maildrop — mail delivery filter/agent
maildrop [option...] [-d user] [arg...]
maildrop [option...] [filename] [arg...]
maildrop is a replacement local mail delivery agent that includes a mail filtering language. The system administrator can either replace the existing mail delivery agent with maildrop, or users may run maildrop using the 'forward to program' mechanism of the existing mail delivery agent.
maildrop first reads the E-mail message on standard input. Trailing carriage return characters are automatically stripped. An E-mail message consists of header lines, followed by a blank line, followed by the contents of the message.
maildrop does not accept an mbox-style
From_
line before the first header line.
maildrop does not accept leading empty lines before
the first non-blank header line. If the message can possibly start with
empty lines, and a
From_
line, use reformail -f0 to remove any initial
empty lines, and replace a From_ line with a
proper “Return-Path:” header; then pipe it to
maildrop.
If the file /etc/courier/maildroprc exists, mail
delivery or mail filtering instructions are read from that file.
maildrop's delivery/filtering instructions may direct
maildrop to save the
message in specific mailbox, discard it, return it to sender, or forward it to
a different E-mail address.
If /etc/courier/maildroprc does not exist, or
its mail delivery instructions do not completely dispose of this message,
maildrop then reads the mail delivery instructions
from $HOME/.mailfilter.
If it doesn't exist, or its mail delivery instructions do not completely
dispose of the message, maildrop
then saves the E-mail message
in the default mailbox.
maildrop knows how to deliver mail to an standard mailbox
files; it
also knows how to deliver to maildirs. A maildir is a
directory-based mail format used by the
Courier
and
Qmail
mail servers.
Many other mail servers also know how to read maildirs.
When delivering to mailbox files, maildrop will lock the
mailbox for the duration of the delivery.
This is the general mail delivery behavior. There are minor differences in behavior depending on maildrop delivery mode, which is determined based on how maildrop was started. maildrop uses three different primary operating modes:
A file containing filtering instructions -
filename
is specified as an argument to the maildrop command.
maildrop reads this
filename
and follows the instructions in it.
Unless the message is explicitly forwarded, bounced,
deleted, or delivered to a specific mailbox, it will be delivered to the
user's system mailbox.
maildrop
is the mail server's mail delivery agent.
maildrop runs in
delivery mode when no filename
is specified on the command line.
maildrop changes the current directory to the user's
home directory, then reads /etc/courier/maildroprc,
then $HOME/.mailfilter.
maildrop functions as a part of another application. The embedded mode is used by the Courier mail server to integrate mail filtering directly into the process of receiving mail from a remote mail relay, thus rejecting unwanted mail before it is even accepted for local mail delivery. Embedded mode is used when either the -m, or the -M, option is specified, and is described below. See below for a more extensive description of the embedded mode.
It is safe to install maildrop as a root setuid program.
The Courier mail server
installs maildrop as a root setuid program by default,
in order to be able to use maildrop in embedded mode.
If root runs maildrop (or it is setuided to root)
the -d option may be used to
specify the message's recipient. maildrop
immediately resets its userid to the one specified by the
-d option. The
user's
$HOME/.mailfilter
is read (if it exists), and the message
is delivered to the indicated user.
The system administrator can configure maildrop to restrict
the -d option for everyone except the mail system
itself.
If in delivery mode the user's home directory has the sticky bit set,
maildrop immediately terminates with an exit code of
EX_TEMPFAIL, without doing anything.
Mail servers interpret the
EX_TEMPFAIL exit code as a request to reschedule the
message
for another delivery attempt later. Setting the sticky bit allows
$HOME/.mailfilter to be edited while
temporarily holding all incoming mail.
maildrop also terminates with EX_TEMPFAIL if the user's home directory has world write permissions.
maildrop immediately terminates with
EX_TEMPFAIL if the filename is
not owned by the user,
or if it has any group or world permissions. This includes read permissions.
The permissions on
$HOME/.mailfilter may only include read and
write privileges to the user.
When using the special embedded mode (see below)
maildrop
immediately terminates with the exit code set to
EX_TEMPFAIL if
$HOME/.mailfilters
is not owned by the user, or if it has any
group or world permissions.
maildrop is heavily optimized and tries to use as little resources as possible. maildrop reads small messages into memory, then filters and/or delivers the message directly from memory. For larger messages, maildrop accesses the message directly from the file. If the standard input is not a file, maildrop writes the message to a temporary file, then accesses the message from the temporary file. The temporary file is automatically removed when the message is delivered.
