Example 5–19 shows the syntax of a start-of-authority (SOA) resource record.
name class SOA origin person-in-charge ( serial number refresh retry expire ttl)
The SOA record designates the start of a zone. The zone ends at the next SOA record. The SOA record fields are described below.
This field indicates the name of the zone. Note that the zone name must end with a trailing dot. For example: doc.com. is correct, while doc.com is wrong.
This field is the address class. For example, IN for Internet (the most commonly used class).
This field is the type of this resource record.
This field is the name of the host where this data file resides. Note that this host name must end in a trailing dot. For example, dnsmaster.doc.com. is correct, but dnsmaster.doc.com is wrong.
This field is the email address of the person responsible for the name server. For example, kjd.nismaster.doc.com. Again, this name must end with a trailing dot.
This field is the version number of this data file. You must increment this number whenever you make a change to the data: slave servers use the serial field to detect whether the data file has been changed since the last time they copied the file from the master server.
This field indicates how often, in seconds, a slave name server should check with the master name server to see if an update is needed. For example, 7200 indicates a period of two hours.
This field indicates how long, in seconds, a slave server is to retry after a failure to check for a refresh.
This field is the upper limit, in seconds, that a slave name server is to use the data before it expires for lack of getting a refresh.
This field is the default number of seconds to be used for the time-to-live field on resource records that do not have a ttl specified elsewhere.
There should only be one SOA record per zone. Example 5–20 is a sample SOA resource record.
;name class SOA origin person-in-charge doc.com. IN SOA dnsmaster.doc.com. root.nismaster.doc.com. ( 101 ;Serial 7200 ;Refresh 3600 ;Retry 432000 ;Expire 86400) ;Minimum )