Saturday, September 24, 2011

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What does serial, refresh, retry, expire, minimum and TTL mean?

Details of SOA parameters used in DNS records:

Serial — The zone serial number, incremented when the zone file is modified, so the slave and secondary name servers know when the zone has been changed and should be reloaded. 

Refresh — This is the number of seconds between update requests from secondary and slave name servers. 

Retry — This is the number of seconds the secondary or slave will wait before retrying when the last attempt has failed. 

Expire — This is the number of seconds a master or slave will wait before considering the data stale if it cannot reach the primary name server. 

Minimum — Previously used to determine the minimum TTL, this is used for negative caching. This is the default TTL if the domain does not specify a TTL.

TTL (time to live) - The number of seconds a domain name is cached locally before expiration and return to authoritative nameservers for updated information.