Types of DNS Records

The following Domain Name System (DNS) record types are supported within the MSU central domain name system. Each record type is described below.

Host (A and PTR) Records

Host records are the most common DNS record type within the MSU system. Each host record will generate an A record, which maps a domain name to an IP address. The first Host record with a given IP address will also generate a PTR record, which maps the IP address back to a domain name. Host records can also generate CNAME and MX records. (See more details in the description below.)

The following format is used for Host records. This is based on the Unix /etc/hosts format, with one line per host, as follows: ip-address domain-name alias-domain-name […] # opsys: cpu: {keyword} comments

“IP-address” is a four-part dotted IP address, as described above. “Domain-name” is a fully-qualified domain name, also as described above.

One or more aliases may be listed on the “Host” record. Aliases may also be listed on separate CNAME records. There is no functional difference in using one form or the other.

The punctuation must be as shown; the “#” separates the host name and any aliases from the comments. The “opsys” (operating system) and “cpu” (computer system type) are optional, but must be followed by colons if included. Colons should not be used for other purposes in the comments. One or more optional keywords enclosed in { }, may be provided; see the description below.

For example: office123.unit.msu.edu # Main office, room 213

Host Record Comments

It can be useful to provide a comment on a Host Record which includes the room number, person’s name, or a short description. Including the CPU (system hardware) type and the operating system has been depreciated.

If included, the CPU type (“cpu”) is typically a generic system type, such as PC, Mac, Sun, or RS/6000. It can also include a specific system model (e.g. “Gateway 386/33”, “Mac IIsi”, “Sun 4/230”, “RS/6000-350”), although this is discouraged, since the model can change fairly often, and it requires more diligence to maintain an accurate list.

The operating system (opsys) is the name of the base system software, such as Windows XP and other Windows variants, MAC-OSX, Linux, or Solaris.

One or more special keywords may be listed in the comments field on a Host Record. The most common keywords are:

• {mx} Generate self-referential MX record. This is the equivalent of providing the MX record:

  MX: mail-domain-name: 0: domain-name
• {shuffle} Allows multiple Host Records to be specified for a given domain-name. The IP addresses on each Host Record will be delivered on a “round-robin” basis to requesting hosts.

CNAME (Alias) Records

CNAME records are of the form: CNAME: alias-domain-name: domain-name

It is important to note that the “alias-domain-name” as given on a CNAME record must NOT appear on any other DNS record. The CNAME record will map any and all requested record types (e.g. A, MX, TXT) to the corresponding record for the target domain name.

MX (Mail Exchange) Records

Mail Exchange (MX) records provide a mapping between a mail domain name and the mail server or servers which handle incoming mail for the domain.

“Mail-domain-name” is the right hand side of an email address, i.e., user@dept.msu.edu. The “mail-domain-name” may be the same as a host (A) domain-name, if desired. It may NOT be the same as a CNAME alias-domain-name. The server for a mail-domain-name may be different from the server for the identical host domain-name. This is useful, for example, to direct mail for “user@dept.msu.edu” (mail-domain-name “dept.msu.edu”) to a mail server, while directing web access for http://dept.msu.edu/ (host domain-name “dept.msu.edu”) to a separate server.

It is strongly recommended that one or more MX records be provided for all systems which receive external SMTP mail.

Mail exchange records records are formatted as follows: MX: mail-domain-name: precedence: domain-name

For example:

  MX: msu.edu: 10: sys23.mail.msu.edu
  MX: msu.edu: 20: mail.msu.edu

Multiple MX records may be specified for a single mail domain. The “precedence” is a decimal number, with lower values indicating higher precedence (will be tried first). If multiple MX records have the same precedence, each listed system will be tried in a “round-robin” fashion.

MX records may also be specified with the {mx} keyword on the Host (A) record, for the case where the mail-domain-name is identical to the host’s own domain-name.

TXT (Text) Records

The TXT record may be used to place any desired text string into the domain name system. TXT records are seldom used.

SRV (Server) Records

SRV (Server) records are most often used in conjunction with Windows 2000 or XP server systems. Server records are generally maintained automatically within the Windows 2000 Active Directory system, but are sometimes applicable within the central campus DNS system.

The SRV record is formatted as follows: SRV: service-domain-name: priority weight port server-domain-name

Priority, weight, and port are all numeric values; server-domain-name is the domain name of the server, as specified in a separate host (A) record.

Submitting DNS Updates

Send your DNS updates periodically or as additions and changes occur. Updates will be made to the campus database and Internet domain name service periodically, usually within one to two working days of receipt. Submit an IP Address or host name update under DNS Services.

DNS updates should be sent as plain text only. DNS updates sent as spreadsheet or word processor attachments, or HTML encoded email messages, may be delayed or rejected. Each entry should be complete on a single line – use long lines rather than “wrapped” lines if at all possible.

If you have a time-critical host name or IP address change, be sure to notify MSU IT at least one working day in advance, so that a suitable time can be arranged for the change.

Please send ONLY the changes (new entries, changes, and/or deletions), rather than a complete replacement listing each time. If you have mixed additions, changes, and/or deletions, preface each entry or set of entries with a SEPARATE line indicating add, change, or delete.

For changes, the new entry should be in the same format as described above for additions; the old entries need not be included as long as it is clear what is changing (IP address, host name, or comments).

For deletions, it is not necessary to include the entire host entry (although you may do so) – either the name or IP address is generally sufficient.