Failover and pools with single machines
dhcp1 at thehobsons.co.uk
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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