Книга: Fedora™ Unleashed, 2008 edition
The Mail Transport Agent
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The Mail Transport Agent
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.
Sendmail
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."
Postfix
Postfix has its origins as the IBM Secure Mailer, but was released to the developer community by IBM. Compared to Sendmail, it is much easier to administer and has a number of speed advantages. Postfix offers a pain-free replacement for Sendmail, and you are able to replace Sendmail with Postfix without the system breaking a sweat. In fact, when you install Postfix in place of Sendmail, applications that relied on Sendmail automatically use Postfix instead and carry on working correctly. Postfix uses a Sendmail wrapper, which deceives other programs into thinking that Postfix is Sendmail. This wrapper, or more correctly, interface, makes switching to Postfix extremely easy.
CAUTION
Fedora provides Postfix version 2.4, which uses a slightly different configuration than the earlier version. If you are upgrading Postfix from an earlier Fedora or Red Hat version, check your configuration files.
Fedora also now compiles Postfix and Sendmail against version 2.1 of the Cyrus SASL library (an authentication library). The Release Notes contain detailed information on file location and option changes that affect you if you use these libraries.
For enhanced security, many Postfix processes used to use the chroot facility (which restricts access to only specific parts of the file system) for improved security, and there are no setuid components in Postfix. With the current release of Fedora, a chroot configuration is no longer used and is, in fact, discouraged by the Postfix author. You can manually reconfigure Postfix to a chroot configuration, but that is no longer supported by Fedora.
If you are starting from scratch, Postfix is considered a better choice than Sendmail.
Qmail and Exim
Qmail is a direct competitor to Postfix but is not provided with Fedora. Postfix is designed to be easier to use than Sendmail, as well as faster and more secure. However, Qmail isn't a drop-in replacement for Sendmail, so migrating an existing Sendmail installation to Qmail is not quite as simple as migrating from Sendmail to Postfix. Qmail is relatively easy to administer, and it integrates with a number of software add-ons, including web mail systems and POP3 servers. Qmail is available fromhttp://www.qmail.org/.
Exim is yet another MTA, and it is available using yum. Exim is considered faster and more secure than Sendmail or Postfix, but is much different to configure than either of those. Exim and Qmail use the maildir format rather than mbox, so both are considered "NFS safe" (see the following sidebar).
MDIR Versus Mailbox
Qmail also introduced maildir, which is an alternative to the standard UNIX method of storing incoming mail. maildir is a more versatile system of handling incoming email, but it requires your email clients to be reconfigured, and it is not compatible with the traditional UNIX way of storing incoming mail. You have to use mail programs that recognize the maildir format. (Modern programs do.)
The traditional mbox format keeps all mail assigned to a folder concatenated as a single file and maintains an index of individual emails. With maildir, each mail folder has three subfolders: /cur, /new, and /tmp. Each email is kept in a separate, unique file. If you are running a mail server for a large number of people, you should select a file system that can efficiently handle a large number of small files. mbox does have one major disadvantage. While you are accessing the monolithic mbox file that contains all your email, suppose that some type of corruption occurs, either to the file itself or to the index. Recovery from this problem can be difficult. The mbox files are especially prone to problems if the files are being accessed over a network and can result in file corruption; you should avoid accessing mbox mail mounted over NFS, the network file system, because sudden connection loss can seriously corrupt your mbox file.
Depending on how you access your mail, maildir does permit the simultaneous access of maildir files by multiple applications; mbox does not.
The choice of a mail user agent, or email client, also affects your choice of mail directory format. For example, the pine program does not cache any directory information and must reread the mail directory any time it accesses it. If you are using pine, maildir would be a poor choice. More advanced email clients perform caching, so maildir might be a good choice, although the email client cache can get out of synchronization. It seems that there is no perfect choice.
Fedora provides you with mail alternatives that have both strong and weak points. Be aware of the differences among the alternatives and frequently reevaluate your selection to make certain that it is the best one for your circumstances.
Оглавление книги
- The Mail Delivery Agent
- Mail Routing on the Internet
- Defining mail transport protocols
- Analyzing Mail Statistics
- Transport Layer
- V-mail
- 4.4.4 The Dispatcher
- About the author
- Chapter 7. The state machine
- Appendix E. Other resources and links
- Example NAT machine in theory
- The final stage of our NAT machine