Domain Name System (DNS)

Start of Authority [SOA] Parameters

Start of Authority [SOA] Parameters identify the single authoritative server of the domain name. Each Zone contains one SOA Record, which holds the following parameters for the Zone -

Name of Primary DNS Server - The domain name of the Primary DNS Server for the Zone. The Zone should contain a matching NS Record.
Mailbox of the Responsible Person - The email address of the person responsible for maintenance of the Zone.
Serial Number - Used by Secondary DNS Servers to check if the Zone has changed. If the Serial Number is higher than what the Secondary Server has, a Zone Transfer will be initiated. This number is automatically increased by the server when changes to the Zone or its Records are made.
Refresh Interval - How often Secondary DNS Servers should check if changes are made to the zone.
Retry Interval - How often Secondary DNS Server should retry checking, if changes are made - if the first refresh fails.
Expire Interval - How long the Zone will be valid after a refresh. Secondary Servers will discard the Zone if no refresh could be made within this interval.
Minimum (Default) TTL - Used as the default TTL for new records created within the zone. Also used by other DNS Server to cache negative responses (such as record does not exist, etc.).