[Snort-sigs] byte_test and buffer cursor

Y M snort at ...3751...
Thu Feb 9 17:00:38 EST 2017


Thank you Alex.


I did go through the manual and I guess I must have missed it, at least in the byte_test section.


Using the value I am expecting in the second byte_test did not yield a match:


content:"Content-Length|3A 20|"; http_raw_header; byte_test:3,>,160,0,relative; byte_test:3,=,165,0,relative;


Keeping only the first byte_test yields a match:


content:"Content-Length|3A 20|"; http_raw_header; byte_test:3,>,160,0,relative;


Using a single byte_test with the value I am expecting does not yield a match:


content:"Content-Length|3A 20|"; http_raw_header; byte_test:3,=,165,0,relative;


tshark -nn -r capture.pcap -Y 'http.request.method == POST' -T fields -e http.content_length | sort | uniq


124
165
89
93


Just to verify my understanding, the first content match should set the cursor to the next byte right after the matched content, correct? I guess this is not the best to do it since the Content-Length is a lot more variable than I thought + historical data. Back to the drawing board.


Thanks again Alex.

YM

________________________________
From: Alex McDonnell <amcdonnell at ...435...>
Sent: Friday, February 10, 2017 12:34:23 AM
To: Y M
Cc: snort-sigs
Subject: Re: [Snort-sigs] byte_test and buffer cursor

I'm pretty sure that's in the Snort Manual, byte_test does not move the cursor, byte_extract does. You could also test for the value you expect in your second test to verify ;)

Alex McDonnell
TALOS

On Thu, Feb 9, 2017 at 4:30 PM, Y M <snort at ...3751...<mailto:snort at ...3833.....>> wrote:

Does a byte_test move the cursor into a buffer the number of bytes it tests? Does is depend on the buffer against which the byte_test is operating?


In essence, I am trying to use byte_test to validate that the decimal value of X number of bytes fall within a decimal range. Something similar to:


http_raw_header; byte_test:3,>,160,0,relative; byte_test:3,<,170,0,relative;


If the first byte_test above moves the cursor 3 bytes into the buffer, how can I reset the cursor back to the beginning of the buffer of the last/relative content match (http_raw_header)? Or am I just doing it all wrong?


YM



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