Start of Authority (SOA)

The Start of Authority (SOA) record is the first section in every zone. This record contains information about a particular zone. This section also specifies that the server is authoritative for a particular zone. The various attributes that contain information about a zone are discussed briefly in the following list:

• Serial number. Used for controlling versions. Serial numbers represent the full date plus two more digits for sequential revisions during a single day.
• Refresh. This attribute is the delay time that slave name servers should wait before checking the master name server's serial number.
• Retry. This attribute is the delay time that a slave name server should wait before refreshing its database after a refresh has failed.
• Expire. This attribute is the upper limit that a slave name server should use in serving DNS information for a lack of a refresh from the master name server.
• Minimum time to live. This attribute specifies the number of seconds for which the records in the zone are valid. All cached entries for this record in various DNS servers should expire after this time limit.

An example of an SOA section is shown in the following code sample:

@   IN 	  SOA 	  xyz.com   root.xyz.com   (
				    1997022700 ;  Serial
				    28800   ; 	  Refresh
				    14400   ; 	  Retry
				    3600000 ; 	  Expire
				    86400 ) ; 	  Minimum time
						  to live

Round Robin Load Sharing

All configurations mentioned in the previous sections are stored in one single database file. If a domain consists of multiple hosts or there is a logical separation between domains, a domain can be divided into multiple domains. Multiple domains can be implemented by performing the following steps:

1. Configure the subdomain as though it were a normal zone in the named.conf file.
2. Specify the main domain in its database file.
3. Create a subdomain database file in /var/named.

In addition, you can use round robin load sharing for heavily loaded servers. This allows duplication of A records to distribute evenly all incoming requests. An example of round robin load sharing is shown in the following sample:

www 	0 	IN 	A 	192.102.44.1
www 	0 	IN 	A 	192.102.44.2
www 	0 	IN 	A 	192.102.44.3

NOTE
The Web server traffic is increased depending on the A records duplication. At the same time, the chance of server failure also increases by the same factor.