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Resource Records Connected: An Internet Encyclopedia
Resource Records

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Up: Programmed Instruction Course
Up: Section 2 - Domain Naming
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Next: Introduction to DNS

Resource Records

Resource Records Resource records are the data elements that define the structure and content of the domain name space. All DNS operations are ultimately formulated in terms of resource records.


Resource Records (RRs) are the DNS data records. Their precise format is defined in RFC 1035 §3.2.1. The most important fields in a resource record are Name, Class, Type, and Data. Name is a domain name, Class and Type are two-byte integers, and Data is a variable-length field to be interpreted in the context of Class and Type. Almost all Internet applications use Class 1, the Internet Class. For the Internet Class, many standard Types have been defined. The complete list can be found in the current Assigned Numbers RFC. Only those most important to DNS operation are shown here.

Address (A) RRs

Address (A) records match domain names to IP address, and are both the most important and the most mundane aspect of DNS. See RFC 1035 §3.4.1 for a more detailed description of the A RR, though there is really very little to describe. The data section consists entirely of a 32-bit IP address. Most DNS operations are queries for A records matching a given domain name. Since hosts can have multiple IP addresses, corresponding to multiple physical network interfaces, so it is permissible for multiple A records to match a given domain name. Normally, only the first one is used, so chose a host's most reliable IP address and put it first when constructing name server databases.

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    ADDRESS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where: 

ADDRESS         A 32 bit Internet address.

Canonical Name (CNAME) RR

Canonical Names (CNAMEs) are the DNS equivalent of aliases or symbolic links. The data field contains another fully-qualified DNS name, which should be used as the target of another DNS operation to acquire the desired information. However, a second lookup is rarely required, since most name servers will provide the additional records as part of the reply. See RFC 1035 §3.3.1 for a more detailed description of the CNAME RR.

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     CNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where: 

CNAME           A <domain-name> which specifies the canonical or primary
                name for the owner.  The owner name is an alias.

Pointer (PTR) RR

Pointers (PTRs) are like CNAMEs in their format - the data area contains a domain name. The difference between CNAMEs and PTRs is purely one of semantics. A CNAME specifies an alias, a PTR merely points to another location in the domain name space. The most important use of PTRs is to construct the in-addr.arpa domain, used to convert IP addresses to DNS names (the reverse of the normal process). See RFC 1035 §3.3.12 for a more detailed description of the PTR RR, and RFC 1035 »3.5 for a explanation of the in-addr.arpa domain.

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   PTRDNAME                    /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where: 

PTRDNAME        A <domain-name> which points to some location in the
                domain name space.

Start of Authority (SOA) RR

A Start of Authority SOA RR marks the beginning of a DNS zone, and is typically seen as the first record in a name server for that domain. The encyclopedia's discussion of name servers explains the various fields. See RFC 1035 §3.3.13 for a more detailed description of the SOA RR.

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     MNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                     RNAME                     /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    SERIAL                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    REFRESH                    |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     RETRY                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    EXPIRE                     |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                    MINIMUM                    |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where: 

MNAME           The <domain-name> of the name server that was the
                original or primary source of data for this zone.

RNAME           A <domain-name> which specifies the mailbox of the
                person responsible for this zone.

SERIAL          The unsigned 32 bit version number of the original copy
                of the zone.  Zone transfers preserve this value.  This
                value wraps and should be compared using sequence space
                arithmetic.

REFRESH         A 32 bit time interval before the zone should be
                refreshed.

RETRY           A 32 bit time interval that should elapse before a
                failed refresh should be retried.

EXPIRE          A 32 bit time value that specifies the upper limit on
                the time interval that can elapse before the zone is no
                longer authoritative.

MINIMUM         The unsigned 32 bit minimum TTL field that should be
                exported with any RR from this zone.

Name Server (NS) RR

An NS RR marks the beginning of a DNS zone and supplies the domain name of a name server for that zone. It is typically seen in two places - at the top of a zone, just after the SOA; and at the start of a subzone, where an NS (and often a paired A) are all that is required to perform delegation. See RFC 1035 §3.3.11 for a more detailed description of the NS RR.

    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    /                   NSDNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where: 

NSDNAME         A <domain-name> which specifies a host which should be
                authoritative for the specified class and domain.


Next: Introduction to DNS

Connected: An Internet Encyclopedia
Resource Records

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