4. Terminology

This section provides definitions for the special terms used in the document.

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 RFC 2119 [RFC2119].

Authoritative server: A DNS server that can answer authoritatively a given DNS request.

DNS64: A logical function that synthesizes DNS resource records (e.g., AAAA records containing IPv6 addresses) from DNS resource records actually contained in the DNS (e.g., A records containing IPv4 addresses).

DNS64 recursive resolver: A recursive resolver that provides the DNS64 functionality as part of its operation. This is the same thing as "DNS64 in recursive-resolver mode".

DNS64 resolver: Any resolver (stub resolver or recursive resolver) that provides the DNS64 function.

DNS64 server: Any server providing the DNS64 function. This includes the server portion of a recursive resolver when it is providing the DNS64 function.

IPv4-only server: Servers running IPv4-only applications and servers that can only use IPv4, as well as cases where only IPv4 connectivity is available to the server.

IPv6-only hosts: Hosts running IPv6-only applications and hosts that can only use IPv6, as well as cases where only IPv6 connectivity is available to the client.

Recursive resolver: A DNS server that accepts requests from one resolver, and asks another server (of some description) for the answer on behalf of the first resolver. Full discussion of DNS recursion is beyond the scope of this document; see [RFC1034] and [RFC1035] for full details.

Synthetic RR: A DNS resource record (RR) that is not contained in the authoritative servers' zone data, but which is instead synthesized from other RRs in the same zone. An example is a synthetic AAAA record created from an A record.

IPv6/IPv4 translator: A device that translates IPv6 packets to IPv4 packets and vice versa. It is only required that the communication initiated from the IPv6 side be supported.

For a detailed understanding of this document, the reader should also be familiar with DNS terminology from [RFC1034] and [RFC1035] and with current NAT terminology from [RFC4787]. Some parts of this document assume familiarity with the terminology of the DNS security extensions outlined in [RFC4035]. It is worth emphasizing that while DNS64 is a logical function separate from the DNS, it is nevertheless closely associated with that protocol. It depends on the DNS protocol, and some behavior of DNS64 will interact with regular DNS responses.