International DNS Propogation

Hi Everyone,

I have a client that I host a pretty large scale web application for in our data center. We recently changed ISPs and will need to change the IP address for the primary web server on Saturday morning. My concern is, that this application is used by clients all over the world and I've heard horror stories about DNS changes taking several days to propagate to caching international name servers. I need the application to be down for the 30 minutes that I move the server from one cage to another and then for it to come back up as quickly as possible. Even internationally.

Anything I can do NOW to ensure that the international clients will be able to access the system remotely after the change?

Who is Participating?
Chris DentConnect With a Mentor PowerShell DeveloperCommented:
24 - 48 hours is completely unsubstantiated. This 24 - 48 hours thing is a nasty rumour that floats around the Internet and never goes away. It's a myth, it has no basis except regurgitated hearsay.

DNS record changes propagate based on the TTL value for the record(s) concerned. If a record is not cached, an authoritative answer is sought, and no delay is incurred.

A very (very) small number of providers may choose to override the TTL, applying their own, longer value but you should not have to account for those (the majority of those are very much in the "used to, at some point in the past" category). Equally, a small number of ISPs may proxy HTTP requests and cache those connections (not to be confused with DNS caching).

Obviously it remains safer to provide the service on both old and new IP, that way every step is in your hands rather than relying on something you cannot track or fully control, like propagation of DNS changes. But if that is not an option, you should take reasonable steps to ensure that propagation time is kept to a minimum by reducing the TTL of the record(s).

Chris DentPowerShell DeveloperCommented:

Reduce the TTL for any records associated with the application, drop them down to 5 minutes (perhaps). All TTL changes thave to be done in advance, because the current TTL must have time to expire before you make the change.

Do note that this does not guarantee the service will be available to everyone quickly, even if it catches the vast majority. The only real way to be sure is to make the service available on both IP addresses (by whatever method you can).

Generally the DNS resolving takes 24-48 hours depending on the respective ISP. But I would suggest you to point the old IPs to the new IP. So for those location to whom the dns is not resolved, they will be routed to the new dns through the old IPs.

This way you can do you migration activity without any hassle.
Ditto what Chris-Dent said.  The key is the TTL, but if you currently have a long value there, you'll need to change it well in advance, too.  Remember that the current value is what determines how quickly the DNS servers will come back to refresh the entry and pick up the new TTL.  Once the new (shorter) TTL has propagated, your real change should propagate fairly quickly, as most all servers would have reached the end of the TTL the next time a request is made, and then check back with the authoritative server for the new information.  

Using this method, let's say your current TTL is 24 hours.  That means it could take up to 24 hours for a change to completely propagate.  Your changing the TTL to 5 minutes is a change that might take up to 24 hours to propagate.  Once that has happened, though, the next change would only take 5 minutes to propagate (at least with the servers using the TTL, which is the vast majority).  Don't forget, after the dust settles from the changeover, to switch the TTL back to its prior value so that you reduce the lookup traffic back to normal levels.  I wouldn't do that, though, until everything is working well.
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