maildropfilter(7)
NAME
maildropfilter - maildrop's filtering language
SYNOPSIS
/etc/maildroprc, $HOME/.mailfilter, $HOME/.mailfilters/*, and friends...
DESCRIPTION
This manual page describes the language used by maildrop to filter
E-mail messages. The mail filtering instructions are read from a file.
The language is loosely structured, it is based on pattern matching.
The language has a distinct lexical and syntactical structure, very
similar to Perl's, but it is important to note that it is not Perl, and
is very different from Perl, in certain cases.
If the filtering instructions do not exist, maildrop delivers the
message to the default mailbox without doing any additional processing,
making it indistinguishable from the usual mail delivery agent.
It is important to note that maildrop reads and parses the filter file
before doing anything. If there are any errors maildrop prints an error
message, and terminates with the exit code set to EX_TEMPFAIL. A
compliant mail transport agent should re-queue the message for a later
delivery attempt. Hopefully, most simple syntax errors will not cause
mail to be bounced back if the error is caught and fixed quickly.
Environment
maildrop uses variables to access and manipulate messages. Variables
are arbitrary text accessed by referring to the name of the variable,
such as HOME, or DEFAULT. Text is placed into a variable by using an
assignment statement, such as:
FILE="IN.junk"
This statement puts the text "IN.junk" (without the quotes) into a
variable whose name is FILE. Later, the contents of a variable are
accessed by using the $ symbol and the name for the variable. For
example:
to $FILE
This will deliver the current message to the mailbox file (or a maildir
directory) named "IN.junk".
maildrop initially creates variables from the environment variables of
the operating system, UNLESS maildrop runs in delivery mode. Each
operating system environment variable becomes a maildrop variable. When
running in delivery mode, maildrop does not import the environment for
security reasons, except for the environment variables that define the
process locale (LANG, LANGUAGE, and LC_*), which are still imported.
In all cases maildrop resets the following variables to their default
values: HOME, DEFAULT, SHELL, PATH, LOCKEXT, LOCKREFRESH, LOCKSLEEP,
LOCKTIMEOUT, MAILDIRQUOTA, SENDMAIL and LOGNAME.
There's one exception to this rule which applies to the version of
maildrop that comes with the Courier mail server[1]. The following does
not apply to the standalone version of maildrop: when running in
delivery mode, if the -d flag was not used, or if it specifies the same
userid as the one that's running maildrop: the following variables are
automatically imported from the environment: HOME, SHELL, LOGNAME and
MAILDIRQUOTA. These environment variables are initialized by the
Courier mail server prior to running maildrop. Additionally, the
initial value for the DEFAULT maildrop variable is imported from the
MAILDROPDEFAULT environment variable. This is because the Courier mail
server overloads the DEFAULT environment variable to store the
defaulted portion of the local mailbox address. See the
dot-courier(5)[2] man page in the Courier mail server distribution. You
can get the Courier mail server's DEFAULT value by using the import
command. Note, however, that this will clobber the old contents of
DEFAULT, which is probably not what you want. The right way to do this
would be something like this:
SAVEDEFAULT=$DEFAULT
import DEFAULT
LOCALDEFAULT=$DEFAULT
DEFAULT=$SAVEDEFAULT
All internal variables are exported back as environment variables when
maildrop runs an external command. Changes to internal variables, made
by the filter file, are reflected in the exported environment.
Lexical structure
Most whitespace is generally ignored. The # character introduces a
comment running to the end of the line, which is also ignored. Unlike
other mail filters, maildrop parses the filter file before taking any
action with the message. If there are syntax errors in the file,
maildrop displays an error message, and returns EX_TEMPFAIL. That
should cause the mail message to remain in the queue, and, hopefully
allow the problem to be corrected, without bouncing any mail.
Note
In maildrop, the end of line is a lexical token. In order to
continue a long statement on the next line, terminate the line with
a backslash character.
Literal text
Literal text in the maildrop filtering language is surrounded by either
single or double quotes. In order to enter a single quote into a text
literal surrounded by single quotes, or a double quote into a literal
surrounded by double quotes, prefix it with a backslash character. Use
two backslash characters characters to enter one backslash character in
the text literal.
Note
A backslash followed by either a backslash, or a matching quote, is
the only situation where the backslash character is actually
removed, leaving only the following character in the actual text
literal. If a backslash character is followed by any other
character, the backslash is NOT removed.
Multiple text literals in a row are automatically concatenated, even if
they use different quotes. For example:
FOOBAR="Foo"'bar'
SAVEDEFAULT=$DEFAULT
import DEFAULT
LOCALDEFAULT=$DEFAULT
DEFAULT=$SAVEDEFAULT
This sets the variable FOOBAR to the text "Foobar".
Variable substitution
Variable substitution is performed on text literals that's surrounded
by double quotation marks. The "$" character, followed by a variable
name, is replaced by that variable's contents.
