Chapter 1 What’s New for FortiOS 5.2.1 : Other New Features : Changing the Header Name of Load Balanced HTTP/HTTPS Traffic
  
Changing the Header Name of Load Balanced HTTP/HTTPS Traffic
A header name can now be configured for HTTP and HTTPS traffic that flows through a virtual server, rather than using the default X-Forward-For header.
In order to use this feature, the HTTP IP header must be enabled.
Syntax
config firewall vip
edit <name>
set type server-load-balance
set server-type {http | https}
set http-ip-header enable
set http-ip-header-name <name>
end
end