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<h1><em><a href="http://www.ijs.si/software/amavisd/">amavisd-new</a></em>
documentation bits and pieces</h1>
<p>The most recent version of this document is available at
<a href="http://www.ijs.si/software/amavisd/amavisd-new-docs.html">
http://www.ijs.si/software/amavisd/amavisd-new-docs.html</a></p>
<ul>
<li><a href="#checks">performing mail checks</a></li>
<li><a href="#actions">acting on mail checks results</a></li>
<li><a href="#tagkill">tag, tag2 and kill levels</a></li>
<li><a href="#quarantine">quarantine</a></li>
<li><a href="#quar-release">releasing from a quarantine</a></li>
<li><a href="#addrext">redirecting malware to a different
mailbox -- plus addressing</a></li>
<li><a href="#wblist">hard black- and whitelisting senders regarding spam</a></li>
<li><a href="#score_sender">soft black- and whitelisting
senders regarding spam -- @score_sender_maps</a></li>
<li><a href="#confvars">configuration variables</a></li>
<li><a href="#pbanks">policy banks</a></li>
<li><a href="#pbanks-ex">putting policy banks to good use -- examples</a></li>
<li><a href="#max_requests">$max_requests</a></li>
<li><a href="#dkim">setting up DKIM (and DomainKeys) mail signing
and verification</a></li>
</ul>
<h2><a name="checks">Performing mail checks</a></h2>
<p>The following checks on mail are available</p>
<ul>
<li>mail header validity checks</li>
<li>banned names and types checks</li>
<li>virus checks</li>
<li>spam checks</li>
<li>is sender white- or blacklisted (regarding spam)</li>
</ul>
<p>Although checks are presently not performed in parallel, it is
best to consider the order of their evaluation unspecified (unknown).
Besides possible future parallel implementation, another reason is
the caching of results, where subsequent mail with the same contents
may benefit from earlier checks if validity of these check results
has not yet expired -- so a check result may be instantly available,
regardless of whether it has been asked for or not.</p>
<p>Using configuration variables @bypass_virus_checks_maps,
@bypass_banned_checks_maps, @bypass_header_checks_maps and
@bypass_spam_checks_maps each recipient (or administrator on their
behalf) may suggest that certain tests are not needed, primarily
for performance reasons. Although the @bypass_*_checks_maps pertain
to individual recipients, a mail check is an operation done on the
whole message, regardless of the number of recipients and their individual
preferences. Suggestion by some of the recipients that certain check is
not needed (is to be bypassed) does not guarantee the test will not be
performed.</p>
<p>Similarly the (hard) blacklisting or whitelisting of sender address
may make running spam check unnecessary, but it does not guarantee
the spam check result will not be available for subsequent decisions.</p>
<p>There are two primary reasons why a check result may still be available
despite the bypass hint or a sender being black- or whitelisted:</p>
<ul>
<li>a check result from some previous mail with the same contents
has been cached and is still valid;</li>
<li>when mail has multiple recipients and not all of them agree
that a check should be bypassed.</li>
</ul>
<p>The amavisd-new program is allowed to skip some check for performance
reasons if all recipients agree that a check is not necessary (that it may
be bypassed), or if the outcome of a check to be skipped could not influence
further mail processing and delivery/non-delivery of the message (as is
the case of a sender being black- or whitelisted regarding spam check).</p>
<p>For example spam checks may be skipped if it is already known that
a mail is infected. This is an implementation and optimization issue,
and no guarantee is given about interdependency of checks.
Future version may use a different strategy of performing checks
(e.g. some checks may be performed in parallel), as long as a change
does not affect the final outcome.</p>
<h2><a name="actions">Acting on mail checks results</a></h2>
<p>Based on the outcome of mail checks performed during mail analysis
or cached from previous mail with the same contents, and based on
global settings and individual recipient preferences, the program now
decides what action to perform next. As described in the previous section,
not all results of checks are necessarily known (e.g. if all recipients
voted for some check to be bypassed). For the purpose of deciding further
actions, unknown results of a check are considered equivalent to
negative (false) results, i.e. skipped virus check is treated the same
as non-infected mail, bypassed spam check is equivalent to low spam score
(ham).</p>
<p>The following decisions are made at this stage:</p>
<ul>
<li>whether a mail should be quarantined and how;</li>
<li>whether an administrator (and which administrator)
should receive a notification (and which notification);</li>
<li>whether recipients should receive a notification;</li>
</ul>
<p>and regarding mail delivery and/or sender (non)delivery notifications:</p>
<ul>
<li>whether a mail should be delivered to each recipient or not;</li>
<li>whether delivered mail should be modified (header edits, defanging);</li>
<li>whether a sender should receive a (non)delivery notification (bounce);</li>
<li>what should be the final status code returned to the mailer
(reject/pass).</li>
</ul>
<p>For the purpose of deciding on these actions, a mail is classified
based on all available checks results. It is quite possible that more
than one check results would be positive (e.g. virus and banned and
bad header, or spam and bad header, or virus and spam), yet a mail
is considered to be only in one category. The logic is currently
hard-wired into the program and can not be influenced by configuration
variables. The following order is used, the first condition met
decides the outcome:</p>
<ol>
<li>a virus is detected: mail is considered infected;</li>
<li>contains banned name or type: mail is considered banned;</li>
<li>spam level is above kill level for at least one recipient,
or a sender is blacklisted: mail is considered spam;</li>
<li>bad (invalid) headers: mail is considered as having a bad header.</li>
</ol>
<p>This decision order explains why amavisd-new is not free to skip
(to optimize away) virus checks if a presence of a banned name or a bad
header is already known or can easily be determined. The order was chosen
with the intention that a more informative or a stronger assertion is the
one to base further mail delivery on, and to be quoted in notifications
and in the log. Even at the expense of possibly longer processing time,
it is more important to declare a mail infected than complain about
a bad header, a banned executable or spamy contents.</p>
<p>The determined mail category now governs further action.
