Chapter 10 Install and System Administration for FortiOS 5.0 : Interfaces : Probing Interfaces
  
Probing Interfaces
Server probes can be used on interfaces. In order for this to occur, the probe response must first be enabled and configured, then the probe response must be allowed administrative access on the interface. Both steps must be done through the CLI.
Enabling and configuring the probe
config system probe-response
set http-probe-port <port>
set http-probe enable
end
Allowing the probe response to have administrative access to the interface
config system interface
edit <port>
set allowaccess probe-response
end
end