Bind 9 Administrator Reference Manual

Chapter 6. BIND 9 Configuration Reference
RR TTLs Each RR can have a TTL as the second field in the RR, which will control
how long other servers can cache the it.
All of these TTLs default to units of seconds, though units can be explicitly specified, for example,
1h30m.
6.3.4. Inverse Mapping in IPv4
Reverse name resolution (that is, translation from IP address to name) is achieved by means of the
in-addr.arpa domain and PTR records. Entries in the in-addr.arpa domain are made in least-to-most
significant order, read left to right. This is the opposite order to the way IP addresses are usually written.
Thus, a machine with an IP address of 10.1.2.3 would have a corresponding in-addr.arpa name of
3.2.1.10.in-addr.arpa. This name should have a PTR resource record whose data field is the name of the
machine or, optionally, multiple PTR records if the machine has more than one name. For example, in
the [example.com] domain:
$ORIGIN 2.1.10.in-addr.arpa
3 IN PTR foo.example.com.
Note: The $ORIGIN lines in the examples are for providing context to the examples only-they do not
necessarily appear in the actual usage. They are only used here to indicate that the example is
relative to the listed origin.
6.3.5. Other Zone File Directives
The Master File Format was initially defined in RFC 1035 and has subsequently been extended. While
the Master File Format itself is class independent all records in a Master File must be of the same class.
Master File Directives include $ORIGIN, $INCLUDE, and $TTL.
6.3.5.1. The $ORIGIN Directive
Syntax: $ORIGIN domain-name [ comment]
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