Section 23.1. The Postfix MTA

23.1. The Postfix MTA

Several MTAs are available for Linux. Historically, the most common MTA on Unix has been sendmail, which has been around for a long time. It is generally considered somewhat more difficult to use than the alternatives, but it is thoroughly documented in the book sendmail, by Bryan Costales with Eric Allman (O'Reilly).

Postfix is a newer MTA, developed by security guru Wietse Venema as a replacement for sendmail. It's designed to be compatible with sendmail but to provide a higher level of security and be easier to configure.

Postfix is a highly flexible and secure piece of software that contains multiple layers of protection against would-be attackers. Postfix was also written with performance in mind, and employs techniques to limit slower activities such as creating new processes and accessing the filesystem. It is one of the easier email packages to configure and administer because it uses straightforward configuration files and simple lookup tables for address rewriting. It is remarkable in that it is simple to use as a basic MTA, yet still able to handle much more complicated environments.

Many Linux distributions have Postfix built in, so you may already have it installed on your system. If not, you can find prebuilt packages or compile it yourself from the source code. The Postfix home page (http://www.postfix.org) contains links to download both the source code ("Download") and packages for different Linux distributions ("Packages and Ports").

Postfix has two different release tracks: official and experimental. The experimental releases contain all the latest patches and new features, although these might change before they are included in the official release. Don't be put off by the term "experimental"; these releases are very stable and have been tested thoroughly. If you are looking for a feature that is available only in the experimental release, you should feel more than comfortable using it. Read the release notes for both tracks to know what the current differences are.

23.1.1. A Word About DNS

Before setting up Postfix , you should understand that if your system is going to receive mail from others across the Internet, the DNS for your domain has to be configured correctly. DNS is discussed in Chapter 13.

Let's assume for this discussion that you are configuring a host called halo in the domain example.org and that you have a user account michael on your system. Regardless of how you want to receive mail, your host halo.example.org must have a DNS A record that maps its hostname to its IP address.

In this example your email address is going to be either [e-mail] or [e-mail]. If you want to use the first form, configuring the DNS A record is enough for messages to reach you.

If your system is going to receive all mail for example.org (including [e-mail]), the domain should have a DNS MX record pointing to your host halo.example.org. If you are configuring the DNS for your domain yourself, make sure you read the documentation to understand how it works; otherwise, speak to your DNS administrator or ISP about routing mail to your system.

Postfix frequently uses DNS in its normal operation, and it uses the underlying Linux libraries to perform its DNS queries. Make sure your system is configured correctly to perform DNS lookups (see "Configuring DNS" in Chapter 13). Postfix usually has to find an MX record to make its deliveries. Don't assume that if Postfix reports a DNS problem with an address, and you find that the domain resolves correctly, that email delivery should therefore succeed. If Postfix reports a problem, you can be almost certain there is a problem.

23.1.2. Installing Postfix

Although prepackaged distributions are available, you may want to build the package yourself if you want to use any of the add-on libraries or functions that are not included in your distribution. You might also want to get the latest version to obtain a new feature that has not yet been included in your distribution.

Before you install Postfix, be aware that it includes the three commands /usr/bin/newaliases , /usr/bin/mailq , and /usr/sbin/sendmail that are normally used by sendmail. Postfix provides replacements that work with the Postfix system rather than with sendmail. You should rename your existing sendmail commands so that the Postfix installation doesn't overwrite them in case you ever want to use the original sendmail binaries again:

 # mv /usr/bin/newaliases /usr/bin/newaliases.orig # mv /usr/bin/mailq /usr/bin/mailq.orig # mv /usr/sbin/sendmail /usr/sbin/sendmail.orig

Postfix uses Unix database files to store its alias and lookup table information. You must, therefore, have the db libraries installed on your system before building Postfix. These libraries are contained within the db-devel RPM package or the Debian libdb4.3-dev package. If you are not using a package manager, you can obtain them directly from Sleepycat Software (http://www.sleepycat.com/). If you are using RPM, execute the following command to see if the necessary libraries have been installed on your system:

 # rpm -qa | grep db-devel db-devel-4.3.27-3

You should see a line similar to the second line in the preceding command that displays the db-devel package with a version number. If rpm returns nothing, you must install the libraries before installing Postfix.

On Debian, you can use dpkg to see if the libraries are installed:

 # dpkg -l libdb4.3-dev

If you download a prepackaged Postfix, use your package manager (described in Chapter 12) to install it. If you download the source postfix-2.2.5.tar.gz, move that file to a suitable directory (such as your home directory) to unpack it. The numbers in the name of the file represent the version of this release. Your file may have different numbers depending on the current release when you download it.

