dnssec-keygen -a HMAC-MD5 -b 128 -r /dev/urandom -n USER DDNS_UPDATE
-a algorithm
Selects the cryptographic algorithm. For DNSSEC keys, the value of algorithm must be one of RSAMD5, RSASHA1, DSA, NSEC3RSASHA1, NSEC3DSA, RSASHA256, RSASHA512, ECDSAP256SHA256, ECDSAP384SHA384, ED25519 or ED448. For TKEY, the value must be DH (Diffie Hellman); specifying his value will automatically set the -T KEY option as well.