11.4. Mail Delivery Agents
Red Hat Enterprise Linux includes two primary MDAs, Procmail and mail. Both of the applications are considered LDAs and both move email from the MTA's spool file into the user's mailbox. However, Procmail provides a robust filtering system.
This section details only Procmail. For information on the mail command, consult its man page.
Procmail delivers and filters email as it is placed in the mail spool file of the localhost. It is powerful, gentle on system resources, and widely used. Procmail can play a critical role in delivering email to be read by email client applications.
Procmail can be invoked in several different ways. Whenever an MTA places an email into the mail spool file, Procmail is launched. Procmail then filters and files the email for the MUA and quits. Alternatively, the MUA can be configured to execute Procmail any time a message is received so that messages are moved into their correct mailboxes. By default, the presence of /etc/procmailrc or of a .procmailrc file (also called an rc file) in the user's home directory invokes Procmail whenever an MTA receives a new message.
Whether Procmail acts upon an email message depends upon whether the message matches a specified set of conditions or recipes in the rc file. If a message matches a recipe, then the email is placed in a specified file, is deleted, or is otherwise processed.
When Procmail starts, it reads the email message and separates the body from the header information. Next, Procmail looks for /etc/procmailrc and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Procmail then searches for a .procmailrc file in the user's home directory. Many users also create additional rc files for Procmail that are referred to within the .procmailrc file in their home directory.
By default, no system-wide rc files exist in the /etc/ directory and no .procmailrc files exist in any user's home directory. Therefore, to use Procmail, each user must construct a .procmailrc file with specific environment variables and rules.
11.4.1. Procmail Configuration
The Procmail configuration file contains important environmental variables. These variables specify things such as which messages to sort and what to do with the messages that do not match any recipes.
These environmental variables usually appear at the beginning of .procmailrc in the following format:
<env-variable>="<value>" |
In this example, <env-variable> is the name of the variable and <value> defines the variable.
There are many environment variables not used by most Procmail users and many of the more important environment variables are already defined by a default value. Most of the time, the following variables are used:
DEFAULT — Sets the default mailbox where messages that do not match any recipes are placed.
The default DEFAULT value is the same as $ORGMAIL.
INCLUDERC — Specifies additional rc files containing more recipes for messages to be checked against. This breaks up the Procmail recipe lists into individual files that fulfill different roles, such as blocking spam and managing email lists, that can then be turned off or on by using comment characters in the user's .procmailrc file.
For example, lines in a user's .procmailrc file may look like this:
MAILDIR=$HOME/Msgs INCLUDERC=$MAILDIR/lists.rc INCLUDERC=$MAILDIR/spam.rc
If the user wants to turn off Procmail filtering of their email lists but leave spam control in place, they would comment out the first INCLUDERC line with a hash mark character (#).
LOCKSLEEP — Sets the amount of time, in seconds, between attempts by Procmail to use a particular lockfile. The default is eight seconds.
LOCKTIMEOUT — Sets the amount of time, in seconds, that must pass after a lockfile was last modified before Procmail assumes that the lockfile is old and can be deleted. The default is 1024 seconds.
LOGFILE — The file to which any Procmail information or error messages are written.
MAILDIR — Sets the current working directory for Procmail. If set, all other Procmail paths are relative to this directory.
ORGMAIL — Specifies the original mailbox, or another place to put the messages if they cannot be placed in the default or recipe-required location.
By default, a value of /var/spool/mail/$LOGNAME is used.
SUSPEND — Sets the amount of time, in seconds, that Procmail pauses if a necessary resource, such as swap space, is not available.
SWITCHRC — Allows a user to specify an external file containing additional Procmail recipes, much like the INCLUDERC option, except that recipe checking is actually stopped on the referring configuration file and only the recipes on the SWITCHRC-specified file are used.
VERBOSE — Causes Procmail to log more information. This option is useful for debugging.
Other important environmental variables are pulled from the shell, such as LOGNAME, which is the login name; HOME, which is the location of the home directory; and SHELL, which is the default shell.
A comprehensive explanation of all environments variables, as well as their default values, is available in the procmailrc man page.
