Discuss FTP proxy transfer through firewall

FTP provides the ability for a client to have data transferred from one FTP server to another FTP server. Several justifications for such a transfer exist, including:
To transfer data from one host to another when direct access to the two hosts are not possible.

To bypass a slow client connection.
To bypass a firewall restriction.
To reduce the amount of traffic within the client’s network The process of setting up a proxy transfer begins with the use of a proxy open command.

 Any FTP command can then be sent to the proxy server by preceding the command with proxy. For example, executing the dir command lists the files on the primary FTP server. Executing the proxy dir command lists the files on the proxy server. The proxy get and proxy put commands can then be used to transfer data between the two hosts.

1. The FTP client opens a connection and logs on to the FTP server A.
2. The FTP client issues a proxy open command, and a new control connection is established with FTP server B. 

3. The FTP client then issues a proxy get command (though this can also be a proxy put). 

4. A data connection is established between server A and server B. Following data connection establishment, the data flows from server B to server A.