According to the man page ddns-hostnameddns-hostname does not b]needneed to be set.
The ddns-hostname statement ddns-hostname name; The name parameter should be the hostname that will be used in set- ting up the client's A and PTR records. If no ddns-hostname is spec- ified in scope, then the server will derive the hostname automati- cally, using an algorithm that varies for each of the different update methods.