RE: Same source/dest

["Brei, Matt" <mbrei () medclaiminc com>]

How do I go about adding a BPF, and what is a BPF as long as I'm asking
how to add one?  Thank you.

-----Original Message-----
From: Erek Adams [mailto:erek () snort org] 
Sent: Wednesday, April 02, 2003 11:59 AM
To: Brei, Matt
Cc: snort-users
Subject: RE: [Snort-users] Same source/dest

On Wed, 2 Apr 2003, Brei, Matt wrote:

> That's exactly what I did.  I'll refer you to my first post seen
below.
>   pass ip 10.13.110.254 53 -> 10.13.110.254 1026 (msg:"BAD TRAFFIC
> > same SRC/DST"; sameip; reference:cve,CVE-1999-0016;
> > reference:url,www.cert.org/advisories/CA-1997-28.html;
> > classtype:bad-unknown; sid:527; rev:3;)

Remove the extra stuff.  It's not needed, and you're 'reusing' a SID
which
you shouldn't do.  You can shorten all that to:

        pass ip 10.13.110.254 53 -> 10.13.110.254 1026

If 1026 is what port it always hits on.  If it varries, then change it
to:

        pass ip 10.13.110.254 53 -> 10.13.110.254 any

I'm assuming that this is DNS traffic.  To reduce the chance of
something
bad slipping by you could make it:

        pass udp 10.13.110.254 53 -> 10.13.110.254 any

One thing to think about:  If you're seeing a lot of traffic of this
type,
instead of using a pass rule, use a BPF filter.  By using the BPF
filter,
you are stopping the packets from ever getting into Snort.  As minor as
that sounds, that can save you CPU cycles which is a good thing.  It
eliminates the need for the reading and parsing the pass rules, and the
comparisions to see if it should be passed.  On a heavily loaded
network,
that could be a significant savings.

Cheers!

-----
Erek Adams

   "When things get weird, the weird turn pro."   H.S. Thompson


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