In Active FTP who sends the PORT command?

In an Active FTP session, the client establishes the control connection to the server's port 21. For any subsequent data transfer (like a file listing or download), the client must inform the server how to connect back to it. The client achieves this by opening a high-numbered port locally and sending the PORT command over the control channel. This command contains the IP address and the specific port number the client is listening on, instructing the server to initiate the data connection from its port 20 to that client-specified endpoint.

B. The FTP Server: The server receives the PORT command and initiates the data connection to the client; it does not send this command in Active mode.

C. Both: Only the client sends the PORT command to specify its listening port for the data channel. The server only responds to it.

D. There is no PORT command in Active FTP: The PORT command is the fundamental mechanism used by the client to establish the data channel in Active FTP. The PASV command is used in Passive FTP.

1. IETF RFC 959 - FILE TRANSFER PROTOCOL (FTP):

Section 4.1.2

DATA CONNECTION MANAGEMENT COMMANDS

PORT (PORT): "The argument is a 32-bit internet host address and a 16-bit TCP port address... This address information is for the server to use to establish a second connection with the client for data transfer." This explicitly states the command provides information for the server

implying it is sent by the client.

Section 5.2

Establishing a Data Connection: This section details the sequence where the user-PI (client) sends a PORT command

and the server-PI initiates the data connection to the specified data port.

2. Fortinet Document Library - FortiOS Handbook 7.4.0

Security Profiles > Application Control:

Technical Tip: How the FTP protocol works: "In active mode

the client sends a PORT command to the server. This command contains the IP address and port number that the client is listening on for the data connection. The server then initiates a connection from its data port (port 20) to the client's data port."

3. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson.

Chapter 2

Section 2.1.4

File Transfer Protocol: FTP: The text and accompanying diagrams (e.g.

Figure 2.5) clearly illustrate the FTP control and data connection process. For "out-of-band" control

it describes the client sending a PORT command to the server to indicate the port number for the server to send data to. This is the defining characteristic of Active FTP.