max-cache-size 50000000; 

recursive-clients 500

acl our-nets { 192.168.5.0/24; };
acl bogusnets {109.18.3.0/24; };

options {
            ...
    
     allow-query { our-nets; };
     allow-recursion { our-nets; };
     blackhole { bogusnets; };
};

zone "this.com" {
  type master;
  file "this.com";
  allow-query { any; };
};