TCP/IP Tutorial and Technical Overview - IBM Redbooks

command would receive an error when trying to open a connection to the
indicated IP address and port. However, by using passive mode, the client
initiates the connection from within the network, and the firewall allows the data
transfer to proceed. Other methods exist to resolve problems when FTPing
through a firewall, including proxy transfers (see 14.1.5, “Using proxy transfer” on
page 522) and the use of EPSV (see 14.1.8, “Using FTP with IPv6” on
page 525).
14.1.5 Using proxy transfer
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. This process is illustrated in Figure 14-5 on
page 523.
522 TCP/IP Tutorial and Technical Overview