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Re: Question regarding distances after a byte_jump...
From: Joel Esler <jesler () sourcefire com>
Date: Fri, 17 Dec 2010 02:23:38 -0500

On Dec 17, 2010, at 1:20 AM, evejou <girl () techn0ev3 net> wrote:

Grr.. I suddenly realized why my signature was all messed up; my fatal mistake was forgetting that from_beginning 
meant: "from the VERY BEGINNING, from the HTTP header in my packet, which I totally forgot was there." Suddenly all 
of my results make sense. :P

Thanks Joel... Sorry to bother.


No problem. I'm stuck on a train at 2 in the morning because electric lines have shorted out our system!


Also, what's the difference (if there is any) between setting "post_offset 2" and using "distance:2"?

In your case, nothing that I can tell. Keep it simple for yourself though. 

J





On Thu, Dec 16, 2010 at 7:37 PM, Joel Esler <jesler () sourcefire com> wrote:
Two things that I see right away that you might want to try and make your life easier.

from_beginning's function is to start it's packet jumping at the beginning of the packet, as opposed to where your 
pointer is, and I am not sure that's what you are trying to do from reading your email.

Also, post_offset can confuse the novice, so you might want go make it simpler for you.

content:"|MM MM|"; byte_jump:3,0,relative; content:"|AA AA|"; distance:2; within:2;


From reading your email, that might be what you are trying to do, please let me know?

Joel

On Dec 16, 2010, at 5:55 PM, evejou wrote:



I was trying to write a signature for Snort v2.6.1.5. I have a question about using the distance/within tags after 
a byte_test, if that's even proper use for it.

Oops. I meant, byte_jump.



On Thu, Dec 16, 2010 at 5:54 PM, evejou <girl () techn0ev3 net> wrote:
Hi,

I was trying to write a signature for Snort v2.6.1.5. I have a question about using the distance/within tags after a 
byte_test, if that's even proper use for it.

Say there's a packet that looks kind of like this:

MM MM OO OO OO [....] TT XX XX AA AA ...

(MM -- magic number)
(OO -- offset value that points to the TTs; this offset counts from the beginning of the file)
(XX XX -- 2 bytes that I don't care about)

I was trying to figure out where the pointer would be after a byte_jump, so I tried to write the following to see if 
it would trigger:
      content:"|MM MM|"; byte_jump:3,0,relative,from_beginning,post_offset 2; content:"|AA AA|"; distance:0; 
within:2;
I noticed that this didn't trigger, but that it did when I removed the "within:2" part.


And then I tried the following:
      content:"|MM MM|"; byte_jump:3,0,relative,from_beginning,post_offset 2; content:"|OO OO OO|"; distance:0; 
within:3;
and this triggered as well.

My first question is whether this is expected behavior (or am I doing something wrong?), and adjunctly to that, how 
I could get a hit on that second content tag (the |AA AA| part)...


Thanks,
Alice

-- 
---
girl () techn0ev3 net

Finché c'è vita, c'è speranza.
As long as there is life, there is hope. 



-- 
---
girl () techn0ev3 net

Finché c'è vita, c'è speranza.
As long as there is life, there is hope. 
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As long as there is life, there is hope. 
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