Failover and pools with single machines

Tina Siegenthaler tina at
Thu Dec 14 15:35:07 UTC 2006

Am 14.12.2006 um 16:24 schrieb Simon Hobson:

> 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.

Does this mean I can just use the same config file (or that part of  
it with the single-host pools) on 2 dhcp servers at the same time?  
Won't they get each other in the way? So, if I understand this  
correctly, a host will issue a broadcast and then take the IP address  
from the first dhcp server that responds. And it doesn't matter if  
there is a second dhcp server which would also be able to assign the  
host an address, since it's already got one. Right?
And then I use failover for the dynamic pools only. Great!

> 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
> server.
> 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.

I don't think we use dynamic DNS, so this won't be a problem.

Thanks a lot, Tina

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