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Mr. Monkey
Asked: 2011-03-08 09:28:45 +0800 CST

Changing ISP - Want to Minimize Downtime on DNS Change

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I do basic IT for our company, but have started getting into more server-level stuff. So I really need a dumbed down answer if possible.

We just recently switched ISPs. We are using a FatPipe system that bonds two different WAN sources into one LAN. Right now we have both plugged in and working fine. We have two connections bonded T1's and a local wireless internet company. Right now our A record for mail.company.com is pointed to our wireless ISP IP address. I would like to change it to be pointed to our T1's.

Looking at our domain hosting company, our www.company.com MX record is pointed to mail.company.com. That mail.company.com has an A record that goes to our wireless IP. To use our T1 I would just replace that A record with a different IP address, right?

I want to minimize downtime as much as possible. I have been searching on here for different solutions, but all seem to include using different servers. All we are doing is switching our mail from one IP to another. What is the best way to do this with minimum downtime?

Thanks in advance.

domain-name-system ip mx-record a-record

3 Answers

  1. Best Answer

    Provided your email server is accessable on both IP's there will be no downtime. You would just need to update the MX record with the new IP. Perhaps a better way would be to add the T1 IP address as a ne.w record with a higher priority (a lower number in the mx record equals higher priority) . This means if there is a problem with the first ip address any incoming email will fall back to your other IP address.

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  2. Prior to the switch over cut the TTL (time to live) on the address to a short period (say an hour). You can override the TTL for individual addresses. This should be done 2*TTL in advance of the change over. A couple of hours before the changeover you can cut the TTL to a shorter period (5 to 10 minutes).

    When you do the cutover, try to notify all your secondaries of the change so that the pull the change immediately. Verify all your secondaries have the change.

    When you are satisfied with the change, increase the TTL on the address. The easiest way to do this is to remove the TTL from the address and let it use the TTL for the domain.

    Due to the use of fast flux DNS for malware distribution and botnet control some DNS sites may limit the minimum TTL they use internally. You may want to watch for your changes to show up on OpenDNS and Google before shutting down service on the old IP address. Alternatively, you can just watch the logs for traffic to drop off.

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  3. The thing to keep in mind with DNS is that it propagates pretty slowly, depending on the TTL (time-to-live) set on the records.

    With this in mind, the best way to handle a cut-over on changing the address is to make sure that everything is up and running and ready to receive traffic on the new address, while keeping it up and running on the old.

    Once this is ready, you can make the change in the DNS for mail.company.com - after this point, connections may be coming in on one address or the other for several hours; since the services are up and running on both addresses, you don't need to care where they're coming in. After, say, 12 hours, everything should be coming in on the new address, and you're safe to drop the services from the old IP.

    EDIT: To switch over the mail server, you will want to ensure you have PTR records to your mail servers name on both addresses. You could also just rename you MX from smtp.example.com to mail.example.com and publish the new records. Add a second MX for the new address. Publish the address 2 * TTL in advance of the switchover. Then switch over ip address binding and mail server name on outgoing email. Finish by removing the old A and MX records from DNS.

    While you are doing this review the comments for Email delivery management grievances.

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