Caching-only is ready after installation of ISPadmin. It is sufficient to use IP of ISPadmin as DNS server, and the system will then contact DNS servers in the Internet.
Type 0.0.0.0/0; (or type specific networks for which the DNS server should serve as cache) in /etc/bind/named.conf.options. We recommend that you list specific networks for which you want to use the ISPadmin as DNS server. If you allow to use ISPadmin for all networks (0.0.0.0/0) its services may be abused by anybody for e.g. DDOS attacks.

nano /etc/bind/named.conf.options

allow-query { 192.168.0.0/16; 10.0.0.0/8; 127.0.0.1; 0.0.0.0/0; };

Then restart bind

/etc/init.d/bind9 restart

If ISPadmin is part of internal network, outcoming ports 53/TCP and 53/UDP must be enabled in a terminal router.

nano /etc/resolv.conf

nameserver 127.0.0.1

nameserver IP_DNS_of_used_server

Set DNS servers from the Internet here. If your customers will use ISPadmin as DNS server, the system will work as Cache, and DNS queries will be translated by servers listed here. If IP address of name server is unknown, use Google name server (8.8.8.8).