Chapter 11 IPsec VPN for FortiOS 5.0 : Dynamic DNS configuration : Dynamic DNS topology : Assumptions
  
Assumptions
You have administrator access to both FortiGate units.
Both FortiGate units have interfaces named wan1 and internal. (If not, you can use the alias feature to assign these labels as “nicknames” to other interfaces to follow this example.)
Both FortiGate units have the most recent firmware installed, have been configured for their networks, and are currently passing normal network traffic.
The branch_2 FortiGate unit has its wan1 interface defined as a dynamic DNS interface with the domain name of example.com.
A basic gateway-to-gateway configuration is in place (see “Gateway-to-gateway configurations”) except one of the FortiGate units has a static domain name and a dynamic IP address instead of a static IP address.
The FortiGate unit with the domain name is subscribed to one of the supported dynamic DNS services. Contact one of the services to set up an account. For more information and instructions about how to configure the FortiGate unit to push its dynamic IP address to a dynamic DNS server, see the System Administration handbook chapter.