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- Herausgeber: Packt Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This book starts with setting up a qmail server and takes you through virtualization, filtering, and other advanced features like hosting multiple domains, mailing lists, and SSL Encryption. Finally, it discusses the log files and how to make qmail work faster.
Qmail is a secure, reliable, efficient, simple message transfer agent. It is designed for typical Internet-connected UNIX hosts. Qmail is the second most common SMTP server on the Internet, and has by far the fastest growth of any SMTP server. Qmail's straight-paper-path philosophy guarantees that a message, once accepted into the system, will never be lost. Qmail also optionally supports maildir, a new, super-reliable user mailbox format.
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Seitenzahl: 246
Veröffentlichungsjahr: 2007
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Table of Contents
Qmail Quickstarter
Kyle Wheeler
Qmail Quickstarter
Install, Set Up, and Run your own Email Server
Copyright © 2007 Packt Publishing
All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews.
Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the author, Packt Publishing, nor its dealers or distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.
Packt Publishing has endeavored to provide trademark information about all the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information.
First published: June 2007
Production Reference: 1040607
Published by Packt Publishing Ltd.
32 Lincoln Road
Olton
Birmingham, B27 6PA, UK.
ISBN 978-1-847191-15-1
www.packtpub.com
Cover Image by Vinayak Chittar (<[email protected]>)
Credits
Author
Kyle Wheeler
Reviewer
Russell Nelson
Development Editor
Nanda Padmanabhan
Assistant Development Editor
Rashmi Phadnis
Technical Editor
Saurabh Singh
Code Testing
Ankur Shah
Project Manager
Patricia Weir
Editorial Manager
Dipali Chittar
Project Coordinator
Abhijeet Deobhakta
Indexer
Bhushan Pangaonkar
Proofreader
Chris Smith
Production Coordinator
Manjiri Nadkarni
Cover Designer
Manjiri Nadkarni
About the Author
Kyle Wheeler is a PhD candidate at the University of Notre Dame in the Computer Science and Engineering Department. Having co-authored papers both nationally and internationally, he received an M.S.C.S.E. from Notre Dame in 2005 and expects to receive his doctorate in the field of scalable computing in 2008. As part of his PhD research, he interned at Sandia National Laboratories from 2006 through 2007.
Kyle began setting up and maintaining qmail-based email servers working for NetSeats Inc. in 2000. Since then, his client base has expanded to include the Philadelphia chapter of Notre Dame Alumni, the Church of Epiphany in the Archdiocese of Louisville, and several other groups, both large and small. He is also a frequent contributor to the qmail mailing list, which supports qmail users and administrators internationally.
I'd like to thank my family and my fiancée for their constant support while writing this book.
About the Reviewer
Russell Nelson has been a postmaster for twenty years, about half of them using qmail. In a previous life, he was Mr. Packet Driver, but people still remember him that way. Russell blogs at http://blog.russnelson.com/.
Preface
Qmail is one of the most popular email servers. The current release was published in 1998, and has stood unchanged ever since. It has withstood the test of time surprisingly well, and a devoted community has grown around it to contribute experience, ideas, and patches to provide new features. While there is some dispute over the claim that qmail has no security flaws yet discovered, it cannot be denied that its security track record over the last ten years is unparalleled. Qmail includes several applications, including an SMTP server, a POP3 server, a QMTP server, and several related utilities for manipulating email and email storage. Qmail has been used by or is currently used by Hotmail, Yahoo!, NetZero, Speakeasy, Qwest, PayPal, Aruba.it, and others. You can learn more about qmail at http://cr.yp.to/qmail.html and http://www.qmail.org/.
This book treats qmail more as an architecture than a mail server, and from that perspective guides the reader through the installation, administration, customization, and deployment of a qmail-based server. The book begins with a quick, minimal, step-by-step walkthrough of a bare-bones qmail server, and then introduces and explains the basic design of the qmail architecture in easily understood terms.
The later chapters of the book are devoted to using qmail to provide specific features through customization and optimization. Alternative methods of providing each feature are compared, and a plethora of example scripts are provided to demonstrate the concepts and techniques.
What This Book Covers
Chapter 1 provides a quick step-by-step guide to installing a basic qmail server on a computer without an existing mail server, using ucspi-tcp and tcpserver to provide some of the basic services that a qmail SMTP server relies upon. At the end of the chapter is an overview of the qmail architecture that is explained in the following chapters to understand how qmail works and how the structure lends itself to customization.
Chapter 2 and Chapter 3 mirror each other: the former details how email enters the qmail queue, and the latter details how email leaves the qmail queue. The discussion of inbound mail includes the basic architectural details as well as discussion of authentication and the two protocols that qmail supports: SMTP and QMTP. The discussion of outbound mail also includes the basic architectural details and expands into basic filtering, the definition of users and mailboxes, and remote delivery.
Chapter 4 examines in detail the storage formats that qmail supports. Specifically, it covers the factors that influence the choice of the format to be used for a given situation, using mbox, Maildir, and MH as examples. One of the most common things to do with email is to retrieve it from a central server either via the POP3 or IMAP protocols, or via a webmail interface. The latter half of this chapter covers all three, and discusses the reasons for choosing one protocol over the other, how to choose an IMAP server package, and how to set up qmail's own POP3 server.
Chapter 5 begins the more advanced section of the book with a discussion of server virtualization. Multiple kinds of virtualization are discussed, including qmail's built-in virtual domain and virtual user framework, virtual domain management software, and the possibilities provided by having multiple qmail installations.
Chapter 6 unleashes the full power of the qmail architecture's design. By altering the flow of mail through the architecture—or changing the architecture itself—qmail can be made to perform virtually any task. Sending mail without a queue, blocking viruses, detecting spam, validating recipients, and using SPF and/or DomainKeys are all used as examples in this chapter. Both lightweight and heavyweight methods of spam and virus prevention are also discussed in detail.
Chapter 7 looks at some advanced features that don't quite fit into other, larger categories, such as SSL support and optimization for mailing-list delivery.
Chapter 8 covers ongoing maintenance, monitoring, and good administrative behavior. It provides a detailed description of the log files and how to interpret them and use the qmailanalog package to get statistical analysis of qmail's behavior. Expanding the log files to contain more information, identifying and recognizing problems (including "silly-qmail" syndrome), and using information in the logs to improve qmail's performance are all explored in this chapter.
What You Need for This Book
Qmail works on practically all UNIX systems: AIX, BSD/OS, FreeBSD, HP/UX, Irix, Linux, NetBSD, OpenBSD, OSF/1, SunOS, Solaris, etc. It automatically adapts itself to new UNIX variants.
Qmail does not support Windows.
Who This Book is For
This book is targeted at System Administrators familiar with Linux/UNIX and DNS servers who need to set up qmail.
Conventions
In this book, you will find a number of styles of text that distinguish between different kinds of information. Here are some examples of these styles, and an explanation of their meaning.
There are four styles for code. Code words and files in text that are not editable are shown as follows: "Qmail's SMTP server, for example, cannot talk to the network by itself; this ability is provided by software like inetd or tcpserver or similar." Code words and files in text that are editable are shown as follows: "Qmail comes with a set of minimal install instructions, in a file named INSTALL."
A block of code will be set as follows:
Any command-line input and output is written as follows:
New terms and important words are introduced in a bold-type font. Usernames have been introduced in an italicized format as follows: "However, qmail does not deliver mail to the real root user."
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You can contact us at <[email protected]> if you are having a problem with some aspect of the book, and we will do our best to address it.
Chapter 1. Basic Qmail
If you're diving into this section, chances are you're either entirely new to qmail, or you're looking to get a feel of this book. There are many very good qmail installation guides and tutorials that are available for free on the Internet. The current version of qmail was published on June 15th, 1998. Since then what has changed the most about the qmail experience is the accumulation of expertise and experience in using and tailoring it for the most common situations and even some uncommon ones.
Without delving into the architecture, qmail is extremely modular. In many ways, qmail is less of a mail server and more of mail server architecture. Pieces of qmail can be replaced, rearranged, filtered, and extended as necessary to achieve virtually any feature the administrator desires. However, along the same lines, qmail requires certain assistance to provide some features one ordinarily expects. Qmail's SMTP server, for example, cannot talk to the network by itself; this ability is provided by software like inetd or tcpserver or similar. This design makes qmail's components secure and much simpler and easier to verify. This design also makes the details of how the qmail components are hooked together a vital part of the system configuration, as opposed to a single monolithic server with a complex configuration file that can achieve the same thing.
To get the most out of this book, you're going to need a basic understanding of UNIX-style operating system conventions and features, simple command-line operations, and how to edit text files.
The Minimum Qmail System
Qmail comes with a set of minimal install instructions, in a file named INSTALL. It contains eighteen relatively basic steps for compiling qmail on most systems and for getting it running. These are somewhat simple, but can be trimmed even further if you're not trying to replace an existing mail server.
Compiling and Installing
Compiling qmail is generally very easy. Before compiling qmail, first obtain the prerequisites:
Having a case-sensitive filesystem is important because during installation, qmail uses several files that are different only in the capitalization of their name. For example, INSTALL is a text file describing basic installation procedures, while install is a script for putting files in the correct places with the correct permissions. The qmail distribution can be modified to work around that problem, but that is a little outside the purview of this book.
