Chapter 3 Authentication for FortiOS 5.0 : Configuring authenticated access : Authentication in security policies : Access to the Internet
  
Access to the Internet
A policy for accessing the Internet is similar to a policy for accessing a specific network, but the destination address is set to all. The destination interface is the one that connects to the Internet Service Provider (ISP). For general purpose Internet access, the Service is set to ANY.
Access to HTTP, HTTPS, FTP and Telnet sites may require access to a domain name service. DNS requests do not trigger authentication. You must configure a policy to permit unauthenticated access to the appropriate DNS server, and this policy must precede the policy for Internet access. Failure to do this will result in the lack of a DNS connection and a corresponding lack of access to the Internet.