[Snort-sigs] No need for content modifier 'within'

Matt Olney molney at ...435...
Thu Jun 10 12:00:21 EDT 2010


Your Chodeship,

I'm at home taking a couple of days off, and haven't tested it, but here is
my understanding:


For the string:

"I wrote hello world"

content:"hello"; content:"wrote"; distance: 0; depth:5;

would fire because "depth" and "offset" force a flag of non-relative to the
match.  I may be getting this backwards, and it may be that "distance" sets
a relative flag instead, but one way or another those two terms are bound
together.  I'll do some testing

I'll try and set some time aside Monday to test this, if no one else gets to
it, but in the meantime, please follow the recommendations.

Matt

On Thu, Jun 10, 2010 at 11:32 AM, L0rd Ch0de1m0rt
<l0rdch0de1m0rt at ...2420...>wrote:

> I understand what you are saying but I'm not sure it is congruent with
> current reality.  Try this -- run these two rules:
>
> alert tcp any any -> any any (msg:"DEPTH TEST!";
> flow:established,to_server; content:"hello"; nocase; content:"world";
> distance:4; depth:5; classtype:trojan-activity; sid:20104568; rev:1;)
>
> alert tcp any any -> any any (msg:"WITHIN TEST!";
> flow:established,to_server; content:"hello"; nocase; content:"world";
> distance:4; within:5; classtype:trojan-activity; sid:20104569; rev:1;)
>
> And then browse to http://www.google.com/hello1234world
>
> Do they both alert?  If so, that would seem to indicate that depth is
> relative, no?
>
> I'd also encourage you to test http://www.google.com/hello123world and
> http://www.google.com/hello12345world and verify that those don't
> cause alerts.
>
> Cheers,
>
> -L0rd Ch0de1m0rt
>
> On 6/10/10, Joel Esler <jesler at ...435...> wrote:
> > I apologize I mean depth instead of distance.  My fingers typed the wrong
> > word.
> >
> > You can do a distance:0, but within allows you to tell the search where
> to
> > stop.  Distance is relative to the last match, within is relative to this
> > match.
> >
> > Depth tells Snort how far to read into a packet to search for a pattern
> > Offset tells Snort how far to read into a packet to START searching for a
> > pattern
> > Distance tells Snort how far into a packet Snort should skip relative to
> the
> > end of the previous content match
> > Within makes sure that there is "x" amount of bytes between pattern
> matches.
> >
> > You can't use a depth with a distance, as depth references the offset,
> not
> > the relative distance from last match (that's what distance is for)
> >
> > There are reasons for all four, we've had this debate for years.
> >
> > Plus with distance, you can do negative relativity, you can't do that
> with
> > offset.  Just FYI.
> >
> >
> > 11:04 AM, on Jun 10, 2010, wrote:
> >
> >> What?  Are you confusing distance and offset?  According to your blog
> >> post according to the Snort manual, "The distance keyword allows the
> >> rule writer to specify how far into a packet Snort should ignore
> >> before starting to search for the specified pattern relative to the
> >> end of the previous pattern match."
> >>
> >> Distance is relative and I'm saying we don't really need the 'within'
> >> keyword since we can just do distance:0; and then use depth since
> >> depth is relative as well.
> >>
> >> Hope this helps.
> >>
> >> -L0rd Ch0de1m0rt
> >>
> >> On 6/10/10, Joel Esler <jesler at ...435...> wrote:
> >>> Distance tells Snort how far to read into a packet to search for a
> >>> pattern
> >>> Within makes sure that at most "x" amount of bytes are between pattern
> >>> matches.
> >>>
> >>> Within is relative, distance is not.
> >>>
> >>>
> >>> 10:39 AM, on Jun 10, 2010, wrote:
> >>>
> >>>> Hello.  Not trying to beat a dead horse here but I was reading
> >>>>
> http://blog.joelesler.net/2010/03/offset-depth-distance-and-within.html
> >>>> and came to a part where it said, "Offset goes with Depth, distance
> >>>> goes with within.  Don’t mix them."  I'm not sure I agree with this
> >>>> and I'm not much of an Blogger/Internet Exhibitionist so I'm posting
> >>>> this here.
> >>>>
> >>>> We all know, offset tells Snort how far into the payload (starting
> >>>> from the beginning of the payload) to start looking for a content
> >>>> match.  Distance tells Snort how far into the payload (starting from
> >>>> the previous content match) to start looking for a content match.
> >>>> Depth *and* within tell Snort where to stop looking based on where it
> >>>> started looking.  So you can have distance and use depth if you want
> >>>> and it is perfectly OK to do this.  Do not be afraid.  The only reason
> >>>> within exists is so if you have a situation where you don't use
> >>>> distance but want to make sure no more than N bytes are between
> >>>> content matches.  But within isn't really necessary. In fact, we could
> >>>> get rid of within in the case described and just add distance:0; and
> >>>> use depth.
> >>>>
> >>>> Hope this helps clarify a few things about the within content
> modifier.
> >>>>
> >>>> Cheers,
> >>>>
> >>>> -L0rd Ch0de1m0rt
> >>>>
> >>>>
> ------------------------------------------------------------------------------
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> >>>
> >>> --
> >>> Joel Esler
> >>>
> >>>
> >>>
> >
> > --
> > Joel Esler
> >
> >
> >
>
>
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