Switch from Single DHCP to Master-Slave DHCP setup

Steven Carr sjcarr at gmail.com
Mon Jul 29 21:55:35 UTC 2013

On 29 July 2013 22:35, Gregory Sloop <gregs at sloop.net> wrote:

> I've recently setup DHCP+DNS [DDNS] on a single master server.
> I'm about to turn the switch on a master + slave DHCP updating the
> master DNS, [with the second/slave DHCP server having a slave DNS
> server getting updates from the master.]
> [i.e. DHCP + Single master DNS + Slave DNS]
> Ignoring the DNS configuration - is there anything special I need to
> do when I start load-balancing the DHCP servers, or do I just change
> the configs on both machines appropriately and the single will pass
> off what needs passing to the new slave and load balancing happens
> "automagically?" [i.e. Do I need to copy the dhcpd.leases file or
> other steps.]
> I've done some searches on this, but haven't found anything - I'm glad
> to be pointed in the right direction. I'd assume this discussion has
> occurred many times before, but I can't find it.
> (My plan is to turn on the Slave DHCP, and once it's stable and working
> fine, then turn on Master/Slave DNS - to do everything just one step
> at a time.)

So you don't need to do anything with the leases file, it will manage all
of that itself. Your "split" setting should be 128 which will balance the
DHCP lease loading between the two servers.

When you want to make a change to the DHCP config update both of the
configuration files and then restart the DHCP services on both systems,
they will automatically sync back up, you don't need to do anything with
the OMAPI interface in the normal running. I usually find it easier to
configure the DHCP networks that are being managed by the failover
association in a separate file to the normal DHCP configuration, that way
you can simply copy over the failover networks configuration to the second
server easily when any changes have been made.

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.

Hope this helps.

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