MAILBOX="$HOME/Mailbox"
This sets the variable MAILBOX to the contents of the variable HOME
followed by "/Mailbox". Variable names must begin with an uppercase
letter, a lowercase letter, or an underscore. Following that, all
letters, digits, and underscores are taken as a variable name, and its
contents replace the $ sign, and the variable name. It is possible to
access variables whose name includes other characters, by using braces
as follows:
MAILBOX="${HOME-WORD}/Mailbox"
Inserts the contents of the HOME-WORD variable. If the variable does
not exist, the empty text literal is used to replace the variable name.
It is not possible to access variables whose names include the }
character.
If the $ character is not followed by a left brace, letter, or an
underscore, the $ character remains unmolested in the text literal. A
backslash followed by the $ character results in a $ character in the
text literal, without doing any variable substitution.
Variable substitution is not done in text literals which are surrounded
by single quotes (apostrophes).
Command line arguments
maildrop initializes special variables: $1, $2, and so on, with
additional parameters specified on the maildrop command line. A filter
file may use those variables just like any other variables.
Predefined variables
The following variables are automatically defined by maildrop. The
default values for the following variables may be changed by the system
administrator. For security reasons, the values of the following
variables are always reset to their default values, and are never
imported from the environment:
DEFAULT
The default mailbox to deliver the message to. If the filter file
does not indicate a mailbox to deliver this message to, the message
is delivered to this mailbox. The default mailbox is defined by the
system administrator.
FROM
Message envelope sender. This is usually the same address as what
appears in the From: header, but may not be. This information may
or may not be available to maildrop on your system. The message
envelope sender is usually specified with the -f option to
maildrop. If the -f option is not given, maildrop looks for the
Return-Path: header in the message. As the last resort, FROM
defaults to "MAILER-DAEMON". Note that FROM may be empty - the
message envelope sender is empty for bounce messages.
HOME
Home directory of the user running maildrop.
HOSTNAME
Network name of the machine running maildrop. Obtained from
gethostname(3).
LOCKEXT
Extension for dot-lock files (default: .lock).
LOCKREFRESH
Refresh interval, in seconds, for dot-locks (default: 15). When
maildrop dot-locks a mailbox, maildrop tries to refresh the lock
periodically in order to keep other programs from removing a stale
dot-lock. This is only required if a dot-lock exists for a
prolonged period of time, which should be discouraged anyway.
LOCKSLEEP
Number of seconds to wait to try again to create a dot-lock file,
if one already exists (default: 5).
LOCKTIMEOUT
Number of seconds to wait before removing a stale dot-lock file
(default: 60). If a dot-lock file still exists after LOCKTIMEOUT
seconds, maildrop assumes that the process holding the lock no
longer exists, and the dot-lock file can be safely removed. After
removing the dot-lock file, maildrop waits LOCKSLEEP seconds before
trying to create its own dot-lock file, in order to avoid a race
condition with another process which is also trying to remove the
same stale dot-lock, at the same time.
LOGNAME
Name of the user to who the message is being delivered.
MAILDROP_OLD_REGEXP
Revert to using the old legacy pattern matching engine. Versions of
maildrop prior to version 2.0 (included in the Courier mail server
0.51, and earlier), used a built-in pattern matching engine,
instead of using the PCRE library (see the "Patterns" section).
maildrop 1.x used a different syntax for patterns, which is no
longer described in this manual page. The old pattern matching
engine is still available, by setting MAILDROP_OLD_REGEXP to "1".
Setting this variable will use the legacy pattern matching engine
for the rest of the maildrop recipe file.
The pattern matching engine will be removed completely in a future
version of maildrop. This setting provides for a transitional
period of converting old recipes. MAILDROP_OLD_REGEXP can be set
to "1" in the global maildroprc file, then reset to "0" in each
individual maildrop recipe file, after it gets converted to the new
syntax.
MAILFILTER
This is the name of the original filter file that was given to
maildrop on the command line. This is mostly useful to
-defaultfilter files, it allows them to obtain the value of the -M
option[3] specified on the command line.
PATH
Command execution path. maildrop resets PATH to the system default
(usually /bin:/usr/bin:/usr/local/bin).
SENDMAIL
The mail delivery agent. When maildrop is instructed to deliver the
message to a mailbox whose name begins with the ! character, this
is interpreted as a request to forward the message. The SENDMAIL
command is executed to forward the message.
SHELL
The login shell. The shell is used to execute all commands invoked
by maildrop.
VERBOSE
Current Debug level (default: 0). Setting VERBOSE to progressive
higher values, between 1 and 9, produces debugging output on
standard error. maildrop ignores the VERBOSE variable in delivery
mode (in order not to confuse the mail transport agent).
UMASK
The file creation mode mask, in octal. The default setting of 077
creates mailboxes that are readable and writable by the owner only.
Use 007 to create mailboxes that are readable/writable by both
owner and the group. Use 037 to create mailboxes that are readable
by both owner and group, but writable by owner only. Permissions on
existing mailboxes are not changed, this setting affects only new
mailboxes. When delivering to maildirs this setting sets the
permissions on new messages only. Access permissions on messages in
maildirs are also affected by the permissions on the maildir
directories.
