1. DNS/RFC/4697

について、ここに記述してください。

An iterative resolver MUST NOT send a query 
for the NS RRSet of a non-responsive zone 
to any of the name servers for that zone's parent zone.  

   For the purposes of this injunction, a non-responsive zone is defined as 
   a zone for which every name server listed in the zone's NS RRSet:

   1.  is not authoritative for the zone (i.e., lame), or

   2.  returns a server failure response (RCODE=2), or

   3.  is dead or unreachable according to Section 7.2 of RFC 2308 [4].

Negative Caching of DNS Queries (DNS NCACHE)

https://tools.ietf.org/html/rfc2308#section-7.2 Dead / Unreachable servers

RCODE=5 (Refused)については、書かれていない。-- ToshinoriMaeno 2020-08-06 04:42:10