Chapter 23 WAN Optimization, Web Cache, Explicit Proxy, and WCCP : The FortiGate explicit web proxy : Proxy chaining (web proxy forwarding servers)
  
Proxy chaining (web proxy forwarding servers)
For the explicit web proxy you can configure web proxy forwarding servers to use proxy chaining to redirect web proxy sessions to other proxy servers. Proxy chaining can be used to forward web proxy sessions from the FortiGate unit to one or more other proxy servers on your network or on a remote network. You can use proxy chaining to integrate the FortiGate explicit web proxy with an web proxy solution that you already have in place.
A FortiGate unit can forward sessions to most web proxy servers including a remote FortiGate unit with the explicit web proxy enabled. No special configuration of the explicit web proxy on the remote FortiGate unit is required.
You can deploy the explicit web proxy with proxy chaining in an enterprise environment consisting of small satellite offices and a main office. If each office has a FortiGate unit, users at each of the satellite offices can use their local FortiGate unit as an explicit web proxy server. The satellite office FortiGate units can forward explicit web proxy sessions to an explicit web proxy server at the central office. From here the sessions can connect to web servers on the Internet.
FortiGate proxy chaining does not support authenticating with the remote forwarding server.
This section also describes:
Adding a web proxy forwarding server
Web proxy forwarding server monitoring and health checking
Grouping forwarding servers and load balancing traffic to them
Adding proxy chaining to an explicit web proxy policy