Other special variables
The following variables are automatically used by maildrop when the
filter file is being processed:
EXITCODE
Return code for maildrop. When maildrop successfully delivers a
message, it terminates with this exit code, which defaults to 0.
When the to or the cc command is used to deliver the message to an
external process, via a pipe, maildrop will set this variable to
the exit code of the external process. Since maildrop immediately
terminates after completing the to command this means that
maildrop's exit code will be the exit code of the external process.
If the to command does not deliver the message to a process you
must set EXITCODE before the to command, since maildrop terminates
immediately after finishing the delivery.
FLAGS
The FLAGS variable is used only when delivering a message to a
maildir, and may contain only the following letters: "D", "F", "R",
and "S". They may appear in any order. When the message gets
delivered to the maildir, the message will be marked with a draft,
flag, replied, or seen, attribute, correspondingly.
FLAGS must be set before the message is delivered to a maildir. The
contents of FLAGS are ignored, when delivering on an mbox folder.
KEYWORDS
The KEYWORDS variable is used only when delivering a message to a
maildir, and implements the optional IMAP keyword extension as
implemented in the Courier IMAP server[1]. It may be optionally
initialized to contain a comma-separate list of keywords. The to,
or the cc command, delivers the message to the maildir normally,
but also associated the list of keywords in KEYWORDS with the newly
delivered message.
KEYWORDS must be set before the message is delivered to a maildir.
The contents of KEYWORDS are ignored, when delivering on an mbox
folder.
LINES
Number of lines in the current message. Note that this may be an
approximation. It may or may not take into account the -A option.
Use this as criteria for filtering, nothing more.
MAILDIRQUOTA
Set this variable in order to manually enforce a maximum size on
ANY maildir where the message is delivered. This is an optional
feature that must be enabled by the system administrator, see
maildirquota(8)[4] for more information.
RETURNCODE
This variable is set when maildrop runs the xfilter[5] command, or
a command that's specified within a pair of backtick characters (
command substitution ). The RETURNCODE variable will be set to the
exit code of the command, after it completes.
SIZE
Number of bytes in the message. This may or may not include the -A
option. Use this as a criteria for filtering, nothing more.
Unquoted text
All text strings in filter files should be in single, or double quotes.
However, for convenience sake, quotes can be omitted under certain
circumstances.
Text that includes ONLY letters, digits, and the following characters:
_-.:/${}@ may appear without quotes. Note that this does not allow
spaces, or backslashes to be entered, however the text is still
variable-substituted, and the substituted text may contain other
characters.
Also, note that patterns (see below) begin with the slash character.
Normally, anything that begins with the slash is interpreted as a
pattern. However, text immediately after "VARIABLE=" is interpreted as
a string even if it begins with a slash. This is why something like:
MAILDIR=/var/mail
works as expected. Using quotes, though, is highly recommended. You
must use quotes to set a variable to a lone slash, because an unquoted
slash is interpreted as a division sign.
Long double or singly-quoted text can be broken across multiple lines
by ending the line with a lone backslash character, like this:
TEXT="This is a long \
text string"
The backslash, the newline, and all leading whitespace on the next line
is removed, resulting in "This is a long text string".
Command substitution
Text enclosed in back-tick characters is interpreted as a shell
command. The shell command is executed as a child process by maildrop.
Its output is used in place of the command. For example:
DIR=`ls`
places the names of the files in the current directory into the DIR
variable.
The output of the command will have all newline characters replaced by
spaces, and leading and trailing spaces will be stripped (multiple
spaces are not removed, though). Also, the contents of the message
being delivered is made available to the command on standard input.
Patterns
The pattern syntax in maildrop is similar to the grep command's syntax,
with some minor differences. A pattern takes the following form in the
filter file:
/pattern/:options
pattern specifies the text to look for in the message, in the UTF-8
codeset. pattern must not begin with a space, otherwise the leading
slash will then be interpreted as a division sign. If you must search
for text that starts with a space, use something like "/[ ] ... /".
The general syntax of maildrop's patterns is described in the
pcrepattern(3) manual page, with certain exceptions noted below.
maildrop uses the PCRE[6] library to implement pattern matching. Not
all features in PCRE are available in maildrop, and the "options" part,
which follows the pattern specification, changes the pattern matching
further. Consult the pcrepattern(3) manual page for more information,
but note the following exceptions:
* Internal options settings are not supported (but see the "D"
maildrop option, below). Do not include option settings in the
pattern, doing so will lead to undefined results.
* Named subpatterns are not implemented. Numbered subpatterns are
implemented, see "Pattern Match Results", below.
* The search pattern gets executed not against the raw message text,
but the message transcoded into a canonical UTF-8-based format.
This process involves transcoding any non-UTF-8 message content
into UTF-8. Additionally, message headers get converted into a
canonical format before the search pattern gets executed.
For structured headers with email addresses, the process involves
removing extraneous punctuation, or adding missing ones (in
situations where a missing punctuation character can be deduced).