Administrators are notified if enabled for the category,
mail is quarantined if quarantining if enabled for the category,
recipients are notified if enabled for the category.</p>
<p>Next a mail delivery is attempted. A decision to deliver depends
on mail category and on global and individual recipient preferences.
The global setting $final_*_destiny=D_PASS or a per-recipient setting
@*_lovers_maps ensure mail delivery for corresponding mail category
even if mail would otherwise be blocked for being infected or banned
or spam or having a bad header.</p>
<p>A mail that is decided to be passed to an individual recipient
undergoes some simple header editing which happens on-the-fly during
mail forwarding. Certain mail header fields may be inserted or removed,
or an existing header field (e.g. Subject) may be modified. This header
editing may be different for each recipient even in multi-recipient
messages. If necessary, a multi-recipient mail is split into more than
one forwarding transaction, grouping (clustering) recipients with same
settings into one SMTP transaction.</p>
<p>Based on decisions to forward or to block mail to each recipient,
and on the global setting for the mail category ($final_*_destiny=D_BOUNCE
or D_REJECT), the sender (non)delivery notification is now prepared
in case of D_BOUNCE, and MTA receives a 2xx status (success); or in
case of D_REJECT the MTA receives a 5xx (reject) status and preparing
sender notifications is thus delegated to MTA (not recommended in
post-queue or dual-MTA content filtering setup).</p>
<p>Even in cases of mail non-delivery when a (non-)delivery status
notification (DSN) for the sender should have been prepared and sent,
there are certain exceptions where the DSN is suppressed, which makes mail
effectively lost as far as the sender and the recipient are concerned
(but quarantining is not affected):</p>
<ul>
<li>when $final_*_destiny=D_DISCARD;</li>
<li>when mail is infected and the detected virus name matches the
@viruses_that_fake_sender_maps (unconditionally true by default);</li>
<li>when spam score exceeds level determined by @spam_dsn_cutoff_level_maps
for all recipients;</li>
<li>when mail is coming from a mailing list, as determined by
examining a mail header <i>Precedence:</i> for containing
string 'bulk' or 'list' or 'junk';</li>
</ul>
<h2><a name="tagkill">tag, tag2 and kill levels</a></h2>
<p>When SpamAssassin is called upon to analyze a mail message, it returns
a spam score (spam level, hits), which is a numeric representation of
spaminess. The higher the number, the more spamy the message is considered.
Small numbers near zero or negative indicate a clean message, colloquially
called ham. Spam score is a characteristic of the whole message,
and does not depend on recipient preferences. SpamAssassin is called
only once for each message regardless of the number of recipients.</p>
<p>To determine further course of action, amavisd-new compares the spam score
to three numeric values: tag level, tag2 level and kill level. These values
may be different for each recipient, and further actions may be different
for each recipient. If necessary, the mail forwarding is split into more
than one transaction to cater for different recipient preferences.</p>
<dl>
<dt>tag level</dt>
<dd>if spam score is at or above tag level, spam-related header fields
(X-Spam-Status, X-Spam-Level) are inserted for local recipients;
undefined (unknown) spam score is interpreted as lower than any
spam score;</dd>
<dt>tag2 level</dt>
<dd>if spam score is at or above tag2 level, spam-related header fields
(X-Spam-Status, X-Spam-Level, X-Spam-Flag and X-Spam-Report)
are inserted for local recipients, and X-Spam-Flag and X-Spam-Status
bear a YES; also recipient address extension (if enabled) is tacked onto
recipient address for local recipients; for these actions to have any
effect, mail must be allowed to be delivered to a recipient;</dd>
<dt>kill level</dt>
<dd>if spam score is at or above kill level, mail is blocked; and
sender receives a nondelivery notification unless spam score exceeds
dsn cutoff level.</dd>
</dl>
<p>The general idea is that kill level is what controls the main actions
as far as MTA and amavisd-new is concerned (regardless of what recipients'
MUA later does with the mail).</p>
<p>Reaching kill level for at least one recipient controls the following:</p>
<ul>
<li>mail gets quarantined (unless disabled)</li>
<li>spam administrator gets a notification (unless disabled)</li>
<li>ContentSpamMsgs counter is incremented</li>
<li>sender gets a notification if warnspamsender
is true and $final_spam_destiny is D_PASS</li>
<li>if message is not delivered, sender gets a nondelivery
notification (suppressed under certain conditions)</li>
<li>the main log entry says: Passed/Blocked SPAM.</li>
</ul>
<p>On the other hand the tag2 level just adds some mark to the passed
mail (only for local recipients), which recipient or his MUA may decide
to act on or not. Specifically:</p>
<ul>
<li>Subject header field is modified (unless disabled)</li>
<li>X-Spam-Flag and X-Spam-Status header field get a Yes</li>
<li>address extension for spam gets tacked on the recipient address</li>
<li>spam defanging is done (unless disabled)</li>
<li>the main log entry says: Passed/Blocked SPAMMY.</li>
</ul>
<p>For mail below kill level, if a recipient (or his MUA) decides
to discard a message based on tag2 marking, there is no way to
retrieve it later from a quarantine, the sender is never notified,
spam administrator is never notified. As far as the MTA and amavisd-new
are concerned, the message was successfully delivered. Whatever MUA
does with the mail is entirely the responsibility and jurisdiction
of the recipient and his LDA and MUA.</p>
<h2><a name="quarantine">Quarantine</a></h2>
<p>Mail quarantining is attempted when enabled for a given contents category,
which usually includes infected or banned or spam mail with score for
at least one of its recipients at or above his kill level. It is also
possible to enable quarantining of clean messages for archiving or
troubleshooting purposes. The <i>*quarantine_to</i> for each recipient
(when nonempty), along with a global corresponding <i>*_quarantine_method</i>,
determines where the quarantine location should be.</p>
<p>The <i>*_quarantine_method</i> can be considered a static and a site-wide
setting, generally controlling a format and location of the quarantine
on the system. The <i>*quarantine_to</i> can be considered a dynamic
part of the quarantine location, possibly affected by per-recipient settings
and the class of malware (contents category). It serves to fully specify
the final location, e.g. a file or a mailbox.</p>
<p>Depending on mail contents category (type of malware), the following
variables specify the quarantine method: <tt>$virus_quarantine_method</tt>,
<tt>$spam_quarantine_method</tt>, <tt>$banned_files_quarantine_method</tt>,
and <tt>$bad_header_quarantine_method</tt>. One way to globally disable
quarantine is to specify undef or an empty string as a value of these
variables. A nonempty string should follow a syntax:</p>
<ul>
<li><tt>local:</tt><i>filename-template</i></li>
<li><tt>bsmtp:</tt><i>filename-template</i></li>
<li><tt>smtp:[</tt><i>IPv4-or-IPv6-address-or-hostname</i><tt>]:</tt><i>port</i></li>
<li><tt>smtp:</tt><i>hostname</i><tt>:</tt><i>port</i></li>
<li><tt>smtp:</tt><i>/path/to/a/unix/socket</i></li>
<li><tt>lmtp:[</tt><i>IPv4-or-IPv6-address-or-hostname</i><tt>]:</tt><i>port</i></li>
<li><tt>lmtp:</tt><i>hostname</i><tt>:</tt><i>port</i></li>
<li><tt>lmtp:</tt><i>/path/to/a/unix/socket</i></li>
<li><tt>pipe:</tt><i>argv=command args...</i></li>
<li><tt>sql:</tt><i>anything</i></li>
</ul>
<p>The <tt>local:</tt>, <tt>bsmtp:</tt> and <tt>sql:</tt> methods are the
usual methods for quarantining. The <tt>smtp:</tt> or <tt>lmtp:</tt> methods
are only useful for quarantining if quarantine location is some dedicated
mailbox instead of a local file or directory. The <tt>smtp:</tt>, <tt>lmtp:</tt>
and <tt>pipe:</tt> methods are more often used for forwarding and notifications.
