Re: an effect of ignoring BCP38

* Pekka Savola · * Thu Sep 11 13:59:59 2008

On Thu, 11 Sep 2008, Jo Rhett wrote:

On Sep 11, 2008, at 10:10 AM, [email protected] wrote:

By the time you walk our list of upstreams to any of the '5 biggest
anything', you've gotten to places where our multihomed status
means you can't filter our source address very easily (or more
properly, where you can't filter multihomed sources in general).

I don't agree with this statement. I hear this a lot, and it's not really
true. Being multihomed doesn't mean that your source addresses are likely to
be random. (or would be valid if they were)
A significant portion of our customers, and *all* of the biggest paying ones,
are multihomed. And they might have a lot of different ranges, but we know
what the ranges are and filter on those.

If you can manage ACLs for these customers, that's fine. But maybe
your multihomed customers and '5 biggest anything' customers are
different. Maybe your multihomed customer has 5 prefixes. The big
ones could have 5000. That's a pretty big ACL to manage.
This is where Strict URPF (+feasible paths) helps a lot because in
some cases you don't need to manage those ACLs by hand. But this gets
broken if the customer advertises more specific routes from another
provider, but doesn't advertise these to you -- your route to those
more specifics goes through another ISP, and if the site is sourcing
packets from those more specifics through you, the strict uRPF would
drop them.
There are ways to work around this, e.g. by requiring that the
customers advertise the more specifics to you as well but mark them so
that you don't propagate them, but I suspect this is not feasible in
the higher tiers of ISP business.
http://tools.ietf.org/html/draft-savola-bcp84-urpf-experiences Section
3 discusses this a bit.
FWIW: we use feasible-paths strict uRPF on all customer and LAN
interface without exception. Multihomed ones as well, though it's a
bit more work.
--
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings