It's been said that you aren't a real Unix system administrator until you've edited a sendmail.cf file. It's also been said that you're crazy if you've attempted to do so twice.
sendmail is an incredibly powerful mail program. It's also incredibly difficult to learn and understand. Any program whose definitive reference (sendmail, by Bryan Costales and Eric Allman, published by O'Reilly) is 1,050 pages long scares most people off. Information on the sendmail reference is contained in the bibliography at the end of this book.
Fortunately, new versions of sendmail are different. You no longer need to directly edit the cryptic sendmail.cf file; the new version provides a configuration utility that will create the sendmail.cf file for you based on much simpler macro files. You do not need to understand the complex syntax of the sendmail.cf file; the macro files don't require you to. Instead, you need only list items, such as the name of features you wish to include in your configuration, and specify some of the parameters that determine how that feature operates. A traditional Unix utility called m4 then takes your macro configuration data and mixes it with the data it reads from template files containing the actual sendmail.cf syntax, to produce your sendmail.cf file.
In this chapter we introduce sendmail and describe how to install, configure and test it, using the Virtual Brewery as an example. If the information presented here helps make the task of configuring sendmail less daunting for you, we hope you'll gain the confidence to tackle more complex configurations on your own.
The sendmail mail transport agent is included in prepackaged form in most Linux distributions. Installation in this case is relatively simple. Despite this fact, there are some good reasons to install sendmail from source, especially if you are security conscious. The sendmail program is very complex and has earned a reputation over the years for containing bugs that allow security breaches. One of the best known examples is the RTM Internet worm that exploited a buffer overflow problem in early versions of sendmail. Most security exploits involving buffer overflows rely on all copies of sendmail on different machines being identical, as the exploits rely on data being stored in specific locations. This, of course, is precisely what happens with sendmail installed from Linux distributions. Compiling sendmail from source yourself can help reduce this risk. Modern versions of sendmail are less vulnerable because they have come under exceedingly close scrutiny as security has become a more widespread concern throughout the Internet community.
The sendmail source code is available via anonymous FTP from ftp.sendmail.org.
Compilation is very simple bceause the sendmail source package directly supports Linux. The steps involved in compiling sendmail are:
# cd /usr/local/src # tar xvfz sendmail.8.9.3.tar.gz # cd src # ./Build
You need root permissions to complete the installation of the resulting binary files using:
# cd obj.Linux.2.0.36.i586 # make install
You have now installed the sendmail binary into the /usr/sbin directory. Several symbolic links to the sendmail binary will be installed into the /usr/bin/ directory. We'll talk about those links when we discuss common tasks in running sendmail.
Overview of Configuration Files
Traditionally, sendmail was set up through a system configuration file (typically called /etc/mail/sendmail.cf, or in older distributions, /etc/sendmail.cf, or even /usr/lib/sendmail.cf ) that is not anything close to any language you've seen before. Editing the sendmail.cf file to provide customized behavior can be a humbling experience.
Today sendmail makes all configuration options macro driven with an easy-to-understand syntax. The macro method generates configurations to cover most installations, but you always have the option of tuning the resultant sendmail.cf manually to work in a more complex environment.