Additionally certain pre-RFC822 obsolete header formats get
converted to canonical form.
This means that header search patterns that include punctuation
character may appear not to work against obviously-matching message
text. Use "reformime -u <message.txt", with message.txt containing
the sample message, to see exactly the actual text that gets
searched by patterns.
Pattern options
Following /pattern/, there may be an optional colon, followed by one.
or more options. The following options may be specified in any order:
h
Match this pattern against the message header.
b
Match this pattern against the message body.
D
This is a case sensitive match. Normally the patterns match either
uppercase or lowercase text. /john/ will match "John", "john", or
"JOHN". Specify the D option for a case-sensitive search: lowercase
letters in the pattern must match lowercase letters in the message;
ditto for uppercase.
If neither 'h' or 'b' is specified, the pattern is matched against the
header only. Specifying the 'b' option causes the pattern to be matched
against the message body. Specifying both causes the pattern to be
matched against the entire message.
Normally, each line in the message gets matched against the pattern
individually. When applying patterns to a header, multi-line headers
(headers split on several lines by beginning each continuation line
with whitespace) are silently combined into a single line, before the
pattern is applied.
MIME encoding
The pattern must be a valid text string in the UTF-8 codeset, and
maildrop should handle messages that use MIME encodings in other known
character sets. Options that specify a message header search result in
maildrop searching the initial message headers, and any headers of
additional MIME sections, in a multipart MIME message. Options that
specify a message body search will search through all "text" MIME
content.
For a MIME search to succeed, the message must be a well-formed MIME
message (with a Mime-Version: 1.0 header).
Weighted scoring
Patterns are evaluated by maildrop as any other numerical expression.
If a pattern is found, maildrop's filter interprets the results of the
pattern match as number 1, or true, for filtering purposes. If a
pattern is not found the results of the pattern search is zero. Once a
pattern is found, the search stops. Second, and subsequent occurrences
of the same pattern are NOT searched for.
maildrop can also do weighted scoring. In weighted scoring, multiple
occurrences of the same pattern are used to calculate a numerical
score.
To use a weighted search, specify the pattern as follows:
/pattern/:options,xxx,yyy
where xxx and yyy are two numbers. yyy is optional -- it will default
to 1, if missing.
The first occurrence of the pattern is evaluated as xxx. The second
occurrence of the pattern is evaluated as xxx*yyy, the third as
xxx*yyy*yyy, etc... All occurrences of the pattern are added up to
calculate the final score.
Note
maildrop does not recognize multiple occurrences of the same
pattern in the same line. Multiple occurences of the same pattern
in one line count as one occurence.
Pattern Match Results
After a pattern is successfully matched, the actual text that is
matched is placed in the MATCH variable. For example:
/^From:.*/
matches a line of the form:
From: postmaster@localhost
Here the variable MATCH will be set to "From: postmaster@localhost",
which can be used in subsequent statements.
If the pattern contains subpatterns, the portions of the text that
match the first subpattern is placed in the MATCH1 variable. The second
subpattern, if any, is placed in MATCH2, and so on:
/^From:\s+(.*)@(.*)/
matched against the same line will set MATCH to "From:
postmaster@localhost", MATCH1 to "postmaster", and MATCH2 to
"localhost". Of course, in real world the "From:" header is usually
much more complicated, and can't be handled that easily. This is just
an illustrative example.
Note
Subpatterns are not processed in the foreach statement.
Conversion of maildrop 1.x patterns to 2.0
Although the new PCRE-based pattern matching code in maildrop is
completely different from the built-in pattern matching code in
maildrop 1.x, very few changes will be required to convert recipes to
the new syntax. The only major differences are:
* The subexpression format has changed. Any pattern that uses
subexpression needs to be converted. Additionally, references to
MATCH2 must be replaced with MATCH1, MATCH3 to MATCH2, and so on.
References to plain old MATCH will remain the same.
* The "w" pattern option is no longer possible, with PCRE. The very
few recipes that use this option, if any actually exist, will have
to be rewritten in some other fashion.
Expressions
Although maildrop evaluates expressions numerically, results of
expressions are stored as text literals. When necessary, text literals
are converted to numbers, then the results of a mathematical operation
is converted back into a text literal.
Operators
The following operators carry their usual meaning, and are listed
in order from lowest precedence, to the highest:
||
&&
< <= > >= == != lt le gt ge eq ne
|
&
+ -
* /
=~ /pattern/
/pattern/ ! ~ function()
Variable assignment
VARIABLE=expression
Assigns the result of the expression to VARIABLE (note no leading $
in front of variable).
Note
If VARIABLE is NOT surrounded by quotes, then it may contain
only letters, numbers, underscores, dashes, and a selected few
other characters. In order to initialize a variable whose name
contains non-standard punctuation marks, surround the name of
the variable with quotes.
cc - deliver a copy of the message
cc expression
The cc statement is very similar to the to statement, except that
after delivering the message maildrop continues to process the
filter file, unlike the to statement which immediately terminates
maildrop after the delivery is complete. Essentially, the message
is carbon copied to the given mailbox, and may be delivered again
to another mailbox by another cc or to statement.
