[Snort-devel] fast_pattern not always longest content string by default?

Mike Cox mike.cox52 at ...2499...
Thu Oct 23 13:23:27 EDT 2014


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 at ...3461...> wrote:

> Legacy, kinda.  But more efficient performance wise.  :)
>
> > On Oct 22, 2014, at 9:18 PM, "Joshua Kinard" <kumba at ...2185...> 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 at ...2499...<mailto:mike.cox52 at ...2499...>>
> >> 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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