[Kea-users] Overwrite option-data fields with client-classes in reservations

[Darren Ankney]

Hello,

This is due to precedence which is documented here:
https://kea.readthedocs.io/en/kea-2.6.1/arm/dhcp4-srv.html#standard-dhcpv4-options

where it says: "The current Kea options precedence order is (from most
important to least): host reservation, pool, subnet, shared network,
class, global."

Kea will use the first option content that it finds for a particular
option in that above order.  Note that subnet comes before class.

You can use the flex_option to override option content based on class
membership: https://kea.readthedocs.io/en/kea-2.6.1/arm/hooks.html#libdhcp-flex-option-so-flexible-option-actions-for-option-value-settings

Thank you,
Darren Ankney

On Tue, Mar 4, 2025 at 11:08 AM Walter Reiner via Kea-users
<kea-users at lists.isc.org> wrote:
>
> Hi,
>
> I'm trying to overwrite option-data fields, e.g. domain-name-servers but also others, for specific reservations identified by MAC address. As the hosts are not separable by different pools, I thought the best option would be to create client-classes with the option-data I want to override and assign them through reservations to the clients so I don't have to duplicate this information through option-data blocks for every client. For clients without a reservation the idea was to set default option-data values on the subnet level.
>
> I have the following configuration:
>
> ~~~
> {
> "Dhcp4": {
>     ...
>     "client-classes": [
>       {
>         "name": "google-ns",
>         "option-data": [
>           { "name": "domain-name-servers", "data": "8.8.8.8" }
>         ]
>       }
>     ],
>     "subnet4": [
>         {
>             "id": 1,
>             "subnet": "192.168.24.0/24",
>             "pools": [ { "pool":  "192.168.24.6 - 192.168.24.100" } ],
>             "option-data": [
>                 { "name": "routers", "data": "192.168.24.243" },
>                 { "name": "domain-name-servers", "data": "192.168.24.2" },
>               ],
>             <?include "/etc/kea/reservations.json"?>
>         }
>     ],
>     ...
> }
> ~~~
>
> Which includes the following reservations:
>
> ~~~
> "reservations": [
>     {
>         "hw-address": "aa:bb:cc:dd:ee:ff",
>         "ip-address": "192.168.24.31",
>         "hostname": "dhtc01",
>         "client-classes": ["google-ns"]
>     },
> ]
> ~~~
>
> In this example, clients however always get the subnets domain-name-servers option instead of the one configured in the assigned client-class. When I explicitly add the option-data to the reservation, the overwrite works.
>
> I'm using isc-kea-dhcp4-server 2.6.1-isc20240725093407 on Debian 12.
>
> Any pointers are greatly appreciated.
>
> Thank you!
>
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