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Re: [OPSEC] Reminder: Call for WG adoption of draft-sriram-opsec-urpf-improvements

Gert Doering <gert@space.net> Wed, 18 April 2018 08:30 UTC

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Date: Wed, 18 Apr 2018 10:30:10 +0200
From: Gert Doering <gert@space.net>
To: Barry Greene <bgreene@senki.org>
Cc: Jeff Haas <jhaas@juniper.net>, "opsec@ietf.org" <opsec@ietf.org>, "draft-sriram-opsec-urpf-improvements@ietf.org" <draft-sriram-opsec-urpf-improvements@ietf.org>
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Subject: Re: [OPSEC] Reminder: Call for WG adoption of draft-sriram-opsec-urpf-improvements
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Hi,

On Wed, Apr 18, 2018 at 08:21:48AM +1200, Barry Greene wrote:
> Then you have this statement "It is well known that this method has limitations when networks are multi-homed and there is asymmetric routing of packets.??? That is false. BCP84 is wrong. uRPF has been deployed with multi-homed downstream customers. It work _if_ you configure it correctly (i.e. use BGP Weights).

... and *if* your customer announces all their prefixes symmetrically to 
all upstreams...

So generally speaking, for multihoming BGP customers, there are too many
failure modes to rely on uRPF - but it's fairly easily remediated if your
tool that deploys BGP prefix-filters also builds matching interface ACLs
with it.  So "whatever prefix the customer *might* announce, we'll accept
the packet".

Of course this assumes that BGP downstreams are actually filtered, but 
this particular source of depression is not really in scope :-)

Gert Doering
        -- NetMaster
-- 
have you enabled IPv6 on something today...?

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