Makes the Courier Authentication Library usage mandatory, i.e. maildrop will throw a temporary error code if the call to the authlib mechanism fails for some reason, such as authdaemon being inaccessible.
This setting may already be the default, depending on maildrop's configuration.
Header: value"Adds an additional header to the message. Specifying -A "Foo: Bar" effectively adds this header to the message being delivered.
The mail transport agent usually adds additional headers when delivering a message to a local mailbox. The way it's usually done is by the mail transport agent sending the message using a pipe to the local delivery agent - such as maildrop - and adding some additional headers in the process. Because maildrop receives the message from a pipe, maildrop must either save the message in memory or write the message into a temporary file.
The -A option enables the file containing the message
to be provided to maildrop directly, as standard input,
and the additional headers specified on the command line.
Because the standard input is a file,
maildrop will not need a temporary file.
Multiple -A options may be specified.
userRun maildrop in delivery mode for this user ID.
The system administrator may optionally restrict the
-d option to be available to the mail system only, so
it may not be available to you.
In all cases, the -d option is allowed if
user is the same user who is running
maildrop.
Also, for the -d option to work at all,
maildrop must be executed by root, or
maildrop must be a root-owned program with the setuid bit
set.
Absence of a filename
on
maildrop's command line
implies the -d option for the user running
maildrop.
If -d is not specified, the first argument following all the
options is
a name of the file containing filtering instructions. The remaining
arguments, if any, are assigned to the variables
$1,
$2, and so on (see
"Environment" and
"Variable substitution").
address
Sets the FROM variable (message envelope sender) to
address.
The system administrator may optionally disable the
-f option for users, so it may not be available to you.
Run maildrop in embedded mode.
It's possible to use both the -m, and the
-d options, but it doesn't make much sense to do so.
Even if you really wanted to run your message through someone else's
.mailfilter,
that .mailfilter probably has at least one
instruction which is not allowed in the embedded mode.
The filename argument to maildrop
should be specified.
filename is a file that includes filtering
instructions to be processed in embedded mode.
The -m option is used for debugging filter files which
are later placed in
$HOME/.mailfilters, and used with the
-M option.
filterfile
Run maildrop in a special embedded mode.
The -d
option is implied when -M is used, and if absent it defaults
to the userid running maildrop.
All the requirements for the -d option apply.
maildrop must either
be executed by root, or the maildrop program must be owned
by root with the setuid bit set.
maildrop immediately gives up root
privileges by changing its user ID to the one specified by -d,
then reads
$HOME/.mailfilters/. For security reasons the name of the file may not begin with a slash or include
periods.
maildrop is very paranoid:
both filterfile$HOME/.mailfilters, and
$HOME/.mailfilters/
must be owned by the user, and may not have any group or world
permissions.filterfile
The -M option allows for some friendly cooperation between
the user
running the application, and the user who provides a filter for the
embedded mode. The user running the application can use someone else's
canned filter and be assured that the filter is not going to run amok and
start sending mail or create files all over the place. The user who
provides the filter can be assured that the environment variables are
clean, and that there are no surprises.
maildrop supports the concept of "default" filter files.
If the
file specified by the -M option cannot be found in
$HOME/.mailfilters,
maildrop will try to open
$HOME/.mailfilters/.
filterfileprefix-defaultfilterfileprefix is the initial part
of filterfile up until the last '-' character in
filterfile.
If
$HOME/.mailfilters/
does not exist, and there are any other dashes left in
filterfileprefix-defaultfilterfileprefix, maildrop
removes the last dash and everything
following it, then tries again.
As a last resort maildrop
tries to open $HOME/.mailfilters/default.
For example, if the parameter to the -M option is
mailfilter-lists-maildrop, maildrop will try to open the following files, in order:
$HOME/.mailfilters/mailfilter-lists-maildrop
$HOME/.mailfilters/mailfilter-lists-maildrop-default
$HOME/.mailfilters/mailfilter-lists-default
$HOME/.mailfilters/mailfilter-default
$HOME/.mailfilters/default
Note that maildrop looks for -default files ONLY if
-M is used.
uuu/gggThis option is reserved for use by the version of maildrop that comes integrated with the Courier mail server.
level
Initialize the VERBOSE variable to
level.
Because maildrop parses the
entire file before
running it, this option is used to produce debugging output in the
parsing phase. Otherwise, if filename
has syntax errors, then
no debugging output is possible because the VERBOSE variable
is not yet set.
-V is ignored when maildrop runs in
delivery mode.
