SOA (Start Of Authority record) is a special entry that must appear in each zone file just once. It is a kind of building block that contains the following information:

• MNAME – the name of the primary DNS server for the zone
• RNAME – the contact of the zone file administrator – there is an e-mail address, where the @ is replaced by a full stop (because the @ has a special value in DNS)
• SERIAL – the zone serial number – this is a numeric value that indicates the version of the zone file; when the records change, the number increases and the secondary DNS servers compare the number to that which they have stored and find out that there has been a change and that the data needs to be updated.
• REFRESH – the number of seconds after which the secondary server checks the serial number since the last check or retrieval of the zone from the primary DNS
• RETRY – after starting to find the serial number from the previous point, it repeats the request to the primary DNS server after RETRY seconds, if the previous request failed (if the server did not respond)
• EXPIRE – if the serial number cannot be downloaded from the primary DNS and EXPIRE seconds have elapsed since the last successful attempt, the zone is considered invalid and the secondary DNS server should exclude it from its records (forget it)
• MINIMUM – an item with many historically different meanings:
  • the minimum TTL value for all records in the zone (the original meaning)
  • the default TTL for records where this value is not specified (the second meaning)
  • the TTL for negative caching – i.e. the period, after the which the caching DNS servers remember that a record does not exist (the current meaning)