcap through a
> system a bunch of times test performance..)
>
> So we'd need to perf test it on a live network in parallel.  Mike, care to
> do so?
>
> --
> Joel Esler
> Senior Research Engineer, VRT
> OpenSource Community Manager
> Sourcefire
>
> On Nov 8, 2012, at 3:52 PM, Russ Combs <rcombs at ...435...> wrote:
>
> Good question.  Do we have perf data for content:"foo" vs. pcre:"foo" ?
> That would probably be indicative.
>
> On Thu, Nov 8, 2012 at 3:38 PM, Joel Esler <jesler at ...435...> wrote:
>>
>> We are looking at this Mike, we think this is an interesting idea,
>>
>> However, implementing a pcre like "pcre:"/bad\.pdf$/";"  shouldn't have
>> that much of an impact.
>>
>> --
>> Joel Esler
>> Senior Research Engineer, VRT
>> OpenSource Community Manager
>> Sourcefire
>>
>> On Nov 7, 2012, at 4:22 PM, Mike Cox <mike.cox52 at ...2420...> wrote:
>>
>> AFIK, it isn't possible to do this without a PCRE but I though I'd
>> ask: is is possible to tell a preprocessor content buffer (like
>> http_uri) to match at the end (or beginning) of the buffer without
>> using a PCRE?
>>
>> For example, let's say I want to match the URI 'bad.pdf".  I know this
>> will be at the end of the URI (and thus the end of the http_uri
>> buffer) and I want to match that specifically so I don't also get
>> alerts on things like "/bad.pdfoobar/index.aspx".
>>
>> Normally I'd just do this:
>>
>> content:"/bad.pdf"; http_uri;
>>
>> But I know that this will be at the end of the URI buffer and I don't
>> want to do a PCRE as well to ensure this due to performance concerns.
>>
>> It seems like this ability would be moderately easy to build into the
>> engine and computationally trivial as far as performance goes.  Maybe
>> have something like, "http_uri:end", "http_uri:beginning",
>> "http_uri:beginning,end", http_cookie:end", etc. or have special
>> characters (that would otherwise have to be escaped) to indicate that
>> you want to match on the beginning or end of the buffer.
>>
>> Just a thought since you guys are re-writing the http-inspect
>> preprocessor :)  Joel, feel free to send to snort-dev, I don't think
>> I'm on that list.
>>
>> Thanks!
>>
>> -Mike Cox
>>
>>
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