ClearOS cannot resolve/forward DNS of internet domain

My setup of ClearOS including PPPoE on the WAN side and PCs on the LAN side.  After setup, although ClearOS can route packet from LAN to WAN, the built in DNS server at ClearOS cannot forward the DNS query to ISP's DNS server.  Only internal hostnames can be resolved.

The solution is to add ISP's DNS addresses, or Google's public DNS server (8.8.8.8) into  /etc/resolv.conf

nameserver 192.168.1.1

nameserver 8.8.8.8 (or your ISP's DNS)