1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

"Negative caching" - the storage of knowledge that something does not exist. We can store the knowledge that a record has a particular value. We can also do the reverse, that is, to store the knowledge that a record does not exist. It is the storage of knowledge that something does not exist, cannot or does not give an answer that we call negative caching.

"QNAME" - the name in the query section of an answer, or where this resolves to a CNAME, or CNAME chain, the data field of the last CNAME. The last CNAME in this sense is that which contains a value which does not resolve to another CNAME. Implementations should note that including CNAME records in responses in order, so that the first has the label from the query section, and then each in sequence has the label from the data section of the previous (where more than one CNAME is needed) allows the sequence to be processed in one pass, and considerably eases the task of the receiver. Other relevant records (such as SIG RRs [RFC2065]) can be interspersed amongst the CNAMEs.

"NXDOMAIN" - an alternate expression for the "Name Error" RCODE as described in [RFC1035 Section 4.1.1] and the two terms are used interchangeably in this document.

"NODATA" - a pseudo RCODE which indicates that the name is valid, for the given class, but are no records of the given type. A NODATA response has to be inferred from the answer.

"FORWARDER" - a nameserver used to resolve queries instead of directly using the authoritative nameserver chain. The forwarder typically either has better access to the internet, or maintains a bigger cache which may be shared amongst many resolvers. How a server is identified as a FORWARDER, or knows it is a FORWARDER is outside the scope of this document. However if you are being used as a forwarder the query will have the recursion desired flag set.

An understanding of [RFC1034], [RFC1035] and [RFC2065] is expected when reading this document.