If you are chaining multiple Web proxy servers together, you can specify that requests containing FTP URLs are to be sent to a chained Web proxy server rather than directly to the FTP server. To specify a chained proxy server for FTP requests, in the Configuration and Administration forms, select Proxy Configuration -> Proxy Chaining and Non-Proxy Domains. The http:// protocol scheme is used to specify the URL of the chained proxy, even when chaining requests for an ftp:// protocol scheme.
To configure FTP chaining by using the proxy configuration file, see the reference section on ftp_proxy -- Specify another proxy server for FTP requests.