The DEFAULTIPADDR setting is a very simple but effective security feature.
[DNS-Settings] DEFAULTIPADDR=192.168.17.18 ; Default: -
If DEFAULTIPADDR is not set then the DNS server serves everybody as expected with resolved names. If it is set, then requesters with an unknown IP address get always the default ip address as an answer to all name resolution requests. The intention of this feature is to forward all unknown clients to a predefined default address (e.g. a web server with registration facility). Since V1.9.3, this can be also defined on a per client basis. Please see AllowDNSQuery for details.
Please note that the PC running the DHCP server always resolves DNS requests regardless of the DEFAULTIPADDR setting.