I'm trying to make our set of servers more fault tolerant. I have a web server, a mail server and a PBX server that uses SIP. I know that for each service, I can have the DNS return multiple A, MX and SRV records respectively. However I'm not sure if the DNS will indicate which record the client should use first or second.
For instance, if my primary mail server is reachable, I want mail clients to always connect to that one and never the back up. Like wise, if the primary is unreachable, I want the client to then try the back up.
I know that their are other ways to implement fault tolerance (I'm looking into them), but I want to know specifically if DNS resolution will work this way.