Multiple subnetted-and-failovered DHCP servers?

November 15, 2007 9:34 PM

Can ISC's dhcpd run in both failover and multiple subnet modes?

I'd like to run two DHCP servers, each in their own physical subnets.

I would also like to run them in failover mode, so that, if a DHCP server were to fail in subnet A, the other DHCP server in subnet B would take over for the failed DHCP server and serve up addresses for subnets A and B.

Any advice and/or sample configuration files? Again, not looking for failover or multiple subnet support alone — but both together. Thanks for any tips.

posted by Blazecock Pileon to computers & internet (3 answers total)

I don't believe it can be done the way you describe, as the load-balancing features and failover features are intertwined. Both DHCP servers would have to be authoritative (and actively answering for) both subnets.

However, you can accomplish this by putting both DHCP servers on one of the subnets (or on a third subnet entirely) and using a DHCP relay to pass the requests along to your server.

My most recent setup involved two servers providing DHCP for 4 different subnets. The servers themselves lived on my "servers only" subnet which did not have any DHCP clients. Our core switch was configured to relay DHCP requests to the servers, tagging the request packets with their IP so the servers would know which subnet it should pull the IP from.
posted by insyte at 10:09 PM on November 15, 2007

Of course, you could do the same thing with VLANs instead of DHCP relay. That wouldn't require DHCP relay support on your router or a dhcp-relay box.
posted by insyte at 10:11 PM on November 15, 2007

DHCP Relay/Helper will do it if your network devices support it and is probably your best option for keeping this simple.

If your DHCP service is smart enough to bind to a specific interface you can try dual homing the servers so they each have a present in both subnets.
posted by iamabot at 10:24 PM on November 15, 2007