Re: fast_pattern not always longest content string by default?

[Mike Cox <mike.cox52 () gmail com>]

Thanks for the replies.  I don't disagree with this behavior but I do think
the Snort manual should make this clear because I've been using Snort for
many years and this is the first I've heard of it.

-Mike Cox

On Wed, Oct 22, 2014 at 9:34 PM, Steve Sturges (ststurge) <
ststurge () cisco com> wrote:

> Legacy, kinda.  But more efficient performance wise.  :)
>
> > On Oct 22, 2014, at 9:18 PM, "Joshua Kinard" <kumba () gentoo org> wrote:
> >
> > I'll wager that this is a relic of Snort's early days as primarily an
> HTTP
> > traffic sniffer, before it became a more generic deep-packet inspection
> tool.
> > Something like this should get a mention in the Snort manual, though
> there are
> > several places where it states that the longest content match is the
> default,
> > yet doesn't differentiate between a normal content match and a content
> match
> > modified by an HTTP keyword.  So, not a quick fix w/o refactoring the
> lingo in
> > a few spots.
> >
> > --J
> >
> >
> > > On 10/22/2014 16:30, Josh Rosenbaum (jrosenba) wrote:
> > > Hi Mike,
> > >
> > > Sorry for this unfortunate news, but it looks like you will need tweak
> those
> > > sigs.  I can confirm that if a fast_pattern keyword is not specified
> for a
> > > given rule, the default fast pattern is the longest HTTP buffer content.
> > > If no HTTP buffer content is present, then the fast pattern is the
> longest
> > > content.
> > >
> > > Josh
> > >
> > >
> > > From: Mike Cox <mike.cox52 () gmail com<mailto:mike.cox52 () gmail com>>
> > > Date: Wednesday, October 22, 2014 at 8:16 AM
> > > Subject: [Snort-devel] fast_pattern not always longest content string
> by default?
> > > Hi All,
> > >
> > > I was looking thru some of my sigs with 'debug-print-fast-pattern'
> turned on
> > > and noticed that the fast pattern string was not always the longest
> content
> > > match by default.  Specifically, it appears that content matches in
> (valid
> > > for fast_pattern) HTTP Inspect buffers (e.g. http_header, http_uri,
> etc.)
> > > are taking priority.  For example, consider this sig:
> > >
> > > alert tcp any any -> any $HTTP_PORTS (msg:"FP Test";
> > > flow:established,to_server; content:"twitter.com<http://twitter.com>";
> > > http_header; content:"hellow Twitter tweet"; sid:1234567;)
> > >
> > > The longest content match is "hellow Twitter tweet" but when I look at
> the
> > > fast pattern debug output, the fast pattern used is
> > > "twitter.com<http://twitter.com>".
> > >
> > > Having the HTTP Inspect buffers take priority makes sense because they
> will
> > > be smaller than the entire packet and thus more efficient.  However, I
> do
> > > not see this behavior documented in the manual which says, "the default
> > > behavior of fast pattern determination is to use the longest content in
> the
> > > rule..."
> > >
> > > Can someone comment/confirm this?  It is looking like I may have to
> > > review/tweak a plethora of sigs.... :(
> > >
> > > Thanks!
> > >
> > > -Mike Cox
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