N
The -w N option places a warning message
into the maildir if the maildir has a quota setting, and after
the message was successfully delivered the maildir was at least
N percent full.
filename
Copy the warning message from filename, or from
/etc/courier/quotawarnmsg if this option is not specified,
with the addition of the
"Date:" and
"Message-Id:" headers.
The warning is repeated every 24 hours (at least), until the maildir drops
below
N percent full.
socket
This option is available if maildrop is compiled with optional
Dovecot authentication support. socket
specifies the location of Dovecot master authentication socket,
for example /var/run/dovecot/auth-master.
If a filename is not specified on the command line, or
if the -d
option is used, maildrop will run in delivery mode.
In delivery mode,
maildrop changes to the home directory of the user
specified by the -d
option (or the user who is running maildrop if the
-d option was not
given) and reads $HOME/.mailfilter for filtering
instructions. $HOME/.mailfilter must be owned by the
user, and have no group
or global permissions (maildrop terminates if it
does).
If $HOME/.mailfilter does not exist,
maildrop will
simply deliver the message to the user's mailbox.
If the file /etc/courier/maildroprc exists,
maildrop reads filtering instructions from this file first,
before reading
$HOME/.mailfilter. This allows the system administrator
to provide global filtering instructions for all users.
/etc/courier/maildroprc is read only in
delivery mode.
The -d option can also specify a name of a virtual account
or mailbox. See the
makeuserdb(1)
manual page in the Courier
Authentication library's documentation
for more information.
The embedded mode is used when maildrop's filtering
abilities are
desired, but no actual mail delivery is needed. In embedded mode
maildrop is executed by another application, and
is
passed the ‐m or the ‐M option. maildrop reads the
message, then runs the filtering rules specified in
filename.
filename
may contain any filtering instructions EXCEPT the following:
Text strings delimited by back-tick characters (run shell command) are not allowed.
The cc command is not allowed in embedded mode.
The dotlock command is not allowed in embedded mode.
The flock command is not allowed in embedded mode.
In embedded mode, GDBM databases may be opened only for reading.
The log command is not allowed in embedded mode.
The logfile command is not allowed in embedded mode.
The system command is not allowed in embedded mode.
The to command is not allowed in embedded mode.
The xfilter command is not allowed in embedded mode.
Normally when the filename does not explicitly delivers
a message,
maildrop will deliver the message to the user's default mailbox. This is also disabled in embedded mode.
The filename may communicate with the parent application
by
using the echo statement and the
EXITCODE environment variable.
/etc/courier/maildroprcsIf maildrop encounters an
include
statement where the filename
starts with /etc/courier/maildroprcs/,
the normal restrictions
for the embedded mode are suspended while executing the filter file in the
/etc/courier/maildroprcs directory. The restrictions are
also
suspended for any additional filter files that are included from
/etc/courier/maildroprcs. The restrictions resume once
maildrop finishes executing the file from
/etc/courier/maildroprcs.
This allows the system administrator to have a controlled environment for running external commands (via the backticks, the system or the xfilter commands).
The name of the file may not contain any periods (so that a creative individual can't write include "/etc/courier/maildroprcs/../../home/user/recipe").
Before executing the commands in the
/etc/courier/maildroprcs
file, maildrop automatically resets the following variables to their
initial values: DEFAULT, HOME,
LOCKEXT, LOCKSLEEP,
LOCKTIMEOUT, LOCKREFRESH,
LOGNAME, PATH,
SENDMAIL, and SHELL.
Please note that the previous
values of these variables (if they were changed) will NOT be restored once
maildrop finishes executing the commands from
/etc/courier/maildroprcs.
maildrop has a watchdog timer that attempts to abort runaway filtering. If filtering is not complete within a predefined time interval (defined by the system administrator, usually five minutes), maildrop terminates.
/etc/passwdSets user's home directory, and related variables. If NIS/YP is install, that will be used as well.
/etc/courier/maildroprcGlobal filtering instructions for delivery mode.
/var/mailSystem mailbox (actual directory defined by the system administrator).
/usr/lib/sendmailProgram to forward mail (exact program defined by the system administrator).
$HOME/.mailfilterFiltering instructions in delivery mode.
$HOME/.mailfiltersDirectory containing files used in special embedded mode.
lockmail(1), maildropfilter(7), makedat(1), maildropgdbm(7), maildropex(7), reformail(1), makemime(1), reformime(1), egrep(1), grep(1), , courier(8), sendmail(8).