Directives begin with the dollar sign character ($) followed by the name of the directive. They usually appear at the top of the zone file.
The following are commonly used directives:
$INCLUDE — Configures named to include another zone file in this zone file at the place where the directive appears. This allows additional zone settings to be stored apart from the main zone file.
$ORIGIN — Appends the domain name to unqualified records, such as those with the hostname and nothing more.
For example, a zone file may contain the following line:
$ORIGIN example.com.
Any names used in resource records that do not end in a trailing period (.) are appended with example.com.
The use of the $ORIGIN directive is unnecessary if the zone is specified in /etc/named.conf because the zone name is used as the value for the $ORIGIN directive by default.
$TTL — Sets the default Time to Live (TTL) value for the zone. This is the length of time, in seconds, a zone resource record is valid. Each resource record can contain its own TTL value, which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to proliferate resource record changes.