One down side to FXP is that alot of routers now provide NAT (Network address translations) for seamless protocol forwarding.
So when the client send the PORT command the router maps the port locally, changing the original value. This provides transparent connectivity, and without it you would be forced to use PASV mode.
However the router is not smart enough to know the difference between local and FXP. Thus breaking FXP support.
The only solution I am aware of is to turn off NAT or ask the site owner to use a non-standard port. Most routers only monitor port 21 for ftp traffic. So if the FTP server is on port 414 the router wont modify the protocol.
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