Network Operators and smurf

Karl Denninger karl at
Fri Apr 24 23:11:06 UTC 1998

On Fri, Apr 24, 1998 at 06:06:50PM -0500, John A. Tamplin wrote:
> On Fri, 24 Apr 1998, Karl Denninger wrote:
> > Well, there is a simple knob for this:
> > 
> > If the Knob is turned "ON", then any packet from a source address which is 
> > not routed to the interface it came in on is dropped.
> > 
> > This works for static, dynamic, and all other kinds of routing.    It will
> > solve the problem and is trivial to implement - if any of the vendors care.
> It doesn't work for asymmetric routing as you describe it above. If you
> modify your criteria to be that there are no valid routes out that
> interface, you would only break transient routing conditions, but
> depending on how the router stores routes it may not be possible (or
> desirable due to memory requirements) to implement. 
> John Tamplin					Traveller Information Services
> jat at Traveller.COM				2104 West Ferry Way
> 205/883-4233x7007				Huntsville, AL 35801


That a route is *valid* doesn't mean its the best path or the one that the
router will use.  It means that the path is *valid*.  You're confusing
"valid" with "best".

Besides, the issue here isn't transport level circuits (where such things
matter); it is end-customer attachments, which are typically not multihomed 
and even if they are, they're also typically static routed.  Someone running
BGP4 with you isn't going to have this enabled on either end of that

I don't expect this to be usable on a backbone circuit.  Then again, that's
not where the problems are originating.

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