Bind updating serial number for zones
(Although it's possible to access some sites using only the IP address of their web server, this is not the case for virtual websites, which require that their hostname be included in the HTTP request header.
Without a working DNS record, virtual websites are completely inaccessible.) But I've found that DNS is something that is not well understood by many website operators.
If you control the secondary name servers and the zone file doesn't change that often, then you might want to set this to as long as day (86400 seconds), especially if you can force an update on the secondary name servers if needed.
But if your secondary name servers are not under your control, then you'll probably want to set this to somewhere between 30 minutes (1800 seconds) and 2 hours (7200 seconds) to ensure any changes you make are propagated in a timely fashion.
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.
If your primary name server goes offline for some reason, you want the secondary name names to keep answering DNS queries for your domain until you can get the primary back online.
So that one could have a better idea of when the zone file was actually changed, it's recommended (but not required) that you use the format YYYYMMDDnn, where YYYY is the year, MM is the month, DD is the day, and nn is the revision number (in case the zone file is changed more than once in a single day).If the serial number from the primary name server is greater than their serial number, they will do a zone update transfer. If you make a change to the zone file on the primary name server and forget to increment the serial number, the change will not be propagated to the secondary name servers even if you attempt to force a zone update transfer.The primary and secondary name servers will remain out of sync until the serial number is incremented on the primary name server.The basics of creating A records (which translate a hostname to an IP address) are simple enough, but when it comes to understanding how changes are propagated in DNS, this is often something of a mystery.There is a widely held belief that any change made the DNS zone file of a domain is instantly seen throughout the Internet. When advising that changes be made to a zone file to fix a problem, I routinely add the following caveat: Changes to a zone file are almost never instantaneous regardless of how despreate you are that they be instantaneous.
A good retry value would be between 10 minutes (600 seconds) and 1 hour (3600 seconds), depending on the length of the Refresh Interval.