static IP allocations in dynamic ranges -why not?

doug doug at
Thu Jun 3 02:16:37 UTC 2010

On 06/02/2010 08:21 PM, Dorsey, Chris wrote:
> My question:
> What is the technical reason why dhcpd cannot mix static addresses 
> within dynamic ranges?  I've been pouring over the list archives, and 
> there are snippets from various folks that say to avoid this because 
> 'bad things will happen', especially related to the dhcpd IP allocator 
> giving out your static IP to other dynamic clients possibly causing IP 
> conflicts.
> It seems to me that dhcpd knows about both previous dynamic and static 
> allocations so it should be able to prevent allocating the given 
> static IP for MAC address 'A' out to dynamic clients that do not have 
> the MAC address 'A'. (i.e. before handing out a dynamic IP, check with 
> the previously allocated dynamic IPs as well as static 'fixed-address' 
> allocations for the given set of subnet(s))  This would help allow 
> other scenarios including allowing a host to do static on one subnet 
> and dynamic on others based solely on its MAC address.

Static leases do not go through the same process as dynamic, therefore 
the server has no idea about their state or their existence. They are 
not tracked nor processed in any way. This is why static IPs do not 
produce updated for DNS. A quick search of the list for that subject 
will give explanations in great detail.

See Simon's excellent explanation from earlier today as to the history 
and evolution of  the ISC server for some insight as to why things are 
done the way they are.

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