Hello Bill:

Our upcoming feature release, 4.4.0 that is due out 1/31/2018 and for which the beta is now available, has some new DDNS functionality that may solve your issue.

DDNS updates for dual-stack clients are problematic when the v4 clients do not comply by embedded the DUID in the client id as per RFC 4705.  As you are probably aware ISC DHCP has two modes of DDNS updates, "interim" which uses TXT records for conflict detection, and "standard" which uses DHCID records.  

4.4.0 adds a mode of conflict detection we call "DDNS Dual Stack Mixed Mode" ask DSMM.  DSMM allows the two modes to co-exist so you can configure all of your DHCPv4 servers to use one update mode (interim or standard) and all of your DHCPv6 servers to use the other mode. This allows your clients to use their v4 identity for their v4 DNS entries and their DUIDs for their v6 DNS entries.

DSMM allows you skirt around trying to devise mechanisms to manufacture v6 IDs in the v4 realm, and was added to address what is becoming a common real world scenario.  Ultimately, clients should be compliant but until or unless they are this is a way around it.

Regards,

Thomas Markwalder
ISC Software Engineering