RRs containing compression pointers in the RDATA part cannot be
treated transparently, as the compression pointers are only
meaningful within the context of a DNS message. Transparently
copying the RDATA into a new DNS message would cause the compression
pointers to point at the corresponding location in the new message,
which now contains unrelated data. This would cause the compressed
name to be corrupted.
To avoid such corruption, servers MUST NOT compress domain names
embedded in the RDATA of types that are class-specific or not well-
known. This requirement was stated in [RFC1123] without defining the
term "well-known"; it is hereby specified that only the RR types
defined in [RFC1035] are to be considered "well-known".
The specifications of a few existing RR types have explicitly allowed
compression contrary to this specification: [RFC2163] specified that
compression applies to the PX RR, and [RFC2535] allowed compression
in SIG RRs and NXT RRs records. Since this specification disallows
compression in these cases, it is an update to [RFC2163] (section 4)
and [RFC2535] (sections 4.1.7 and 5.2).
Receiving servers MUST decompress domain names in RRs of well-known
type, and SHOULD also decompress RRs of type RP, AFSDB, RT, SIG, PX,
NXT, NAPTR, and SRV (although the current specification of the SRV RR
in [RFC2782] prohibits compression, [RFC2052] mandated it, and some
servers following that earlier specification are still in use).
Future specifications for new RR types that contain domain names
within their RDATA MUST NOT allow the use of name compression for
those names, and SHOULD explicitly state that the embedded domain
names MUST NOT be compressed.
As noted in [RFC1123], the owner name of an RR is always eligible for
compression.
