Andrew Hudson - Fedora™ Unleashed, 2008 edition

Mon Sep 3 02:35:35 2001 1 helios 8 /var/ftp/pub/tftp-server- 0.17-14.i386.rpm a o a testing@test.com ftp 0 * c

Mon Sep 3 02:35:35 2001 1 helios 8 /var/ftp/pub/wu-ftpd-2.6.1- 22.i386.rpm a o a testing@test.com ftp 0 * c

You use these commands to install, configure, and manage FTP services in Fedora:

► epiphany — A graphical GNOME browser supporting FTP

► ftp — A text-based interactive FTP command

► ftpcopy — Copy directories and files from an FTP server

► ftpcp — Retrieve data from a remote FTP server, but do not overwrite existing local files

► gftp — A graphical FTP client for GNOME

► konqueror — KDE's graphical web browser

► lftp — An advanced text-based FTP program

► nautilus — Red Hat's graphical file explorer and browser

► ncftp — A sophisticated, text-based FTP program

► sftp — Secure file transfer program

► smbclient — Samba FTP client to access SMB/CIFS resources on servers

► system-config-services — Red Hat's system service GUI admin utility

► vsftpd — The Very Secure FTP daemon

► webcam — A webcam-oriented FTP client included with xawtv

► http://www.wu-ftpd.org/— wu-ftpofficial website.

► http://www.cert.org/— Computer emergency response team.

► http://www.openssh.com/— OpenSSH home page and source for the latest version of OpenSSH and its component clients, such as sftp.

► http://www.cert.org/tech_tips/anonymous_ftp_config.html— CERT anonymous FTP configuration guidelines.

► http://vsftpd.beasts.org/— Home page for the vsftdFTP server.

► ftp://vsftpd.beasts.org/users/cevans/— Download site for the latest releases of the vsftpdserver.

Email is still the dominant form of communication over the Internet. It is fast, free, and very easy to use. However, much of what goes on behind the scenes is extremely complicated and would appear scary to anyone who does not know much about how email is handled. Fedora comes equipped with a number of powerful applications to help you build anything from a small email server right through to large servers handling thousands of messages.

This chapter shows you how to configure Fedora to act as an email server. We look at the options available in Fedora, as well as the pros and cons of each one. You will also learn how mail is handled in Linux, and to a lesser extent, UNIX.

Email is transmitted as plain text across networks around the world using SMTP (Simple Mail Transfer Protocol) . As the name implies, the protocol itself is fairly basic, and it has been extended to add further authentication and error reporting/messaging to satisfy the growing demands of modern email. Mail transfer agents , or MTAs, work in the background, transferring email from server to server, allowing emails to be sent all over the world. You might have come across such MTA software such as Sendmail, Postfix, Fetchmail, Exim, or Qmail.

SMTP enables each computer through which the email passes to forward it in the right direction to the final destination. When you consider that there are millions of email servers across the world, you have to marvel at how simple it all seems.

Here is a simplified example of how email is successfully processed and sent to its destination:

1. andrew@hudson.orgcomposes and sends an email message to paul@hudzilla.org.

2. The MTA at hudson.orgreceives andrew's email message and queues it for delivery behind any other messages that are also waiting to go out.

3. The MTA at hudson.orgcontacts the MTA at hudzilla.orgon port 25. After hudzilla.orgacknowledges the connection, the MTA at hudson.orgsends the mail message. After hudzilla.orgaccepts and acknowledges receipt of the message, the connection is closed.

4. The MTA at hudzilla.orgplaces the mail message into paul's incoming mailbox; paulis notified that he has new mail the next time he logs on.

Of course, several things can go wrong during this process. Consider these examples:

► What if pauldoes not exist at hudzilla.org? In this case, the MTA at hudzilla.orgrejects the email and notifies the MTA at hudson.orgof what the problem is. The MTA at hudson.orgthen generates an email message and sends it to andrew@hudson.org, informing him that no paulexists at hudzilla.org(or perhaps just silently discards the message and gives the sender no indication of the problem, depending on how the email server is configured).

► What happens if hudzilla.orgdoesn't respond to hudson.org's connection attempts? (Perhaps the server is down for maintenance.) The MTA at hudson.orgnotifies the sender that the initial delivery attempt has failed. Further attempts will be made at intervals decided by the server administrator until the deadline is reached, and the sender then is notified that the mail is undeliverable.

Several MTAs are available for Fedora, each with pros and cons to being used. Normally they are hidden under the skin of Fedora, silently moving mail between servers all over the world with need for little or no maintenance. Some MTAs are extremely powerful, and are able to cope with hundreds of thousands of messages each day, whereas others are geared toward smaller installations. Other MTAs are perhaps not as powerful, but are packed full with features. The next section takes a look at some of the more popular MTAs available for Fedora.

The overwhelming majority of emails transmitted over the Internet today are handled by Sendmail, which just so happens to be the default MTA supplied with Fedora. It is extremely popular across the Linux/UNIX/BSD world and is well supported. There is a commercial version available, which has a GUI interface for ease of configuration.

As well as being popular, Sendmail is particularly powerful compared to some of the other MTAs. However, it is not without its downsides, and other MTAs can handle more email per second in a larger environment. The other issue with Sendmail is that it can be extremely complicated to set it up exactly as you want it. In fact, the level of complexity associated with Sendmail often leads to system administrators replacing it with one of the other alternatives that is easier to configure. There are a few books available specifically for Sendmail, but the most popular one has more than a thousand pages, reflecting the complex nature of Sendmail configuration.

The good news, however, is that the default configuration for Sendmail works fine for most basic installations out of the box, making further configurations unnecessary. Even if you want to use it as a basic email server, you have to do only some minor tweaks. We take a look at some basic Sendmail configuration later in this chapter in the section titled "Basic Sendmail Configuration and Operation."