ddns-update-on-renew and cache-threshold still produce ddns updates
If a Kea server (2.4.1) has these settings:
...
"cache-threshold": 0.25,
"ddns-update-on-renew": true,
...and a client renews their lease at under 25% of the lease length, ddns updates are still sent.
$ sudo tail -f /var/log/kea/kea-dhcp4.log | egrep 'DHCP4_LEASE_REUSE|DHCPSRV_DHCP_DDNS_NCR_SENT'
2024-02-15 16:46:35.032 INFO [kea-dhcp4.leases/1192.140241126283008] DHCP4_LEASE_REUSE [hwtype=1 c8:7f:54:9e:cf:c8], cid=[01:c8:7f:54:9e:cf:c8], tid=0x17c6ef6f: lease 192.168.20.20 has been reused for 24845 seconds
2024-02-15 16:46:35.033 DEBUG [kea-dhcp4.dhcpsrv/1192.140241211900608] DHCPSRV_DHCP_DDNS_NCR_SENT NameChangeRequest sent to kea-dhcp-ddns: Type: 1 (CHG_REMOVE)
2024-02-15 16:46:35.033 DEBUG [kea-dhcp4.dhcpsrv/1192.140241211900608] DHCPSRV_DHCP_DDNS_NCR_SENT NameChangeRequest sent to kea-dhcp-ddns: Type: 0 (CHG_ADD)
2024-02-15 16:46:40.807 INFO [kea-dhcp4.leases/1192.140241159853824] DHCP4_LEASE_REUSE [hwtype=1 c8:7f:54:9e:cf:c8], cid=[01:c8:7f:54:9e:cf:c8], tid=0xfbfead04: lease 192.168.20.20 has been reused for 24840 seconds
2024-02-15 16:46:40.807 DEBUG [kea-dhcp4.dhcpsrv/1192.140241211900608] DHCPSRV_DHCP_DDNS_NCR_SENT NameChangeRequest sent to kea-dhcp-ddns: Type: 1 (CHG_REMOVE)
2024-02-15 16:46:40.808 DEBUG [kea-dhcp4.dhcpsrv/1192.140241211900608] DHCPSRV_DHCP_DDNS_NCR_SENT NameChangeRequest sent to kea-dhcp-ddns: Type: 0 (CHG_ADD)The customer who brought this to our attention notes:
While looking at some customer logs I noticed that we were both reusing a lease and doing DDNS update for that reused lease. It seems like if a lease is being reused and therefore it doesn't have any changes to the client DNS name that Kea shouldn't redo the DDNS even if the configuration has update on renew enabled. As soon as the device renews outside of the threshold window I would expect it to do a DDNS update based on the update on renew option. I've looked at the code and the design. It appears that in dhcp4_srv.cc:assignLease() the call to createNameChangeRequests() is called without checking the threshold and the threshold isn't checked within that function. The design doesn't explicitly say anything but seems to suggest that the DDNS update shouldn't be done if a lease is reused.
I am unsure if this is a feature request or a bug report as I am not sure of the intended behavior here. It seems like it would be preferable to reduce load by not sending ddns updates on a reused lease.