11.4.2. Procmail Recipes
New users often find the construction of recipes the most difficult part of learning to use Procmail. To some extent, this is understandable, as recipes do their message matching using regular expressions, which is a particular format used to specify qualifications for a matching string. However, regular expressions are not very difficult to construct and even less difficult to understand when read. Additionally, the consistency of the way Procmail recipes are written, regardless of regular expressions, makes it easy to learn by example. To see example Procmail recipes, refer to Section 11.4.2.5 Recipe Examples.
Procmail recipes take the following form:
:0<flags>: <lockfile-name> * <special-condition-character> <condition-1> * <special-condition-character> <condition-2> * <special-condition-character> <condition-N> <special-action-character><action-to-perform> |
The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed after the zero to control how Procmail processes the recipe. A colon after the <flags> section specifies that a lockfile is created for this message. If a lockfile is created, the name can be specified by replacing <lockfile-name>.
A recipe can contain several conditions to match against the message. If it has no conditions, every message matches the recipe. Regular expressions are placed in some conditions to facilitate message matching. If multiple conditions are used, they must all match for the action to be performed. Conditions are checked based on the flags set in the recipe's first line. Optional special characters placed after the * character can further control the condition.
The <action-to-perform> specifies the action taken when the message matches one of the conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used here to direct matching messages into that file, effectively sorting the email. Special action characters may also be used before the action is specified. Refer to Section 11.4.2.4 Special Conditions and Actions for more information.
11.4.2.1. Delivering vs. Non-Delivering Recipes
The action used if the recipe matches a particular message determines whether it is considered a delivering or non-delivering recipe. A delivering recipe contains an action that writes the message to a file, sends the message to another program, or forwards the message to another email address. A non-delivering recipe covers any other actions, such as a nesting block. A nesting block is a set of actions, contained in braces { }, that are performed on messages which match the recipe's conditions. Nesting blocks can be nested inside one another, providing greater control for identifying and performing actions on messages.
When messages match a delivering recipe, Procmail performs the specified action and stops comparing the message against any other recipes. Messages that match non-delivering recipes continue to be compared against other recipes.
11.4.2.2. Flags
Flags are essential to determine how or if a recipe's conditions are compared to a message. The following flags are commonly used:
A — Specifies that this recipe is only used if the previous recipe without an A or a flag also matched this message.
a — Specifies that this recipe is only used if the previous recipe with an A or a flag also matched this message and was successfully completed.
B — Parses the body of the message and looks for matching conditions.
b — Uses the body in any resulting action, such as writing the message to a file or forwarding it. This is the default behavior.
c — Generates a carbon copy of the email. This is useful with delivering recipes, since the required action can be performed on the message and a copy of the message can continue being processed in the rc files.
D — Makes the egrep comparison case-sensitive. By default, the comparison process is not case-sensitive.
E — While similar to the A flag, the conditions in the recipe are only compared to the message if the immediately preceding the recipe without an E flag did not match. This is comparable to an else action.
e — The recipe is compared to the message only if the action specified in the immediately preceding recipe fails.
f — Uses the pipe as a filter.
H — Parses the header of the message and looks for matching conditions. This occurs by default.
h — Uses the header in a resulting action. This is the default behavior.
w — Tells Procmail to wait for the specified filter or program to finish, and reports whether or not it was successful before considering the message filtered.
W — Is identical to w except that "Program failure" messages are suppressed.
For a detailed list of additional flags, refer to the procmailrc man page.
11.4.2.3. Specifying a Local Lockfile
Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a message simultaneously. Specify a local lockfile by placing a colon (:) after any flags on a recipe's first line. This creates a local lockfile based on the destination file name plus whatever has been set in the LOCKEXT global environment variable.
Alternatively, specify the name of the local lockfile to be used with this recipe after the colon.
11.4.2.4. Special Conditions and Actions
Special characters used before Procmail recipe conditions and actions change the way they are interpreted.
The following characters may be used after the * character at the beginning of a recipe's condition line:
! — In the condition line, this character inverts the condition, causing a match to occur only if the condition does not match the message.
< — Checks if the message is under a specified number of bytes.
> — Checks if the message is over a specified number of bytes.
The following characters are used to perform special actions:
! — In the action line, this character tells Procmail to forward the message to the specified email addresses.
$ — Refers to a variable set earlier in the rc file. This is often used to set a common mailbox that is referred to by various recipes.
| — Starts a specified program to process the message.
