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.
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:
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.
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.
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.
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.
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.
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.
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 https://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:
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.
With this example, any messages sent by
[email protected] 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
[email protected] 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.
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:
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 |
