On 2000-09-05 at 11:31 -0400, Richard Welty gifted us with:
> RFC 1123, Requirements for Internet Hosts -- Application and Support,
[snip]
1123 Requirements for Internet hosts - application and support. R.T.
Braden. Oct-01-1989. (Format: TXT=245503 bytes) (Updates RFC0822)
(Updated by RFC2181) (Also STD0003) (Status: STANDARD)
^^^^^^^^^^^^^^^^^^
2181 Clarifications to the DNS Specification. R. Elz, R. Bush. July
1997. (Format: TXT=36989 bytes) (Updates RFC1034, RFC1035, RFC1123)
(Updated by RFC2535) (Status: PROPOSED STANDARD)
(RFC 2535 is Security Extensions)
Proposed Standard is admittedly less than Standard, but it's fairly
helpful to refer to the latest Standards Track RFC.
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11. Name syntax
Occasionally it is assumed that the Domain Name System serves only
the purpose of mapping Internet host names to data, and mapping
Internet addresses to host names. This is not correct, the DNS is a
general (if somewhat limited) hierarchical database, and can store
almost any kind of data, for almost any purpose.
The DNS itself places only one restriction on the particular labels
that can be used to identify resource records. That one restriction
relates to the length of the label and the full name. The length of
any one label is limited to between 1 and 63 octets. A full domain
name is limited to 255 octets (including the separators). The zero
length full name is defined as representing the root of the DNS tree,
and is typically written and displayed as ".". Those restrictions
aside, any binary string whatever can be used as the label of any
resource record. Similarly, any binary string can serve as the value
of any record that includes a domain name as some or all of its value
(SOA, NS, MX, PTR, CNAME, and any others that may be added).
Implementations of the DNS protocols must not place any restrictions
on the labels that can be used. In particular, DNS servers must not
refuse to serve a zone because it contains labels that might not be
acceptable to some DNS client programs. A DNS server may be
configurable to issue warnings when loading, or even to refuse to
load, a primary zone containing labels that might be considered
questionable, however this should not happen by default.
Note however, that the various applications that make use of DNS data
can have restrictions imposed on what particular values are
acceptable in their environment. For example, that any binary label
can have an MX record does not imply that any binary name can be used
as the host part of an e-mail address. Clients of the DNS can impose
whatever restrictions are appropriate to their circumstances on the
values they use as keys for DNS lookup requests, and on the values
returned by the DNS. If the client has such restrictions, it is
solely responsible for validating the data from the DNS to ensure
that it conforms before it makes any use of that data.
See also [RFC1123] section 6.1.3.5.
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