DNSSEC defines a canonical form and ordering for RRs [RFC2535]
(section 8.1). In that canonical form, domain names embedded in the
RDATA are converted to lower case.
The downcasing is necessary to ensure the correctness of DNSSEC
signatures when case distinctions in domain names are lost due to
compression, but since it requires knowledge of the presence and
position of embedded domain names, it cannot be applied to unknown
types.
To ensure continued consistency of the canonical form of RR types
where compression is allowed, and for continued interoperability with
existing implementations that already implement the [RFC2535]
canonical form and apply it to their known RR types, the canonical
form remains unchanged for all RR types whose whose initial
publication as an RFC was prior to the initial publication of this
specification as an RFC (RFC 3597prop).
As a courtesy to implementors, it is hereby noted that the complete
set of such previously published RR types that contain embedded
domain names, and whose DNSSEC canonical form therefore involves
downcasing according to the DNS rules for character comparisons,
consists of the RR types NS, MD, MF, CNAME, SOA, MB, MG, MR, PTR,
HINFO, MINFO, MX, HINFO, RP, AFSDB, RT, SIG, PX, NXT, NAPTR, KX, SRV,
DNAME, and A6.
This document specifies that for all other RR types (whether treated
as unknown types or treated as known types according to an RR type
definition RFC more recent than RFC 3597prop), the canonical form is such
that no downcasing of embedded domain names takes place, and
otherwise identical to the canonical form specified in [RFC2535]
section 8.1.
Note that the owner name is always set to lower case according to the
DNS rules for character comparisons, regardless of the RR type.
The DNSSEC canonical RR ordering is as specified in [RFC2535] section
8.3, where the octet sequence is the canonical form as revised by
this specification.
