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DNS Name Server Data Storage: Resource Records and Classes
(Page 3 of 4)
Common Resource Records Types
The main DNS standards, RFC 1034
and 1035, defined a number of resource record types. Over time, the
list has changed, with new RR types being created in subsequent standards
and the use of others changed. Like other Internet parameters, the list
of DNS RR types is maintained in a file at IANA.Also
like other Internet parameters, there are in fact several dozen defined
RRs in DNS, but only a few are commonly used; others are now obsolete,
used for special purposes or experimental in nature.
Table 166
summarizes the most important resource record types. For each, I have
shown the numeric Type value for the record, which is used to
identify the resource record type in message exchanges, and also the
text code used for the RR in master files. I have also provided a brief
description of each.
Table 166: Summary Of Common DNS Resource Records
RR Type
Value
|
RR Text
Code
|
RR Type
|
Description
|
1
|
A
|
Address
|
Contains a 32-bit IP address.
This is the meat and potatoes of DNS, since it is where
the address of a node is stored for name resolution purposes.
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2
|
NS
|
Name
Server
|
Specifies the
name of a DNS name server that is authoritative for the zone. Each zone
must have at least one NS record that points to its primary name server,
and that name must also have a valid A (Address) record.
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5
|
CNAME
|
Canonical
Name
|
This resource record is used
to allow aliases to be defined that point to the real name of a node.
The CNAME record provides a mapping between this alias and the
canonical (real) name of the node. The CNAME resource
record is commonly used to hide changes in the internal DNS structure
from outside users, by letting them use an unchanging alias while the
internal names are modified based on the needs of the organization.
See
the topic on name resolution for an example.
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6
|
SOA
|
Start
Of Authority
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The SOA resource
record is used to mark the start of a DNS
zone and provide important information
about it. Every zone must have exactly one SOA record, which contains
details such as the name of the zone, its primary (master) authoritative
server name, and technical details such as the e-mail address of its
administrator and parameters for how often slave (secondary) name servers
are updated.
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12
|
PTR
|
Pointer
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Provides a pointer to another
location in the name space. These records are best known for their use
in reverse
resolution through the IN-ADDR.ARPA domain.
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15
|
MX
|
Mail
Exchange
|
Specifies the
location (device name) that is responsible for handling
e-mail sent to the domain.
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16
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TXT
|
Text String
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Allows arbitrary additional text
associated with the domain to be stored.
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All of these resource records
are used in different ways to define zones and devices within them,
and then permit name resolution and other functions to take place. We'll
see how they are used in more detail in the
section on name resolution. You can also
find a more lengthy description of some of them in the
topic devoted to resource record field formats.
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Version 3.0 - Version Date: September 20, 2005
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