Follow this basic procedure to build Postfix. Note that you'll have to be the root user to create the user and group and to install the package.

1. Rename your sendmail binaries as described earlier.

2. Create a user account called postfix and a group called postdrop. See "Managing User Accounts" in Chapter 11 for information on setting up accounts and groups.

3. Run gunzip on the compressed file to produce a file named postfix-2.2.5.tar.

4. Execute

    tar -xvf postfix-2.2.5.tar

   
   

   to unpack the source into a directory called postfix-2.2.5.

5. Move to the directory created when you unpacked the file. You'll find a file called INSTALL with detailed instructions about building your Postfix system. In most cases, building Postfix should be as simple as typing make in the directory.

6. If your build completes without any errors, type make install to install Postfix on your system. You should be able to accept all the defaults when prompted by the installation script.

After installation, you will have Postfix files in the following directories:

/usr/libexec/postfix

This directory contains the various Postfix daemons. Postfix uses a split architecture in which several discrete programs handle separate tasks. The master daemon is started first. It deals with starting other programs as they are needed. For the most part, you don't need to worry about any of the programs here. Stopping and starting Postfix is handled with the postfix command found in the /usr/sbin directory.

/etc/postfix

Typically this directory contains dozens of Postfix configuration files, but only master.cf and main.cf and a few lookup tables are used by Postfix. The rest of the files are examples that document the various parameters used for configuration.

The master.cf file controls the various Postfix processes. It includes a line for each component of Postfix. The layout of the file is described by comments in the file itself. Usually, you shouldn't have to make any changes to run a simple Postfix installation.

The main.cf file is the global SMTP configuration file. It includes a list of parameters set to one or more values using the format

 parameter = value

Comments are marked with a hash mark (#) at the beginning of the line. You cannot put comments on the same line as parameters. Commented lines can begin with whitespace (spaces or tabs), but they must appear on lines by themselves.

Multiple values for parameters can be separated by either commas or whitespace (including newlines), but if you want to have more than one line for a parameter, start the second and subsequent lines with whitespace. Values can refer to other parameters by preceding the parameter name with a dollar sign ($).

Here's an example of an entry that includes comments, multiple lines, and a parameter reference:

 # Here are all the systems I accept mail from. mynetworks = $myhostname     192.168.75.0/24     10.110.12.15

/usr/sbin

All the Postfix commands are located in /usr/sbin and have names starting with post. There are commands to create index files, manage the mail queue, and otherwise administer your Postfix system. The postfix command, which is used to stop and start Postfix (described later), is found here.

/var/spool/postfix

The Postfix queue manager is an important component of the Postfix system that accepts incoming email messages and arranges with other Postfix components to deliver them. It maintains its files under the /var/spool/postfix directory. The queues it maintains are shown next. Postfix provides several tools to manage the queues, such as postcat, postsuper, and mailq, but you might also use the usual Linux commands, such as find and cat, to inspect your queue.

/var/spool/postfix/incoming

All incoming messages, whether from over the network or sent locally.

/var/spool/postfix/active

Messages that the queue manager is delivering or preparing to deliver.

/var/spool/postfix/deferred

Messages that could not be delivered immediately. Postfix will attempt to deliver them again.

/var/spool/postfix/corrupt

Messages that are completely unreadable or otherwise damaged and not deliverable are stored here for you to look at if necessary to figure out the problem. This queue is rarely used.

/usr/local/man

Postfix installs documentation in the form of manpages on your system. The documentation includes information on command-line utilities, daemons, and configuration files.

As mentioned earlier, Postfix also installs replacements for /usr/bin/newaliases, /usr/bin/mailq, and /usr/sbin/sendmail.

23.1.3. Postfix Configuration

Before you start Postfix for the first time, you have to make sure that the aliases table is formatted correctly and that a few of the critical configuration parameters are set correctly for your system.

Historically, sendmail has used the file /etc/aliases to map one local username to another. Postfix continues the tradition. The /etc/aliases file is a plain-text file that is used as input to create an indexed database file for faster lookups of aliases on your system. There are at least two important aliases on your system that must be set in your /etc/aliases file. If you have been running sendmail on your system, these aliases are probably already set correctly, but make sure your file has entries for root and postmaster pointing to a real account that receives mail on your system. Once you have verified the aliases, execute the command newaliases to rebuild the index file in the correct format for Postfix.

