KNOCKDNS - No Knock Edition is a dynamic DNS service that lets you update a hostname to your current IP address simply by fetching a secret URL. When you register, you receive a unique secret URL via email. Whenever you want to update your DNS record, just fetch that URL from the IP address you want the hostname to point to — using wget, curl, or any HTTP client. No port knocking software required.
Each update creates an address record (A for IPv4, AAAA for IPv6). An optional MX record can also be enabled at registration if you need to receive email at the hostname. You can get both A and AAAA records by fetching the URL from each address type.
For further technical information see the Wikipedia entry on Dynamic DNS.