slocalmh - Online in the Cloud

PROGRAM:

NAME

slocal - asynchronously filter and deliver new mail

SYNOPSIS

/usr/lib/mh/slocal [-addr address] [-info data] [-sender sender] [-user username]
[-mailbox mbox] [-file file] [-maildelivery deliveryfile] [-verbose | -noverbose]
[-suppressdup | -nosuppressdup] [-debug] [-version] [-help]

DESCRIPTION

Slocal is a program designed to allow you to have your inbound mail processed according to
a complex set of selection criteria. You do not normally invoke slocal yourself, rather
slocal is invoked on your behalf by your system's Message Transfer Agent (such as
sendmail) when the message arrives.

The message selection criteria used by slocal is specified in the file “.maildelivery” in
the user's home directory. You can specify an alternate file with the -maildelivery file
option. The syntax of this file is specified below.

The message delivery address and message sender are determined from the Message Transfer
Agent envelope information, if possible. Under sendmail, the sender will obtained from
the UUCP “From:” line, if present. The user may override these values with the -addr and
-sender switches.

The message is normally read from the standard input. The -file switch sets the name of
the file from which the message should be read, instead of reading stdin. This is useful
when debugging a “.maildelivery” file.

The -user switch tells slocal the name of the user for whom it is delivering mail. The
-mailbox switch tells slocal the name of the user's maildrop file.

slocal is able to detect and suppress duplicate messages. To enable this, use the option
-suppressdup. slocal will keep a database containing the Message-ID's of incoming
messages, in order to detect duplicates. Depending on your configuration, this database
will be in either ndbm or Berkeley db format.

The -info switch may be used to pass an arbitrary argument to sub-processes which slocal
may invoke on your behalf.

The -verbose switch causes slocal to give information on stdout about its progress. The
-debug switch produces more verbose debugging output on stderr. These flags are useful
when creating and debugging your “.maildelivery” file, as they allow you to see the
decisions and actions that slocal is taking, as well as check for syntax errors in your
“.maildelivery” file.

Message Transfer Agents
Most modern MTAs including sendmail, postfix and exim support a .forward file for
directing incoming mail. You should include the line

“| /usr/lib/mh/slocal -user username”

in your .forward file in your home directory. This will cause your MTA to invoke slocal
on your behalf when a message arrives.

The Maildelivery File
The “.maildelivery” file controls how slocal filters and delivers incoming mail. Each
line of this file consists of five fields, separated by whitespace or comma. Since
double-quotes are honored, these characters may be included in a single argument by
enclosing the entire argument in double-quotes. A double-quote can be included by
preceding it with a backslash. Lines beginning with `#' and blank lines are ignored.

The format of each line in the “.maildelivery” file is:

header pattern action result string

header:
The name of a header field (such as To, Cc, or From) that is to be searched for a
pattern. This is any field in the headers of the message that might be present.

The following special fields are also defined:

source the out-of-band sender information

addr the address that was used to cause delivery to the recipient

default this matches only if the message hasn't been delivered yet

* this always matches

pattern:
The sequence of characters to match in the specified header field. Matching is case-
insensitive, but does not use regular expressions.

action:
The action to take to deliver the message. When a message is delivered, a
“Delivery-Date: date” header is added which indicates the date and time that message
was delivered.

destroy
This action always succeeds.

file, mbox, or >
Append the message to the file named by string. The message is appended to the
file in mbox (uucp) format. This is the format used by most other mail clients
(such as mailx, elm). If the message can be appended to the file, then this
action succeeds.

mmdf
Identical to file, but always appends the message using the MMDF mailbox format.

pipe or |
Pipe the message as the standard input to the command named by string, using the
Bourne shell sh to interpret the string. Prior to giving the string to the
shell, it is expanded with the following built-in variables:

$(sender) the out-of-band sender information

$(address) the address that was used to cause delivery to the recipient

$(size) the size of the message in bytes

$(reply-to) either the “Reply-To:” or “From:” field of the message

$(info) the out-of-band information specified

qpipe or ^
Similar to pipe, but executes the command directly, after built-in variable
expansion, without assistance from the shell. This action can be used to avoid
quoting special characters which your shell might interpret.

folder or +
Store the message in the nmh folder named by string. Currently this is handled
by piping the message to the nmh program rcvstore, although this may change in
the future.

result:
Indicates how the action should be performed:

A Perform the action. If the action succeeds, then the message is considered
delivered.

R Perform the action. Regardless of the outcome of the action, the message is not
considered delivered.

? Perform the action only if the message has not been delivered. If the action
succeeds, then the message is considered delivered.

N Perform the action only if the message has not been delivered and the previous
action succeeded. If this action succeeds, then the message is considered
delivered.

The delivery file is always read completely, so that several matches can be made and
several actions can be taken.

Security of Delivery Files
In order to prevent security problems, the “.maildelivery” file must be owned either by
the user or by root, and must be writable only by the owner. If this is not the case, the
file is not read.

If the “.maildelivery” file cannot be found, or does not perform an action which delivers
the message, then slocal will check for a global delivery file at /etc/nmh/maildelivery.
This file is read according to the same rules. This file must be owned by root and must
be writable only by root.

If a global delivery file cannot be found or does not perform an action which delivers the
message, then standard delivery to the user's maildrop is performed.

Example Delivery File
To summarize, here's an example delivery file:

#
# .maildelivery file for nmh's slocal
#
# Blank lines and lines beginning with a '#' are ignored
#
# FIELD PATTERN ACTION RESULT STRING
#

# File mail with foobar in the “To:” line into file foobar.log
To foobar file A foobar.log

# Pipe messages from coleman to the program message-archive
From coleman pipe A /bin/message-archive

# Anything to the “nmh-workers” mailing list is put in
# its own folder, if not filed already
To nmh-workers folder ? nmh-workers

# Anything with Unix in the subject is put into
# the file unix-mail
Subject unix file A unix-mail

# I don't want to read mail from Steve, so destroy it
From steve destroy A -

# Put anything not matched yet into mailbox
default - file ? mailbox

# always run rcvtty
* - pipe R /usr/lib/mh/rcvtty

Sub-process environment
When a process is invoked, its environment is: the user/group-ids are set to recipient's
ids; the working directory is the recipient's home directory; the umask is 0077; the
process has no /dev/tty; the standard input is set to the message; the standard output and
diagnostic output are set to /dev/null; all other file-descriptors are closed; the
environment variables $USER, $HOME, $SHELL are set appropriately, and no other environment
variables exist.

The process is given a certain amount of time to execute. If the process does not exit
within this limit, the process will be terminated with extreme prejudice. The amount of
time is calculated as ((size / 60) + 300) seconds, where size is the number of bytes in
the message (with 30 minutes the maximum time allowed).

The exit status of the process is consulted in determining the success of the action. An
exit status of zero means that the action succeeded. Any other exit status (or abnormal
termination) means that the action failed.

In order to avoid any time limitations, you might implement a process that began by
fork()-ing. The parent would return the appropriate value immediately, and the child
could continue on, doing whatever it wanted for as long as it wanted. This approach is
somewhat risky if the parent is going to return an exit status of zero. If the parent is
going to return a non-zero exit status, then this approach can lead to quicker delivery
into your maildrop.

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