Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DNS services : DNS settings
  
DNS settings
Basic DNS queries are configured on interfaces that connect to the Internet. When a web site is requested, for example, the FortiGate unit will look to the configured DNS servers to provide the IP address to know which server to contact to complete the transaction.
DNS server addresses are configured by going to System > Network > DNS. Here you specify the DNS server addresses. Typically, these addresses are supplied by your ISP. An additional option is available if you have local Microsoft domains on the network, by entering a domain name in the Local Domain Name field.
In a situation where all three fields are configured, the FortiGate unit will first look to the local domain. If no match is found, a request is sent to the external DNS servers.
If virtual domains are enabled, you create a DNS database in each VDOM. All of the interfaces in a VDOM share the DNS database in that VDOM.
See Also
Dynamic DNS
DNS services
Additional DNS CLI configuration
DNS server
Recursive DNS