Switch from Single DHCP to Master-Slave DHCP setup

[sthaug at nethelp.no]

> One thing you can't do is do it step by step, when enabling a failover
> association between two DHCP servers it has to be done on both systems at
> the same time (otherwise neither system will issue leases as neither will
> have been able to reconcile/balance the lease pools).
> 
> Make sure your network IP helper configuration is pointing at both DHCP
> servers and for DDNS make sure both DHCP servers are able and allowed to
> update DNS on the master.

Also, just to have mentioned this (it should be pretty obvious): Make
sure that the servers in a failover pair run the same (or at least
*compatible*) versions of dhcpd. Specifically, 3.0.x is *incompatible*
with anything recent.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no