Table of Contents
Resource Records
Resource Records comprise the data within a DNS zone. Although there is no fixed limit to the number of Resource Records a zone may own, in general, a zone may own one or more Resource Records of a given type, or none. There are some exceptions depending on the types involved.
All Resource Records have the following required entries:
- Name---The name (host) that owns the record, such as example.com.
- Class (not required for all formats)---DNS supports only the IN (Internet) class of record.
- TTL (Time To Live)---Amount of time the record can be stored in cache. It is expressed in seconds. If you do not include a TTL, Network Registrar uses the zone default TTL, defined in the SOA resource record.
- Type---The type of the record. There are many types defined by various RFCs, although 10 or fewer are in common use.
- Record data---Data types whose format and meaning varies with record type.
The following are descriptions of the most common Resource Records.
The Address (A) record provides the name-to-address mapping for the zone. It contains an IP address in dotted decimal form. There must be at least one A record for each host address.
The name specified is the host name expressed as FQDN (ns.example.com.). Note the trailing dot. The data is an IP address.
The presence of one or more A records indicates that the owner's name is the name of a host.
The Canonical Name (CNAME) record is used for aliases or nicknames. The data portion is the official or canonical name. You cannot have any other Resource Records associated with a CNAME.
Aliases are useful when you want the outside world to know a single, easily remembered name. You can also use aliases when a host changes its name. In that case, make sure that you have a CNAME pointer so that when people use the original name it can be resolved to the newer name.
The Host Info (HINFO) record provides information about a particular host. The data contains a description of the hardware and software. The hardware description contains the name of the manufacturer and the model number. The software description contains the name of the operating system.
The Mail Exchange (MX) record specifies where the mail for a domain name should be delivered. You can have several MX records for a single domain name, and they can be ranked in order of preference.
The Name Server (NS) record lists the name of the machine that provides domain service for the particular domain. Machines that provide name service do not have to reside in the named domain. For each domain you must have at least one NS record. NS records for a domain must exist in both the zone that delegates the domain and in the domain itself. You must also have a corresponding A record mapped to the hostname specified in the data field. The name is the name of the zone (for example, myexample.com) and the data is the name of the host machine on which the name server is running.
NS record names must have an equivalent A record (that is, they cannot point to an alias).
The Pointer (PTR) record enables you to point to some other location in the domain tree. They are used for reverse mapping, specifically in the IN-ADDR.ARPA zones for translation of addresses to names. PTRs use official names not aliases. The name in a PTR record is the local IP address portion of the reverse name
The Start of Authority (SOA) record designates the start of a zone. Every zone must have a single SOA record. The SOA record indicates the name of the machine with master data (the primary name server). The record also contains the e-mail address of the zone's administrator. It informs secondary name servers about the timing of zone transfers.
This record contains:
- The name of the zone
- The name of the primary server
- The e-mail address of the person responsible for the zone
- The serial number of the zone file
- The secondary refresh time
- The secondary retry time
- The secondary expire time
- The minimum Time To Live (TTL)
The SRV record allows administrators to use several servers for a single domain, to move services from host to host with little difficulty, and to designate some hosts as primary servers for a service and others as backups. Clients ask for a specific service or protocol for a specific domain and receive the names of any available servers. For more information, refer to RFC 2052bis.
The Text (TXT) record contains any string of less than or equal to 256 characters that can contain any type of information. The name is the hostname (equivalent A record) and data.
The Well Known Services (WKS) record lists the Well Known Services a host provides on a particular IP protocol. The common protocols are TCP and UDP. The common services are TIME, TELNET, FTP, or SMTP.
Table A-1 lists all the Resource Record types Network Registrar supports.
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Table A-1: Resource Records
| Name
| Data Type
| Description
|
|---|
A
| 1
| Host address
|
AAAA
| 28
| SIPv6 Address
|
AFSDB
| 18
| AFS cell database
|
CNAME
| 5
| Canonical name
|
HINFO
| 13
| Host information
|
ISDN
| 20
| ISDN calling address
|
LOC
| 29
| Location information
|
MB
| 7
| Mailbox domain name
|
MG
| 8
| Mail group member
|
MINFO
| 14
| Mailbox information
|
MR
| 9
| Mail rename name
|
MX
| 15
| Mail routing information
|
NS
| 2
| Authoritative server
|
NSAP
| 22
| NSAP address
|
PTR
| 12
| Domain name pointer
|
PX
| 26
| X.400 mail mapping
|
RP
| 17
| Responsible person
|
RT
| 21
| Router
|
SOA
| 6
| Start of authority
|
SVR
| 33
| Server selection
|
TXT
| 16
| Text strings
|
WKS
| 11
| Well known service
|
X25
| 19
| X.25 calling address
|
Posted: Thu Nov 18 13:45:46 PST 1999
Copyright 1989-1999©Cisco Systems Inc.
