A Domain Name System (DNS) zone file is a text file that describes a DNS zone. A DNS zone is a subset, often a single domain, of the hierarchical domain name structure of the DNS.
The zone file contains mappings between domain names and IP addresses and other resources,
organized in the form of text representations of resource records (RR).
A zone file may be either a DNS master file, authoritatively describing a zone, or it may be used to list the contents of a DNS cache.
The format of a zone file is defined in RFC 1035 (section 5) and RFC 1034 (section 3.6.1).
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.
A zone file is a sequence of entries for resource records. Each line is a text description that defines a single resource record (RR).
The description consists of several fields separated by white space (spaces or tabs).
The first field is the domain name, called the owner of the record, but if left blank, defaults to the owner of the previous record.
The domain name is followed by the
- time to live field,
- the record class,
- the record type, and
- one or possibly several fields of type-specific data.
The time-to-live field specifies the time after which a domain name client must discard the record and perform a new resolution operation to obtain fresh information.
The record class indicates the namespace of the record's information. The most commonly used namespace is that of the Internet, indicated by parameter IN.
The type of the resource record is a short abbreviation for the type of information stored in the record and determines the number of parameters needed.
The type also provides the name of each record. For example, an address record, having the abbreviation A for IPv4 and AAAA for IPv6, maps the domain name in the first field to an IP address in the fourth field,
and a mail exchanger record (type MX) specifies the Simple Mail Transfer Protocol (SMTP) mail host for a domain.
Resource records may occur in any order in a zone file. For formatting convenience, resource records may span several lines by enclosing in parentheses
a set of parameters that spans several lines, but belongs to the same record. The file may contain comment text by preceding such text with a semicolon, either at the beginning of a line, or after the last field on any line, or on a blank line.
Comments end at the end of a line. The zone file may contain any number of blank lines with or without comments.
The zone file may also contain various directives that are marked with a keyword starting with the dollar sign character. The most notable is the $ORIGIN keyword,
which specifies the starting point for the zone in the DNS hierarchy.
If this keyword is omitted from a zone file, the origin is inferred by the server software from the reference to the zone file in its server configuration.