Depending on your DHCP and DNS servers, they might already have this built in. dnsmasq will maintain DNS entries for the DHCP leases it hands out, and can also add AAAA records if it sees a SLAAC message for a MAC address it gave a DHCP lease to. BIND’s servers (dhcpd, named, kea) can be set up so that for each DHCP lease assigned, a DDNS update is sent.