11.4. Mail Delivery Agents
Red Hat Linux includes two primary MDAs, Procmail and mail. Both of the applications are considered Local Delivery Agents 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 so the MUA can find it, 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 a .procmailrc file in the user's home directory will invoke Procmail whenever an MTA receives a new message.
The actions Procmail takes with an email are dependent upon instructions from particular recipes, or rules, that messages are matched against by the program. If a message matches the recipe, then the email will be placed in a certain file, deleted, or 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. Then, Procmail looks for a .procmailrc file in the user's home directory to find rules specific to that user. Many users also create additional rc files of their own for Procmail that are referred to by their .procmailrc file, but may be turned on or off quickly if a mail filtering problem develops.
By default, no system-wide rc files exist in the /etc directory, and no user .procmailrc files exist. To begin using Procmail, construct a .procmailrc file with particular environment variables and rules for certain types of messages.
In most configurations, the decision as to whether Procmail starts and attempts to filter the email is based on the existence of a user's .procmailrc file. To disable Procmail, but save the work on the .procmailrc file, move it to a similar file's name using the mv ~/.procmailrc ~/.procmailrcSAVE command. When ready to test Procmail again, change the name of the file back to .procmailrc. Procmail will begin working again immediately.
11.4.1. Procmail Configuration
Procmail configuration files, most notably the user's .procmailrc, contain important environmental variables. These variables tell Procmail which messages to sort, what to do with the messages that do not match any recipes, and so on.
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.
Many environment variables are 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 will be used:
DEFAULT — Sets the default mailbox where messages that do not match any recipes will be 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 would like to turn off Procmail filtering of their email lists but leave spam control in place, they could 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 location and file to contain any Procmail information or error messages.
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 will pause 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 figure out what is happening.
A thorough explanation of regular expressions is beyond the scope of this section. The structure of Procmail recipes is more important and useful sample Procmail recipes can be found at various places on the Internet (including http://www.iki.fi/era/procmail/links.html). The proper use and adaptation of the regular expressions found in these recipe examples depend upon an understanding of Procmail recipe structure. Introductory information specific to basic regular expression rules can be found in the grep man page.
A Procmail recipe takes 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 optionally be placed after the zero to control what Procmail does when processing this recipe. A colon after the <flags> section specifies that a lockfile is created for this message. If a lockfile is to be created, specify its name in the <lockfile-name> space.
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 in order to facilitate a match with a message. If multiple conditions are used, they must all match in order for an 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 what is to happen to a message if it 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.
11.4.2.1. Delivering vs. Non-Delivering Recipes
The action used if the recipe matches a particular message determines whether the recipe is considered delivering or non-delivering. 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 when a nesting block is used. A nesting block is an action contained in braces { } that designates additional actions to perform on messages that match the recipe's conditions. Nesting blocks can be nested, providing greater control for identifying and performing actions on messages.
Delivering recipes that match messages causes Procmail to perform the action specified and stop comparing the message against any other recipes. Messages that match conditions in non-delivering recipes continue to be compared against other recipes in the current and following rc files. In other words, non-delivering recipes cause the message to continue through the recipes after the specified action is taken on it.
11.4.2.2. Flags
Flags are very important in determining 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.
To ensure that the action on this last previous matching recipe was successfully completed before allowing a match on the current recipe, use the a flag instead.
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 this recipe are only compared to the message if the immediately preceding recipe without an E flag did not match. This is comparable to an else action.
Use the e flag if this recipe is checked only when the preceding recipe matched but the action failed.
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.
To ignore "Program failure" messages when deciding whether a filter or action succeeded, use the W option instead.
Additional flags can be found in 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 certain message at the same time. 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
Particular 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 to see if the message is under the specified number of bytes.
> — Checks to see 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.
| — The pipe character tells Procmail to start a specific program to deal with this 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 a mailbox where the message should be written.
11.4.2.5. Recipe Examples
Procmail is an extremely flexible program, which matches messages with very specific conditions and then perform detailed actions on them. As a result of this flexibility, however, composing a Procmail recipe from scratch to achieve a certain goal 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. The following very basic examples exist to serve as a demonstration of the 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 starts the recipe by specifying that a local lockfile is to be created but does not specify a name, leaving Procmail to use the destination file name and the LOCKEXT to name it. No condition is specified, so every message matches this recipe and, therefore, 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.
This basic recipe could be placed at the end of all rc files to direct messages to a default location. A more complicated example might grab messages from a particular email address and throw them away, as can be seen in this example.
:0 * ^From: [email protected] /dev/null |
With this example, any messages sent by [email protected] are immediately moved to /dev/null, deleting them.
 | Caution |
|---|---|
Be very careful that a rule is working correctly before moving messages that are matched to /dev/null, are permanently deleted. If the recipe conditions inadvertently catch unintended messages, they disappear without a trace, making it hard 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 in order to look for false positives, or messages that inadvertently matched the conditions. Once satisfied that no messages are accidentally being matched, delete the mailbox and direct the action to send the messages to /dev/null. |
Procmail is primarily used as a filter for email, automatically placing it in the right place to prevent manual sorting. The following recipe grabs email sent from a particular mailing list and puts it in the correct folder.
:0: * ^(From|CC|To).*tux-lug tuxlug |
Any messages sent from the [email protected] mailing list will be placed in the tuxlug mailbox automatically for the MUA. Note that the condition in this example will match 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 from Section 11.6 Additional Resources to see 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, and a spam-tracking database 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 |
In order for the SpamAssassin daemon to start when the system is booted, use an initscript utility, such as the Services Configuration Tool (redhat-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 or for a system-wide configuration, place it in /etc/procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc |