Bind updating serial number for zones
Unless you are manually editing the zone files (something that is not uncommon when using BIND), most DNS servers or frontend DNS applications will increment the serial number for you.
But if you find that updates are not being propagated to the secondary name servers, the serial number is the first thing you should check.
Time in seconds that a secondary name server should wait before trying to contact the primary name server again after a failed attempt to check for a zone file update.
There are all kinds of reasons why a zone file update check could fail, and not all of them mean that there is something wrong with the primary name server.
Time in seconds that a secondary name server should wait between zone file update checks.
The value should not be so short that the primary name server is overwhelmed by update checks and not so long that propagation of changes to the secondary name servers are unduely delayed.
In the early days of DNS, the serial number was just that — a number that was incremented by 1 each time the zone file was changed.
Time in seconds that a secondary name server will treat its zone file as valid when the primary name server cannot be contacted.
Any change requires time before it will be seen everywhere on the Internet.
But what many don't understand is that how fast or slow these updates are propagated is actually under their direct control through the SOA record. Although you have control over the speed that updates are propagated throughtout the Internet, they will never, ever, be instantenous! Your only control is over how short or long this delay will be.
It uses a special format where the "@" character is replaced with a "." (period) character and the email address ends with a period. Serial number of the zone file that is incremented each time a change is made.
The secondary name servers compare the serial number returned by the primary name server with the serial number in their copy of the zone file to determine if they should update their zone file.