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 both an address record and an MX record pointing to it, so your hostname works for both web and email. If you connect from an IPv4 address an A record is created; from an IPv6 address an AAAA record is created. You can get both by fetching the URL from each address type.
For further technical information see the Wikipedia entry on Dynamic DNS.