Organization

sendmail configuration is a means to an end. sendmail is configured in order to perform specific functions effectively. This book focuses on the proper configurations for those functions. sendmail configuration language elements are secondary. The configuration code is explained in sufficient detail for you to fully understand it, so that you can tune the code as necessary for your system. But the goal is to provide useable solutions for common problems. This goal dictates a book that is organized around sendmail's functions

The first chapter defines the basic framework upon which all of the other chapters are built. It provides recipes for downloading and installing the sendmail distribution, for recompiling sendmail to support a variety of features, for building the sendmail configuration, and for testing a new configuration. Starting with Chapter 2 and running through Chapter 10, sample solutions for properly configuring important sendmail functions are given. An overview of the functions and commands used in each chapter are provided by the chapter's Introduction.

The recipes in Chapter 2 through Chapter 10 standalone. An administrator can jump directly to an individual solution and come away with enough information to solve a specific problem. Most of these solutions address only one problem. Your configuration will probably need to address multiple problems. You can jump around in this book selecting just those items you need. For example, your server might need to accept mail for several clients, and it might need to hide the hostnames of those clients on outbound mail. Such a configuration would need recipes from Chapter 2 and Chapter 4. Select just those things that suit your needs.

A quick synopsis of the chapters in this book follows :

Chapter 1, describes the files and directories used to build a sendmail configuration. It provides how-to recipes for downloading, installing, and compiling sendmail and for building and testing a sendmail configuration.

Chapter 2, focuses on mail delivery. The sendmail configuration controls what mail is accepted for delivery. Accepting the correct mail is essential for creating a server, particularly one that acts as a mail exchanger . Additionally, only mail that is accepted for delivery can be aliased or forwarded. Properly configured aliasing supports clients and creates mailing lists.

Chapter 3, focuses on mail relaying. When mail is not accepted for delivery by the local server, sendmail must decide if it should be relayed to another server for delivery. Properly configuring relaying is essential to creating a fully functional server, and it is a primary ingredient in controlling spam. A mistake in relay configuration can get your server blacklisted!

Chapter 4, describes why and how the true identity of the end system in a mail exchange is hidden. Masquerading hides the source address of outgoing mail. Both basic masquerading and the genericstable database are covered. Configurations that hide both the host portion and the user portion of the source address are given.

Chapter 5, describes how the administrator controls mail routing through the use of sendmail databases. The mailertable is used to route mail to a specific mailer for special processing. The mailertable provides access to the wide variety of mailers provided by sendmail. Virtual mail domains allow a single mail server to process mail for many different domains. Using the virtusertable database to handle virtual mail domains is covered. The ldap_routing feature, which reads intranet mail routing information from an LDAP server, is also covered.

Chapter 6, describes how to configure sendmail to reduce the problem of unsolicited commercial email. The chapter discusses how the access database is used to control spam; how sendmail is configured to use procmail for personal, local, and outbound mail filtering; how blackhole listing services are used; and how to build your own DNS blackhole list. Custom header processing and sendmail.cf regular expressions are also covered.

Chapter 7, provides solutions for configuring sendmail to act as an SMTP AUTH server or client. Sample sendmail configurations are provided, as are the necessary SASL configuration files. How the access database is used with AUTH authentication is also covered.

Chapter 8, covers STARTTLS, the sendmail feature used to encrypt the mail transport. The sendmail configuration of STARTTLS is given along with the required configuration of the SSL tools used by sendmail. Creating a private sendmail CA, client and server certificates, and signing certificates are all covered. Recipes that control when encryption is used are provided, as are examples of using STARTTLS with the access database.

Chapter 9, covers sendmail configurations that are particularly useful for systems with large mail queues. Creating multiple queues, defining queue groups, and using queue groups with the access database are all covered.

Chapter 10, provides recipes that can increase sendmail security. By default, sendmail has tight security settings. Some configurations loosen sendmail security to increase flexibility. The recipes in this chapter take the opposite approach. These recipes are for those willing to sacrifice flexibility for additional security.