This format was originally used by the Berkeley Internet Name Domain (BIND) software package, but has been widely adopted by other DNS server software – though some of them (e.g. NSD, PowerDNS) are using the zone files only as a starting point to compile them into database format, see also Microsoft DNS with Active Directory-database integration.
A zone file is a sequence of line-oriented entries, each of which is either a directive or a text description that defines a single resource record (RR).
An entry is composed of fields separated by any combination of white space (tabs and spaces), and ends at a line boundary except inside a quoted string field value or a pair of enclosing formatting parentheses.
The ttl field specifies the number of seconds after which a caching client must discard the record and perform a new resolution operation to obtain fresh information.
It may be omitted, in which case the resulting value will be set from the default TTL (if defined) or from the preceding record.
in the above example) are fully qualified while those that do not end with a full stop are relative to the current origin (which is why www in the above example refers to www.example.com).
Some server software automatically configures resource records for specially recognized domains or hostnames, such as localhost, but a customized zone master file may be used.