You can hold downtime to a few minutes for a good chunk of your traffic.

How can you speed up the time it takes for DNS to propagate? Set your TTL low now, before you the move. (Set it to about 300 seconds.) This means that many DNS servers will query your DNS box every 5 minutes, rather than waiting for a day or two.

You cannot know when propogation is "officially complete" as DNS servers around the world may not respect your TTL and serve old results for a long bit.

What you can do is put up a blanket webpage at the old host saying, "Hey folks, we are moving hosts. If you want to visit the new site click here..." and you can use the new IP address if you have your host configured properly.

If you don't "have" nameservers, you'll want to find a company like dnsmadeeasy.com that can do this for you. Many hosts will do this though. Slicehost, a VPS, has a DNS tab in their admin area. Find out if they can do this for you.

So the steps:

1) Get your new site set up. (Use step 5 liberally to test if it is working correctly without doing any switching yet.)

2) If your VPS does DNS, set up your records. If not, get one like DNSMadeEasy.com and set up your records. Remember to set the TTL to 300 seconds. BUT set the records to point to the IP address of your OLD host.

3) Go to your domain registrar like GoDaddy and change the nameserver to whatever DNS site you used in step 2. They will TELL you the names to enter, it'll be like: ns1.namehost.com, ns2.namehost.com.

4) Give it a couple days for the switchover.

5) Ok, it's the day to switch. First update your hosts file on your PC, to do a last minute test of your new site before the switch. (Add the domain and the new IP address, just google to see how this is done. This allows you to test in advance of the move.)

5) Now switch the A records from your old host to your new host. Tail the log file. You should see traffic in a few minutes.

6) Put up a webpage on the old host pointing to the new host.

I had written up a long comment about what to do, but this more or less nails it. Here's what I had to say from a more DNS centric perspective:

You can do this with no downtime if you don't have any other outside circumstances forcing your hand, such as your current web hosting expiring very soon.

Hint: find a web based DNS test tool. Google "DNS Report", the dnsstuff.com one used to be really nice but they got too ambitious with a bunch of crap on their site and I think it's a pay service now, the dnscolos.com one looks like it's pretty good, I can't tell about the checkdns.net one. Also if you'll be running your own DNS services (as opposed to outsourcing them where someone else maintains the hosts) you'll want to find a DNS server package that has good documentation and good tutorials. My personal bias is toward BIND to get things going right away and then migrate to something else (PowerDNS, maybe) after you get it working because BIND is very, um, featureful, and needs updated a lot. :)

As you've determined, DNS servers are like web servers for static content in that you can have two different sets of them running that think they're the owners of the domain. If you give your new servers data identical to the old ones, it won't matter which set is serving your records, as the client will end up with the same data.

There is a notion, as you've probably read by now, of a TTL, or Time-To-Live. This is a number that goes along side your DNS response and tells the computer that your record will be staying the same for the next X seconds, so it doesn't have to ask again. Managing this in all of the records is crucial to a smooth migration.

You need to know the TTL of your current records. It sounds like you have a UNIXy machine at your disposal in the VPS. Log in and execute:

dig -t NS google.com

You will see a bunch of goop, but you really only care about:

;; ANSWER SECTION: google.com. 345600 IN NS ns4.google.com. ...

The 345600 is the number of seconds that the current name servers can be cached out on the internet. You won't be able to cleanly move your services faster than that if you don't control your current nameservers.

dig google.com @ns4.google.com

;; ANSWER SECTION: google.com. 300 IN A 64.233.187.99 ...

if you care about your email you're going to need to track your MX records as well, using:

dig -t MX google.com @ns4.google.com

wherein you will see:

;; ANSWER SECTION: google.com. 10800 IN MX 10 smtp4.google.com. ...

;; ADDITIONAL SECTION: smtp1.google.com. 3600 IN A 209.85.237.25

Again, in all of these answers field #2 (300, 10800, 3600) are the number of seconds that each record type (A, MX, A for smtp) are cacheable by servers across the internet. You will control these on your new server.

You will want to set the TTL of your A (and MX) records very low on your new server. 300 seconds would be a 5 minute migration from site to site.

Once you have your DNS servers properly set up (dig example.com @new.srvr.ip.addr) update your information with your registrar, giving them your new name servers as the only nameservers for the account. Then the NS TTL countdown begins. In our google test case you'd have four days before you could move the site cleanly. I'm not sure if all registrars push updates hourly or not -- it used to be that they only did it once daily. I'd wait NS TTL + 24 hours to be safe, if you have the time.

Once everyone is visiting your new name servers, you now can migrate your non-DNS servers. All you need to do is properly update the IP addresses, which can include incrementing the zone serial numbers if you're using a BIND-alike. After updating the A (and MX) IPs you have their old TTL seconds to wait before the migration is complete and everyone should be viewing things on your new servers.

After your migration is complete you can (at your option) turn your TTLs back up so that the load is reduced on your DNS servers. It's not really critical to do this, but it can carry you through if your DNS servers ever have a short network connectivity outage.

Good luck.

I've glossed over a bunch of stuff (some root NS stuff, DNS is badly implemented in a lot of places, some things don't honor TTLs, and so on) but these steps should be enough to do an upgrade in the common case with no problems.

Yeah, DNS is a bitch.

Best approach would be to host your DNS completely outside of your hosting company. This provides a certain level of resiliency.

Changing just about anything with DNS can take 6-24+ hours to be fully reflected on the Intartubes.

