Understanding DNS...

[lazyz]

I just moved several virtual sites over to a dedicated server. Easy enough until it comes to the DNS...

I've changed the DNS information on several sites within a period of one hour. However, some of the site's DNS propogates within a day some are taking longer. Why wouldn't all of them just click in around the same time?

[jmendenhall]

It all depends on the domain(s) and how each domain is set up. It also depends on the usage of the domain.

For each domain, there is the concept of a 'time-to-live' for the information contained in the domain zone. If the 'time-to-live' is set for a week, and other attributes are set up in a similar manner, there could be some DNS caches on the internet that contain the old data for up to a week.

Thus, best practice is to know about a change before the previous 'time-to-live' window starts, set the 'time-to-live' values to, say, oh about an hour or even 5 minutes, wait for the window to expire of the previous 'time-to-live', make the changes, and reset the 'time-to-live' values to your standard values.

Setting the 'time-to-live' values implies understanding the various SOA values for the domain. And, setting it too low on a "busy" domain could swamp your DNS boxes.

Hope this helps!

JohnM