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.

3.6.1. Textual expression of RRs

RRs are represented in binary form in the packets of the DNS protocol,
and are usually represented in highly encoded form when stored in a name
server or resolver.  In this memo, we adopt a style similar to that used




 
RFC 1034             Domain Concepts and Facilities        November 1987


in master files in order to show the contents of RRs.  In this format,
most RRs are shown on a single line, although continuation lines are
possible using parentheses.

The start of the line gives the owner of the RR.  If a line begins with
a blank, then the owner is assumed to be the same as that of the
previous RR.  Blank lines are often included for readability.

Following the owner, we list the TTL, type, and class of the RR.  Class
and type use the mnemonics defined above, and TTL is an integer before
the type field.  In order to avoid ambiguity in parsing, type and class
mnemonics are disjoint, TTLs are integers, and the type mnemonic is
always last. The IN class and TTL values are often omitted from examples
in the interests of clarity.

The resource data or RDATA section of the RR are given using knowledge
of the typical representation for the data.

For example, we might show the RRs carried in a message as:

    ISI.EDU.        MX      10 VENERA.ISI.EDU.
                    MX      10 VAXA.ISI.EDU.
    VENERA.ISI.EDU. A       128.9.0.32
                    A       10.1.0.52
    VAXA.ISI.EDU.   A       10.2.0.27
                    A       128.9.0.33

The MX RRs have an RDATA section which consists of a 16 bit number
followed by a domain name.  The address RRs use a standard IP address
format to contain a 32 bit internet address.

This example shows six RRs, with two RRs at each of three domain names.

Similarly we might see:

    XX.LCS.MIT.EDU. IN      A       10.0.0.44
                    CH      A       MIT.EDU. 2420

This example shows two addresses for XX.LCS.MIT.EDU, each of a different
class.

3.6.2. Aliases and canonical names

In existing systems, hosts and other resources often have several names
that identify the same resource.  For example, the names C.ISI.EDU and
USC-ISIC.ARPA both identify the same host.  Similarly, in the case of
mailboxes, many organizations provide many names that actually go to the
same mailbox; for example Mockapetris@C.ISI.EDU, Mockapetris@B.ISI.EDU,




 
RFC 1034             Domain Concepts and Facilities        November 1987


and PVM@ISI.EDU all go to the same mailbox (although the mechanism
behind this is somewhat complicated).

Most of these systems have a notion that one of the equivalent set of
names is the canonical or primary name and all others are aliases.

The domain system provides such a feature using the canonical name
(CNAME) RR.  A CNAME RR identifies its owner name as an alias, and
specifies the corresponding canonical name in the RDATA section of the
RR.  If a CNAME RR is present at a node, no other data should be
present; this ensures that the data for a canonical name and its aliases
cannot be different.  This rule also insures that a cached CNAME can be
used without checking with an authoritative server for other RR types.

CNAME RRs cause special action in DNS software.  When a name server
fails to find a desired RR in the resource set associated with the