Chapter 10 Install and System Administration for FortiOS 5.0 : FortiGuard : Web filtering
  
Web filtering
The multiple FortiGuard data centers around the world hold the entire categorized URL database and receive rating requests from customer-owned FortiGate units, typically triggered by browser-based URL requests. When these rating requests are responded to with the categories stored for specific URLs, the requesting FortiGate unit will then use its own local profile configuration to determine what action to take, for example blocking, monitoring, or permitting the URL request.
Rating responses can also be cached locally on the FortiGate unit, providing a quicker response time while easing load on the FortiGuard servers and aiding in a quicker response time for less common URL requests. This is a very effective method for common sites such as search engines and other frequently visited sites. Other sites that are less frequently visited can also be cached locally for a determined amount of time.
By default, the web filtering cache is enabled. The cache includes a time-to-live value, which is the amount of time a URL will stay in the cache before expiring. You can change this value to shorten or extend the time between 300 and 86400 seconds. For a site such as Google, the frequency of its access can keep it in the cache, while other sites can remain in the cache up to 24 hours, or less depending on the configuration.
See Also
Web Filtering and Email Filtering Options
URL verification
Antivirus and IPS Options
FortiGuard Services
Antivirus and IPS
Web filtering
Email filtering
Security tools
Troubleshooting