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Reliable, flexible, and configurable enough to solve the mail routing needs of any web site, sendmail has withstood the test of time, but has become no less daunting in its complexity. Even the most experienced system administrators have found it challenging to configure and difficult to understand. For help in unraveling its intricacies, sendmail administrators have turned unanimously to one reliable source--the bat book, or sendmail by Bryan Costales and the creator of sendmail, Eric Allman. Now in its third ...
Reliable, flexible, and configurable enough to solve the mail routing needs of any web site, sendmail has withstood the test of time, but has become no less daunting in its complexity. Even the most experienced system administrators have found it challenging to configure and difficult to understand. For help in unraveling its intricacies, sendmail administrators have turned unanimously to one reliable source--the bat book, or sendmail by Bryan Costales and the creator of sendmail, Eric Allman. Now in its third edition, this best-selling reference will help you master the most demanding version of sendmail yet.The new edition of sendmail has been completely revised to cover sendmail 8.12--a version with more features and fundamental changes than any previous version of the Unix-based email routing program. Because the latest version of sendmail differs so significantly from earlier versions, a massive rewrite of this best-selling reference was called for.The book begins by guiding you through the building and installation of sendmail and its companion programs, such as vacation and makemap. These additional programs are pivotal to sendmail's daily operation. Next, you'll cover the day-to-day administration of sendmail. This section includes two entirely new chapters, "Performance Tuning" to help you make mail delivery as efficient as possible, and "Handling Spam" to deal with sendmail's rich anti-spam features. The next section of the book tackles the sendmail configuration file and debugging. And finally, the book wraps up with five appendices that provide more detail about sendmail than you may ever need. Altogether, versions 8.10 through 8.12 include dozens of new features, options, and macros, and this greatly expanded edition thoroughly addresses each, and provides and advance look at sendmail version 8.13 (expected to be released in 2003).With sendmail, Third Edition in hand, you will be able to configure this challenging but necessary utility for whatever needs your system requires. This much anticipated revision is essential reading for sendmail administrators.
The post office transports real letters in real envelopes, whereas sendmail transports electronic letters in electronic envelopes. If your friend (the recipient) is in the same neighborhood (on the same machine), only a single post office (sendmail running locally) is involved. If your friend is distant, the mail message will be forwarded from the local post office (sendmail running locally) to a distant one (sendmail running remotely) for delivery. Although sendmail is similar to a post office in many ways, it is superior in others:
This would be like the post office using United Parcel Service to deliver an overnight letter.
This analogy between a post office and sendmail will break down as we explore sendmail in more detail. But the analogy serves a role in this introductory material, so we will continue to use it to illuminate a few of sendmail's more obscure points.
A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office. Usually, there is only one MTA on a machine. The sendmail program is an MTA. Others include MMDF, Smail 3.x, and Zmailer, but we'll cover only sendmail in this book.
The sendmail program needs to transport mail between a wide variety of machines. Consequently, its configuration file is designed to be very flexible. This concept allows a single binary to be distributed to many machines, where the configuration file can be customized to suit particular needs. This configurability contributes to making sendmail complex.
Consider, for example, when mail needs to be delivered to a particular user. The sendmail program decides on the appropriate delivery method based on its configuration file. One such decision process might include the following steps:
Rewriting rules and rule sets also appear in the configuration file. They transform a mail address into another form that may be required for delivery. They are perhaps the single most confusing aspect of the configuration file. Because the configuration file is designed to be fast for sendmail to read and parse, rules can look cryptic to humans:
R$+@$+ $:$1<@$2> focus on domain
R$+<$+@$+> $1$2<@$3> move gaze right
But what appears to be complex is really just succinct. The R at the beginning of each line, for example, labels a rewrite rule. And the $+ expressions mean to match one or more parts of an address. With experience, such expressions (and indeed the configuration file as a whole) soon become meaningful.