Dhcp Failover behind load balancers

[Glenn Satchell]

>Date: Fri, 12 May 2006 13:00:00 -0400
>From: anthony <acqant at optonline.net>
>To: dhcp-users at isc.org
>Subject: Dhcp Failover behind load balancers
>
>I'm running DHCP failover on two servers behind a load balancer.  I have
>15 remote sites forwarding the dhcp requests to the virtual ip and they
>get sent to only one of the two dhcp servers at a time.
>
>For awhile it looked like if a request came to one server it would
>forward it to the other.
>
>Now for some reason I have a pool out of wack and the "peer holds all
>leases" error.
>
>I have two questions:
>
>1) Do I need to send the dhcp traffic to both servers behind the load
>balancers?
>	vip 10.0.0.1 ---> goes to primary, secondary hot spare
>	vip 10.0.0.2 ---> goes to secondary, primary hot spare
>	ip address helper 10.0.0.1,10.0.0.2
>
>2) I'm running the rpm package from Fedora Core3.  I think I need to
>upgrade to at least 3.0.3.

If you're running dhcpd in failover mode then you don't really need the
load balancer.

dhcpd failover is a load sharing setup, ie the two dhcp servers
essentially split the pool of available addresses between them and
notify the other server when they issue a lease or a lease expires.

Both need to receive the broadcast DHCPDISCOVER messages and both need
to receive the renewal requests which are unicast back to the dhcp
server that issued the lease.

So your pools are out of wack because only one dhcp server has been
receiving the client data.

If you want to use a load balancer then you should configure dhcpd to
not use failover and then use some other means to copy the dhcpd.leases
file to the other server periodically (say every couple of minutes or
so).

regards,
-glenn