source filtering

prue at ISI.EDU prue at ISI.EDU
Tue Jan 12 23:54:37 UTC 1999

On Tue, Jan 12, 1999 at 01:11:09PM -0500, Steve Gibbard wrote:
==>On Tue, 12 Jan 1999 danderson at wrote:
==>> I'm not sure what the big issue here is with the smurf attacks. If you set
==>> up some kind of access list that disables incoming icmp traffic, then turn
==>That breaks path MTU discovery (see RFC 1435 for more info on that), among
==>other things.

What I have done is write a C program which looks at netflow records
as they come in,(close to real time).  It checks the source port on the
Cisco, the flow record identifies, and compares the source address to a
list of acceptable source addresses for validity.  I give the program a
list of permit and deny like records with IP address and mask along with the
ordinal port number.  Any that are not legit get printed out.  You can
then beat on your customer until they clean up their act.  If they
don't stop right away you can always put in a filter on the interface
til they do.  We have pretty strong contract verbage which permits us
to cut people off if their connection is being used for predatory
activity.  We haven't had to resort to that yet but being able to say
"Fix it or I turn your connection down til you do" is very effective.

The nice thing is that it doesn't slow the router down at all, as long
as you are doing netflow anyhow.   I modified a copy of fdget which
is Ciscos netflow demonstration software they have available via ftp.
That made the programming task easy for a non programming type like

Walt Prue

More information about the NANOG mailing list