See the to statement[7] for more details. When cc is used to
deliver a message to a process maildrop will set the EXITCODE
variable to the process's exit code.
dotlock - create a manual dot-lock
dotlock expression {
...
}
maildrop automatically creates a lock when a message is delivered
to a mailbox. Depending upon your system configuration, maildrop
will use either dot-locks, or the flock() system call.
The dotlock statement creates an explicit dot-lock file. Use the
flock statement[8] to create an explicit flock() lock.
The expression is a filename that should be used as a lock file.
maildrop creates the indicated dot-lock, executes the filtering
instructions contained within the { ... } block, and removes the
lock. The expression must be the name of the dot-lock file itself,
NOT the name of the mailbox file you want to lock.
Note
With manual locking, it is possible to deadlock multiple
maildrop processes (or any other processes that try to claim
the same locks).
No deadlock detection is possible with dot-locks, and since
maildrop automatically refreshes all of its dot-locks
regularly, they will never go stale. You'll have maildrop
processes hanging in limbo, until their watchdog timers go off,
aborting the mail delivery.
echo - output diagnostic information
echo expression
maildrop will print the given text. This is usually used when
maildrop runs in embedded mode, but can be used for debugging
purposes. Normally, a newline is printed after the text. If text is
terminated with a \c, no newline will be printed.
exception - trap fatal errors
exception {
...
}
The exception statement traps errors that would normally cause
maildrop to terminate. If a fatal error is encountered anywhere
within the block of statements enclosed by the exception clause,
execution will resume immediately following the exception clause.
exit - terminate filtering unconditionally
exit
The exit statement immediately terminates filtering. maildrop's
return code is set to the value of the EXITCODE variable. Normally,
maildrop terminates immediately after successfully delivering the
message[7] to a mailbox. The exit statement causes maildrop to
terminate without delivering the message anywhere.
The exit statement is usually used when maildrop runs in embedded
mode[9], when message delivery instructions are not allowed.
flock - create an manual flock() lock
flock expression {
...
}
maildrop automatically creates a lock when a message is delivered
to a mailbox. Depending upon your system configuration, maildrop
will use either dot-locks, or the flock() system call.
The flock statement creates a manual flock() lock. Use the dotlock
statement[10] to create a manual dot-lock file.
The expression is the name of the file that should be locked.
maildrop creates the lock on the indicated file, executes the
filtering instructions contained within the { ... } block, and
removes the lock.
Note
With manual locking, it is possible to deadlock multiple
maildrop processes (or any other processes that try to claim
the same locks). The operating system will automatically break
flock() deadlocks. When that happens, one of the maildrop
processes will terminate immediately. Use the exception
statement in order to trap this exception condition, and
execute an alternative set of filtering instructions.
foreach - iterate over text sections matched by a pattern
foreach /pattern/:options
{
...
}
foreach (expression) =~ /pattern/:options
{
...
}
The foreach statement executes a block of statements for each
occurrence of the given pattern in the given message, or
expression. On every iteration MATCH variable will be set to the
matched string. All the usual options may be applied to the pattern
match, EXCEPT the following:
,xxx,yyy
Weighted scoring is meaningless, in this context.
( ... )
Subpatterns are not processed. Only the MATCH variable will be
set for each found pattern.
if - conditional execution
if (expression)
{
...
}
else
{
...
}
Conditional execution. If expression evaluates to a logical true
(note - parenthesis are required) then the first set of statements
is executed. The else keyword, and the subsequent statements, are
optional. If present, and the expression evaluates to a logical
false, the else part is executed.
maildrop evaluates all expression as text strings. In the context
of a logical expression, an empty string, or the number 0
constitutes a logical false value, anything else is a logical true
value.
If the if part, or the else part consists of only one statement,
the braces may be omitted.
Note
The grammar of this if statement is stricter than usual. If you
get baffling syntax errors from maildrop, make sure that the
braces, and the if statement, appear on separate lines.
Specifically: the closing parenthesis, the closing braces, and
the else statement, must be at the end of the line (comments
are allowed), and there may not be any blank lines in between
(not even ones containing comments only).
If the else part contains a single if, and nothing else, this may
be combined into an elsif:
if (expression)
{
...
}
elsif (expression)
{
...
}
The above example is logically identical to:
if (expression)
{
...
}
else
{
if (expression)
{
...
}
}
Consecutive elsif sequences are allowed:
if (expression)
{
...
}
elsif (expression)
{
...
}
elsif (expression)
{
...
}
Consecutive occurences of elsif commands eliminate a significant
amount of indentation, and the resulting code is more readable.
import - access original environment variable
import variable
When maildrop starts, it normally imports the contents of the
environment variables, and assigns them to internal maildrop
variables. For example, if there was an environment variable FOO,
the internal maildrop variable FOO will have the contents of the
environment variable. From then on, FOO will be no different than
any other variable, and when maildrop runs an external command, the
contents of maildrop's variables will be exported as the
environment for the command.
