[CentOS] Physically moving a mail server vs. cached DNS

Mon Jul 3 17:16:07 UTC 2017
Warren Young <warren at etr-usa.com>

On Jul 1, 2017, at 3:00 AM, Pete Biggs <pete at biggs.org.uk> wrote:
>> In your experience, what's the "longest" a DNS cache is configured to
>> keep outdated information? A day? A week? A month? Longer?
> That is controlled by the TTL (time to live) entry.

…which is often under your control as the domain owner.

A common practice when moving hosts between providers like this is to temporarily shorten TTL from its normal working value to something much shorter, wait out the original TTL, do the move, wait out the new shorter TTL, and put the TTL back up to its previous value.

For example, if the normal TTL for the domain is 24 hours, then 24+ hours before the move, you could set TTL to 1 hour, then move the host 24+ hours later, so that any client that queries the DNS for that domain will get the 1-hour TTL.  Then 1 or more hours after you’re sure everything is fine, put TTL back to 24 hours.