Failover and pools with single machines

Simon Hobson dhcp1 at
Thu Dec 14 15:24:14 UTC 2006

Tina Siegenthaler wrote:

>We have configured our dhcp server with lots of classes/subclasses/
>pools that contain but one single host. The reason was that using 
>"fixed-address" for assigning fixed addresses to hosts doesn't make 
>those hosts appear in the lease file.
>Now we'd like to implement failover... but what will happen to those 
>pools with just one host? It's difficult to split a range of just one 
>IP equally between two dhcp servers, and that's what failover does, 
>if I understand it correctly. Can we use failover nevertheless, or
>will we run into problems? What happens when one server goes down?

Failover won't work with such small pools. However, since you will 
only have one client/pool then you could simply replicate the config 
and NOT use failover. The client will get the same address regardless 
of which server it took the lease from, and no other client will get 
the same address from the other server.

You would still need to use failover on the general use pools though.

If a server goes down, the client will keep trying to renew it's 
lease from it. Eventually the client will revert to broadcasts to 
find a server, at which point the other server will offer it the same 
address. After this, the client will keep renewing with it's new 

Dynamic DNS updates will be a problem. Once one server has added DNS 
records for a client, the other server will not be able to work with 
them - the key will not match. When the first server comes back up, 
it will start to time out leases where the client is no longer 
renewing with it, and when a lease expires it will remove the DNS 
entries. The client will now be without DNS entries until it next 
renews and the second server is now able to do a DNS update for it.

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