Chapter 15 Unified Threat Management for FortiOS 5.0 : Web filter : FortiGuard Web Filtering Service : FortiGuard Web Filter and your FortiGate unit
  
FortiGuard Web Filter and your FortiGate unit
When FortiGuard Web Filter is enabled in a web filter profile, the setting is applied to all firewall policies that use this profile. When a request for a web page appears in traffic controlled by one of these firewall policies, the URL is sent to the nearest FortiGuard server. The URL category is returned. If the category is blocked, the FortiGate unit provides a replacement message in place of the requested page. If the category is not blocked, the page request is sent to the requested URL as normal.
Figure 311: Webfiltering flowchart