Finding asymmetric path

[attributions lost]
> > >>> I'm reasonable certain a customer of ours who is using one of our
> > >>> netblocks is using a different reverse path to reach us. How might I
> > >>> figure out who is allowing them to source traffic from IPs that belong
> > >>> to us?
> > >> you are implying that they are not allowed to multi-home using the ip
> > >> space you have assigned to them. good way to lose a customer.
> > > Does it count as multihoming when we are the only ones announcing the
> > > space?
> >
> > almost an interesting question. but i think it is playing with words.
> > if i understand your original statement, they are clearly attached to at
> > least two providers.
> >
> > perhaps it is fear of what they, possibly mistakenly, perceive to be
> > your policy regarding announcement of space that keeps them from
> > announcing normally to both, or more, links?

It wasn't clear that the customer was a BGP downstream though by saying
'We are the only ones announcing the space', I think not. Non-BGP
multihoming is broken* and when not done out of ignorance generally is
the smoke pointing to the fire of someone trying to hide something.
Was very common for spammers to abuse no-uRPF networks in the early
days of broadband.

This is still rather common for people to do, at least where there's a
significant cost differential. There are enough networks that can accept
arbitrary traffic that BGP doesn't really play into it at all.

> It could also be something simple like pricing. For example, in a large
> colo facility, you might easily find that a number of providers offer
> low cost transit, but not IP space. For a customer who is heavy on the
> outbound traffic, they might find it more affordable to buy their inbound
> plus IP space from you, and then dump onto Cogent or something like that
> for outbound. Unless your contract specifically prohibits this, you're
> probably not going to be able to prevent it.

I wonder if there is a drift of baseline assumptions between the current
wave of operators and previous ones? To me (and BCP38) it is beyond bad
practice to allow -and if allowed, to make use of- such sloppy edges.

Of course it is.

If the other network truly is practicing bad forwarding hygiene then
they are a security problem for everyone else and IMO would be good for
naming and shaming.

Sure. But it's easy enough to go to, for example, $BACKBONE_OF_CHOICE
and say "I'm delegated from $SMALLISP, I would like you to
accept traffic from that prefix, here's my SWIP'd WHOIS to prove that"
and there are lots of providers for whom that won't be a problem; it is
not the same sort of security problem that a complete lack of filtering
is. Generally speaking, what's needed is a control over what's being
cast into the network.

* for the majority of the cases. I know there are purposeful Non-BGP
  MOAS/anycast purposefully run by those who understand the implications.
  It is unfortunate that their use of lack of inherent BGP path security
  contribute to fuzzing what would otherwise have been a clear indicator
  of 'bad' behavior. But same could be said for the deaggregators
  using longest-match to have everyone else do their TE; water under
  the bridge pushing work onto everyone else.


... JG