File Transfer Protocol (FTP
) is one of the oldest and most commonly used protocols found on the Internet today. Its purpose is to reliably transfer files between computer hosts on a network without requiring the user to log directly into the remote host or have knowledge of how to use the remote system. It allows users to access files on remote systems using a standard set of simple commands.
This section outlines the basics of the FTP
protocol, as well as configuration options for the primary FTP
server shipped with Fedora, vsftpd
.
12.2.1. The File Transfer Protocol
However, because FTP
is so prevalent on the Internet, it is often required to share files to the public. System administrators, therefore, should be aware of the FTP
protocol's unique characteristics.
12.2.1.1. Multiple Ports, Multiple Modes
Unlike most protocols used on the Internet, FTP
requires multiple network ports to work properly. When an FTP
client application initiates a connection to an FTP
server, it opens port 21
on the server — known as the command port. This port is used to issue all commands to the server. Any data requested from the server is returned to the client via a data port. The port number for data connections, and the way in which data connections are initialized, vary depending upon whether the client requests the data in active or passive mode.
The following defines these modes:
- active mode
Active mode is the original method used by the FTP
protocol for transferring data to the client application. When an active mode data transfer is initiated by the FTP
client, the server opens a connection from port 20
on the server to the IP
address and a random, unprivileged port (greater than 1024
) specified by the client. This arrangement means that the client machine must be allowed to accept connections over any port above 1024
. With the growth of insecure networks, such as the Internet, the use of firewalls to protect client machines is now prevalent. Because these client-side firewalls often deny incoming connections from active mode FTP
servers, passive mode was devised.
- passive mode
Passive mode, like active mode, is initiated by the FTP
client application. When requesting data from the server, the FTP
client indicates it wants to access the data in passive mode and the server provides the IP
address and a random, unprivileged port (greater than 1024
) on the server. The client then connects to that port on the server to download the requested information.
While passive mode resolves issues for client-side firewall interference with data connections, it can complicate administration of the server-side firewall. You can reduce the number of open ports on a server by limiting the range of unprivileged ports on the
FTP
server. This also simplifies the process of configuring firewall rules for the server. See
Section 12.2.5.8, “Network Options” for more information about limiting passive ports.