Who does source address validation? (was Re: what's that smell?)

"reachable-via any" means you're only going to drop the packet if you
don't have *ANY* route back to them.

What's a route? An IP RIB instance? A BGP Loc-RIB instance? An IGP LSDB
IP prefix entry? A BGP Adj-RIB-In instance?

I think you mean "if you don't have *ANY* **FIB** entry for the
source address".

If I peer with two large providers on the same router and both
have prefix D.1 behind them and advertise the prefix to me, it's
likely that only one of those two paths is going to make it into
the BGP Loc-RIB (and subsequently, the IP RIB then FIB).

If I use ANY FIB entry as proof that it's a valid source then
that only addresses RFC1918ish space and only suggest that I
first need to generate an invalid BGP route for the prefix, then
spoof the packets. This doesn't fix spoofing with global IP

If I use only entries that occur in the RIB and associate them
with the receiving interface and receive a packet with an SA of
D.1 from the peer whose path wasn't installed in the BGP
Loc-RIB then I'll drop it. (And there's nothing broken with
this configuration -- it's why we have routers with 1 million
BGP paths but only 150K routes/fib entries, as I'm sure you

If you're going to do source address validation then you need
to associated all potential valid paths for a given prefix with
the associated ingress interface, else it's mostly useless.


Yes, but if i continue in my ideal situation of people
(mostly) filter their bgp customers, so they won't announce the
1918 space, or similar. even the large peers filter out each other
so they don't pick up 1918 announcements. Plus people use Robs
"Secure IOS Template" to drop extraneous bgp announcements for
unregistered/unassigned space (from IANA).

  I'm not purporting this as a solution to all problems on
the internet, but if one walks before one runs this is a reasonable
step in the correct direction. Or at least a nice bandaid (duct tape?)
to help keep the network in a bit more sensible shape. And if everyone
did it, it would help with the orignal problem/statistics posted about
how much 1918 space was hitting one specific root server.

  I am interested in hearing other solutions to the problem
including extra validations such as the above, but those aren't
avalable today and what i'm suggesting is in the 12.0S and 12.1E
IOS images and probally others.

  - Jared