[Snort-sigs] Question regarding distances after a byte_jump...

evejou girl at ...3471...
Thu Dec 16 17:54:19 EST 2010


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 at ...3471...

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