zone "acme.example.com" IN {
        type master;
        file "/etc/bind/zones/acme.example.com.zone";

        update-policy {
                grant client1.example.acme. subdomain client1.example.acme.example.com. TXT;
                grant client2.test.acme. subdomain client2.test.acme.example.com. TXT;
        };
};