Types of Resource Records and SOA record VALUE field format
A start of authority (SOA) record is information stored in a domain name system (DNS) zone about that zone and about other DNS records.
A DNS zone is the part of a domain for which an individual DNS server is responsible. Each zone contains a single SOA record. SOA records are defined in IETF RFC 1035, Domain Names - Implementation and Specification.
The FQDN of the host where this SOA record resides; in other words, the authoritative server for this zone.
The email address of the administrator for this zone, written with the usual @ sign replaced by a dot.
A number identifying this version of the zone file. This field is used by secondary servers to decide if the zone file has changed, so it should be incremented every time the file is modified.
How often (in seconds) the secondary servers should reload this zone file to check for changes.
How long (in seconds) the secondary servers should wait after a failed zone file transfer before retrying.
How long (in seconds) the secondary servers should wait between successful zone file transfers before discarding the data.
Tells other servers how long, by default, to cache results from this server.
(SOA ) Start of Authority
An SOA (Start of Authority) record marks the beginning of a zone, a group of resource records located at the same place within the DNS namespace. The data for a DNS domain usually includes at least two zones: one for translating hostnames to IP addresses, called the forward zone, and others that map IP addresses back to hostnames, called reverse zones. Each zone has exactly one SOA record. The SOA record includes the name of the zone, the primary name server for the zone, a technical contact, and various timeout values. Comments are introduced by a semicolon.
Here is an example:
The value of @ is the domain name specified in the zone statement of named.conf or in the zone's name entry in the nsd.conf file.
This value can be changed from within the zone file with the $ORIGIN parser directive
This example has no ttl field. The class is IN for Internet, the type is SOA, and the remaining items form the data field. The numerical parameters in parentheses are timeout values and are often written on one line without comments. "ns1.atrust.com." is the zone's master name server.11
hostmaster.atrust.com. is the email address of the technical contact in the format "user.host." rather than the standard user@host. Just replace that first dot with an @ and remove the final dot if you need to send mail to a domain's administrator.
Sites often use an alias such as admin or hostmaster in place of an actual login name. The sysadmin responsible for hostmaster duties may change, and it is easier to change one entry in the aliases file (see page 756) than to change all your zone files when you need to update the contact person.