Site to Site transfers are not compatible with most home network solutions. This includes most software/hardware that uses Network Address Translations (NAT).
NAT will effect site to site transfers in both instances.
FTP SERVER > NAT > INTERNET or FTP CLIENT > NAT > INTERNET
The problem arises from NAT's protocol helpers, which monitor most common protocols for common commands that often send the wrong IP address to the server. In the case of the FTP protocol this happens to be the PORT command. When performing normal FTP transfers the PORT command contains your local machine IP, behind a NAT firewall/router this IP is incorrect so NAT modifies the IP to that of the router/firewall. However when the same rules are applied to site to site transfers it causes them to fail. One of FTP server ends up trying to connect to your router/firewall and not the other FTP server.
The only solution I am aware of is to run both FTP servers on a non-standard port, Most NAT firewall/router only monitor port 21, so if the FTP server was run on port 2121 it would not be effected. But this also has a catch 22, now that you're running the FTP server on port 2121 you will not be able to use the PORT command for file listing without NAT.
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Home Network and FXP problem
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