Just in summary, the DNS flow looks something like this:

Client asks DNS server what IP is www.foo.com DNS Server has no idea (it could be in cache, but this is worst-case). DNS Server looks up the root name server for foo.com and sees that NS1.BAR.COM is the nameserver for foo.com. DNS server finds glue record for NS1.BAR.COM and finds that it has IP address 1.2.3.4 DNS asks 1.2.3.4 what IP is www.foo.com DNS Caches answer based on TTL in response AND/OR sometimes its own settings to override too-low default TTLs DNS Server tells Client PC www.foo.com == 1.2.3.4

Moving your domain from being handled by NS1.BAR.COM to NS1.BAZ.COM can take 6-12 hours to take effect, and then another 6-24 hours for most other servers to expire their caches.

Changing www.foo.com to move from 10.10.10.10 to 10.12.13.14 takes effect immediately on the nameserver (NS1...), but various other servers along the way (often including the client PC) will have cached the old information, and may not go out to see if the IP has changed for some period of time.

All in all, a "seemless" DNS move takes about 1 week end to end and goes through a few iterations in the middle.

I personally would probably setup a new server somewhere with HAProxy setup to shoot connections back to my "old" server. Then make nameserver changes to move DNS someplace better (this step might be able to be eliminated). Stage site on new server. Change DNS to send traffic to haproxy IP. Set haproxy to push traffic to new site. If things work as expected, set DNS to reflect IP of new server, and watch haproxy logs to see when traffic stops coming in (3-5 days typically).

The reason for the haproxy bit is that you can "steer" the traffic at will and immediately, no propagation changes. Many people have gotten burned on a new site/server where something doesn't work and DNS changes aren't immediate, so you're stuck with all these hits that can't be fulfilled.

Sorry if my answer is non-readable... I'm writing this while on a conference call :)

I can help you more if you want, email in my profile...

You should not need to endure any downtime, even though DNS changes can take days to propagate. The trick is to get DNS working at your new host BEFORE you point your domains at the new host's servers. This way any given person will be hitting the new server or the old server, but it won't matter -- both should be serving the same information.

Other suggestions in this thread, such as reducing the TTL beforehand, will help drastically if any issues arise.

DNS and BIND, Third Edition By Paul Albitz, Cricket Liu Third Edition Pages: 499 ISBN 10: 1-56592-512-2 | ISBN 13: 9781565925120

Or http://cr.yp.to/djbdns.html --- no book required, and you avoid a really spotty security track record.

DJBDNS will hurt a bit if you're not Unix-y, but it will hurt less than managing zone files if you're not BIND-y. No sequence numbers, TTLs, or cryptic "IN A blah blah terminating period" syntax.

I assume you're using your shared host's DNS servers and that a move to a VPS also requires a DNS move. If it's possible to keep the same nameservers it's just a matter of changing the record and everything will work right away.

Switching nameservers is what takes a long time.

It can take 24 hours, but it's usually much faster. There have been times that I've seen the change take place in just a few minutes, but that only means that the nameservers I'm using have picked up the change. Other nameservers may take longer. The result is that you're not dealing with a total outtage. Some users will get pointed to the new location while others will get pointed to the old one.

You obviously want to do this when you expect usage to be at it's lowest and it wouldn't be a bad idea to have a message on the old site letting users know what's going on.

Also, as long as you're moving it's probably worth looking into a more permanent DNS solution so that you don't need to switch nameservers again next time you switch hosts.

If you know you need to make to your network, you can set TTLs to be lower, several days in advance. Problem solved.

(It's true that some crappy DNS resolvers don't honor TTLs, so you'll need to retain control of old servers for at least several days afterwards, to set up some automagic forwarding, receive mail from senders who haven't caught up with new MX records, etc. But none of this should be too difficult.)

True, but if you have a reasonably popular site SOMEONE has always broken DNS TTL's along the way. I've been through this more times than I can count... If your site is still small/early, it's less of an issue.

Right. So, like I said, you need to control the old servers for at least a few days after the cutoff date.

Uhm... Lots of good suggestions here. Given the innomence of your question, I'll take the liberty of assuming you won't take offense to me questioning some of the more fundamental assumptions. Why do you need to host your own DNS? Honestly, it's a real pain. And if you're asking questions like this, you probably don't need anything fancy. The control panels at the major registrars are pretty advanced these days.

Apologies if you're a DNS rockstar, no offense intended.

Definitely NOT hosting my own DNS. I've got my domains registered at invisiondomains.com and am hosting someplace else. Over at invisiondomains, I set up the nameservers and aimed my domains at the new nameservers. My initial post was an attempt to find out if the downtime I was experiencing was normal or just my host and registrar giving me a line. Not it seems that what I'm seeing is normal for someone who doesn't take some of these more intricate measures beforehand. All of this is really useful info. Like I said, I was looking for a primer - to know if I should call bullshit on my host and registrar or not.

I just did this recently. I reduced the TTL a few days before the move and I also set up Apache on the old server to proxy all requests to the new server. Look up Reverse Proxy for Apache. I turned that on when I changed the IP addresses in the DNS records. So there was no downtime. I left this up for a week and then shut it down. Worked great. Note: This site doesn't use SSL. That would make it a little more complicated.

Honestly, the "high end" DNS hosting services cost ~$15/mo. If you DIY, during setup you're going to incur many times more than that in time and effort costs, and you'll do it again every time you need to move DNS. If I were you, I'd just use a good hosting service with a web interface.

Ok. So I'm not crazy. It's incredibly helpful to know that 2 days of downtime doesn't mean my host is a lemon per se and that I should reconcile myself to some serious downtime over the weekend (which is the best time to be down for me).