DNS and changing domain host

[#1]

Hi all,
I'm not an expert at the inner workings of DNS so I'm wondering if someone can help me here please. I have a domain which I registered with 1&1 but which is currently hosted by Plusnet, however, I would soon like to move the domain's hosting back to 1&1. (Well, sort of – I'm glossing over some details, but they won't matter here.) Now, I know that normally when making a DNS change – e.g. adding or removing individual records – one has to allow 48 hours (or more!) for the change to propagate around the internet's DNS servers. However, what I'd like to know is, does a change of the domain's hosting – i.e. the actual nameservers it uses – still involve that period of flux? Put another way, does the change happen close enough to the root of the DNS ‘path’ for the change to be picked up on quickly, or will I still have to wait a while for caches around the internet to update?
Thanks in advance for any light shed on the topic.

[#2]

Hello Be3G  sorry i dont have exact reference but computeractive covered this a few weeks ago so........its in.......

http://forums.computeractive.co.uk
[Moderator's note by Dick (Strat) URL fixed.

[#3]

Hmm well I looked back a couple of months and couldn't find anything relevant, though thanks anyway.

[#4]

For what it's worth, I've just read Wikipedia's article on name servers and it's given me enough information to conclude that caching would still occur so… query solved.

[#5]

Yes, in one sense, there's still a period of propagation because caching nameservers will still associate the domain with the ip address they got from your old Plusnet nameservers. If the Plusnet nameservers and new 1&1 nameservers are both pointing at the same 1&1 ip address then the propagation should be invisible.
That still leaves a propagation period for the A records during which some requests will come to the Plusnet address and some will go to 1&1. If it's critical that you minimise the time that your domain points at two different places, you could ask Plusnet about lowering the TTL (time to live) temporarily, but I don't know if they'd be amenable to that. Even so, not all caching nameservers respect the TTL, so you may still get requests coming to Plusnet for a while.
(With the caution that Friday afternoon is my optimum time for talking twaddle.)
Gabe

[#6]

I don't know the answer to this but this is what I would have thought should happen.
When someone wants to browse your website, their web browser will request a DNS lookup of your domain.
Your domain DNS record resides at 1&1, but you have pointed it at the Plusnet web servers where your site is hosted.
Presumably to move your site a 1&1, then you copy your whole web site across to the 1&1 servers.
Then you access your 1&1 account, change the DNS to point at the 1&1 version of your website.
The 1&1 DNS record normally updates in about five minutes.
Now because the location of your DNS record has not moved from 1&1 then you should not get the 48 hour catch-up delay.
In the five minute update period, somebody browsing would either catch the old or new site.
So from that moment anyone making a new connection to your site would immediately be seeing the new copy running on 1&1.
The remaining problem is people like me who run our own caching DNS, or those who never close their Firefox session (as that now caches too), will still be pointed at the previous server.
I don't know what the best way to force those users to do a DNS refresh is, but these possibilities come to mind -
1)  Remove all traces of the original website so browsing fails to find anything, so should force a DNS lookup.
2)  Replace the old website with a holding page which redirects the browser to the new fully qualified website path.
3)  Run both sites in parallel but leaving the html pages on Plusnet alone, but change all the sql/cgi/... references to run from the 1&1 servers - so you only have one copy of your database etc.
I'm sure if I am wrong then some bright spark will be along soon to correct me  

[#7]

Thanks both. This is actually about e-mail rather than web hosting, which is why I'm more concerned about caching, as I don't want e-mails to end up in strange places or get lost altogether. What's happening is I'm moving the hosting of the domain back to 1&1, where I'll change the DNS records to point to Google's servers as I'm setting myself up with Google Apps. I am aware that I could simplify the process by keeping PN as the host and changing their DNS records to point to Google instead, but the whole reason for this change of e-mail host is that I'm trying to make my domain/e-mail setup independent of PN, so I don't have to worry about them when switching ISPs as I think might be happening in the not-too-distant future.

Quote from: Gabe

Even so, not all caching nameservers respect the TTL, so you may still get requests coming to Plusnet for a while.

This is something I've been concerned about; when researching the subject yesterday I spotted someone saying that they'd encountered three-week-old DNS records. What I think I'm going to have to do is for a little while after changing the nameservers at 1&1, I'll continue to let Plusnet ‘think’ they're hosting the domain, so that if any e-mails are sent their way owing to cached DNS results then they will still be stored away safely in my old mailboxes. This does mean however that I'll have to then manually move over any such e-mails to Google to keep things together. I did consider using redirects (in fact I'll want to use them eventually anyway to send my @username.plus.com e-mails over to Google), but of course the domain at the ‘end’ of the redirects (i.e. now genuinely hosted by Google) would be the same one that Plusnet would think they're still hosting, which I imagine would have the potential to cause a mess, possibly including an infinite loop of e-mail redirection. So the use of redirects will have to wait until I've removed the domain's hosting from PN, which in turn will have to wait until I believe the DNS has sufficiently propagated for PN to not be receiving any more of my e-mails, I think.
I can see why I've never wanted to do this before!
Edit: I see I posted this at 13:37… which kind of tickles me as this post is about as ‘1337’ as I get!

[#8]

email is something else entirely !
Consider the worst case scenario - If all you did was redirect your domain's DNS record to point at the 1&1 email server AND you disable your Plusnet email.
So any machine that checks DNS before sending email, will pick up the new server and the emails arrive immediately  - good so far.
Any machine that uses the previous DNS value will try and send the email to Plusnet - AND FAIL.
Now nearly all genuine SMTP email servers, will keep trying to get the message through - but eventually give up after three or four weeks !.
So sender keeps trying, and failing, until their local DNS catches up, at which point the email gets delivered.
Therefore you should NOT lose any genuine (i.e. non SPAM) emails, just that some might arrive a bit later than expected.
If you were also to redirect your Plusnet messages to your 1&1 host, then that would work similarly and should catch those out of date emails more quickly, until everything catches up.

[#9]

Quote from: purleigh

email is something else entirely ! Now nearly all genuine SMTP email servers, will keep trying to get the message through - but eventually give up after three or four weeks !.

Don't believe no SMTP server will try to deliver that long, you may be getting confused with MX backup server which will accept mail if the main server is down and will deliver when the server comes back on line. But most will only try to deliver for 2 weeks I have not came across any that will try for 4 weeks. My own email server will only try to deliver for 2 hours, every 10 minutes, before giving up. Hence the reason why most domains have more then 1 MX entry in the DNS records.
In the past I have use twisted4life for a MX backup but their do have a neat DNS test. http://www.twisted4life.com/tools.php