Chapter 12 Load Balancing for FortiOS 5.0 : Configuring load balancing : HTTP and HTTPS load balancing, multiplexing, and persistence : HTTP and HTTPS multiplexing : Preserving the client IP address
  
Preserving the client IP address
Select preserve client IP from the web‑based manager or enable the http-ip-header option from the CLI to preserve the IP address of the client in the X-Forwarded-For HTTP header. This can be useful in an HTTP multiplexing configuration if you want log messages on the real servers to the client’s original IP address. If this option is not selected, the header will contain the IP address of the FortiGate unit.