Review of Linux Sendmail Administration
December 5, 2001
- Linux Sendmail Administration
- Craig Hunt
While sendmail remains the most popular way to get email from
one side of the Internet to the other, its configuration and usage is
one of the most daunting tasks a professional system administrator may
face. There are many reasons for this, but in my opinion the three predominant
ones are that sendmail has an enormous number of configuration
options which must be sifted through to discover how one might make the
program behave in the required fashion, that sendmail is one of
the oldest utilities in wide use on the Internet and in order to support
historical configurations it suffers from having features added to
it piecemeal over the years, and that its configuration file format is
extremely complex and the procedure by which these are generated (as well
as how the software is compiled) are unique among Open Source packages.
Despite its complexity, because sendmail is ubiquitous on Unix
systems, a lot has been written
both in print and on the Internet about how to make it function properly
under many different circumstances.
Because of this, one might ask many questions about Craig
Hunt's Linux Sendmail Administration in order to determine
whether or not it is worth the investment in time and money.
- Isn't the Bat Book good enough? The Bat Book, more formally
known as sendmail, 2nd edition by Brian Costales, has been
the standard reference on sendmail since it's original publication.
It's an excellent book that, in my opinion, should be available to
every sendmail administrator. However, it has two flaws:
(1) It's old. It was last updated in 1997 and only covers up through
sendmail version 8.8. As of this writing, the latest
sendmail version is 8.12, and enormous changes to the code have
occurred in the last few years.
Costales has been working on a 3rd edition of the
Bat Book, but there is no telling when it will become available. (2) While
the Bat Book covers a great deal of information on sendmail
configuration files and options, it does not focus on the "right" way
to build these configuration files, that is, using the M4 macro language.
Too many people have gotten the idea from the Bat Book that editing
sendmail rulesets by hand is a good idea, when, in fact, it is
almost never the right thing to do.
- But aren't explanations of the new features provided in the
with sendmail? A great deal of it is, if one looks hard
enough, but in the sendmail documentation scattered throughout
the sendmail source tree it's not always clear
where one should look for the necessary information, and often the
explanations are quite terse. A more methodical, thorough, and detailed
of the way sendmail works that includes realistic examples could
be quite helpful.
- Is the book so Linux-specific that it won't help me maintain my
(other Unix variant) system? The Short answer is, no. While several
Linux specific considerations are mentioned in the book, such as the
location of the sendmail.cf and associated files, and the use
of Linux-specific package utilities, the vast majority of this book is
easily extendible to other Unix-based operating systems such as Solaris,
FreeBSD, HP-UX, etc.. The reader should not be strongly dissuaded from
buying this book for use in a non-Linux environment.
- What information is in this book that I might not know?
Many of the new features available in sendmail are well described
in this book. Just as one example, the author describes how to set up and use
the SMTP AUTH and STARTTLS features that have been made available starting
with sendmail version 8.11, including how to become your own
Certification Authority in order to use self-signed certificates, although
I think the author downplays the strong benefits to doing this rather
than going with commercial certification. This is the first description
of this material I have seen clearly written anywhere. Many other
topics are also covered for the first time in book form such as
using LDAP with
sendmail, sendmail's DontBlameSendmail security
options, and hoststat information, just to
name a few.
While Linux Sendmail Administration is by no means perfect,
there are several points on which I'd be willing to take minor issue
with the author, it is very good in accomplishing what it sets out to
do, providing an explanation of how to build and configure recent
releases of sendmail on Linux systems. It provides this information
in a way that is currently more relevant than the venerable Bat Book,
and its usefulness is by no means limited to Linux-based systems.
At this moment
in time, this is the best place for a system administrator to learn the
"right" way to configure sendmail, and I believe that all but
the most experienced sendmail administrators will find this
At this time, Linux Sendmail Administration is the best place
to learn how to build and configure sendmail on Linux email servers.
The contents of this book are easily adaptable to other Unix flavors.
Craig Hunt's emphasis on M4-based configuration and information on
recent sendmail versions makes this book useful to all but the
most experienced sendmail administrators. I recommend it.
Click here to return to the index of reviews.