MAILDROP(1)

NAME
maildrop - mail delivery filter/agent

SYNOPSIS

maildrop [option...] [-d user] [arg...]

maildrop [option...] [filename] [arg...]

DESCRIPTION

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/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/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[1] 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:

Manual mode
    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.

Delivery mode
    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/maildroprc, then $HOME/.mailfilter.

Embedded mode
    maildrop functions as a part of another application. The embedded mode is used by the Courier[1] 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.

SECURITY

It is safe to install maildrop as a root setuid program. The Courier mail server[1] 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.

TEMPORARY FILES

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.

OPTIONS

-a
    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.

        Note
        This setting may already be the default, depending on maildrop's configuration.

-A "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.

-d user
    Run 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"[2] and "Variable substitution"[3]).

-f 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.

-m
    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.

-M 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/filterfile. For security reasons the name of the file may not begin with a slash or include periods. maildrop is very paranoid: both $HOME/.mailfilters, and $HOME/.mailfilters/filterfile must be owned by the user, and may not have any group or world permissions.

    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-default. filterfileprefix is the initial part of filterfile up until the last '-' character in filterfile.

    If $HOME/.mailfilters/filterfileprefix-default does not exist, and there are any other dashes left in filterfileprefix, 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:

    Note that maildrop looks for -default files ONLY if -M is used.

-D uuu/ggg
    This option is reserved for use by the version of maildrop that comes integrated with the Courier mail server[1].

-V 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.

-w 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.

-W filename
    Copy the warning message from filename, or from /etc/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.

-t 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.

DELIVERY MODE

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/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.

    Note

    /etc/maildroprc is read only in delivery mode.

VIRTUAL ACCOUNTS

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.

EMBEDDED MODE

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.[4] 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.

cc[5]
    The cc command is not allowed in embedded mode.

dotlock[6]
    The dotlock command is not allowed in embedded mode.

flock[7]
    The flock command is not allowed in embedded mode.

gdbmopen[8]
    In embedded mode, GDBM databases may be opened only for reading.

log[9]
    The log command is not allowed in embedded mode.

logfile[9]
    The logfile command is not allowed in embedded mode.

system[10]
    The system command is not allowed in embedded mode.

to[11]
    The to command is not allowed in embedded mode.

xfilter[12]
    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[13] statement and the EXITCODE environment variable.

  /etc/maildroprcs

If maildrop encounters an include[14] statement where the filename starts with /etc/maildroprcs/, the normal restrictions for the embedded mode are suspended while executing the filter file in the /etc/maildroprcs directory. The restrictions are also suspended for any additional filter files that are included from /etc/maildroprcs. The restrictions resume once maildrop finishes executing the file from /etc/maildroprcs.

This allows the system administrator to have a controlled environment for running external commands (via the backticks, the system[10] or the xfilter[12] commands).

The name of the file may not contain any periods (so that a creative individual can't write include "/etc/maildroprcs/../../home/user/recipe").

Before executing the commands in the /etc/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/maildroprcs.

WATCHDOG TIMER

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.

FILES

/etc/passwd
    Sets user's home directory, and related variables. If NIS/YP is install, that will be used as well.

/etc/maildroprc
    Global filtering instructions for delivery mode.

/var/mail
    System mailbox (actual directory defined by the system administrator).

/usr/sbin/sendmail
    Program to forward mail (exact program defined by the system administrator).

$HOME/.mailfilter
    Filtering instructions in delivery mode.

$HOME/.mailfilters
    Directory containing files used in special embedded mode.

SEE ALSO

lockmail(1)[15], maildropfilter(7)[16], makedat(1)[17], maildropgdbm(7)[8], maildropex(7)[18], reformail(1)[19], makemime(1)[20], reformime(1)[21], egrep(1), grep(1), , courier(8)[22], sendmail(8).

AUTHOR

Sam Varshavchik
    Author

NOTES

1.  Courier
    https://www.courier-mta.org

2.  "Environment"
    http://www.courier-mta.org/maildrop/maildropfilter.html#environment

3.  "Variable substitution"
    http://www.courier-mta.org/maildrop/maildropfilter.html#varsubst

4.  is passed the -m or the -M option.
    http://www.courier-mta.org/maildrop/#options

5.  cc
    http://www.courier-mta.org/maildrop/maildropfilter.html#cc

6.  dotlock
    http://www.courier-mta.org/maildrop/maildropfilter.html#dotlock

7.  flock
    http://www.courier-mta.org/maildrop/maildropfilter.html#flock

8.  gdbmopen
    http://www.courier-mta.org/maildrop/maildropgdbm.html

9.  log
    http://www.courier-mta.org/maildrop/maildropfilter.html#log

10. system
    http://www.courier-mta.org/maildrop/maildropfilter.html#system

11. to
    http://www.courier-mta.org/maildrop/maildropfilter.html#to

12. xfilter
    http://www.courier-mta.org/maildrop/maildropfilter.html#xfilter

13. echo
    http://www.courier-mta.org/maildrop/maildropfilter.html#echo

14. include
    http://www.courier-mta.org/maildrop/maildropfilter.html#include

15. lockmail(1)
    http://www.courier-mta.org/maildrop/lockmail.html

16. maildropfilter(7)
    http://www.courier-mta.org/maildrop/maildropfilter.html

17. makedat(1)
    http://www.courier-mta.org/maildrop/makedat.html

18. maildropex(7)
    http://www.courier-mta.org/maildrop/maildropex.html

19. reformail(1)
    http://www.courier-mta.org/maildrop/reformail.html

20. makemime(1)
    http://www.courier-mta.org/maildrop/makemime.html

21. reformime(1)
    http://www.courier-mta.org/maildrop/reformime.html

22. courier(8)
    http://www.courier-mta.org/maildrop/courier.html