DHCP-Failover with 2 OS X Servers, how?

I believe you are looking for ISC DHCP as a replacement to Apple's included bootpd. It has the ability to configure failover.

R-

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Apple have effectively removed all high-availability features from their server software although to be fair they have also removed most of the price as well.

Some people have done their own solution which basically boils down to -

Having a live server with two network interfaces, one for 'real' work and one for checking the health of the server

A second standby server also has two network interfaces but initially only the health-check interface which is connected to the live server equivalent health-check interface is active

The standby server runs a script to check the live server is still responding, this could be a simple ping although this is not necessarily robust enough, or something a bit more sophisticated to confirm the live server is working properly, if this determines the live server is now 'dead' then it will enable the standby servers equivalent 'real' interface and startup the services like in this case DHCP that are supposed to be running

It depends on the services in question how complicated this needs to be, for DHCP things are quite simple as you can probably cope with not syncing changes between the two servers for other services like Profile Manager then with the lack of facilities provided by Apple you will probably find it is not worth the grief.

I certainly would like to see Apple address this issue properly and I would be happy to pay more for Server.app for it.

Note: As a result of Apple's weakness in this area most larger organisations use other server platforms if at all possible.

I use a pair of firewall/gateways in a failover configuration which also act as my DHCP servers. If the live one fails then the standby automatically takes over basically like described above except this is all built-in as standard.

There is one other approach for DHCP only that you can use, if you have two DHCP servers both live all the time, but make sure they have non-overlapping address ranges then clients will use whichever answers first and if one fails then only the other will answer. You would have the same DNS and Gateway and Subnet settings on both but would have a different range of IP addresses on each.