DNS/qname-minimisation/RFCについて、ここに記述してください。

https://tools.ietf.org/html/rfc7816 /Appendix A

3. Possible Issues

Some broken name servers do not react properly to QTYPE=NS requests.

For instance, some authoritative name servers embedded in load balancers reply properly to A queries but send REFUSED to NS queries.

• This behaviour is a protocol violation, and there is no need to stop improving the DNS because of such behaviour.

However, QNAME minimisation may still work with such domains,

• since they are only leaf domains (no need to send them NS requests).

Such a setup breaks more than just QNAME minimisation.

• It breaks negative answers, since the servers don't return the correct SOA, and it also breaks anything dependent upon NS and SOA records existing at the top of the zone.

Another way to deal with such incorrect name servers would be to try

• with QTYPE=A requests (A being chosen because it is the most common and hence a QTYPE that will always be accepted, while a QTYPE NS may ruffle the feathers of some middleboxes).

Instead of querying name servers with a query "NS example.com",

• we could use "A _.example.com" and see if we get a referral.