Introducing FTP - HTML

File Transfer Protocol was created to easily move files between systems on the Internet. Dating back to the very early days of TCP/IP and the Internet, FTP hasn’t evolved much in the years it has been in service. FTP encapsulates several functions to transfer files, view files on both sides of the connection, and more.

FTP servers use TCP/IP ports 20 and 21. These ports are unique to the FTP service, allowing a server to run a Web server (port 80), an FTP server (ports 20 and 21), as well as other services at the same time. The FTP server sits patiently waiting for a client to request a connection on port 21. The client opens an unprivileged port greater than port 1024 and requests a connection from the server.

After the connection is authenticated the client can initiate commands. When data is transferred between the client and server, the server initiates the connection, using port 20—the client uses one port higher than the port used for commands. Figure shows a graphical representation of the connection and port arrangement. One problem with the traditional FTP process is that the server must initiate the data connection. This requires that the server be able to access the requisite port on the client to initiate the connection.

If the client is using a firewall, this could present a problem, as the firewall might prevent the server from accessing the correct port. Because the client port isn’t consistent, configuring the firewall to allow access is problematic. To solve this problem, a new mode of FTP was created. Passive mode (typically referred to as PASV) allows the client to initiate both connections.

A typical FTP connection.

A typical FTP connection.

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