must be of increasing value for secondary to recognize the file has been modified
one of the most common problems in DNS is not properly updating this field for changes to propagate
Refresh
Time in seconds for the secondary to contact the primary to compare serial number in SOA record
Retry
Time in seconds for the secondary to retry contacting primary if the primary did not respond at refresh minutes
Expire
How long the secondary should continue to answer queries for the zone if it loses contact with the primary
Minimum TTL
The time in seconds that a nameserver can cache a resource record for use in queries before needing to contact one of the authoritative nameservers
