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

Russ Combs rcombs at ...435...
Thu Nov 8 15:52:29 EST 2012


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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