41.1. Introduction to WU-FTPD

WU-FTPD (the WU stands for Washington University) is probably the most popular UNIX FTP server on the Internet and is included by default with most UNIX and Linux operating systems. Its large number of configurable options make it superior to the “classic,” or BSD FTP, server that is still used by some flavors of UNIX, but it is not as flexible or cleanly implemented as ProFTPD, which is covered in Chapter 40. That chapter also has a brief general introduction to the FTP protocol, which you should read before going further if you are unfamiliar with concepts such as FTP clients and servers.

In its normal default configuration, WU-FTPD will allow any UNIX user (except for system users) to log in with their standard usernames and passwords and upload, download, and manipulate files on the server system with the same permissions that they would have if connected via telnet or SSH. It can also be set up to support anonymous logins, so that anyone can connect without needing a valid UNIX account, although anonymous clients are typically restricted to a certain directory and prevented from uploading files.

WU-FTPD's primary configuration file is called /etc/ftpaccess, but it also makes use of several other files such as /etc/ftpusers and /etc/ftphosts. The ftpaccess file contains a series of directives—one per line—each of which has a name and several values. Each directive sets a single option, such as the path to a message file or a directory alias.

Like ProFTPD, WU-FTPD can be run either as a permanent standalone daemon process or from a super server like inetd or xinetd. Typically the latter option is used, as this removes the need for an additional server process to be running at all times waiting for an FTP connection. As far as clients and the configuration file are concerned, there is no difference between either mode other than performance.