What is the proper way to delegate to a private / hidden sub-domain?

[John Levine]

In article <mailman.372.1588806226.942.bind-users at lists.isc.org> you write:
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>On 5/6/20 4:12 PM, John Levine wrote:
>> Since they can't access the root servers, how do you expect them to 
>> do DNS lookups at all?
>There is a copy of the root zone in the environment.
>
>There is also enough net zone for the needed tests.
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>DNSSEC is obviously not in play with doctored zones in the labs.

Oh, in that case, why don't you just put some adjusted NS entries in
your stub .net zone pointing at your internal name servers?  Seems a
lot easier than fooling around with routing.