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

Crook, Parker Parker_Crook at ...2899...
Thu Jun 10 16:01:53 EDT 2010


Hello good sirs,



I wanted to chime in here and say that 'depth' is 'relative', but only to the 'offset' flag, and if no 'offset' flag is set, the beginning of the payload is used as the starting point.  With that in mind, 'within' is a useful content matching option as every time you want to constrain a match to a certain field after your previous match, you don't have to move your offset (The workaround to this would be an ugly hack of using distance+depth).



Now I think this is much clearer when you run a test on a rule that is not a simple composite, but rather a complex, ie, more than 3 content matches:



alert tcp any any -> any any (msg:"DEPTH TEST-passed!"; flow:established,to_server; content:"hello"; nocase; content:"3"; distance:2; depth:3; content:"world"; nocase; distance:1; depth:5; classtype:trojan-activity; sid:20104568; rev:1;)



vs



alert tcp any any -> any any (msg:"WITHIN TEST-passed!"; flow:established,to_server; content:"hello"; nocase; content:"3"; within:3; content:"world"; within:6; classtype:trojan-activity; sid:20104569; rev:1;)



or a composite of the two types:



alert tcp any any -> any any (msg:"COMBO TEST-passed!"; flow:established,to_server; content:"hello"; nocase; content:"3"; within:3; content:"world"; distance:1; depth:5; classtype:trojan-activity; sid:20104570; rev:1;)



I have verified that all three of the aforementioned rules trigger.  It would seem to me that 'within' is a much more elegant solution if you are trying to ensure that a value is scattered somewhere with a range of possibilities, but if you want to be surgical with your rules 'distance' + 'depth' would be better suited, but really at this point you're double-dipping with options:  distance is going to set your match at the beginning and depth is going to set your end of the field, so you might just be better off using distance at that point, ie:



alert tcp any any -> any any (msg:"CONTENT TEST-passed!"; flow:established,to_server; content:"hello"; nocase; content:"3"; distance:2; content:"world"; nocase; distance:1; classtype:trojan-activity; sid:20104571; rev:1;)



I think both depth and within deserve their place in rules, as sometimes room for padding needs to be made whereas other times exact placement of payload is necessary.



I hope I haven't confused anyone.  I tried to be as clear as I could, and if any of it needs clarification, I'll be happy to provide it.

-Parker



  _____

From: Matt Olney [mailto:molney at ...435...]
Sent: Thursday, June 10, 2010 12:00 PM
To: L0rd Ch0de1m0rt
Cc: snort-sigs at lists.sourceforge.net
Subject: Re: [Snort-sigs] No need for content modifier 'within'



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 ...3420....<mailto: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...<mailto: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...<mailto: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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