We are going to be moving from one ISP to another in the next month or so, and I will be changing the TTL's down to 60 seconds on the DNS servers that I have authority for, but our current ISP has authority for the reverse DNS zone.

Since this is the first time I have ever been through something like this, I'm wondering what to expect, and when I should do things (most likely the weekend, when email traffic is slow)

I know how to do all the changes on my side as far as DNS goes, both at our registrar, and our authoritative DNS server, as well as making sure our secondary DNS servers get the latest zone transfers. However, since the IP addresses of our current netblock is from our ISP, I am wondering what the best approach for this would be. I'm thinking something along the lines of this:

1) Request that the TTL's for our current IP's be reduced to 60 seconds.

2) Give all the information to our NEW ISP for what IP's will have for the hostnames, and tell them to hold tight.

3) On the day of the 'move', tell our old ISP to change the hostnames from what they are now, back to their 'default' hostnames. And then, tell our new ISP to make the new hostnames 'live' as far as the PTR records go.

Never having done this before, I'm just wondering what other people do, to make this as seamless as possible. I'm confident in my ability to reduce the TTL's of our current DNS records, and on the day of the move, to change the IP's to the new ones for our hostnames, but I don't have any control over the PTR records, and will be relying on 2 different ISP's to make changes.

What do other folks do in these situations?

Many thanks,



You don't need to do anything complicated.

As soon as your new ISP has told you what your new IP range will be you can request which PTR records should be put on each IP address.

Once you've moved, you can tell the old ISP that you no longer require them to maintain PTR records for you.

That's it. There's no "harm" in having both sets of IPs having the same PTR records on them for a while. So long as the IPs in use at the time have the right PTR records on them it doesn't matter that some other (now unused) IP address also has the same PTR record.


You can have the old ISP and new ISP delegate the reverse DNS zones to you now (the new ISP may not want to do this until the movetime).
Add the reverse records for the old and new IP addresses to your DNS server. This ensures that your PTR will remain present and accurate throughout the switch. After the TTL expires on the A for the old IP, you can remove its PTR record.

  • Sounds like my old ISP doesn't want to give or even have a way of delegating the reverse DNS zones to me. They say to open a ticket up for any reverse DNS changes. Call me a control freak, but I hate having to rely on anyone for stuff like this. At least if things get messed up, then it's my butt, and I can take it, but when it's someone else, it's still my butt, and I can't do much about it...
    – bgarlock
    Oct 29 '10 at 15:17
  • See if the new ISP will - it's a lot easier when you can change records by editing something on your server, as opposed to calling them. Doesn't help much with the switch though. Oct 29 '10 at 15:19

Depending on your network setup, is it possible to have both ISPs routed to the same equipment for a breif amount of time? When I did this a couple years back I had the luxury of being able to bring the new T1 into the same DMZ as the existing ISPs T1. To describe the setup more in-depth, I had a DMZ network, and then two routers, one for the one T1 and one for the new T1. I then applied both sets of IPs to the network interfaces of my equipment. So the IIS server had two IPs that it responded on, the mail server had two IPs, etc.

If this is an option for you, I would suggest considering it. It allowed for a much smoother and more gradual rollover between the two ISPs. We used this overlapping network for 2 weeks until we were able to determine that nothing was coming in through the old T1.

  • Hmmm.. This may be an option for us. I actually have the new (dual T1's) installed and turned up. Maybe I'll try this first.
    – bgarlock
    Oct 29 '10 at 15:15

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