Connected: An Internet Encyclopedia
3.6. Resource Records

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3.6. Resource Records

3.6. Resource Records

A domain name identifies a node. Each node has a set of resource information, which may be empty. The set of resource information associated with a particular name is composed of separate resource records (RRs). The order of RRs in a set is not significant, and need not be preserved by name servers, resolvers, or other parts of the DNS.

When we talk about a specific RR, we assume it has the following:

owner           which is the domain name where the RR is found.

type            which is an encoded 16 bit value that specifies the type
                of the resource in this resource record.  Types refer to
                abstract resources.

                This memo uses the following types:

                A               a host address

                CNAME           identifies the canonical name of an

                HINFO           identifies the CPU and OS used by a host

                MX              identifies a mail exchange for the
                                domain.  See [RFC-974 for details.

                the authoritative name server for the domain

                a pointer to another part of the domain name space

                identifies the start of a zone of authority]

class           which is an encoded 16 bit value which identifies a
                protocol family or instance of a protocol.

                This memo uses the following classes:

                IN              the Internet system

                CH              the Chaos system

TTL             which is the time to live of the RR.  This field is a 32
                bit integer in units of seconds, an is primarily used by
                resolvers when they cache RRs.  The TTL describes how
                long a RR can be cached before it should be discarded.

RDATA           which is the type and sometimes class dependent data
                which describes the resource:

                A               For the IN class, a 32 bit IP address

                                For the CH class, a domain name followed
                                by a 16 bit octal Chaos address.

                CNAME           a domain name.

                MX              a 16 bit preference value (lower is
                                better) followed by a host name willing
                                to act as a mail exchange for the owner

                NS              a host name.

                PTR             a domain name.

                SOA             several fields.

The owner name is often implicit, rather than forming an integral part of the RR. For example, many name servers internally form tree or hash structures for the name space, and chain RRs off nodes. The remaining RR parts are the fixed header (type, class, TTL) which is consistent for all RRs, and a variable part (RDATA) that fits the needs of the resource being described.

The meaning of the TTL field is a time limit on how long an RR can be kept in a cache. This limit does not apply to authoritative data in zones; it is also timed out, but by the refreshing policies for the zone. The TTL is assigned by the administrator for the zone where the data originates. While short TTLs can be used to minimize caching, and a zero TTL prohibits caching, the realities of Internet performance suggest that these times should be on the order of days for the typical host. If a change can be anticipated, the TTL can be reduced prior to the change to minimize inconsistency during the change, and then increased back to its former value following the change.

The data in the RDATA section of RRs is carried as a combination of binary strings and domain names. The domain names are frequently used as "pointers" to other data in the DNS.

Next: 3.6.1. Textual expression of RRs

Connected: An Internet Encyclopedia
3.6. Resource Records