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
alias
HINFO identifies the CPU and OS used by a host
MX identifies a mail exchange for the
domain. See [RFC-974 for details.
NS
the authoritative name server for the domain
PTR
a pointer to another part of the domain name space
SOA
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
domain.
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.
