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DNS Master File Format
(Page 3 of 4)
Master File Directives
In addition to resource records,
most master file implementations also support the use of directives.
These are commands that specify certain important pieces of information
to guide how the master file is to be interpreted. Three of the most
common directives are:
Syntax Rules for Master Files
- $ORIGIN: Specifies the domain name that
is appended to unqualified specifications; this is the base
used to convert PQDNs to FQDNs. For example, if the origin is xyzindustries.com.,
then a PQDN such as sales will be interpreted as sales.xyzindustries.com..
Once defined, the origin can be referenced by just using @
in place of a name, as we will see in the example at the end of this
- $TTL: Specifies the default Time To
Live value to be used for any resource records that do not specify
a TTL value in the record itself. (This value was formerly specified
by the Minimum field in the Start Of Authority record.)
- $INCLUDE: Allows one master file to include
the contents of another. This is sometimes used to save the duplication
of certain entries that are common between zones.
There are a few other syntax rules
for DNS master files, some of which are intended to save further time
or energy on the part of administrators:
- Multiple-Record Shorthand: If multiple
consecutive records pertain to the same domain, the <domain-name>
is specified for the first one, and can be then be left blank for the
subsequent ones. The server will assume that any resource records without
a <domain-name> indicated apply to the last <domain-name>
- Comments: A semicolon (;)
marks a comment. Any text from the semicolon until the end of the line
- Escape Character: A backslash (\)
is used to escape the special meaning of a character. For
example, a double-quote (") is used to delimit text strings; a literal
double-quote character is indicated by a backslash-double-quote combination
- White Space: Tabs and spaces are used
as delimiters and blank lines are ignored. For readability, most smart
administrators indent using tabs to make more clear which records belong
with which names, and group records using blank lines and comments.
- Case: Like DNS domain names, master file
entries are case-insensitive.
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Version 3.0 - Version Date: September 20, 2005
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