The zone header is a lengthy series of code “shorthand.”
@ – Represents the 'root name' of the zone. E.g. if the zone name is .fun, ‘@’ will take the .fun as the root name of zone.
86400 – 86400 Seconds or 1 Day is the Time to Live (TTL). TTL defines the duration in seconds that the record may be cached.
IN – Defines the class of record, the value IN = Internet.
SOA – Start of Authority, represents the start of the zone.
NS1. – The Primary or Master DNS Server is listed here. This is commonly written as a FQDN and ends with a dot (ns1.example.fun.). Although it can be ended without a dot (ns.example) we strongly recommend using the FQDN.
hostmaster. – Represents the email address of the person responsible. In practice, TLDs do not use a personal email address rather a corporate account, such as, admin or hostmaster. The Format is mailbox-name.domain-name. (admin.nic.fun.). The ‘@’ is replaced by a dot and the email address ends with an dot. Again, the email can be written without the trailing dot (admin.nic). the effect of failing to add the trailing dot, is the system will append the ‘root-name’ of the zone.
2006010100 – The serial number is an unsigned 32 bit value in range 1 to 4294967295 with a maximum increment of 2147483647. This value must change when any resource record in the zone file is updated. The convention is to use a date based value to simplify this task - the most popular being yyyymmddss where yyyy = year, mm = month and dd = day ss = a sequence number in case you update it more than once in the day.
21600 – The refresh is a signed 32 bit time value in seconds (6 hours). Indicates the time when the slave will try to refresh the zone from the master. Please refer to RFC 1912 before changing value.
3600 – The retry value is a signed 32 bit value in seconds (1 hour). It defines the time between retries if the slave or secondary name server(s) fails to contact the master when refresh (above) has expired.
604800 – The expiry is a signed 32 bit value in seconds (1 week). It defines when the zone data is no longer authoritative. This value affects slaves or secondary servers only.
If contact is made before the expiry, the refresh value (21600s) is reset and the cycle restarts. If the slave fails to contact the master it will retry every retry period (3600s) but continue to supply authoritative data for the zone until the expiry value (604800s) is reached at which point it will stop answering queries for the domain.
86400 – The minimum TTL should not be set more than 10800 seconds. RFC 2308 (implemented by BIND 9) redefined this value to be the negative caching time - the time a NAME ERROR or NXDOMAIN record is cached. This applies to queries that try to resolve non-existing resource records (rr) within the zone file.