With those prerequisites, installing a bare-bones version of qmail is a straightforward five-step process as follows:
Simple, isn't it? Let's go into a bit more detail here.
Preparing the System
On most UNIX systems it should be relatively easy to add users and groups, using tools like useradd, adduser, mkuser, or something similar. For example, on many Linux distributions, the commands for preparing the system are as follows:
The users are required as part of qmail's security setup; almost every major portion of qmail runs as a different user. The reason for this is simple—it allows qmail to use standard UNIX user protections to enforce separation between its components, which communicate via tightly-controlled interfaces (namely, pipes and environment variables). This user separation is the backbone of qmail's security model—a model that has done exceedingly well and has been adopted by other security-conscious programs (e.g. OpenSSH). To complete the protection that these users provide, it's a good idea to ensure that each of these users cannot be used by anyone to log into the system. On most modern systems, this is achieved by not giving the user a working shell (e.g. /bin/false).
Compiling and Installing the Necessary Binaries
The second step is the compilation step. Generally, this is the simplest of the steps, provided that the necessary tools (a compiler and the make utility) are available. Qmail will compile on most systems without further configuration, by simply executing the command make setup check. The exceptions are modern Linux systems that use a more recent version of glibc than version 2.3.1. On these systems, it is necessary to edit the conf-cc file that comes with qmail before compiling, so that it looks like the following:
If your compiler cannot be run using the cc command, edit the conf-cc file to contain the correct command to compile files.
Creating the Basic Configuration Files
The third step simply adds the most minimal configuration information that qmail requires for functioning—the Fully Qualified Domain Name (FQDN) of the host computer. The term "fully-qualified" means that the FQDN not only contains the host name, but also the full domain name. For example, to set up a computer named mail as part of example.com's computer network, the FQDN would be mail.example.com. To configure qmail for this computer, the minimal configuration command would then be:
The alternative command, ./config, does the same thing that ./config-fast does, however, it obtains the FQDN by looking up the computer's IP address in DNS. If the system is already set to go with the IP address it will always have, this is a convenient way to avoid extra typing. However, if the system's network configuration is not in its final state, using ./config will probably produce an incorrect set of configuration files. Running either command overwrites any existing configuration files.
Creating the Necessary Minimum Account Aliases
The fourth step adds the accounts that are required by the various email standards documents (in particular, RFC 822). The following accounts are required:
The last one, [email protected], needn't necessarily exist. However, qmail does not deliver mail to the real root user, and the address is commonly assumed to refer to the administrator of the machine (for example, by scripts and monitoring programs) when the administrator needs to be notified of something. Thus, creating an alias for root is generally a good idea.
Aliases are defined by creating files in the home directory of the alias user. If the alias user has been created according to the above instructions, that directory is /var/qmail/alias. The general way of referring to this directory is ~alias/. The alias-defining files in this directory must have very specific names, all beginning with .qmail and ending with the name of the alias. For example, the postmaster alias is established by creating a file named .qmail-postmaster in the directory ~alias/. The mailer-daemon alias is established by creating a file named .qmail-mailer-daemon, and so forth. Capitalization for account names is always converted to lowercase for delivery, so don't use capital letters in .qmail filenames.
The content of these files specifies exactly what should happen to email that is sent to one of these aliases. In general, the syntax is identical to the generic dot-qmail (.qmail) file syntax, which is discussed later in this book, but the exception is the bare minimum: an empty file. If an alias is established with an empty file, it will be delivered as specified by the default delivery mechanism (for more details refer to the Default Mail Delivery section).
The simplest option is to put an account name in those files, which tells qmail to forward all mail sent to these aliases to the account specified. For example, if all email addressed to [email protected] should be delivered to an account named steve, put steve into the ~alias/.qmail-root file.
It is important to note that these files should have very specific permissions—they should be readable by any user, but only writable by the root user. This may not be the default when these files are created. To set the permissions to what they need to be, run a command that looks something like the following:
Default Mail Delivery
The fifth and final step is to tell qmail how to deliver mail by default. Default means how qmail delivers all mail unless told to do something else by a .qmail file. Generally, this is done by selecting a startup script from the /var/qmail/boot directory and copying it to the file /var/qmail/rc.
In the /var/qmail/boot directory, there are several files, each of which can start up qmail with a different default delivery method. The ones that come with qmail are:
Unless you are migrating from an older mail server and have a reason to want the compatibility features, the file to use is either home or proc. The simplest is home.
Basic Configuration
Once all five steps are completed, a working, bare-bones installation of qmail is ready in /var/qmail. However, in many situations, a barebones installation is insufficient.