{ and } — Constructs a nesting block, used to contain additional recipes to apply to matching messages.
If no special character is used at the beginning of the action line, Procmail assumes that the action line is specifying the mailbox in which to write the message.
11.4.2.5. Recipe Examples
Procmail is an extremely flexible program, but as a result of this flexibility, composing Procmail recipes from scratch can be difficult for new users.
The best way to develop the skills to build Procmail recipe conditions stems from a strong understanding of regular expressions combined with looking at many examples built by others. A thorough explanation of regular expressions is beyond the scope of this section. The structure of Procmail recipes and useful sample Procmail recipes can be found at various places on the Internet (such as http://www.iki.fi/era/procmail/links.html). The proper use and adaptation of regular expressions can be derived by viewing these recipe examples. In addition, introductory information about basic regular expression rules can be found in the grep man page.
The following simple examples demonstrate the basic structure of Procmail recipes and can provide the foundation for more intricate constructions.
A basic recipe may not even contain conditions, as is illustrated in the following example:
:0: new-mail.spool |
The first line specifies that a local lockfile is to be created but does not specify a name, so Procmail uses the destination file name and appends the value specified in the LOCKEXT environment variable. No condition is specified, so every message matches this recipe and is placed in the single spool file called new-mail.spool, located within the directory specified by the MAILDIR environment variable. An MUA can then view messages in this file.
A basic recipe, such as this, can be placed at the end of all rc files to direct messages to a default location.
The following example matched messages from a specific email address and throws them away.
:0 * ^From: spammer@domain.com /dev/null |
With this example, any messages sent by spammer@domain.com are sent to the /dev/null device, deleting them.
 | Caution |
|---|---|
Be certain that rules are working as intended before sending messages to /dev/null for permanent deletion. If a recipe inadvertently catches unintended messages, and those messages disappear, it becomes difficult to troubleshoot the rule. A better solution is to point the recipe's action to a special mailbox, which can be checked from time to time to look for false positives. Once satisfied that no messages are accidentally being matched, delete the mailbox and direct the action to send the messages to /dev/null. |
The following recipe grabs email sent from a particular mailing list and places it in a specified folder.
:0: * ^(From|CC|To).*tux-lug tuxlug |
Any messages sent from the tux-lug@domain.com mailing list are placed in the tuxlug mailbox automatically for the MUA. Note that the condition in this example matches the message if it has the mailing list's email address on the From, CC, or To lines.
Consult the many Procmail online resources available in Section 11.6 Additional Resources for more detailed and powerful recipes.
11.4.2.6. Spam Filters
Because it is called by Sendmail, Postfix, and Fetchmail upon receiving new emails, Procmail can be used as a powerful tool for combating spam.
This is particularly true when Procmail is used in conjunction with SpamAssassin. When used together, these two applications can quickly identify spam emails, and sort or destroy them.
SpamAssassin uses header analysis, text analysis, blacklists, a spam-tracking database, and self-learning Bayesian spam analysis to quickly and accurately identify and tag spam.
The easiest way for a local user to use SpamAssassin is to place the following line near the top of the ~/.procmailrc file:
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc |
The /etc/mail/spamassassin/spamassassin-default.rc contains a simple Procmail rule that activates SpamAssassin for all incoming email. If an email is determined to be spam, it is tagged in the header as such and the title is prepended with the following pattern:
*****SPAM***** |
The message body of the email is also prepended with a running tally of what elements caused it to be diagnosed as spam.
To file email tagged as spam, a rule similar to the following can be used:
:0 Hw * ^X-Spam-Status: Yes spam |
This rule files all email tagged in the header as spam into a mailbox called spam.
Since SpamAssassin is a Perl script, it may be necessary on busy servers to use the binary SpamAssassin daemon (spamd) and client application (spamc). Configuring SpamAssassin this way, however, requires root access to the host.
To start the spamd daemon, type the following command as root:
/sbin/service spamassassin start |
To start the SpamAssassin daemon when the system is booted, use an initscript utility, such as the Services Configuration Tool (system-config-services), to turn on the spamassassin service. Refer to Section 1.4.2 Runlevel Utilities for more information about initscript utilities.
To configure Procmail to use the SpamAssassin client application instead of the Perl script, place the following line near the top of the ~/.procmailrc file. For a system-wide configuration, place it in /etc/procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc |