SOA (Start Of Authority record) is a special entry that must appear in each zone file just once. This is the kind of header that contains the following information:
- MNAME – the name of the primary DNS server for the zone
- RNAME – contact of the zone file administrator – there is noted e-mail address, where is @ replaced by a dot-sign (because @ has a special value in DNS)
- SERIAL – zone serial number – this is a numeric value that indicates the version of the zone file; during the change in the number of records will increase and secondary DNS servers when compared with the number that is stored, it finds that there has been a change and it is necessary to update data
- REFRESH – the number of seconds after which the secondary server performs a check of the serial numbers
- RETRY – after the start of discovery of the serial number is repeater request after RETRY count of seconds, if the previous request failed (if the server did not respond)
- EXPIRE – If you can not download serial number from the primary DNS and since the last successful attempt passed EXPIRE seconds, zone is considered invalid and secondary DNS server have to discard from records (forget it)
- MINIMUM – item with many historically different meanings:
- minimum TTL value for all records in the zone (the original meaning)
- default TTL for records where this value is not specified (the second meaning)
- TTL for negative caching – ie. period, after the which the caching DNS servers remember that a record does not exist (meaning current)
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