On Wed, Mar 6, 2013 at 10:40 AM, Tim Evans tkevans@tkevans.com wrote:
We are replacing four servers, running mail, web, ftp, and dns, respectively, with a single server to run all four services.
The new server will have a new IP address.
It seems fairly straightforward to redirect mail, web, and ftp services to the new server via DNS CNAMES,
SMTP mail needs to use MX records, not CNAMEs, and in fact the target of the MX can't be a CNAME. Pop/imap connections want an A record or CNAME. You can use multiple MX records and they don't all have to work - smtp deliveries are supposed to try them all before failing so that can be handy during transitions.
but I'm not quite sure about how to do the change for the DNS service itself.
Is there a need to maintain the old DNS server's IP address during a transition, or longer? Via a virtual IP with the old DNS server's IP address on the new machine, perhaps? Or a second NIC with the old address? Or just have the router redirect incoming DNS requests?
Is this a 'public' DNS server? These must be registered (and there should be at least 2 so you can move them and the registration one at a time). Or a local resolving server? These have the IPs configured into every client and/or will be handed out via DHCP. Again, there should be at least 2 but failures cause fairly long timeouts so breakage is a bad thing. Assuming these are all in the same subnet, the simple fix is to use IP aliases to keep the old addresses on the new server until you are sure that everything knows the new one. More drastically, but sometimes easier, you could also convert the old machines to VMs running on a single physical host which would let you consolidate the hardware without many logical changes.