The following features became available with version 2.5.0: the <tt>lmtp:</tt>
method, support for IPv6, and specifying a Unix socket to a <tt>smtp:</tt> or
<tt>lmtp:</tt> method.</p>
<p>Depending on the method specified (local/bsmtp/smtp/sql) a per-recipient
setting <i>*quarantine_to</i> adopts different semantics and syntax,
possibly modified by the configuration variable <tt>$QUARANTINEDIR</tt>.</p>
<table border="1">
<tr>
<th>method</th>
<th>quarantine_to</th>
<th><tt>$QUARANTINEDIR</tt></th>
<th>effect</th></tr>
<tr>
<td>anything</td>
<td>empty or undef</td>
<td>anything</td>
<td>not quarantined</td></tr>
<tr>
<td>empty or undef</td>
<td>anything</td>
<td>anything</td>
<td>not quarantined</td></tr>
<tr>
<td><tt>local:</tt></td>
<td>pseudo-alias mapped through %local_delivery_aliases</td>
<td>directory</td>
<td>stored as an individual file
below the directory <tt>$QUARANTINEDIR</tt>, file name comes
from the template specified in the <i>*_quarantine_method</i>
</td></tr>
<tr>
<td><tt>local:</tt></td>
<td>pseudo-alias mapped through %local_delivery_aliases</td>
<td>filename of a mailbox</td>
<td>appended to a file <tt>$QUARANTINEDIR</tt> in mbox format</td></tr>
<tr>
<td><tt>local:</tt></td>
<td>pseudo-alias mapped through %local_delivery_aliases</td>
<td>empty or undef</td>
<td>not quarantined</td></tr>
<tr>
<td><tt>local:</tt></td>
<td>e-mail address containing '@'-sign</td>
<td>anything</td>
<td>sent via SMTP to a mailer for storage,
uses $notify_method to specify how to deliver to MTA;
much like a newer '<tt>smtp:</tt>' entry below</td></tr>
<tr>
<td><tt>smtp:</tt></td>
<td>e-mail address</td>
<td>anything</td>
<td>sent via SMTP to a mailer for storage,
uses the specified IP address and port,
or a Unix socket for delivery; formerly
a '<tt>local:</tt>' method was used for
this purpose</td></tr>
<tr>
<td><tt>lmtp:</tt></td>
<td>e-mail address</td>
<td>anything</td>
<td>sent via LMTP to a mailer for storage,
uses the specified IP address and port,
or a Unix socket for delivery</td></tr>
<tr>
<td><tt>bsmtp:</tt></td>
<td>anything (nonempty)</td>
<td>anything</td>
<td>stored in a file specified in the <i>*_quarantine_method</i>
in BSMTP format
(if file name is absolute, i.e. starts with a "/")</td></tr>
<tr>
<td><tt>bsmtp:</tt></td>
<td>anything (nonempty)</td>
<td>directory</td>
<td>stored in a file specified in the <i>*_quarantine_method</i>
in BSMTP format
(file name relative to <tt>$QUARANTINEDIR</tt>)</td></tr>
<tr>
<td><tt>sql:</tt></td>
<td>anything (nonempty)</td>
<td>anything</td>
<td>stored into SQL database specified by @storage_sql_dsn</td></tr>
<tr>
<td><tt>sql:</tt></td>
<td>empty or undef</td>
<td>anything</td>
<td>not quarantined</td></tr>
</table>
<p>The <i>*quarantine_to</i> is currently quite limited in functionality,
it is often used only to turn off the quarantining for some user or local
subdomain. The reason for this limited functionality is a more vulnerable
nature of this value, as it may come from SQL or LDAP lookups where
non-careful access controls to these databases might permit users to enter
any value in the <i>*quarantine_to</i> field, which is why we do not let
it control the directory or the exact file name of the quarantine file.
