[Kea-users] KEA allocates an IP while it should not

[Veronique Lefebure]

Dear Francis, thanks for confirming.

Indeed, https://kea.readthedocs.io/en/latest/arm/dhcp4-srv.html?highlight=8.3#global-reservations-in-dhcpv4 clearly says:

"global reservations that assign addresses bypass the whole topology determination provided by the DHCP logic implemented in Kea. It is very easy to misuse this feature and get a configuration that is inconsistent" .

So, in order to get the same behaviour with KEA than the one we had with ISC DHCP, we now need 2 host-reservations for one mac address:
- #1 : one host-reservation in the subnet, where we reserve an IP that matches the subnet
- #2 : one additional host-reservation, at the global level, with no reserved IP, for the client to get a dynamic IP in case it is not connected to the expected subnet.

In ISC DHCP, host-reservation #1 was enough to get both.
Am I correct ?

Thanks again,
Veronique

> On 30/08/2022 17:41 Francis Dupont <fdupont at isc.org> wrote:
> 
>  
> Veronique Lefebure writes:
> >  I would like to understand why KEA allocates an IP which should not be alloc
> > ated. 
> >
> >
> >  I have, on purpose (for testing KEA behaviour), declared the following globa
> > l host reservation: 
> 
> => the answer is here: you use a global host reservation with a reserved
> address so as it is documented this breaks localization aka subnet selection
> so you can get exactly the kind of results you got...
> 
> >  My question is: 
> >  ISC DHCP would not do that. 
> 
> in ISC DHCP there is no global reservations for an address even it is not
> obvious from the syntax: the reservation is attached to the subnet the
> address belongs to.
> 
> >  Is it expected that KEA does not behave the same way ? 
> 
> => yes, host reservation model is very different in KEA.
> 
> >  Is there any tuning that can be used so that KEA behaves the same way as ISC
> >  DHCP used to behave ? 
> 
> => put reservations with an address in a subnet the address belongs to.
> Note you can still use global reservations for other things as KNOWN /
> UNKNOWN classification, option setting, etc. With last versions of KEA
> you have also optional early global reservation lookup too.
> 
> Regards
> 
> Francis Dupont <fdupont at isc.org>