The first resource record in any Domain Name System (DNS) Zone file shouldbe a Start of Authority (SOA) resource record. The SOA resource recordindicates that this DNS name server is the best source of information forthe data within this DNS domain.
The SOA resource record contains the following information:
Source host - The host where the file was created.
Contact e-mail - The e-mail address of the person responsible foradministering the domain's zone file. Note that a "." is used instead ofan "@" in the e-mail name.
Serial number - The revision number of this zone file. Increment thisnumber each time the zone file is changed. It is important to incrementthis value each time a change is made, so that the changes will bedistributed to any secondary DNS servers.
Refresh Time - The time, in seconds, a secondary DNS server waits beforequerying the primary DNS server's SOA record to check for changes. When therefresh time expires, the secondary DNS server requests a copy of thecurrent SOA record from the primary. The primary DNS server complies withthis request. The secondary DNS server compares the serial number of theprimary DNS server's current SOA record and the serial number in it's ownSOA record. If they are different, the secondary DNS server will request azone transfer from the primary DNS server. The default value is 3,600.
Retry time - The time, in seconds, a secondary server waits before retryinga failed zone transfer. Normally, the retry time is less than the refreshtime. The default value is 600.
Expire time - The time, in seconds, that a secondary server will keeptrying to complete a zone transfer. If this time expires prior to asuccessful zone transfer, the secondary server will expire its zone file.This means the secondary will stop answering queries, as it considers itsdata too old to be reliable. The default value is 86,400.
Minimum TTL - The minimum time-to-live value applies to all resourcerecords in the zone file. This value is supplied in query responses toinform other servers how long they should keep the data in cache. Thedefault value is 3,600.
The following is an example of a Microsoft DNS server generated default SOAresource record:
@ IN SOA nameserver.place.dom. postmaster.place.dom. ( 1 ; serial number 3600 ; refresh [1h] 600 ; retry [10m] 86400 ; expire [1d] 3600 ) ; min TTL [1h]
Parentheses allow the SOA record to wrap to multiple lines.
In the above example:
Source Host = nameserver.place.dom.
Contact Email = postmaster.place.dom.