This may be somewhat relaxed in the future.</p>
<p>In common setups the quarantine location (e.g. a directory or a dedicated
mailbox) is the same for all recipients. If at least one recipient specifies
a nonempty <i>*quarantine_to</i> specifying this location, the message is
quarantined (stored) there once, regardless of the number of recipients.</p>
<p>The general algorithm is: the <i>*quarantine_to</i> value associated with
each recipient is looked up. Empty or undef values are ignored and duplicates
are discarded. A mail to be quarantined is then stored/sent to each
unique location remaining on the list.</p>
<p>The "bsmtp:" quarantine method is somewhat special in that the quarantine
file location is entirely determined by the <i>*_quarantine_method</i> setting,
and the value of per-recipient <i>*quarantine_to</i> settings do not influence
the quarantine location, as long as this value is nonempty.</p>
<p>When using the "bsmtp:" quarantine method and versions of amavisd-new
earlier than 2.2.0, the <i>*_quarantine_to</i> was completely ignored,
which made it impossible to turn off quarantining selectively for certain
users by specifying an empty or undef value. Since 2.2.0, an empty
<i>*_quarantine_to</i> turns off quarantine for a recipient regardless
of the quarantine method. A nonempty string in <i>*_quarantine_to</i>
(the exact value is ignored) must now be used even with "bsmtp:" to
enable quarantining.</p>
<h2><a name="quar-release">Releasing from a quarantine</a></h2>
<p>The utility <i>amavisd-release</i> tells the amavisd daemon to fetch
a mail from a local quarantine, and send it to MTA through its regular
channels ($notify_method), bypassing re-checking.</p>
<p>By default it connects to socket /var/amavis/amavisd.sock, on which
amavisd should be listening for AM.PDP protocol, but one can use inet socket
instead of a Unix socket if there is a need to run <i>amavisd-release</i>
from a remote host.</p>
<p>In the amavisd.conf the following should be added:</p>
<pre>
$unix_socketname = "$MYHOME/amavisd.sock"; # listen on Unix socket
# alternatively (less common):
# $inet_socket_port = [10024, 9998]; # listen on listed inet tcp ports
# apply policy bank AM.PDP-SOCK on a Unix socket:
# (note that this precludes the use of old amavis-milter
# helper program (with sendmail) on the same socket)
$interface_policy{'SOCK'} = 'AM.PDP-SOCK';
# apply policy bank AM.PDP-INET to some inet tcp socket, e.g. tcp port 9998:
$interface_policy{'9998'} = 'AM.PDP-INET';
$policy_bank{'AM.PDP-SOCK'} = {
protocol => 'AM.PDP', # select Amavis policy delegation protocol
auth_required_release => 0, # don't require secret_id for amavisd-release
};
$policy_bank{'AM.PDP-INET'} = {
protocol => 'AM.PDP', # select Amavis policy delegation protocol
inet_acl => [qw( 127.0.0.1 [::1] )], # restrict access to these IP addresses
# auth_required_release => 0, # don't require secret_id for amavisd-release
};
</pre>
<p>Setting of $auth_required_release decides whether the requestor needs
to specify secret_id in addition to mail_id to authorize a mail release.
The secret_id is stored in SQL table msgs when logging to SQL is enabled,
otherwise this information is not accessible.</p>
<p>Note that turning off $auth_required_release check is safe as long as
access to the socket is restricted, like with file protections on a Unix
socket, or restricted with inet_acl to specific IP addresses. Enabling or
disabling $auth_required_release is a management / setup decision and
convenience.</p>
<p>To release a mail message an exact quarantine location from a log file
should be specified as an argument to amavisd-release, e.g.:</p>
<pre>
amavis[29297]: (29297-01-6) Blocked SPAM,
... <xxx> -> <yyy>,
quarantine: <i>spam/U/UM3XM3XDbN52.gz</i>,
Message-ID:<...>, mail_id: UM3XM3XDbN52, Hits: 13.365,
$ amavisd-release <i>spam/U/UM3XM3XDbN52.gz</i>
250 2.6.0 Ok, id=rel-UM3XM3XDbN52,
from MTA([193.2.4.66]:10025): 250 2.0.0 Ok: queued as F137717B88B
</pre>
<p>The <i>amavisd-release</i> utility also accepts <i>mail_id</i> from STDIN
if releasing more than one message in one go is more convenient:</p>
<pre>
$ amavisd-release -
<i>spam/U/UM3XM3XDbN52.gz</i>
<i>spam/g/gnwKVFKiuey3.gz</i>
<i>spam/X/Xpkj9mLLBHTR.gz</i>
</pre>
<h2><a name="addrext">Redirecting malware to a different
mailbox -- plus addressing</a></h2>
<p>Amavisd-new can tag passed malware by appending an address extension
to a recipient address. An address extension is usually a short string
(such as 'spam') appended to the local part of the recipient address,
delimited from it by a single character delimiter, often a '+'
(or sometimes a '-'). This is why address extensions are also known as
"plus addressing". Examples of such mail addresses belonging to user
<i>jim@example.com</i> are: <i>jim+spam@example.com</i>,
<i>jim+cooking@example.com</i>, <i>jim+health@example.com</i>,
<i>jim+postfix@example.com</i>.</p>
<p>Most mailers (MTA), including Postfix and sendmail, have some provision
to put address extensions to good use. Similarly, local delivery agents (LDA)
such as Cyrus or LDAs that come with MTA, can be configured to recognize
and make use of address extensions.</p>
<p>The most common application for address extensions is to provide
additional information to LDA to store mail into a separate mail folder.
Users may for example choose to use this feature to let LDA automatically
file messages from mailing lists to a dedicated subfolder, or to file
spam to a spam folder, just by letting LDA simply and quickly examine
the envelope recipient address, without having to parse mail header
or having to configure and run filters such as procmail or Sieve.</p>
<p>Mailers (MTA and LDA) usually attempt first to examine (to check for
validity, to lookup in virtual or aliases maps) a full unmodified recipient
address. If the attempt is unsuccessful, they strip away the extension part,
and try again. This way a presence of some unknown address extension is
simply ignored. For example, a delivery for <i>jim+health@example.com</i>
would deliver the mail to the main Jim's inbox if he hasn't provided
a subfolder <i>health</i> in his mailbox.</p>
<p>For this fallback to work (to ignore unknown extensions), it is important
that all components that need to deal with address extensions (MTA, LDA,
content filters) have the same notion of the delimiter in use on the
system. For Postfix the configuration option is <tt>recipient_delimiter=+</tt>
(see also propagate_unmatched_extensions), for amavisd-new the option is
<tt>$recipient_delimiter='+';</tt> for Cyrus the delimiter is hardcoded
as '+', see <a href="http://cyrusimap.web.cmu.edu/imapd/faq.html">Cyrus
IMAP FAQ -> plus addressing</a>.</p>
<p>The amavisd-new configuration options for adding address
extensions are @addr_extension_virus_maps, @addr_extension_spam_maps,
@addr_extension_banned_maps, @addr_extension_bad_header_maps.