Certain variables, like HOME and PATH, are always reset to fixed
defaults, for security reasons. Also, in delivery and embedded
modes, the environment is not imported at all (with the exception
of system locale environment variables), and maildrop starts with
only the fixed default variables.
The import statement initializes the specified variable with the
contents of the original environment variable when maildrop
started. For example:
echo "PATH is $PATH"
PATH="/bin"
echo "PATH is $PATH"
import PATH
echo "PATH is $PATH"
exit
This results in the following output:
PATH is /bin:/usr/bin:/usr/local/bin
PATH is /bin
PATH is /home/root/bin:/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/bin
This shows that when maildrop starts PATH is set to the fixed
default of /bin:/usr/bin:/usr/local/bin. However, the original
contents of the PATH environment variable we different, and the
import statement shows what it was.
include - execute filtering instructions from another file
include expression
The include statement reads a file, and executes filtering
instructions contained in that file. Note that the include
statement is processed when the current filter file is being
executed. When maildrop reads the initial filter file, any syntax
errors in the filtering instructions are immediately reported, and
maildrop will terminate with a return code of EX_TEMPFAIL. Any
errors in files specified by include statements are NOT reported,
because those files will not be read until the include statement is
itself executed.
If the specified file does not exist, or if there are any syntax
errors in the file, maildrop reports the error, and terminates with
a return code of EX_TEMPFAIL.
log, logfile - log message deliveries
logfile expression
log expression
Logging in maildrop is normally turned off. The logfile statement
specifies the file where maildrop will log how the message has been
disposed of. The parameter is then name of the file. If the file
exists maildrop appends to the file.
For each delivery (the to[7] and cc[11] statements, and default
deliveries) maildrop records the From: and the Subject: fields,
together with the current time, in the log file.
The log statement adds additional logging text to the log file. The
log statement works exactly like the echo statement, except that
the text is written to the logfile, instead of standard output.
to - deliver message to a mailbox
to expression
The to statement delivers the message to a mailbox. expression
must evaluate to a valid mailbox. A valid mailbox is either a
mailbox file, a maildir, or an external program (which includes
forwarding to another address).
The to statement is the final delivery statement. maildrop
delivers message, then immediately terminates, with its return code
set to the EXITCODE variable. If there was an error while
delivering the message, maildrop terminates with the EX_TEMPFAIL
exit code. A properly-written mail transport agent should re-queue
the message, and re-attempt delivery at some later time.
An expression that begins with the "|" character specifies an
external program to run to handle the actual delivery. The SHELL
variable specifies the shell to execute the given command. The
message is provided to the command on standard input. maildrop's
exit code will be the process's exit code.
An expression that begins with an exclamation mark, "!" specifies a
whitespace-delimited list of E-mail addresses to forward the
message to. The program specified by the SENDMAIL variable is run
as an external program, with the list of E-mail addresses provided
as parameters to the program.
Otherwise, expression names the mailbox where maildrop delivers the
message. If expression is a directory, maildrop assumes that the
directory is a maildir directory. Otherwise, maildrop will deliver
the message to a file, formatted in traditional mailbox format.
maildrop will use either dot-locking, or flock()-locking when
delivering the message to the file.
while - repeatedly execute a block of statements
while (expression)
{
...
}
The expression is repeatedly evaluated. Each time it evaluates to a
logical true[12], the statements inside the braces are executed.
When expression evaluates to a logical false, the while loop is
over. Take care to avoid infinite loops.
xfilter - filter message through another program
xfilter expression
expression specifies an external program that maildrop runs to
filter the current message. The current message will be piped to
the filter program as standard input. The output of the filter
program replaces the current message being delivered. The external
program must terminate with an exit code of 0. If the external
program does not terminate with an exit code of 0, or if it does
not read the message from the standard input, maildrop terminates
with an exit code of EX_TEMPFAIL.
|| - logical or
expression1 || expression2
If expression1 evaluates to a logical true, the result of the || is
expression1, otherwise it's expression2, which is evaluated.
maildrop uses the following concept of true/false: an empty text
literal, or a text literal that consists of the single character
"0" is a logical false value. Anything else is a logical true
value.
&& - logical and
expression1 && expression2
If expression1 evaluates to a logical false, the result of the &&
is expression1, otherwise it's expression2, which is evaluated.
maildrop uses the following concept of true/false: an empty text
literal, or a text literal that consists of the single character
"0" is a logical false value. Anything else is a logical true
value.
<, <=, >, >=, ==, != - numerical comparison
expression1 < expression2
expression1 <= expression2
expression1 > expression2
expression1 >= expression2
expression1 == expression2
expression1 != expression2
These operators compare their left hand side expression against
their right hand side. These operators compare the numerical values
of each side, as floating point numbers. If the numbers compare as
indicated, the result of the comparison is the text string "1",
otherwise it is the text string 0.
