OS X client to OD if master is down
You should specifically not try to do any DNS or similar trickery to achieve failover. The way OD failover works is that the OD data includes configuration records listing all of the OD servers (master and replicas). The clients store (&amp; periodically update) copies of this list, so if they ever have trouble contacting their primary server (generally the one they were told to bind to), they have a ready list of other servers to try.

(It's actually more complicated than this if you have locales defined, since that overrides the basic server list. But the same principles apply; just define the locales in OD, and the clients will automatically download the info they need.)