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I support a small office of about 100 users. We run a small virtual environment and host our own on-premises Exchange server and Web Server.

We are in the process of upgrading our internet connection from a 100mpbs line on a 100mbps bearer to 200mbps on 1gbps bearer. I've been told by our ISP that since we are getting a new bearer, we will be getting a new block of static IP addresses.

Since we still host our own internal mail server and web server, we will need to update our public MX and A records accordingly. My question is, is there a way we could pre configure the MX record with the new IP address in advance so as to minimize downtime on our email when we come to install our new ISP provided router?

ie. Can I configure a second MX record with the new IP address before it is live so that when we make the switch, the first MX record dies and the second one works straight away?

I'm trying to avoid a situation where we have to switch the boxes, I change the MX record and it takes 24 hours to go live across public DNS servers. Leaving us with no email during that time.

The same also applies to a web server hosted on site. What is the best way to deal with the switch over in this case? I can only have one A record pointing to www.website.com, right? No way of having two A records configured and if one doesn't work, the other does?

Any help greatly appreciated.

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Working with a second mx is definitively the way to go for you:

Create a second mx entry that has the new IP- address but a lower preference number. As long as the first mx is available, nothing will happen, but DNS- servers will already cache the second mx. As soon as you switch IPs, the first mx will not be available, and ever mail server will -after a short period- try the second one.

Set the dns time to live of your mx records as little as possible, then after the switch you just change the preferences of the mx entries or remove the old IP address completely.

That way the most that can happen is a 5 minute delay in mail delivery, as that is the default time, some systems wait before trying again, but most of the systems try the second mx immediately if the first one is not available.

And regarding DNS- entries: indeed you can configure one A- record to have more than one IP- address. This is called "DNS round robin". BUT: Browsers most of the time use the first IP address in the list and only switch to the second after 30 seconds. This can only be reduced, if both IP- addresses are up at the same time...

  • Great. I assumed I could do something along these lines but wasn't quite sure. Thanks for the help. Any idea of a similar way to keep downtime on our webserver to a minimum? Perhaps just decrease the TTL of the A record in advance? – OPGailey Jul 18 '16 at 14:03
  • Yes keep a low TTL on the web server. As browser retry behavior is not standard, easiest would be to tolerate DNS taking a few minutes to update. – John Mahowald Jul 20 '16 at 0:35

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