Note
Ccomparisons are not associative: "a < b < c" is an error. If
it is absolutely necessary, use "(a < b) < c".
lt, le, gt, ge, eq, ne - text comparison
expression1 lt expression2
expression1 le expression2
expression1 gt expression2
expression1 ge expression2
expression1 eq expression2
expression1 ne expression2
These operators compare their left hand side expression against
their right hand side. These operators compare each side as text
strings (alphabetically, although the text may include anything).
If the text strings compare as indicated, the result of the
comparison is the text string "1", otherwise it is the text string
0.
Note
Comparisons are not associative: "a lt b lt c" is an error. If
it is absolutely necessary, use "(a lt b) lt c". (But why would
you?).
| - bitwise or
expression1 | expression2
This is the bitwise or operator. Its result is a 32 bit integer,
which is a bitwise-or combination of the left hand side and the
right hand side.
& - bitwise and
expression1 & expression2
This is the bitwise and operator. Its result is a 32 bit integer,
which is a bitwise-and combination of the left hand side and the
right hand side.
+, -, *, / - numerical operations
expression1 + expression2
expression1 - expression2
expression1 * expression2
expression1 / expression2
These are numerical, floating point, operators.
=~ /pattern/:options - pattern match against string
expression =~ /pattern/:option
The left hand side of the =~ operator can be any expression. The
right hand side is always a pattern specification. The result of
the operator is the weighted match of the pattern against
expression (if the options do not specify weighted scoring, the
result is simply 1 if the pattern was found, 0 if not).
See "Patterns[13]" for more information.
/pattern/:options - pattern match against message
/pattern/:option
The result of this operator is the weighted match of the pattern
against the current message (if the options do not specify weighted
scoring, the result is simply 1 if the pattern was found, 0 if
not).
See "Patterns[13]" for more information.
!, ~ - logical/bitwise not operator.
! expression
~ expression
The result of the ! operator is a logical opposite of its right
hand side expression. If the right hand side expression evaluated
to a logical true, the result is a logical false. If it evaluated
to a logical false, the result is a logical true.
maildrop uses the following concept of true/false: an empty text
literal, or a text literal that consists of the single character
"0" is a logical false value. Anything else is a logical true
value.
The result of the ~ operator is a bitwise complement of its right
hand side expression. The right hand side expression is evaluated
as a 32 bit integer, and the result of this operator is a bitwise
complement of the result.
escape(string) - escape special characters in a string.
escape(expression)
The escape function returns its sole argument with every occurrence
of a special character prefixed by a backslash. A special character
is any of the following characters:
|!$()[]\+*?.&;`'-~<>^{}"
This can used when matching pattern sections[14], and then taking
one section and matching it again. For example:
if ( /^From:\s*(.*)/ )
{
MATCH1=escape($MATCH1)
if ( /^Subject:.*$MATCH1/ )
{
...
}
}
This example checks if the contents of the From: header can also be
found in the Subject: header. If the escape function were not used,
then any special characters in the From: header that are also used
in regular expressions, such as * or +, would introduce
unpredictable behavior, most likely a syntax error.
The reason why this list of special characters also includes
characters not used in maildrop's regular expressions is to allow
maildrop's variables to be used on the command line of a shell
command executed by the xfilter command, backtick characters, or to
or cc commands.
Although using data from an external data source is dangerous, and
it may result in inadvertent exploits, using the escape function
should hopefully result in fewer surprises.
gdbmopen, gdbmclose, gdbmfetch, gdbmstore - GDBM support in maildrop
These functions provide support for GDBM database files. See
maildropgdbm(5)[15] for more information.
Note
The system administrator can disable GDBM support in maildrop,
so these commands may not be available to you.
getaddr(string) - extract RFC 2822 addresses from a header.
if ( /^From:\s*(.*)/ )
{
ADDR=getaddr($MATCH1)
}
This function is usually applied to a header that contains RFC
2822[16] addresses. It extracts the actual addresses from the
header, without any comments or extraneous punctuation. Each
address is followed by a newline character. For example, if string
contains:
joe@domain.com (Joe Brown), "Alex Smith" <alex@domain.com>, tom@domain.com
The result of the getaddr function is the following string:
joe@domain.com<NL>alex@domain.com<NL>tom@domain.com<NL>
Note
Because getaddr() interprets RFC 2822[17] loosely, it is not
necessary to strip off the "To:" or the "Cc:" header from the
string, before feeding it to getaddr(). For example, the
following snippet of code takes all addresses in the message,
and concatenates them into a single string, separated by
spaces:
ADDRLIST=""
foreach /^(To|Cc): .*/
{
foreach (getaddr $MATCH) =~ /.+/
{
ADDRLIST="$ADDRLIST $MATCH"
}
}
Note
In certain rare situations, RFC 2822[17] allows spaces to be
included in E-mail addresses, so this example is just
educational.
hasaddr(string) - Search for an address.
if ( hasaddr(string) )
{
...
}
"string" is of the form user@domain. The hasaddr function returns 1
if this address is included in any To:, Cc:,Resent-To:, or
Resent-Cc:, header in the message, otherwise this function returns
0.
