Appendix A. Why can't we just use the owner name of the returned SOA?

In this document, we deduce the nonexistence of a domain only for NXDOMAIN answers where the denied name was the exact domain. If a resolver sends a query to the name servers of the TLD example, asking for the mail exchange (MX) record for www.foobar.example, and subsequently receives a NXDOMAIN, it can only register the fact that www.foobar.example (and everything underneath) does not exist. This is true regardless of whether or not the accompanying SOA record is for the domain example only. One cannot infer that foobar.example is nonexistent. The accompanying SOA record indicates the apex of the zone, not the closest existing domain name. So, using the owner name of the SOA record in the authority section to deduce "NXDOMAIN cuts" is currently definitely not OK.

Deducing the nonexistence of a node from the SOA in the NXDOMAIN reply may certainly help with random qnames attacks, but this is out-of-scope for this document. It would require addressing the problems mentioned in the first paragraph of this section. A possible solution is, when receiving a NXDOMAIN with a SOA that is more than one label up in the tree, to send requests for the domains that are between the QNAME and the owner name of the SOA. (A resolver that does DNSSEC validation or QNAME minimization will need to do it anyway.)