The SOA record
SOA is the start of authority record. It contains the information that other name servers querying this one will require, and will determine how the data will be handled by those name servers. Each primary file pointed to in named.boot should have one and only one SOA record.
The two fully qualified names listed after the SOA label are the primary name server for the domain, and where to send problems with the zone or domain via e-mail.
HNOUE Because @ signs cannot be used in this field, the postmaster entry should be read as postmaster@foo.com. If your e-mail address contains a dot, such as E.Marks@foo.com, it would have to be escaped with a backslash, and the entry would read E\.Marks.foo.com.
Note that the zone for which this forward map file is authoritative is not listed. It is strictly located in the named.boot file in the line that refers to this filename. The SOA record has five magic numbers: Serial, Refresh, Retry, Expire, and Minimum.
Serial. The first magic number is Serial, which stands for serial number
Serial is used strictly to determine whether secondaries are up to date. Many people like to use the date with a two-digit extension. We find it much easier to start with the number 1 and increment every time a change is made to the file. If changed address information does not seem to be making it to the secondaries, chances are that you either forgot to update the serial number or forgot to restart named.
TIJ Decide to use sequential numbers or a date+ scheme for your serial numbers, and use it consistently.
Refresh. The time (in seconds) interval within which the secondaries should contact the primaries to compare the serial number in the SOA; in the preceding sample, this is every 3 hours.
Retry. The interval within which the secondary should retry contacting an authoritative server if it did not respond at the refresh interval.
Expire. The amount of time that a secondary will hold information about a zone without successfully updating it or confirming that the data is up to date (i.e., if it loses contact with the primary). This number should be fairly large so that the loss of your primary name server will not affect your network. From experience, we can tell you that it is not unusual to point client machines at the secondaries and have no clients query the primary name server directly.
Minimum. The default time to live (TTL) for all records in a zone not containing explicit TTL values. In other words, this denotes how long the information will be considered good without needing to check with an authoritative server for changes.
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