The /etc/postfix/main.cf file contains many parameters, but there are just a few important ones that you should verify before starting Postfix; we explain these in this section. If you installed Postfix from a prepackaged distribution, these parameters might already be set correctly. It's also possible that the Postfix defaults work for your system, but edit your /etc/postfix/main.cf file to make sure.

myhostname

This is the fully qualified hostname for your system. By default, Postfix uses the name returned by the gethostname function. If this value is not fully qualified, and you have not set this parameter, Postfix will not start. You can check it by executing the command hostname. It's probably a good idea to specify your fully qualified hostname here explicitly:

 myhostname = halo.example.org

mydomain

Specifies the domain name for this system. This value is then used as the default in other places. If you do not set it explicitly, Postfix uses the domain portion of myhostname. If you have set myhostname as shown previously and example.org is correct for your system, you do not have to set this parameter.

mydestination

Specifies a list of domain names for which this system should accept mail. In other words, you should set the value of this parameter to the domain portions of email addresses for which you want to receive mail. By default, Postfix uses the value specified in myhostname. If you are setting up your system to accept mail for your entire domain, specify the domain name itself. You can use the variables $myhostname and $mydomain as the value for this parameter:

 mydestination = $myhostname $mydomain

myorigin

This parameter is used to append a domain name to messages sent locally that do not already include one. For example, if a user on your system sends a message with only the local username in the From: address, Postfix appends this value to the local name. By default, Postfix uses myhostname, but if your system is handling mail for the entire domain, you might want to specify $mydomain instead:

 myorigin = $mydomain

Some Linux distributions that already include Postfix configure it to use Procmail by default. Procmail is a separate mail delivery agent (MDA) that can filter and sort mail as it makes deliveries to individual users on your system. We describe Procmail in more detail later in this chapter. If you need the features it provides, you should study the Procmail documentation carefully to understand how it interacts with Postfix. For many systems that don't filter mail for users at the MTA level, Procmail is an unnecessary additional layer of complexity because Postfix can also make local deliveries and provide some of the same functions. Your distribution might be configured to use Procmail in either the mailbox_command or mailbox_transport parameters. If you want Postfix to handle local deliveries directly, you can safely comment out either of these parameters in your /etc/postfix/main.cf file.

23.1.4. Starting Postfix

Once you have verified the important configuration parameters described earlier and rebuilt your aliases index file, you are ready to start Postfix. As the superuser, execute:

 postfix start

You can stop Postfix by executing:

 postfix stop

Whenever you make changes to either of Postfix's configuration files, you must reload the running Postfix image by executing:

 postfix reload

Once you have Postfix running, all the users on your system should be able to send and receive email messages.

Any of your applications that depend on sendmail should still work, and you can use the sendmail command as you always did. You can pipe messages to it from within scripts and execute sendmail -q to flush the queue. The native Postfix equivalent for flushing the queue is postfix flush. Options to sendmail that deal with it running as a daemon and setting queue delays do not work because those functions are not handled by the sendmail command in Postfix. All the Postfix options are set in its two configuration files. Many parameters deal with the Postfix queue. You can find them in the manpage for qmgr(8).

23.1.5. Postfix Logging

After starting or reloading Postfix, you should check the log to see if Postfix reports any problems. (Most Linux distributions use /var/log/maillog, but you can also check the file /etc/syslog.conf to be sure.) You can see Postfix's most recent messages by running the command tail /var/log/maillog. Since Postfix is a long-running process, it's a good idea to check the log periodically even if you haven't been restarting it. You can execute the following to see if Postfix has reported anything interesting while running:

 egrep '(reject|warning|error|fatal|panic):' /var/log/maillog

In general, Postfix keeps you informed of what is going on with your system by logging lots of good information to syslogd. On Linux, syslogd uses synchronous writes by default, which means that after every write to the logfile, there is also a sync to force everything in memory to be written to the disk. Therefore, the performance of Postfix (and other processes) can suffer. You can change this default by preceding the name of the logfile with a hyphen in /etc/syslog.conf. Your entry in syslog.conf for mail logging should look like the following:

 mail.*                -/var/log/maillog

Be sure to have syslogd reread its configuration file after you make any changes. You can execute killall -HUP syslogd to reinitialize it.

23.1.6. Running Postfix on System Startup

Because of Postfix's compatibility with sendmail, if you have your system configured to start sendmail at system initialization, more than likely Postfix will start correctly when your system boots. However, system shutdown will probably not work correctly. Most Linux distributions shut down sendmail by locating a process called sendmail and then killing that process. The Postfix processes, while in many ways compatible with sendmail, do not run under the name sendmail, so this shutdown fails.