This is more than just a simple text search. Each header is parsed
according to RFC822. Addresses found in the header are extracted,
ignoring all comments and names. The remaining addresses are
checked, and if "string" is one of them, hasaddr returns 1,
otherwise it returns 0.
The comparison is case-insensitive. This actually violates RFC822
(and several others) a little bit, because the user part of the
address may be (but is not required to be) case sensitive.
length (string) - length of a string
if (length(string) > 80)
{
...
}
The length function returns the number of characters in string.
lookup (expr, 'filename', 'options') - read file for patterns
if (lookup(expr, file, "option"))
{
...
}
expr is any expression. filename is a name of a file containing a
list of patterns. Note that filename is relative to the current
directory, which is the home directory of the user when maildrop
runs in delivery mode, or embedded mode. maildrop then reads the
file. Blank lines will be ignored, as well as any lines that begin
with the # character (comments).
Leading whitespace (but not trailing whitespace, take care) is
removed, and the remaining contents of each line are interpreted as
a pattern which is matched against expr. As soon as the match is
found, lookup returns "1". If no match is found after reading the
entire file, lookup returns "0". For example:
if ( /^To:\s*(.*)/ && lookup( $MATCH1, "badto.dat" ))
{
exit
}
The file badto.dat contains the following two lines:
friend@public
^[^@]*$
If a message has a To: header that contains the text
"friend@public", or does not contain at least one @ character, then
the message will be silently dropped on the floor ( maildrop will
terminate without delivering the message anywhere).
options are the pattern matching options to use. The only supported
option is "D" (the rest are meaningless, in this case).
Note
Be careful with discarding messages like that. Pattern matching
can be tricky, and a slight miscalculation can cause mail to be
unintentionally discarded. It is much desirable to first
deliver message to a separate folder or mailbox, and once the
filter is verified to work correctly, change it so the messages
are discarded completely.
substr(string,start [,count]) - return substring
foo=substr($foo, 1, 10)
The substr function extracts characters from string beginning with
character #start. If count is specified, at most count characters
starting at position start are kept, any excess is trimmed.
time - return current time
foo=time
The time function returns the current time, in seconds, since
January 1, 1970. This function is useful when using GDBM files. See
maildropex(7)[18] for an example of using the time function.
tolower(string) - Convert string to lowercase.
foo=tolower(string)
This function returns the string with all uppercase characters
replaced by lowercase characters.
toupper(string) - Convert string to uppercase.
foo=toupper(string)
This function returns the string with all lowercase characters
replaced by uppercase characters.
Statements
The filter file is read by maildrop ($HOME/.mailfilter or another
file), and it contains filtering statements, one per line. The
filtering language used by maildrop has a loosely - defined grammatical
structure.
Statements are listed one per line. Multiple statements may be listed
on the same line by separating them with semicolons. To continue a long
statement on the next line, terminate the line with a backslash
character.
BUGS
If getaddr() or hasaddr() functions are used on broken headers, the results are unpredictable. hasaddr() is completely case insensitive. This actually violates a few RFCs, because the userid portion of the address could be case-sensitive, but it's not in too many cases, so there.
SEE ALSO
lockmail(1)[19], maildrop(1)[20], maildropgdbm(5)[15], maildirquota(8)[4], reformail(1)[21], egrep(1), sendmail(8).
AUTHOR
Sam Varshavchik
Author
NOTES
1. Courier mail server
http://www.courier-mta.org/
2. dot-courier(5)
http://www.courier-mta.org/maildrop/dot-courier.html
3. value of the -M option
http://www.courier-mta.org/maildrop/maildrop.html#moption
4. maildirquota(8)
http://www.courier-mta.org/maildrop/maildirquota.html
5. xfilter
http://www.courier-mta.org/maildrop/#xfilter
6. PCRE
http://www.pcre.org
7. See the to statement
http://www.courier-mta.org/maildrop/#to
8. flock statement
http://www.courier-mta.org/maildrop/#flock
9. embedded mode
http://www.courier-mta.org/maildrop/maildrop.html#embedded
10. dotlock statement
http://www.courier-mta.org/maildrop/#dotlock
11. cc
http://www.courier-mta.org/maildrop/#cc
12. evaluates to a logical true
http://www.courier-mta.org/maildrop/#if
13. Patterns
http://www.courier-mta.org/maildrop/#patterns
14. matching pattern sections
http://www.courier-mta.org/maildrop/#patmatch
15. maildropgdbm(5)
http://www.courier-mta.org/maildrop/maildropgdbm.html
16. RFC 2822
http://www.rfc-editor.org/rfc/rfc2822.txt
17. RFC 2822
http://www.rfc-editor.org/rfc/rfc822.txt
18. maildropex(7)
http://www.courier-mta.org/maildrop/maildropex.html
19. lockmail(1)
http://www.courier-mta.org/maildrop/lockmail.html
20. maildrop(1)
http://www.courier-mta.org/maildrop/maildrop.html
21. reformail(1)
http://www.courier-mta.org/maildrop/reformail.html
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