[Snort-users] Depth vs. offset in rules

Joel Esler (jesler) jesler at ...589...
Fri Mar 6 16:11:49 EST 2015


Just as a followup.  We’re thinking about this right now and some interesting ideas have come into our heads.

--
Joel Esler
Open Source Manager
Threat Intelligence Team Lead
Talos Group

On Mar 3, 2015, at 12:02 PM, Research <research at ...17107...<mailto:research at ...17107...>> wrote:

Hi,

I see.  This makes sense to me, except that offset can be a negative number.  So if I specified an offset of -100, wouldn’t that mean to start looking for data 100 bytes *BEFORE* the start of the data section of the packet ?

On Mar 3, 2015, at 11:54 AM, Joel Esler (jesler) <jesler at ...589...<mailto:jesler at ...589...>> wrote:

Offset tells Snort how far into the data portion of the packet to start the search.


Depth tells Snort how far from the offset to stop searching.  (Offset being one specified, or in case one is not specified, from the beginning of the packet payload)


--
Joel Esler
Open Source Manager
Threat Intelligence Team Lead
Talos Group


On Mar 3, 2015, at 11:39 AM, Research <research at ...17107...<mailto:research at ...17107...>> wrote:

Hi,

In reading chapter 3 of the Snort 2.9.7.0 manual, I have a clarification question for the use of “depth” vs. “offset”.

Depth appears to specify where to start a content match in the packet payload, so if I understand correctly:

depth:5;

…would mean begin content matching 5 bytes into the packet payload.

When compared to offset, does that mean offset relative to the depth ?  So:

depth:5; offset:10; …

…means start at byte 5 in the packet payload and an offset from the depth as a starting location of another 10 bytes ?  I am thinking that is correct because I note that offset can have negative values and a negative starting point for a packet payload would not make sense, but as an offset it would.

Thanks
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