Chapter 23 WAN Optimization, Web Cache, Explicit Proxy, and WCCP : The FortiGate explicit FTP proxy : Explicit FTP proxy configuration overview : Restricting the IP address of the explicit FTP proxy
  
Restricting the IP address of the explicit FTP proxy
You can use the following command to restrict access to the explicit FTP proxy using only one IP address. The IP address that you specify must be the IP address of an interface that the explicit FTP proxy is enabled on. You might want to use this option if the explicit FTP proxy is enabled on an interface with multiple IP addresses.
For example, to require uses to connect to the IP address 10.31.101.100 to connect to the explicit FTP proxy:
config ftp-proxy explicit
set incoming-ip 10.31.101.100
end