[Bro] How to check the length of NDS request packets?

Hongda Li hongdal at g.clemson.edu
Thu Jun 8 13:02:56 PDT 2017

Hi All,

I am going to write a script that detects DNS tunneling.

First the script checks all DNS request packets to see the length.
If the length of a DNS request packet exceeds a threshold, say, 255 bytes,
then this packet will be sent for DPI to check the requested domain name.

The problem is the "dns_request" event does not provide packet length,
which means, for every DNS request, I have to check the requested domain
name. This is expensive.

If I use "raw_packet" or "new_packet" events, then every new packet will
trigger an event, which is also expensive.

Is there a way that only triggers an event for a DNS request packet (e.g.,
based on the protocol and port number), and I can determine whether DPI is
necessary for this DNS request packet based on its length?

I am appreciate for any inputs!

Best regards,
Hongda Li, Graduate Research Assistant
Division of Computer Science, School of Computing
Clemson University
Email: hongdal at clemson.edu
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