Chapter 23 WAN Optimization, Web Cache, Explicit Proxy, and WCCP : The FortiGate explicit FTP proxy : Explicit FTP proxy configuration overview : Restricting the outgoing source IP address of the explicit FTP proxy
  
Restricting the outgoing source IP address of the explicit FTP proxy
You can use the following command to restrict the source address of outgoing FTP proxy packets to a single 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 restrict the outgoing packet source address to 172.20.120.100:
config ftp-proxy explicit
set outgoing-ip 172.20.120.100
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