The configuration must also ensure the malware mail is to be delivered,
otherwise there is nothing to tack an address extension on -- either by
setting kill level sufficiently high, or by declaring spam lovers, or by
<tt>$final_spam_destiny=D_PASS</tt>; an example:</p>
<pre>
$recipient_delimiter = '+';
@addr_extension_spam_maps = ('spam');
$sa_tag2_level_deflt = 6.7 ; # score above which spam extension is added
$sa_kill_level_deflt = 15; # block higher score entirely
$final_spam_destiny=D_DISCARD; # junk all above kill level
</pre>
<p>or provide extension string more selectively for certain users
or subdomains:</p>
<pre>
@addr_extension_spam_maps = (
{ '.sub1.example.com' => 'spam', # an entire subdomain
'user1@example.com' => 'spam', # a particular user
'user2@example.com' => 'malware', # another user wants a different ext.
'.' => '' } # all the rest do not receive an extension
);
</pre>
<p>If one is considering using a quarantine mechanism but wants a per-user
(or perhaps per-subdomain) quarantines, this is not such a good idea,
because quarantined files are not supposed to be directly visible or
handled by recipients: to protect the privacy of the sender, some
header pre-processing must be performed on a quarantined file before
handing it over to a recipient.</p>
<p>The cleanest way to achieve per-user quarantine which may be directly
accessible and/or manipulated by recipients is to turn on adding address
extensions, and configure MTA and/or LDA to store such mail wherever
necessary, either to a user's dedicated subfolder, or perhaps to some
centralized dedicated set of malware mailboxes (per-user or perhaps
per-subdomain).</p>
<p>If it is desired to reroute extension-tagged mail to some mailbox
away from the usual LDA, the virtual alias mapping by MTA is the tool
for the job. With Postfix, a pcre-based virtual map can specify
for example:</p>
<pre>
/^(.*)\+spam@([^@]*)\.example\.com$/ spam-$2-box@example.com
</pre>
<p>which will collect all spam into one mailbox for each subdomain.</p>
<p>For the Postfix local(8) LDA, a presence of a file $HOME/.forward+spam
can redirect mail for user+spam to some dedicated file. For the Postfix
virtual(8) LDA, a virtual_mailbox_maps may contain entries like:</p>
<pre>
user1 mbxfile1
user1+spam mbxspamfile1
user2 mbxfile2
user2+spam mbxspamfile2
</pre>
<h2><a name="wblist">Hard black- and whitelisting senders regarding spam</a></h2>
<p>The blacklisting and the whitelisting are ways of telling that we already
know that a message is spam or is ham (non-spam) just by examining the envelope
sender address and comparing it to lists of known spammers or to lists of
known legitimate senders of ham. It is a quick check, potentially saving us
the trouble of examining the mail contents. It has a big drawback however
in that the sender mail address can be (and often is) faked and there is no
guarantee that the claimed sender address represents the actual sender.</p>
<p>The sender address is usually faked for spam messages, so whitelisting
some sender address is a of questionable value, and often lets in far more
spam than it does good by approving legitimate mail. For a reliable way
of permitting certain sending clients to send spamy mail see <i>policy
banks</i>.</p>
<p>Blacklisting however is still useful: spammer has no desire to pretend
to be some blacklisted sending address, when he can choose any other address.
Genuine sender that is intentionally blacklisted can only avoid being
blocked by falsifying his address (joining spammers in his methods)
<em>and</em> sending non-spamy mail, the later being our objective anyway.
Although amavisd-new does provide blacklisting, it is functionally equivalent
but more effective to blacklist senders at the MTA, preventing such mail
from even entering the mail system.</p>
<p>It should be emphasized that whitelisting (and blacklisting) only affects
spam checks. It has no influence on other checks such as virus, banned or
header checks. Infected mail from whitelisted sender would still be blocked
if our policy is to block viruses.</p>
<p>Another point to bear in mind is that the sender address examined
is the one from the SMTP protocol, exactly as provide by MTA to amavisd-new.
It is known as the envelope sender address or return path. This address
does not necessarily match the mail author's address from the mail header
(From:) or the sender's address from the header (Sender:). This is most
obvious with mail from mailing lists, where the envelope sender address
is usually the address of a mailing list management service, while the
author's address (From:) is the address of a person sending the message.
Using the envelope sender address in most cases makes it easier to black-
or whitelist mail from mailing lists, compared to guessing a sender address
by parsing mail header.</p>
<p>To avoid surprises, whitelisted sender suppresses inserting/editing
the tag2-level header fields (X-Spam-*, Subject), appending spam address
extension, and quarantining, even if we know the message is spam (e.g.
because the spam check result on the same mail contents has been cached from
some earlier mail or known from check on behalf of another recipient).</p>
<p>For mail from blacklisted senders, the effect is as if the spam level
were artificially pushed high, resulting in 'X-Spam-Flag: YES', high
'X-Spam-Level' bar and other usual reactions to spam, including possible
rejection. If the message nevertheless still passes (e.g. for spam loving
recipients), it is tagged as BLACKLISTED in the 'X-Spam-Status' header field,
but the reported spam value and set of tests in this report header field
is not adjusted (if available from SpamAssassin, which may or may not have
been called)</p>
<p>If <em>all</em> recipients of a message either white- or blacklist the
sender, amavisd is free to skip spam scanning (calling the SpamAssassin),
saving on time. There is no guarantee however that spam scanning will
actually and always be skipped.</p>
<p>The following variables (lists of lookup tables) are available,
with the semantics and syntax as specified in README.lookups:
@whitelist_sender_maps, @blacklist_sender_maps, which implement
global policy applicable to all recipients. Similarly there are
$per_recip_blacklist_sender_lookup_tables and
$per_recip_whitelist_sender_lookup_tables, which make possible
for each recipient or subdomain to specify its own set of black-
or whitelisted senders. The per-recipient tables take precedence
over global tables.</p>
<p>For SQL lookups, amavisd-new will first lookup the recipient in table
<i>users</i> in order of descending priority, e.g. user@sub.domain.org,
user, @.sub.domain.org, @.domain.org, @.org, and @. (which can be considered
a catchall). Each matching recipient record may have a list of senders
associated (through join on field <i>users.id</i> and <i>wblist.rid</i>).
