localmailfilter(7)
NAME
localmailfilter - Local mail filtering
SYNOPSIS
echo /usr/bin/maildrop >/etc/courier/maildropfilter
mkdir $HOME/.mailfilters
vi $HOME/.mailfilters/rcptfilter $HOME/.mailfilters/rcptfilter-ext
vi $HOME/.mailfilters/smtpfilter $HOME/.mailfilters/smtpfilter-ext
chmod 700 $HOME/.mailfilters
chmod 600 $HOME/.mailfilters/*
DESCRIPTION
The maildrop mail filter can be used by the Courier mail server as a
mail filtering engine, rejecting unwanted mail on a per-recipient
basis.
The actual filtering interface used by the Courier mail server does not
really require that maildrop must be used as a mail filtering engine,
it just so happens that maildrop has a compatible interface that can be
used right out of the box. The following brief information can be used
to craft a homebrewed mail filter to take maildrop's place.
The local mail filter only works for addresses that correspond to local
accounts. This filtering is not used if the recipient is a remote
address on another mail server. The local mail filter is disabled by
default. To enable local mail filtering you will need to initialize the
/etc/courier/maildropfilter configuration file to contain the pathname
to your local mail filter.
Local mail filtering is performed in two distinct phases:
Recipient filters
When the Courier mail server receives an address naming a local
mail recipient, the local mail recipient's mail filter is executed
before the Courier mail server acknowledges the address. The local
mail filter tells the Courier mail server whether to: A) accept
message unconditionally - the message is whitelisted; B) reject the
message unconditionally - the Courier mail server tells the other
mail server that the recipient address is invalid; or C) accept
this recipient, but run the content mail filter, once the message's
contents are available.
Content filters
After receiving the contents of the message, the mail filter is
executed again for any recipients whose recipient filters used the
third option. The content filter can now examine the contents of
the message, and indicate whether the message should be accepted or
rejected. Content filtering is not available for alias addresses.
It should be noted that mail filtering is executed as an integral part
of receiving a message from a remote mail server. If the message is
rejected, the Courier mail server refuses to accept the message for
delivery.
The local mail filter will be invoked as follows:
HOME=$HOME FILTER -D uid/gid -M filter
The local mail filter will NOT be invoked as root, so if it needs to
access files in the recipient's account, it must be installed setuid to
root (as maildrop is installed by default).
"uid/gid" is the recipient account's system userid and group id,
respectively. The recipient account's home directory is placed in the
HOME environment variable, prior to running FILTER, and "filter" is set
as follows:
rcptfilter
The mail filter is invoked initially when the remote mail server
specifies this address as a recipient. FILTER should terminate
with one of the following exit codes: 0 - this sender is
acceptable; 99 - this sender is acceptable, but I want to run the
content filter for this the message; any other non-zero exit code -
the sender is not acceptable, reject the message.
smtpfilter
If FILTER terminates with exit code 99, FILTER runs again with this
parameter set to the word smtpfilter. FILTER will be invoked once
the message has been received from the remote mail server, but not
yet acknowledged. If FILTER terminates with a non-zero exit code,
the message is rejected. If FILTER terminated with the exit code of
zero, the message is accepted.
rcptfilter-ext, smtpfilter-ext
If the recipient created sub-addresses - see dot-courier(5)[1] - a
dash followed by the subaddress "ext" is appended to the name of
the filter.
rcptfilter-alias-ext
This is how FILTER gets invoked if the address is a locally defined
mail alias (ext is the alias name).
The rcptfilter invocation must terminate with a zero exit code when the
message originates from a mailing list or any other source that should
be considered as "whitelisted". This filtering model does not fit very
well with some mail transfer protocols, so unless trusted sources are
explicitly declared to be whitelisted, there is a remote possibility
that the recipient will be removed from a mailing list because of a
poorly-written mail filter from some other recipient of the same
message. The 0 return exit code (which is the implied default if no
mail filtering is installed) protects the recipient from being
adversely affected, in any way, by anyone else's mail filter.
The mail filters may print a diagnostic message before rejecting a
message. The diagnostic message will be returned to the sending mail
relay, where possible.
The mail filters inherit environment variables that describe the
incoming mail. The following environment variables are provided by
default:
SENDER
The return address on the message.
TCPREMOTEHOST, TCPREMOTEIP
When the message is received via ESMTP, these variables specify the
remote IP address and the corresponding hostname. Hostname is empty
if the IP address does not have a reverse DNS record, or is set to
"softdnserr" if there was a temporary failure while looking up this
IP address.
BLOCK2
The default the Courier mail server configuration sets this
environment variable if the remote IP address is listed in an
unsecured relay blacklist. See /etc/courier/esmtpd for more
information. Other environment variables may also be available. For
mail received via ESMTP, environment variables are usually set in
the /etc/courier/smtpaccess configuration file.
maildrop implementation
Maildrop implements this mail filtering API as follows:
$HOME/.mailfilters
This directory contains the filtering recipes. This directory, and
its contents, cannot have any group or world permissions.
smtpfilter*, rcptfilter*
These mail filtering recipes directly correspond to the events
defined in the previous section. Maildrop's "import" statement can
be used to gain access to the environment variables (these mail
filters are executed in maildrop's embedded mode). The mail
filtering recipes can set the EXITCODE variable appropriately
before terminating, in order to accept or reject the message.
See maildrop(1)[2] for more information.
Filtering mail to aliases
The /etc/courier/aliases configuration file is used to mail aliases,
see makealiases(8)[3]. The system administrator may set aside a
reserved local account that will be used to specify a local mail filter
for messages addressed to aliases. The configuration file
/etc/courier/aliasfilteracct specifies the home directory of the mail
account that will be used to filter alias recipients.
For example, if /etc/courier/aliasfilteracct contains /home/admin, then
the Courier mail server runs the mail filter as follows:
HOME=/home/admin FILTER -D uid/gid -M rcptfilter-alias-name
Here, "uid/gid" is owner uid and gid of the specified directory NOTE:
"name" is a fully qualified address, and the local aliases listed in
/etc/courier/aliases do not typically include the domain name. If
defines an alias called "system", for example, the -M option will
probably be "system@example.com", if example.com is the contents of
/etc/courier/me configuration file.
Unfortunately, currently it is not possible to specify content filters
(a.k.a. smtpfilters) for aliases, only recipient filters.
FILES
/etc/courier/maildropfilter
Local mail filtering engine.
/etc/courier/aliasfilteracct
Account that is used to filter mail to aliases.
SEE ALSO
courierfilter(8)[4], maildrop(1)[2].
AUTHOR
Sam Varshavchik
Author
NOTES
1. dot-courier(5)
[set $man.base.url.for.relative.links]/dot-courier.html
2. maildrop(1)
[set $man.base.url.for.relative.links]/maildrop.html
3. makealiases(8)
[set $man.base.url.for.relative.links]/makealiases.html
4. courierfilter(8)
[set $man.base.url.for.relative.links]/courierfilter.html
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