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DNS Master File Format
(Page 2 of 4)
DNS Common Master File Record Format
Just as all resource records are
stored internally using a common field format, they also use a common
master file format. Each record normally appears on a separate line
of the file. This format is as follows, with optional fields shown in
<domain-name> [<ttl>] <class> <type> <rdata>
The fields are as follows:
- <domain-name>: A DNS domain name,
which may be either a fully-qualified domain name (FQDN) or a partially-qualified
name (PQDN). See below.
- <ttl>: A Time To Live value,
in seconds, for the record. If omitted, the default TTL value
for the zone is used. In fact, most resource records do not have a specified
TTL, just using the default provided by the Start Of Authority
- <class>: The resource record class.
For modern DNS this field is optional, and defaults to IN
- <type>: The resource record type,
specified using a text code such as A or NS,
not the numeric code.
- <rdata>: Resource record data, which
is a set of space-separated entries that depends on the record type.
The <rdata> can
be either a single piece of information or a set of entries, depending
on the record type. In the case of longer record types, especially the
Start Of Authority record, multiple entry <rdata>
fields are spread over several lines and enclosed in parentheses; the
parentheses make all the entries act as if they were on a single line.
Note that if the <ttl> field is present, the order
of it and the <class> field may be switched; this
causes no problems because one is a number and the other text (IN).
Use and Interpretation of Partially-Qualified Domain Names
Domain names may be mixed between
and PQDNs. Partially-qualified names are
used to make master files faster to create and more readable, by cutting
down on the common parts of names; they are sort of the human
equivalent of DNS message compression. A FQDN is shown as a full
domain name ending in a dot (.) to represent the DNS name
tree root. A PQDN is given as just a partial name with no root, and
is interpreted as a FQDN by the software reading the master file (see
the $ORIGIN directive below for more.)
It is important to remember the trailing
dot to mark FQDNs; if the origin is xyzindustries.com and
in its zone file the name bigisp.net appears, the server
will read this as bigisp.net.xyzindustries.comprobably
not what you want. Also, e-mail addresses, such as the <r-name>
field in the SOA record, have the @ of the e-mail
address converted to a dot, following the standard DNS convention.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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