Subject: [DNSOP] CDS and multi-provider DNSSEC

From: Tony Finch <[email-masked]>
Date: Fri, 1 Feb 2019 12:34:57 +0000

I'm working on tools for KSK rollover automation at the moment.

It turns out that CDS records are very useful even if your parent zone
doesn't check them.

KSK rolls work better when the DS records are not simply generated from
the current DNSKEY RRset. You need to be a bit more clever if you want to
minimize interactions with the parent zone, or minimize the DNSKEY RRset
size, or do an algorithm rollover.

So your tool for setting DS records needs some way to ask the key store
what DS records should be. The nice thing about CDS records is that they
provide a standard way to do this, independent of the key store or signing
software. This allows registrar API clients to be decoupled from the
DNSSEC implementation.

This makes me wonder how well this observation generalizes to
multi-provider DNSSEC.

In model 1, the zone owner manages the KSK, so all the CDS/DS logic
remains centralized.

In model 2, each DNS provider has its own KSK, and does its own DNSKEY
RRset management. In order to support CDS/CDNSKEY, I think it is necessary
for each provider to (somehow) generate RRsets that are the union of their
CDS/CDNSKEY RRsets and the other provider's.

In normal cases, I think the "somehow" involves getting the other
provider's RRset, replacing any records corresponding to this provider's
keys with what this provider thinks they should be, and retaining any
records for unknown keys (which presumably belong to the other provider).
There's a mildly awkward risk of zombie records that are copied back and
forth despite neither provider knowing about them, but I suppose that can
be fixed manually if it arises. Or maybe it's simpler if this is done via
an API, like ZSK sharing :-)

Algorithm rollovers are more difficult, because loose consistency will not
work: a new algorithm needs to be introduced into the DS RRset for all
providers at the same time, and same for removing an old algorithm. In any
case, a zone owner will have to co-ordinate an algorithm rollover between
the providers, so it isn't a big problem that CDS records can't help.

Tony.
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