The sender address is then looked up in the associated list of senders
(<i>wblist</i>) in order of descending priority, e.g. sender@sub.example.com,
@.sub.example.com, @.example.com, @.com, and @. . This search stops at the
first matching sender record with a non-NULL field <i>wblist.wb</i>. The value
of a field <i>wblist.wb</i> from the matched record determines if the sender
is considered whitelisted ('W'), blacklisted ('B') or neutral (' ')
for this recipient.</p>
<p>The neutral value is there just as a way to explicitly stop the search,
which may be used by a recipient to overrule site-wide or static
white- or blacklisting defaults for some specific sender, and to
explicitly neither whitelist nor blacklist the sender, letting the
normal spam check determine the spaminess of a mail.</p>
<p>For recipient user@sub.domain.com and sender sender@sub.example.com
the following search is performed:</p>
<pre>
user@sub.domain.org
sender@sub.example.com @.sub.example.com @.example.com @.com @.
user
sender@sub.example.com @.sub.example.com @.example.com @.com @.
@.sub.domain.org
sender@sub.example.com @.sub.example.com @.example.com @.com @.
@.domain.org
sender@sub.example.com @.sub.example.com @.example.com @.com @.
@.org
sender@sub.example.com @.sub.example.com @.example.com @.com @.
@.
sender@sub.example.com @.sub.example.com @.example.com @.com @.
</pre>
<h2><a name="score_sender">Soft black- and whitelisting
senders regarding spam -- @score_sender_maps</a></h2>
<p>Instead of hard black- or whitelisting a sender address (unconditionally
considering mail spam or ham solely based on sender address regardless
of mail contents), a more gentle approach is to add score points (penalties)
to the spam score for mail from certain senders or sending domains.
Positive points lean towards blacklisting, negative towards whitelisting.
This is much like adding SpamAssassin rules or using its white/blacklisting,
except that here only envelope sender addresses are considered (not addresses
in a mail header), and that score points can be assigned per-recipient
(or per-domain or globally), and that the assigned penalties are customarily
much lower than the default SpamAssassin white/blacklisting score.</p>
<p>The table structure of @score_sender_maps is similar to
$per_recip_blacklist_sender_lookup_tables i.e. the first level key is
recipient address, pointing to by-sender lookup tables. The essential
difference is that scores from <em>all</em> matching by-recipient lookups
(not just the first that matches) are summed to give the final score boost.
That means that both the site and domain administrators, as well as the
recipient can have a say on the final score.</p>
<p>For SQL lookups, the mechanism is much like the one described for
hard black- or whitelisting, with the following differences:</p>
<ul>
<li>the field <i>wblist.wb</i> is numeric, representing score points,
instead of containing a character W or B or space;</li>
<li>the search through matching recipients does not stop at the first
match, but traverses all matching recipients, summing up the
corresponding <i>wblist.wb</i> field values.</li>
</ul>
<p>Namely, amavisd will lookup the recipient, e.g. user@sub.domain.org,
user, @.sub.domain.org, @.domain.org, @.org, and @. . Since the search will
not stop at the first recipient match, the search order in this case is
unimportant, although it is actually the same descending-priority order as with
hard b/w listing. Each matching recipient record may have a list of senders
associated (through join on field <i>users.id</i> and <i>wblist.rid</i>).
The sender address is then looked up in the associated list of senders
(<i>wblist</i>) in order of descending priority, e.g. sender@sub.example.com,
@.sub.example.com, @.example.com, @.com, and @. . This search stops at the
first matching sender record with a non-NULL field <i>wblist.wb</i>, but this
does not terminate the outer recipients search. Numeric values of a field
<i>wblist.wb</i> from matched records are summed up across all matching
recipients tables, and the result is added to the spam score as produced
by SpamAssassin.</p>
<p>Unlike static tables, where hard and soft w/b-listing use separate
tables, the SQL-based hard and soft w/b-listing uses the same SQL tables
and the same field <i>wblist.wb</i>. Mixing the 'W', 'B' with numeric values
is somewhat frowned upon, but is supported to facilitate transition.
The search goes like described above as long as only numeric field values
are encountered, summing up the values and adding the accumulated sum
to the final score. If a non-numeric value of field <i>wblist.wb</i>
is encountered during this search, its value (W or B or space) is
interpreted as described for hard w/b listing, and the search stops at
this point.</p>
<h2><a name="confvars">Configuration variables</a></h2>
<p>The behaviour of the amavisd-new is controlled by a set of configuration
variables, which are just normal module-global Perl variables (in package
Amavis::Conf). At daemon startup time these variables are first assigned
an initial value (often just an undefined value, the undef). The default
values of configuration variables are documented in file amavisd.conf-defaults,
which lists all configuration variables.</p>
<p>Next a configuration file amavisd.conf (or other file as specified
by option -c) is read and interpreted by the Perl interpreter itself.
