6.1.3.5 Extensibility

DNS software MUST support all well-known, class-independent formats [DNS:2], and SHOULD be written to minimize the trauma associated with the introduction of new well-known types and local experimentation with non-standard types.

DISCUSSION:

The data types and classes used by the DNS are extensible, and thus new types will be added and old types deleted or redefined. Introduction of new data types ought to be dependent only upon the rules for compression of domain names inside DNS messages, and the translation between printable (i.e., master file) and internal formats for Resource Records (RRs).

Compression relies on knowledge of the format of data inside a particular RR. Hence compression must only be used for the contents of well-known, class-independent RRs, and must never be used for class-specific RRs or RR types that are not well-known. The owner name of an RR is always eligible for compression.

A name server may acquire, via zone transfer, RRs that the server doesn't know how to convert to printable format. A resolver can receive similar information as the result of queries. For proper operation, this data must be preserved, and hence the implication is that DNS software cannot use textual formats for internal storage.

The DNS defines domain name syntax very generally -- a string of labels each containing up to 63 8-bit octets, separated by dots, and with a maximum total of 255 octets. Particular applications of the DNS are permitted to further constrain the syntax of the domain names they use, although the DNS deployment has led to some applications allowing more general names. In particular, Section 2.1 of this document liberalizes slightly the syntax of a legal Internet host name that was defined in RFC-952 [DNS:4].