Something that keeps coming up recently in private discussions is that
there's supposedly an ambiguity in RFC 1034/1035 about NXDOMAINs, that
is practically observed in broken authoritatives on the internet when
implementing RFC 7816 (qname minimization), and that it was only
clarified in RFC 8020 (NXDOMAIN: there really is nothing
underneath). I'm sorry I didn't pay attention when RFC 8020 was being
discussed, and the RFC itself is nice to have.
There really is no ambiguity in RFC 1034/1035 about NXDOMAINs. RFC 1034
doesn't introduce the DNS as a collection of names; names only come
afterwards. The domain name space is introduced as a tree structure
composed of nodes. Each node has an associated label of 1-63 octets
except the root that has a 0 length label. Only then, is a domain name
defined as the concatenation of labels from a node to the
root. Everything in the global DNS is this domain name space. There are
nodes, not names, and names are identifiers for the nodes. A name can't
be "present" without the corresponding node existing in the domain name
space. Due to the tree, it follows that for some node to exist, its
ancestor nodes on the path to the root must exist. For a domain name to
exist, all its superdomain names must exist. Hence if a domain name
(node identifier) does not exist, there can be nothing under it.
There is no ambiguity in RFC 1034/1035, and implementations that return
NXDOMAIN for empty non-terminals are broken against RFC 1034.
Mukund
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