The amavisd.conf is just a normal Perl program, and can in principle
do whatever and however it pleases, but its main purpose is to assign
values to configuration variables.</p>
<p>After execution of amavisd.conf is done, the daemon may correct some
configuration variable values (mainly to maintain backwards compatibility
with earlier version of configuration file), and may assign a default value
to certain variables which are still undefined -- these variables and their
default values are marked "after-defaults" in the documentation file
amavisd.conf-defaults. The main reason for existence of the "after-defaults"
concept is that some default values depend on other configuration variables
and can not be computed before the amavisd.conf is finished. To force such
variables to an off/false/disabled state, one needs to assign some false but
defined value to them, such as '' (an empty string) or a 0 for booleans.</p>
<p>Perl variables always start with a character $, @ or % to indicate a type
of variable. This leading character is part of the variable name for all
practical purposes.</p>
<dl>
<dt>$ (dollar character)</dt>
<dd>indicates a scalar variable (a string, a number, a reference)</dd>
<dt>@ (at sign)</dt>
<dd>indicates an array variable (a list)</dd>
<dt>% (percent character)</dt>
<dd>indicates an associative array (also known as hash),
which maps keys to values</dd>
</dl>
<p>A couple of Perl syntactical elements deserve mention at this point,
as they are often used in the amavisd.conf configuration file.</p>
<dl>
<dt>"...", a double-quoted string</dt>
<dd>is a string; variables within are evaluated, e.g. "$MYHOME/tmp"</dd>
<dt>'...', a single-quoted string</dt>
<dd>is a string; variables within are not evaluated,
the $ and @ loose their special meaning, e.g. 'user@example.com'</dd>
<dt>(...)</dt>
<dd>is a list of comma-separated expressions, e.g. (1,2,"test");
a list is normally assigned to an array variable</dd>
<dt>qw(string)</dt>
<dd>is an operator that interprets its argument as a single string,
splits it on whitespace to words, and returns a list of words (strings);
it is a convenience to avoid some typing,
e.g. qw(user@example.com .example.net .org) is exactly equivalent
to ('user@example.com', '.example.net', '.org');
</dd>
<dt>[...]</dt>
<dd>is a reference to an anonymous list of comma-separated expressions,
e.g. [1,2,"test"]; (note: a reference is a scalar)</dd>
<dt>{...}</dt>
<dd>is a reference to an anonymous associative array,
e.g. {'alfa'=>1, 'beta'=>99, 'other'=>'test'};
(note: a reference is a scalar)</dd>
<dt>\variable</dt>
<dd>is a reference to a variable, e.g. \$virus_admin, \@mynetworks, \%whitelist_sender;
(note: a reference is a scalar)</dd>
</dl>
<p>Historically amavisd-new accessed all configuration variables directly
with their name, e.g. %spam_lovers, @spam_lovers_acl, $spam_lovers_re.
Later it became apparent that certain groups of variables (lookups) are
always used together in the same way, so new array variables like
@spam_lovers_maps were introduced. The program now never accesses old
lookup table variables directly, but always through higher level lists.
The solution is fully backwards compatible, as the default value
for the new lists references the old variables, e.g.:</p>
<pre>
@spam_lovers_maps = (\%spam_lovers, \@spam_lovers_acl, \$spam_lovers_re);
</pre>
<p>Administrator is free to modify or replace the lists in variables like
@spam_lovers_maps, perhaps rearranging the order or loosing all references to
legacy variables, and replacing them with other variables, often anonymous
arrays/lists or anonymous associative maps (hashes), or constants which can
serve as a convenient catchall default value when used last in the list.</p>
<p>Since amavisd-new version 2.0, there is one further generalization step
in the way a program accesses configuration variables. More than a hundred
configuration variables which control amavisd-new operation on a by-message
level (as opposed to by-recipient and truly global settings) are now grouped
in associative array called a <i>policy bank</i>. These configuration variables
are no longer accessed directly by their variable name by the program, but
always through a currently installed policy bank. Administrator is free to
modify the policy bank, normally by providing replacement policy banks and
specifying under what conditions the replacement policy bank is to be
automatically installed.</p>
<h2><a name="pbanks">Policy banks</a></h2>
<p>Policy banks hold sets of configuration variables controlling most
of per-message settings, including: static lookup tables, IP interface
access rules, forwarding address, log level, templates, administrator
addresses, spam trigger levels, quarantine rules, lists of anti-virus
scanner entries (or just a subset), banned names rules, defang settings,
etc. The whole set of these settings may be replaced with another
predefined set based on incoming port number, making it possible for one
amavisd daemon to cope with more diverse needs of served user communities
which could so far only be implemented by running more than one instance
of the amavisd daemon, each with its own configuration file.</p>
<p>This mechanism brings new potentials for the future: in principle policy
banks could be swapped not only based on port number or SMTP client
IP address, but on any characteristics pertaining to a mail message as
a whole (not specific to each of its recipients), or to characteristics
of a connection from a mailer (e.g. the interface address or protocol);</p>
<p>Until a better mechanism is available, a policy bank named 'MYNETS' has
special semantics: this policy bank is loaded (if it exists) whenever MTA
supplies a SMTP client's IP address (through Postfix XFORWARD extension to
the SMTP protocol, or via a new AM.PDP protocol) and that address matches
the @mynetworks list (actually: the list referenced by 'mynetworks_maps'
key in the currently installed policy map).</p>
<p>An associative array %interface_policy is a current mechanism of assigning
a policy bank to an incoming TCP port number (port must be in the list
@$inet_socket_port, otherwise amavisd will not listen on that port).
Whenever a connection from MTA is received, first a built-in policy bank
with an empty name -- the $policy_bank{''} gets loaded, which brings in
all the global/legacy settings. Then it is overlaid by whatever configuration
settings are in the bank named in the $interface_policy{$port} if any,
and finally the policy bank named 'MYNETS' (i.e. settings from
$policy_bank{'MYNETS'}) is overlaid if such policy bank exists and the SMTP
client IP address is known (by XFORWARD SMTP extension command from MTA)
and it matches the current mynetworks_maps. See amavisd.conf-sample for
examples.</p>
<p>When a new policy bank is overlaid over an existing set of configuration
variables, the variables not present in the new policy bank retain their
value. This makes it possible to specify new policy banks which carry
only a minimal set of settings that need to be changed.</p>
<p>The built-in policy bank (with empty name) is predefined, and includes
references to most other variables (the dynamic config variables),
which are accessed only indirectly through the currently installed
policy bank. Overlaying a policy bank with another policy bank may
bring in references to entirely different variables, possibly unnamed,
and may remove references to legacy variables if it so chooses.</p>
<p>Configuration variables are referenced from a policy bank (which
is implemented as a perl associative array, i.e. a hash) by keys of the
same name, e.g. { log_level => \$log_level, inet_acl => \@inet_acl, ...}.
