System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP)

Start-of-Authority record (SOA)

Example 5–19 shows the syntax of a start-of-authority (SOA) resource record.

Example 5–19 SOA Record Format

name class SOA origin person-in-charge ( 
    serial number

The SOA record designates the start of a zone. The zone ends at the next SOA record. The SOA record fields are described below.

The name Field

This field indicates the name of the zone. Note that the zone name must end with a trailing dot. For example: is correct, while is wrong.

The class Field

This field is the address class. For example, IN for Internet (the most commonly used class).

The SOA Field

This field is the type of this resource record.

The origin Field

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, is correct, but is wrong.

The person-in-charge Field

This field is the email address of the person responsible for the name server. For example, Again, this name must end with a trailing dot.

The serial Field

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.

The refresh Field

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.

The retry Field

This field indicates how long, in seconds, a slave server is to retry after a failure to check for a refresh.

The expire Field

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.

The ttl Field

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.

Example 5–20 Sample SOA Resource Record

;name class 		SOA 	origin				 person-in-charge IN		SOA (
							101			;Serial
							7200		;Refresh
							3600		;Retry
							432000		;Expire	
							86400)		;Minimum			 )