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-flexible-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.
--
Peter Davies
Support Engineer
Internet Systems Corporation
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