[Kea-users] Kea 2.4.0 and duplicate host reservations not working as documented

[Darren Ankney]

Hi Kevin,

The setting  "ip-reservations-unique": false, only ensures your config
will pass muster with multiple reservations for the same IP, it
guarantees nothing about the future allocation of the addresses.

This frequently comes up, as it does not behave in the same way that
it did in ISC DHCP.  In ISC DHCP, there was no lease recorded for
reservations made with the "fixed-address" keyword.  Therefore,
whatever client that was assigned the same address and came along and
asked for it, got it.  In Kea, this behavior has changed.  A lease is
always recorded for a leased IP address.  Therefore, what is happening
here is noted in the log is that the address can't be allocated as it
is already in use by your other interface:

"[hwtype=1 90:2e:16:d6:a4:1e],
cid=[00:81:74:fa:e6:94:98:99:8d:79:c0:a3:9c:87:27:72], tid=0x262eb546:
conflicting reservation for address 192.168.88.192 with existing lease
Address:       192.168.88.192"

It then goes on to print details about the existing lease.  You could
possibly use the flex-id hook to overcome this (in certain situations
where there is something else to use as the identifier such as
circuit-id).  Alternatively, you could use the API to delete the lease
for 192.168.88.192 soon after granting so that it is free for your
other interface.  However, this could lead to strangeness should the
original interface try to renew (because the lease will no longer be
found and renewal will fail).  If it's possible, it would be best to
assign a separate address for each interface on the system.

Thank you,

Darren Ankney

On Sun, Oct 22, 2023 at 8:43 AM Kevin P. Fleming
<lists.kea-users at kevin.km6g.us> wrote:
>
> I've got this in my config file for the DHCPv4 server (using Kea 2.4.0:
>
>   "Dhcp4": {
>     "authoritative": true,
>     "valid-lifetime": 86400,
>     "reservations-global": false,
>     "reservations-in-subnet": true,
>     "reservations-out-of-pool": true,
>     "ip-reservations-unique": false,
> ...
>     "subnet4": [
>       {
>         "id": 88,
>         "subnet": "192.168.88.0/24",
>         "pools": [
>           {
>           "pool": "192.168.88.64/26"
>           },
>           {
>           "pool": "192.168.88.128/26"
>           }
>         ],
>         "reservations": [
>           {
>             "hw-address": "90:2e:16:d6:a4:1e",
>             "ip-address": "192.168.88.192",
>             "hostname":  "balrog22",
>             "user-context": {
>               "name": "balrog22"
>             }
>           },
>           {
>             "hw-address": "a8:64:f1:30:88:0b",
>             "ip-address": "192.168.88.192",
>             "hostname":  "balrog22",
>             "user-context": {
>               "name": "balrog22"
>             }
>           },
> ...
>
> In theory this should result in my laptop getting the same IPv4 address whether it is connected to the wired or wireless networks (which are the same VLAN/subnet).
>
> However, I see this in the log:
>
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: WARN  ALLOC_ENGINE_V4_DISCOVER_ADDRESS_CONFLICT [hwtype=1 90:2e:16:d6:a4:1e], cid=[00:81:74:fa:e6:94:98:99:8d:79:c0:a3:9c:87:27:72], tid=0x262eb546: conflicting reservation for address 192.168.88.192 with existing lease Address:       192.168.88.192
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Valid life:    86400
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Cltt:          1697977751
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Hardware addr: a8:64:f1:30:88:0b
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Client id:     01:a8:64:f1:30:88:0b
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Subnet ID:     88
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Pool ID:       0
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: State:         default
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Relay ID:      (none)
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: Remote ID:     (none)
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: INFO  DHCP4_LEASE_ADVERT [hwtype=1 90:2e:16:d6:a4:1e], cid=[00:81:74:fa:e6:94:98:99:8d:79:c0:a3:9c:87:27:72], tid=0x262eb546: lease 192.168.88.64 will be advertised
> Oct 22 08:29:55 core23-a kea-dhcp4[1650]: INFO  DHCP4_LEASE_ALLOC [hwtype=1 90:2e:16:d6:a4:1e], cid=[00:81:74:fa:e6:94:98:99:8d:79:c0:a3:9c:87:27:72], tid=0x262eb546: lease 192.168.88.64 has been allocated for 86400 seconds
>
> I'm running in the default multithreaded mode, so 'reservations-lookup-first' should be enabled (its default).
>
> I must be missing something, as I believe I'm doing exactly what the ARM says I should do to have multiple reservations for the same IP address, but it's not working.
>
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