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The filename is arbitrary.
For more information on the fields of SOA and NS records, see named.hosts(4tcp).
The SOA lists a serial number, which should be changed each time the master file is changed. Note that the serial number can be given as a dotted number, but this is unwise since the translation to normal integers uses concatenation rather than multiplication and addition. You can catenate the year, month, day of month, and a version number in the range 0-99 and still fit inside the unsigned 32-bit size of this field.
Secondary servers check the serial number at intervals specified by the refresh time in seconds; if the serial number changes, a zone transfer will be done to load the new data. If a master server cannot be contacted when a refresh is due, the retry time specifies the interval at which refreshes should be attempted. If a master server cannot be contacted within the interval given by the expire time, all data from the zone is discarded by secondary servers. The minimum time-to-live value is used by records in the file which have no explicit time-to-live value.
Standard Resource Record Format in RFC 1035
@ IN SOA volga.mynet.COM. dave.mynet.COM ( 1993091901 ; Serial number 10880 ; Refresh time 1800 ; Retry time 3600000 ; Expiry time 259200 ) ; Minimum time-to-live IN NS volga.mynet.COM IN NS istanbul.mynet.COM