TCP/IP Tutorial and Technical Overview - IBM Redbooks

14.1.4 The passive data transfer
Contrary to the use of an active data connection, the passive data transfer
reverses the direction of establishment of the data connection. Instead of issuing
a PORT command, the client issues a PASV command, which uses no
parameters. Upon accepting this command, the FTP server sends back a reply
containing an IP address and port number. The client initiates a connection back
to the server on the indicated IP address and port. An example of this sequence
is illustrated in Figure 14-4.
FTP client
IP address
10.1.2.3
>LS
>227 Entering
Passive Mode
(10,4,5,6,8,9)
>125 List
started OK
>file1
>file2
>file3
>250 List
completed
successfully
Figure 14-4 The passive data connection
PASV
(Control Connection)
227 Entering Passive Mode (10,4,5,6,8,9)
(Control Connection)
(Data Connection)
NLST
(Control Connection)
125 List started OK
(Control Connection)
(Data Connection)
250 List completed successfully
(Control Connection)
FTP server
IP address
10.4.5.6
One of the reasons to use a passive data transfer is to bypass firewall
configurations that block active data connections. For this reason, passive mode
is often referred to as “firewall friendly mode.” An example of such a scenario is a
firewall that has been configured to block any inbound attempts to open a
connection. In this example, an FTP server responding to a client’s PORT
Chapter 14. File-related protocols 521