Obsolete RRs

Because DNS is ubiquitous, it is very tempting to use it for many things. Several uses have been suggested, and the following are those uses that have already proven unsuitable.

EID (Endpoint Identifier)

Defined in an Internet Draft, expired as of April 1996.

See the NIMROD record type for further information.

GPOS (Geographical Position)

Defined in RFC 1712.

The RFC has been withdrawn, and the LOC (location) record was defined in RFC 1876. The GPOS record lacks the size and precision fields of the LOC record.

ISDN (ISDN Address)

Defined in RFC 1183, section 3.1.

The ISDN, X25, and RT RRs were meant to hold routing information. They are classified as experimental. In RFC 1183, they appear under the heading "Route Binding." It does not appear to be in use and is not documented in anything but the RFC.

MB (Mailbox)

Defined in RFC 1035:

 domainname     MB host 

The named host holds the mailbox named by the domain name. See the MG description for more information.

MD (Mail Destination)

Defined in RFC 1035, section 3.3.4. See RFC 883 for the full, now historical, description:

 domainname     MD host 

This record used to name hosts with mail agents willing to handle and deliver mail for the domain. This, along with the MF record, was replaced by the MX record. An MTA would issue a MAILA query and receive the MF and MD records associated with the specified name. As described in RFC 973, the MF and MD records had several problems, including problems with caching. A special query, MAILA, was used to retrieve both MD and MF records.

Unfortunately, cached MD and MF records could originate with MD or MF queries, so the cache server could not know whether it had the complete set of MD and MF records when a MAILA query arrived. Additionally, the mail sender does not need to know whether the agent will forward or deliver the mail so the distinction is redundant.

MF (Mail Forwarder 883)

Defined in RFC 1035, section 3.3.4:

 domainname     MF host 

This record defined a host willing to store and forward mail to the named domain, in case the MD host was unavailable. See the MD record for more information.

MG (Mail Group Member)

Defined in RFC 1035:

 domainname     MG mgname 

The MG, MR, and MB records make a triad of records described under the heading "Mailbox biding" in RFC 883. They are not really documented in RFC 1035. The MTA would issue a MAILB query. The query could fail, in which case a MAILA query would be issued to find MD and MF records (see the previous description of the MD record). If an MR record was returned, the mailbox name was rewritten as specified by the MR record. If an MB record was retrieved, mail delivery to the named host proceeded. If MG RRs were returned, each mailbox named received a copy of the mail.

The MAILB query had the same problems with caching as the MAILA query, which is explained in the MD RR description and which contributed to replacing MD and MF with the MX RR. These records are not in any wide use and are not implemented by any of the major UNIX-based MTAs. As with the MINFO record, they are all officially classified as experimental, but I'm risking calling them obsolete.

MINFO (Mailbox or Mail-List Info)

Defined in RFC 1035:

 domainname     MINFO rmailbx emailbx 

This record specifies additional information about the mailing list named by the domain name in particular, the request (or administrative) mailbox and the error mailbox. This record is officially marked as experimental, but it is not used or implemented by any mail user agent (MUA) I know or by any popular MTA. It has been ignored since 1984, and the advent of SPAM makes wide implementation unlikely, so I'll risk calling it obsolete. You need to read RFC 883 to find a proper description of this RR.

MR (Mail Rename)

Defined in RFC 1035:

 domainname     MR mailbox 

Specifies that the mailbox named by the domain name should be replaced by the mailbox named by the record. See the description of the MG record for further explanation.

NIMLOC (NIMROD Locator)

Defined in an Internet Draft, expired as of April 1996.

NIMROD is a Internet routing architecture. The two RRs proposed in the Internet Draft are not even implemented in BIND. The document is not easily understandable without knowledge of NIMROD.

NSAP_PTR (NSAP Variant of PTR Record)

Defined in RFC 1183.

This was meant to be used for reverse lookups of NSAP addresses. In RFC 1706 this was discontinued and the PTR record used instead.

WKS (Well Known Service)

Defined in RFC 1035:

 domainname     WKS 32-bit-ip-address protocol service-list 

This record was used to publish the services a host offered. This was before the practice of giving hosts names or aliases based on their function became widespread (such as one single penguin.bv host answering to both the names ns.penguin.bv and ftp.penguin.bv). Therefore, by examining the WKS records of a zone, you could, in theory, locate the FTP server. No one ever did that, though, and the record fell into disuse. The new SRV RR now enables service location in a much more sensible and powerful manner.

The protocol field must be either TCP or UDP. The service list must be a list of IANA approved TCP or UDP services, which are the services listed in your /etc/services file. The official list is found in the latest "Assigned Numbers" document, known as STD 2. Currently that corresponds to RFC 1700.