[Kea-users] dynamic hostname Option 12 in a kea pool

[Darren Ankney]

Hi Joop,

> Would it be possible to limit the libdhcp_flex_option.so hook to a specific pool
or interface.

You can add the client(s) you want to modify to a class in some way
(see: https://kea.readthedocs.io/en/stable/arm/classify.html) and the
supersede can be limited to a certain class as described here:
https://kea.readthedocs.io/en/stable/arm/hooks.html#libdhcp-flex-option-so-flexible-option-actions-for-option-value-settings
where it says:

> Since Kea 2.1.4, the client-class parameter specifies a class guard, and takes a client-class name. If not empty, the client's packet needs to belong to tne specified class for this entry to be used.

Thank you,
Darren Ankney

On Thu, Dec 18, 2025 at 10:13 AM Joop Boonen <joop.boonen at credativ.de> wrote:
>
> On Wednesday, December 17, 2025 12:04:01 PM Central European Standard Time
> Carsten Strotmann via Kea-users wrote:
> > Hi Joop,
> >
> > On 17 Dec 2025, at 11:29, Joop Boonen wrote:
> > > Dear all,
> > >
> > > I didn't find any option to implement dynamic host-names in kea pool.
> > >
> > > Like this in isc dhcpd.
> > >
> > > subnet 172.26.250.0 netmask 255.255.254.0 {
> > >
> > >   set machex = binary-to-ascii(16, 8, "", substring(hardware, 1, 6));
> > >   set macsuffix = suffix(machex, 6);
> > >   set hstnm = concat("vm-", macsuffix);
> > >   option host-name = hstnm; # Option 12
> > >
> > > -->8--
> > >
> > > i.e.
> > >
> > >   pool {
> > >
> > >     range 172.26.250.64 172.26.251.250;
> > >
> > >    set machex = binary-to-ascii(16, 8, "", substring(hardware, 1, 6));
> > >    set macsuffix = suffix(machex, 6);
> > >    set hstnm = concat("vm-", macsuffix);
> > >    option host-name = hstnm; # Option 12
> > >
> > > -->8--
> > >
> > > Does anyone know if this is possible, in kea?
> >
> > Yes, it is possible, even via multiple ways:
> >
> > 1)
> > via DDNS-Tuning, see the examples at
> > https://kea.readthedocs.io/en/stable/arm/hooks.html#libdhcp-ddns-tuning-so-d
> > dns-tuning
> >
> > 2)
> > via flexible-options (the expression below is build based on your ISC-DHCP
> > code, I've not tested it):
> >
> >       {
> >         "library": "libdhcp_flex_option.so",
> >         "parameters": {
> >             "options": [
> >                 {
> >                    "name": "host-name",
> >                    "supersede":
> > "concat("vm-",substring(hexstring(option[host-name].hex,""),-1,6))",
> > "csv-format": true
> >                 },
> >           ]
> >         }
> >       },
> >
> > Greetings
> >
> > Carsten
>
> Thank you very much for your answer.
>
> I've just tested this solution. Option 12 works like a charm.
>
> Sadly the ddns entries aren't written i.e. they don't end up in the bind9
> zones.
>
> My configuration:
> "Dhcp4": {
>     "hooks-libraries": [
>         {   "library": "libdhcp_flex_option.so",
>             "parameters": {
>                 "options": [
>                     {
>                         "name": "host-name",
>                         "supersede":
> "concat('vm-',hexstring(option[50].hex,''))",
>                         "csv-format": true
>                     }
>                 ]
>             }
>         }
>     ],
>
>
>    "hw-address": "<hw-address>",
>    "ip-address": "<ip-address>",
>    "hostname": "<hostname>"
>
> are written correctly in bind9.
>
> Would it be possible to limit the libdhcp_flex_option.so hook to a specific pool
> or interface.
>
>
> Regards,
>
> Joop Boonen.
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