With multiple IPs in a domain descriptor, how are invalid IPs handled?
I am trying to coordinate the movement of a site from its current server to a new one. My initial plan was to migrate data to a new machine that already exists and the migration scripts have been tested successfully. My plan was to set up the original machine to proxy all requests to the new one to ensure that anyone hitting the original machine before the DNS is completely changed will still get the correct response.
Someone made an alternative suggestion that I add entries for a new machine without actually serving any content. I was told that under these conditions, traffic will go to the original IP address. When new entries are distributed, I am told that I can shutdown the old server and open the new server. I am skeptical about this and my migration is semi-batch. Should I even try or just stick to my original plan?
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There is nothing wrong with your original plan.
It's also a good plan when you don't have control of the DNS server (s) you are using.
[Edit] Also, since this sounds like a production environment, it is never recommended to try something that you are not familiar with and comfortable with in a production environment.
The second idea doesn't sound like that at all, but in this case it is not optimal. Go with what you know works.
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You cannot control how long before the records are distributed, only until you are sure that they are distributed.
The TTL of DNS records is the maximum time at which they can be cached; however, even before that time, people will already attack the site at the new address (for example, if their DNS server did not have an entry in their caches).
This means that any solution will have a window of time where some people land on old addresses and other people land on new addresses. Any migration decision should consider this scenario.
Your initial plan is best as long as you remember the proxy for the IP address (since you can't know what the domain name indicates until the TTL has expired). An alternative plan (if you can take your site offline for some time) - just shut down the old server offline, perform the migration, bring the new server online, change the DNS records and wait for the TTL to end; people will gradually start using the new server as the DNS records in their nameservers and DNS browser caches expire and until then they only see the offline website.
Suggesting to add entries for a new machine without any content serving it is suspicious because once an entry is added some people will start using it.
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