Cutting a zone with DNSSEC

This week we will be delegating newton.cam.ac.uk (the Isaac Newton Institute's domain) to the Faculty of Mathematics, who have been running their own DNS since the very earliest days of Internet connectivity in Cambridge.

Unlike most new delegations, the newton.cam.ac.uk domain already exists and has a lot of records, so we have to keep them working during the process. And for added fun, cam.ac.uk is signed with DNSSEC, so we can't play fast and loose.

In the absence of DNSSEC, it is mostly OK to set up the new zone, get all the relevant name servers secondarying it, and then introduce the zone cut. During the rollout, some servers will be serving the domain from the old records in the parent zone, and other servers will serve the domain from the new child zone, which occludes the old records in its parent.

But this won't work with DNSSEC because validators are aware of zone cuts, and they check that delegations across cuts are consistent with the answers they have received. So with DNSSEC, the process you have to follow is fairly tightly constrained to be basically the opposite of the above.

The first step is to set up the new zone on name servers that are completely disjoint from those of the parent zone. This ensures that a resolver cannot prematurely get any answers from the new zone - they have to follow a delegation from the parent to find the name servers for the new zone. In the case of newton.cam.ac.uk, we are lucky that the Maths name servers satisfy this requirement.

The second step is to introduce the delegation into the parent zone. Ideally this should propagate to all the authoritative servers promptly, using NOTIFY and IXFR.

(I am a bit concerned about DNSSEC software which does validation as a separate process after normal iterative resolution, which is most of it. While the delegation is propagating it is possible to find the delegation when resolving, but get a missing delegation when validating. If the validator is persistent at re-querying for the delegation chain it should be able to recover from this; but quick propagation minimizes the problem.)

After the delegation is present on all the authoritative servers, and old data has timed out of caches, the new child zone can (if necessary) be added to the parent zone's name servers. In our case the central cam.ac.uk name servers and off-site secondaries also serve the Maths zones, so this step normalizes the setup for newton.cam.ac.uk.