1) Follow instructions here:
Mac OS X Server (v10.3 or Later): Network Services Administration (Manual)
2) Set up DNS Server to use ISP's name server to query domains not defined in the DNS settings.
Open /etc/named.conf.
In the options directive, insert the following lines:
forwarders {
               123.456.789.012;
               123.456.789.013;
               123.456.789.014;
          };
     forward only;
