I run PowerDNS as my DNS server. This supports rfc2136 (iirc) which can be configured via the dnsupdate tool for example. and then a webpage that allows to register new names. i added two columns to the powerdns databse “dyndns” as a flag if this domain can be used for ddns and “webkey” as an alternative update mechanism via wget. some small php script that does the sign-up and update in the powerdns database.