Force DHCP server to assign new IP to client

Glenn Satchell glenn.satchell at
Wed Oct 15 01:23:14 UTC 2014

Hi Jeffrey

Perhaps you could give us some more information as to why you want this
type of behaviour? Perhaps there may be a different way to achieve what
you want?


On Wed, October 15, 2014 1:24 am, Simon Hobson wrote:
> Jeffrey Zheng <jeffreyzheng at> wrote:
>> I am working on a project in which I would like to force DHCP server to
>> assign a new IP address to client whenever the client sends an IP
>> request, instead of keeping the current IP address. Is it possible? If
>> yes, can someone please tell me how to do it?
> As Chuck so eloquently put it, it's a recipe for "unhappy" clients. At the
> very least it will terminate all active connections every time the client
> renews it's lease, and with some clients it may cause bigger problems as
> there have been reports on this list from time to time of clients not
> handling changed leases very well.
> As pointed out, it's against the letter and spirit of the RFCs, which the
> ISC coders worked hard to follow. Thus the ISC code goes to great lengths
> to *NOT* change the client's address.
> Apart from techniques like periodically changing the config to make some
> addresses unavailable, one I can think of might go like :
> Write a script that monitors the logs, and every time an address is ACKed
> make note of it.
> The script then needs to update the DHCP config, explicitly making the
> address unavailable - perhaps by adding it to a class and denying that
> class access ot the address pool.
> The next time the client attempts to renew, it's request will be Nacked
> and it'll start over with a Discover to get a new address.
> The script needs to clean up, removing the blocked IPs from the list. A
> simple timeout would suffice but you could lose a lot fo addresses, but
> you could monitor the logs for the DHCP-NAck and set the address to be
> removed a short time later.
> It's a lot of work to go to just to break things and make life difficult
> for your clients.
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