RFC 3597 (rfc3597) - Page 2 of 8
Handling of Unknown DNS Resource Record (RR) Types
Alternative Format: Original Text Document
RFC 3597 Handling of Unknown DNS RR Types September 2003
2. Definition
An "RR of unknown type" is an RR whose RDATA format is not known to
the DNS implementation at hand, and whose type is not an assigned
QTYPE or Meta-TYPE as specified in [RFC 2929] (section 3.1) nor
within the range reserved in that section for assignment only to
QTYPEs and Meta-TYPEs. Such an RR cannot be converted to a type-
specific text format, compressed, or otherwise handled in a type-
specific way.
In the case of a type whose RDATA format is class specific, an RR is
considered to be of unknown type when the RDATA format for that
combination of type and class is not known.
3. Transparency
To enable new RR types to be deployed without server changes, name
servers and resolvers MUST handle RRs of unknown type transparently.
That is, they must treat the RDATA section of such RRs as
unstructured binary data, storing and transmitting it without change
[RFC 1123].
To ensure the correct operation of equality comparison (section 6)
and of the DNSSEC canonical form (section 7) when an RR type is known
to some but not all of the servers involved, servers MUST also
exactly preserve the RDATA of RRs of known type, except for changes
due to compression or decompression where allowed by section 4 of
this memo. In particular, the character case of domain names that
are not subject to compression MUST be preserved.
4. Domain Name Compression
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 [RFC 1123] without defining the
term "well-known"; it is hereby specified that only the RR types
defined in [RFC 1035] are to be considered "well-known".
Gustafsson Standards Track