Hi Peter,
I'm not sure, but it looks like what I want doesn't work.
I have the following kea-dhcp4.conf configuration (snippets):
"Dhcp4": {
"hooks-libraries": [
{
"library": "libdhcp_ddns_tuning.so",
"parameters": {
"hostname-expr" : "'vm-test-' +
split(addrtotext(option[50].hex),'.',1) + '-' +
split(addrtotext(option[50].hex),'.',2) + '-' +
split(addrtotext(option[50].hex),'.',3) + '-' +
split(addrtotext(option[50].hex),'.',4)"
}
},
{
"library": "libdhcp_flex_id.so",
"parameters": {
"identifier-expression": "substring(hexstring(pkt4.mac,''),
0,6)",
"replace-flex-id": true
}
},
{ "library": "libdhcp_flex_option.so",
"parameters": {
"options": [
{
"name": "host-name",
"supersede": "'vm-test-' +
split(addrtotext(option[50].hex),'.',1) + '-' +
split(addrtotext(option[50].hex),'.',2) + '-' +
split(addrtotext(option[50].hex),'.',3) + '-' +
split(addrtotext(option[50].hex),'.',4)",
"csv-format": true
}
]
}
}
],
When I have entry:
"subnet4": [
{
// This defines the whole subnet. Kea will use this information to
// determine where the clients are connected. This is the whole
// subnet in your network.
// Subnet identifier should be unique for each subnet.
"id": 1,
// This is mandatory parameter for each subnet.
"subnet": "10.10.10.0/24",
// Pools define the actual part of your subnet that is governed
// by Kea. Technically this is optional parameter, but it's
// almost always needed for DHCP to do its job. If you omit it,
// clients won't be able to get addresses, unless there are
// host reservations defined for them.
"pools": [ { "pool": "10.10.10.32 - 10.10.10.192" } ],
"reservations": [
// This is a reservation for a specific hardware/MAC address.
// It's a rather simple reservation: just an address and
nothing
// else.
{
"hw-address": "52:54:00:97:8b:89",
"hostname": "dummy-name"
},
It's fine a dns name is sent via Option (12) and the dnsname is written in dns.
IP sent 10.10.10.32
With entry:
{
"flex-id": "'525400'",
"hostname": "dummyvalue"
},
No dns entry is written, the hostname has only be sent via Option(12).
IP sent 10.10.10.32
I also looks like this flex-id hasn't been used. I don't know how to debug
this.
My "severity": "DEBUG" and "debuglevel": 99
It would be great to have an option like:
"pools": [ { "pool": "10.10.10.32 - 10.10.10.192" },
"hostname": "ddns: }
],
So a hostname can be given dynamically to all hosts in a pool.
On Tuesday, January 6, 2026 2:42:46 PM Central European Standard Time you
wrote:
> Hi Joop,
> A host reservations require an identifier; otherwise, Kea would have
> no way
> of knowing which client the reservation is intended for.
>
> The reservation identifier need not be the “hw-address. The "duid",
> "circuit-id",
> and "client-id" may also be used.
>
> The flex-id hooks library enables user-defined identifiers based on the
> contents
> of DHCP request packets.
> See:
> https://kea.readthedocs.io/en/stable/arm/hooks.html#libdhcp-flex-id-so-flexi
> ble-identifier-for-host-reservations
>
> If you know in advance that all clients on a subnet need to have their
> hostname
> processed in this way, it is possible to limit the scope of the
> "hostname-expr"
> to a subnet by adding a “ddns-tuning” “user-context” to the subnet
> definition.
> For example:
> "subnet4": [{
> "subnet": "192.0.2.0/24",
> "pools": [{ "pool": "192.0.2.10 - 192.0.2.20" } ],
> "user-context": {
> "ddns-tuning":
> "hostname-expr" :
> "concat('vm-test-',addrtotext(option[50].hex))" }
> }
> }
> }],
>
>
>
> Perhaps your intended use of host reservations performs a different
> function than
> that normally associated with host reservations. Maybe some sort of
> limiting
> mechanism?
>
> On 06/01/2026 10.48, Joop Boonen wrote:
> > Hi,
> >
> > Thank you for the answer. The addrtotext option is great.
> >
> > I would like to explain what we need.
> >
> > We wouldn't like to use an identifier for every host.
> > 1) Because we want to hand out DNS names to VMs with random MAC addresses.
> > 2) We are using /24 and /23 networks. So that even if we would know the
> > MAC- addresses this would mean a lot of entries in the kea-dhcp*.conf.
> >
> > It would be great dns names could be handed out for a pool in
> > kea-dhcp*.conf for a host that doesn't have an identifier. Or an Asterix
> > identifier.
> >
> > Regards,
> >
> > Joop.
> >
> > On Tuesday, January 6, 2026 9:35:04 AM Central European Standard Time
> > Peter
> >
> > Davies wrote:
> >> Hi Joop,
> >>
> >> A host reservation should have an identifier. If you do not use the
> >>
> >> "hw-address,"
> >> what identifier do you use?
> >>
> >> You may wish to use the following, which appends the IP address to the
> >>
> >> hostname:
> >> "hostname-expr" :
> >> "concat('vm-test-',addrtotext(option[50].hex))" } }
> >>
> >> /Peter
> >>
> >> On 05/01/2026 16.59, Joop Boonen wrote:
> >>> Dear all,
> >>>
> >>> I'm using the option like this.
> >>> "Dhcp4": {
> >>>
> >>> "hooks-libraries": [
> >>>
> >>> {
> >>>
> >>> "library": "libdhcp_ddns_tuning.so",
> >>> "parameters": {
> >>>
> >>> "hostname-expr" : "concat('vm-
> >>>
> >>> test-',hexstring(option[50].hex,''))"
> >>>
> >>> }
> >>>
> >>> },
> >>>
> >>> I notice when I don't have an hw-address entry
> >>>
> >>> {
> >>>
> >>> "hw-address": "<hw-addres>",
> >>> "ip-address": "<ip-address",
> >>> "hostname": "<hostname>"
> >>>
> >>> },
> >>>
> >>> The ddns entries doesn't get Updated.
> >>> For some reason it looks like hostname-expr needs an assigned hostname.
> >>>
> >>> Does someone know how this can be solved.
> >>>
> >>> Regards,
> >>>
> >>> Joop Boonen.
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