The basic questions to answer when configuring an email server on a new system include:
The most common and simplest answers to the first question generally fall into one of the following two categories: either mail should be relayed to a smarter mail server or the mail should be delivered locally.
The second question can often become far more complicated due to spam and viruses and the like, but the most basic answer is generally a list of domain names for which this email server is responsible.
As you can tell already, various answers to these questions can result in wildly different behaviors. For example, if no mail should be accepted from the network, no mail should be delivered locally, and all mail should be forwarded to a specific mail server, then this is considered mini-qmail. In such a situation, many of the more complex features of qmail can be eliminated. In different circumstances, the qmail server may need to accept any and all email and forward it to a central mail server (for example, a mail proxy or a caching forwarder). Or it may need to accept email for a specific domain and deliver it to system-defined users (the standard setup). Or it may need to accept email for a set of domains and deliver it locally via some virtual-domain configuration. There could be any number of additional complications, twists, and turns.
The most basic answers to these questions are specified to qmail via configuration files. Which mail should be accepted is generally specified by files in the /var/qmail/control directory, and what to do with mail that has been accepted is generally specified in a combination of files in the control directory and the rc file (which was set up in Default Mail Delivery section of the installation procedure). Note though, that the rc file is a shell script. Much of qmail configuration is in the form of scripts controlling how qmail and its related binaries are run.
The most basic, most important control files for qmail are: me, rcpthosts, locals, smtproutes, and defaultdomain. The files are not necessarily created by default or by the ./config scripts; but they control qmail's most important functionality. They control, respectively, the name of the server, which domains' mail to accept, which domains are to be considered local once mail addressed to them is accepted for delivery, where to send outbound mail, and which domain to append to bare usernames to transform them into real email addresses. The defaultdomain and me files are simple one-line files. In the case of me, this line is considered the name of the server. In the case of defaultdomain, this line is considered the name to append (for example, example.com) to a bare username (for example, user) to construct a valid email address (for example, [email protected]) when necessary. The rcpthosts and locals files are simply lists of domains, one domain per line in the file. The most complex of the four, smtproutes, is also rather simple. Each line of the file consists of three fields separated by colons. The first field is the domain that needs to be routed this way and the second field is the domain name or IP address (in square brackets) of the server to which matching email must be sent. The third field is the port on the server to connect to, which if not present, defaults to port 25. For example:
This line in the file informs qmail that any email sent to an address ending in @somewhere.com must be forwarded to the IP address 1.2.3.4. The files smtproutes, rcpthosts, and locals can all use prefix-wildcards. A prefix-wildcard is a line that begins with a period, followed by the suffix that must match following the period. For example:
This line in the smtproutes file will match email addresses ending in @here.somewhere.com, @there.somewhere.com, @anywhere.somewhere.com, and so forth, where there is an arbitrary string and a period preceding somewhere.com. Note that it doesn't match the bare @somewhere.com. Emails addressed to matching domains are forwarded to mail.isp.com.
Finally there is the special case, where there is nothing to the left of the first colon as shown in the following example:
This line in the smtproutes file will send all email to the mail.isp.com server listening on port 1000. In the smtproutes file, the first match is the one that is used, and this line will match anything. As such, it's usually at the end of the file.
There are many more files that qmail looks for in the /var/qmail/control directory. Explanations of how they work and what they do can be found in the qmail man pages, however, they are generally for more involved configuration tasks and non-basic qmail installations.
Default delivery instructions are part of simple execution.
Simple Execution
There are two primary architectural segments of qmail involved in setting up a standard SMTP email server. The first is the set of programs that work together to perform mail deliveries, either locally or remotely, and the second is the set of programs that work together to accept messages via the SMTP protocol.
qmail-start
The programs that work together to perform mail deliveries are: qmail-send, qmail-lspawn, qmail-rspawn, and qmail-clean, as well as any program that they spawn to complete their tasks (like qmail-remote, qmail-local, procmail, etc.). Most of these have corresponding users. In particular, qmail-send and qmail-clean operate as the qmails user, and qmail-rspawn (and qmail-remote) operate as qmailr. The qmail-lspawn program runs as root, because it must be able to deliver mail to each user as that user. In any case, all of these programs are spawned by the command qmail-start. This command takes two optional arguments—a default delivery command and a logging command. To understand exactly how this works, take a look at the most basic of the scripts in the /var/qmail/boot directory, home:
The first part of this script is fairly straightforward: using the env command to remove all environment variables before executing qmail-start, it then sets the PATH environment variable to make sure that the qmail bin directory is the first place searched for qmail's binaries. The second part, executing qmail-start with arguments, requires a little more explanation.
