Zones and Zone files

A zone is convenient short-hand for that part of the domain name for which we are configuring the DNS server, for example, BIND and is always an entity for which we are authoritative.

Assume we have a domain name of example.com. This is comprised of a domain-name (example) and a gTLD name (com). The zone in this case is example.com. If we we have a sub-domain which has been delegated to us called us.example.com then the zone is us.example.com.

Zones are described in zone files (sometimes called master files and normally located in /var/named) which can contain Directives used by the DNS software, for example, BIND and Resource Records which describe the characteristics of the zone and individual hosts and services within the zone. Both Directives and Resource records are a standard defined by RFC 1035 so can be read by any self-respecting DNS server software. The single exception to this is the BIND-specific $GENERATE directive. So if you think you will change DNS servers don't use $GENERATE.

Example Zone File

example.com.  IN      SOA   ns1.example.com. hostmaster.example.com.(
                              2003080800 ; se = serial number
                              3h         ; ref = refresh
                              15m        ; ret = update retry
                              3w         ; ex = expiry
                              3h         ; min = minimum
                              )
              IN      NS      ns1.example.com.
              IN      NS      ns2.example.com.
              IN      MX  10  mail.anotherdomain.com.
joe           IN      A       192.168.254.3
www           IN      CNAME   joe 

For a description of what the records mean or to see various sample configurations.

Pro DNS and BIND by Ron Aitchison

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