validity
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... CNAME records relate, what names are legal in what parts of the DNS,
and what is the valid syntax of a DNS name.
...
... applied, whenever a SIG record was included with a response to
validate that response, the SIG records for all other RRSets
associated with the appropriate node ...
... domain name may have different NXT records in
the parent zone and the child zone, and both are valid, and are not
an RRSet. See also section 5.3.2.
...