Answer by Sergey Ponomarev for Dyndns2 protocol specification?

The RFC 2136 spec is an extension of the DNS itself and used by nsupdate command. It's complicated and is used by sysadmins.

Routers are using just a plain HTTP GET requests to some DDNS provider. Their urls are looking the same as original Dyn.com have: /nic/update?hostname=[DOMAIN]&myip=[IP].

See:

• https://openwrt.org/docs/guide-user/services/ddns/client
• https://sourceforge.net/p/ddclient/wiki/protocols/