FTP Inspection in the Zone Policy Firewall

Figure 12-4 revisits the two modes of operation for the File Transfer Protocol (FTP):

• Active mode: The client opens a control connection on TCP port 21 and negotiates a data port using the PORT command. After accepting the port proposed by the client, the server opens a data connection to it (using TCP/20 as source port).

• Passive mode: The client requests Passive mode operation by issuing the PASV command over the control connection on TCP port 21. The server suggests a data port, to which the client must connect (using a second source port, randomly selected).