Well-Known
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... requirement was stated in [RFC1123] without defining the
term "well-known"; it is hereby specified that only the RR types
defined in [RFC1035 ...
... RR types
defined in [RFC1035] are to be considered "well-known".
The specifications of a few existing RR ...
... Receiving servers MUST decompress domain names in RRs of well-known
type, and SHOULD also decompress RRs of type RP ...