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Re: router worms and International Infrastructure

  • From: Pekka Savola
  • Date: Thu Sep 22 13:59:52 2005

On Thu, 22 Sep 2005, Christopher L. Morrow wrote:
Not quite (feasible path check is different from loose RPF) -- I
didn't understand the documentation myself at first we had to open a
case to get this cleared :-(.  I was not sure of your example, but I
think what you're saying..

Let me try to clarify.

Let's assume you have a router with one interface to the customer with and on that link.  If you enable strict uRPF
towards the customer, only packets coming from or are accepted (except if you have a more specific route).

Now, let's consider you have the second interface to the customer
(let's say on the same router, for simplicity) which has the same but different  With simple strict uRPF, only
one of the interfaces is active at the time.  If the traffic is
asymmetric, packets will get dropped coming from the wrong customer's

Feasible RPF (or possibly manual tuning of route preferences to affect
strict RPF) helps here.  It allows packets from from both
interfaces no matter which one is active at the moment.  So, you could
consider feasible path RPF to be "strict RPF++".

The customer cannot send packets (on either interface) from any source
address other than,, and -- even if
you have a route to one of these networks pointing somewhere else.
Meaning the customer also has which they have routed into
your network on some other router on your network, that traffic isn't
accepted on the 2 links above... which is much more like 'strict' mode.
No, I just described here the simple case. The feasible path strict uRPF works if the customer connects to two routers as well. It's just required that all the prefixes are routed towards the customer's each interface (even if the routes would not be active). In the case of BGP, it requires that the customer advertises all the prefixes on each link, and that's it.

What *isn't* supported, however, that forwards traffic to the ISP on a link without proper advertisements (when using BGP or a routing protocol) on *all* links.

As said, the preferences can be adjusted in any suitable manner (this is different from plain strict uRPF), but the consistent advertisement must be there.

Also, consider the cases where customers push packets your way (for uRPF
strict,  which isn't available for JunOS, but is for IOS depending on
platform/code/hardware-rev... ugh!)
Uhh? uRPF strict is definitely availabe for JunOS.. we've used it for
3-4 years towards all the customers..
the documentation again is probably lacking :( it doesn't mention strict,
just 'active' and 'feasible'.
The configuration is done at two different places: under the forwarding options, you configure whether you want "plain" or "feasible". Under interfaces where you activate uRPF, you configure strict (the default) or loose.

and never send you a route for the
traffic back to them? Maybe they are just a transit and don't even hear
the routes for their customer who chose a 'cheaper' path that doesn't
include them nor me directly on this link in question?
For uRPF to work, you have to have a route toward the interface.
With feasible path strict uRPF, the route doesn't need to be active
(e.g., it can be prepended so that it's never used).
often it's never sent on this link at all, I don't pretend to understand
why a customer would do this, they just do.
Yeah, this is one of the things that cannot be fixed other than by educating the user. For example, smuggle it in the contract, and when the packets get dropped, explain the situation ;-)

Pekka Savola "You each name yourselves king, yet the
Netcore Oy kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

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