If you would like your system to shut down cleanly, you should create your own rc script for Postfix, as described in "rc Files" in Chapter 17. The commands you need to include in your script to start and stop Postfix are the same as those you execute on the command line: postfix start and postfix stop. Here's an example of a basic script to get you started. You may want to review other rc scripts on your system to see if you should add more system checks or follow other conventions and then make your adjustments to this example:

 #!/bin/sh PATH="" RETVAL=0 if [ ! -f /usr/sbin/postfix ] ; then     echo "Unable to locate Postfix"     exit 1 fi if [ ! -f /etc/postfix/main.cf ] ; then     echo "Unable to locate Postfix configuration"     exit 1 fi case "$1" in     start)         echo -n "Starting Postfix: "         /usr/sbin/postfix start > /dev/null 2>1         RETVAL=$?         echo         ;;     stop)         echo -n "Stopping Postfix: "         /usr/sbin/postfix stop > /dev/null 2>1         RETVAL=$?         echo         ;;     restart)         echo -n "Restarting Postfix: "         /usr/bin/postfix reload > /dev/null 2>1         RETVAL=$?         echo         ;;     *)         echo "Usage: $0 {start|stop|restart}"         RETVAL=1 esac exit $RETVAL

Place this script in /etc/rc.d/init.d or /etc/init.d, depending on your Linux distribution. Then make the appropriate symbolic links in each of the rcN.d directories for each runlevel in which Postfix should start (see "init, inittab, and rc Files" in Chapter 17). For example, if you want to have Postfix start at runlevels 3 and 5 and stop at runlevels 0 and 6, create symbolic links like those that follow for Red Hat. For Debian, the rcN.d directories are directly below /etc.

 # cd /etc/rc.d/rc3.d # ln -s .../init.d/postfix S97postfix # cd /etc/rc.d/rc5.d # ln -s .../init.d/postfix S97postfix # cd /etc/rc.d/rc0.d # ln -s .../init.d/postfix K97postfix # cd /etc/rc.d/rc6.d # ln -s .../init.d/postfix K97postfix

If you create a Postfix rc script, you should configure your system not to start sendmail at startup.

23.1.7. Postfix Relay Control

The default installation allows any system on the same subnet as yours to relay mail through your mail server. If you want to override the default, you can set the parameter mynetworks to be a list of hosts or networks that you trust to relay mail through your system. You can specify a list of IP addresses or network/netmask patterns, and any connecting SMTP client that matches will be allowed to relay mail. You can list network or IP addresses that reside anywhere. So, for example, if you want to be able to relay mail through your home Postfix system from your work machine, you can specify the IP address of your machine at work in your home Postfix configuration.

Here's an example that allows mail from the local subnet (192.168.75.0/28) and a single host located elsewhere:

 mynetworks = 192.168.75.0/28 10.150.134.15

If you want to allow relaying for mobile users who do not have static IP addresses, you have to use some kind of SMTP authentication mechanism. Postfix can work with SASL Authentication (which requires that Postfix be compiled with additional libraries, and that users' client software be specially configured) and pop-before-smtp (which requires a POP server running on the same system to first authenticate users).

It is important not to open relay access to anyone except users you trust. In the early days of the Internet, open relays were commonplace. Unfortunately, the current prevalence of spam has precluded that kind of freedom. If your MTA is not protected, you leave yourself and other Internet systems vulnerable to abuse. Spammers constantly scan for open relays, and if you place one on the network, it is only a matter of time before it will be found. Fortunately, the default Postfix installation behaves correctly. However, if you make lots of changes to your Postfix configuration (especially in setting up antispam controls, ironically), you may inadvertently open yourself up to relay abusers. There are some online antispam initiatives that offer to test if your server is configured to correctly deny relaying; try, for example, http://www.abuse.net/relay.html.

If you want your own Postfix installation to relay mail through another MTA, specify the IP address of the relay server using the relayhost parameter. Postfix normally figures out where to deliver messages on its own, based on the destination address. However, if your system is behind a firewall, for example, you may want Postfix to hand off all messages to another mail server to make the actual delivery. When you specify a relay server, Postfix normally performs a DNS query to obtain the mail exchanger (MX) address for that system. You can override this DNS lookup by putting the hostname in square brackets:

 relayhost = [mail.example.org]

23.1.8. Additional Configurations

The configuration described here creates a simple Postfix installation to send and receive messages for users on your system. But Postfix is an extremely flexible MTA with many more configuration options, such as hosting multiple virtual domains, maintaining mailing lists, blocking spam, and scanning for viruses. The manpages, HTML files, and sample configuration files that come with Postfix contain a lot of information to guide you in the more advanced configurations.