Olaf Kirch - Linux Network Administrator Guide, Second Edition

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. We touched on this briefly in Chapter 9, TCP/IP Firewall. 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.

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.

The m4 macro processor program generates the sendmail.df file when it processes the macro configuration file provided by the local system administrator. Throughout the remainder of this chapter we will refer to this configuration file as the sendmail.mc file.

The configuration process is basically a matter of creating a suitable sendmail.mc file that includes macros that describe your desired configuration. The macros are expressions that the m4 macro processor understands and expands into the complex sendmail.cf syntax. The macro expressions are made up of the macro name (the text in capital letters at the start), which can be likened to a function in a programming language, and some parameters (the text within brackets) that are used in the expansion. The parameters may be passed literally into the sendmail.cf output or may be used to govern the way the macro processing occurs.

A sendmail.mc file for a minimal configuration (UUCP or SMTP with all nonlocal mail being relayed to a directly connected smart host) can be as short as 10 or 15 lines, excluding comments.

If you're an administator of a number of different mail hosts, you might not want to name your configuration file sendmail.mc . Instead, it is common practice to name it after the host - vstout.m4 in our case. The name doesn't really matter as long as the output is called sendmail.cf . Providing a unique name for the configuration file for each host allows you to keep all configuration files in the same directory and is just an administrative convenience. Let's look at two example macro configuration files so we know where we are heading.

Most sendmail configurations today use SMTP only. It is very simple to configure sendmail for SMTP. Example 18.1 expects a DNS name server to be available to resolve hosts and will attempt to accept and deliver all mail for hosts using just SMTP.

Example 18.1: Sample Configuration File vstout.smtp.m4

divert(-1)

#

# Sample configuration file for vstout - smtp only

#

divert(0)

VERSIONID(`@(#)sendmail.mc 8.7 (Linux) 3/5/96')

OSTYPE(`linux')

#

# Include support for the local and smtp mail transport protocols.

MAILER(`local')

MAILER(`smtp')

#

FEATURE(rbl)

FEATURE(access_db)

# end

A sendmail.mc file for vstout at the Virtual Brewery is shown in Example 18.2. vstout uses SMTP to talk to all hosts on the Brewery's LAN, and you'll see the commonality with the generic SMTP-only configuration just presented. In addition, the vstout configuration sends all mail for other destinations to moria , its Internet relay host, via UUCP.

Example 18.2: Sample Configuration File vstout.uucpsmtp.m4

divert(-1)

#

# Sample configuration file for vstout

#

divert(0)

VERSIONID(`@(#)sendmail.mc 8.7 (Linux) 3/5/96')

OSTYPE(`linux')