Bind 9 Administrator Reference Manual

Chapter 6. BIND 9 Configuration Reference
A for the IN class, a 32 bit IP address.
A6 maps a domain name to an IPv6 address, with a provision for
indirection for leading "prefix" bits.
CNAME a domain name.
DNAME provides alternate naming to an entire subtree of the domain
name space, rather than to a single node. It causes some suffix of
a queried name to be substituted with a name from the DNAME
record’s RDATA.
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 fully qualified domain name.
PTR a fully qualified domain name.
SOA several fields.
The owner name is often implicit, rather than forming an integral part of the RR. For example, many
nameservers 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.
6.3.1.2. 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 nameserver or resolver. In the examples provided in RFC 1034, a
style similar to that used in master files was employed 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.
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