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 chapter outlines the basics of the FTP protocol, as well as configuration options for the primary FTP server shipped with Red Hat Enterprise Linux, vsftpd.
FTP uses a client server architecture to transfer files using the TCP network protocol. Because FTP is an older protocol, it uses unencrypted username and password authentication. For this reason, it is considered an insecure protocol and should not be used unless absolutely necessary. A good substitute for FTP is sftp from the OpenSSH suite of tools. For information about configuring OpenSSH, refer to the chapter titled OpenSSH in Red Hat Enterprise Linux System Administration Guide. For more information about the SSH protocol, refer to Chapter 19 SSH 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.
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 are descriptions of these two modes:
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, 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. Limiting the range of unprivileged ports offered for passive connections in the FTP server's configuration file is one way to limit the number of open ports on a server and simplify the task of creating firewall rules for the server. Refer to Section 14.5.8 Network Options for more about limiting passive ports.