For scalars one level of indirection is allowed, e.g.
a policy bank { log_level => \$log_level }; $log_level=2;
is equivalent to { log_level => $log_level } or to { log_level => 2 },
but in the first example with an indirect reference, the $log_level
may be assigned to even _after_ the policy bank has already been formed.</p>
<p>A word of caution: the syntax of entries within a policy bank hash
is slightly different from assignments to configuration variables.
This is because entries within policy bank are not assignments, but
key=>value pairs as in any Perl associative array. And these pairs are
delimited by commas, unlike statements, which are delimited by semicolons.
Value is separated from its key by '=>' (or by a comma), whereas the
assignment operator is '='. Keys of a policy bank are without leading $
or @ or %, unlike variable names. Values of an associative array can only
be scalars (e.g. strings or numbers or references to arrays or references
to associative array).</p>
<p>Compare:</p>
<ul>
<li>value of a policy bank is a reference to a Perl associative array, e.g.:
<pre>
{ log_level => 3,
forward_method => 'smtp:[127.0.0.1]:10025',
spam_admin_maps => ["spamalert\@$mydomain"],
blacklist_sender_maps => [ [qw(.example.org .example.net)] ],
}
</pre>
</li>
<li>normal assignments look like:
<pre>
$log_level = 3;
$forward_method = 'smtp:[127.0.0.1]:10025';
@spam_admin_maps = ("spamalert\@$mydomain");
@blacklist_sender_maps = ( [qw(.example.org .example.net)] );
</pre>
</li>
</ul>
<p>And a final note: Perl can detect and report typing mistakes in variable
names, but mistyped key is just some unused associative array entry lurking
in a hash, never used and never reported as mistyped/useless.</p>
<h2><a name="pbanks-ex">Putting policy banks to good use -- examples</a></h2>
<p>The sender address can be faked, so comparing envelope sender address
to @local_domains_maps or some other lookup table to base some important
decisions on would not be trustworthy. The only reliable information is
the recipient's e-mail address and information about client SMTP session,
such as the IP address of the sending SMTP client and the server port number
or the interface address. Such information can be made available by MTA to
amavisd-new through a feeding protocol (e.g. XFORWARD extension or via AM.PDP),
or separate MTA paths can be set up for mail that needs to be treated
differently, such as internally originating and externally originating mail,
or perhaps separating authenticated mail from the rest.</p>
<p>Amavisd-new has two ways of receiving such extra information from MTA:</p>
<ul>
<li>it can listen on more than one TCP port and apply different policy banks
to each port (applicable to any dual-MTA setup including Postfix), and/or</li>
<li>it can accept SMTP client's IP address from MTA by XFORWARD extension
to the SMTP protocol (available in Postfix only), which can control loading
of policy bank MYNETS. This is in addition to loading policy banks based
on TCP port number. The MYNETS policy bank is loaded (if applicable)
<i>after</i> (on top of the) the port-assigned policy bank.</li>
</ul>
<p>The following examples illustrate several ways of distinguishing
between different mail origins. For most common purposes the only distinction
that really matters is separating internally originating mail from the rest,
and for this purpose the use of policy bank MYNETS and a sufficiently
recent version of Postfix supporting XFORWARD suffices -- the complication
with multiple ports and multiple interfaces is needed only for more demanding
sites which prefer maximum flexibility.</p>
<h3>Example 1</h3>
<p>As stated earlier, a policy bank named 'MYNETS' is loaded (if it exists)
whenever MTA supplies an original SMTP client's IP address (e.g. via the
Postfix XFORWARD extension) and that address matches the @mynetworks list.
This covers most common needs to distinguish internally-originating mail
from the rest, and allows them to be treated differently, as illustrated
by the following example:</p>
<pre>
$policy_bank{'MYNETS'} = { # mail originating from @mynetworks
virus_admin_maps => ["security\@$mydomain"], # alert of infected local hosts
spam_admin_maps => ["abuse\@$mydomain"], # alert of internal spam
spam_kill_level_maps => [7.0], # slightly more permissive spam kill level
spam_dsn_cutoff_level_maps => [15],
banned_filename_maps => [
new_RE(
# block double extensions in names:
qr'\.[^./]*\.(exe|vbs|pif|scr|bat|cmd|com|cpl|dll)\.?$'i,
# allow any name or type (except viruses) within an archive:
[ qr'^\.(Z|gz|bz2|rpm|cpio|tar|zip|rar|arc|arj|zoo)$' => 0],
# blocks MS executable file(1) types, unless allowed above:
qr'^\.(exe-ms)$',
),
],
};
</pre>
<h3>Example 2</h3>
<p>In the following example some of the external mail is coming in via
fetchmail, the rest of the externally originating mail is coming in via normal
SMTP at tcp port 25, and all internally originating mail is coming to MTA
via mail submission port 587 reserved for that purpose, or via dedicated IP
address accessible only from inside, or through a Postfix pickup service.
We'll use Postfix in this example, although it does not rely on any particular
Postfix capability that wouldn't be available in any general purpose MTA
in some form or another.</p>
<p>Only the specifics of this setup are described here. Missing bits
like the MTA re-entry port 10025 and other options are described in
<a href="./README.postfix">README.postfix</a> and are assumed here.
Specifying additional smtpd restrictions and options may be desired,
and is omitted here for brevity.</p>
<p>To let amavisd-new be able to distinguish between all four
mail entry routes, we let amavisd listen on four TCP ports
(the fifth is for good measure, to be used in the next example):
<tt>$inet_socket_port = [10040,10041,10042,10043,10044];</tt>
(any unused non-privileged TCP ports can be used)</p>
<p>In Postfix configuration file master.cf we attach different
<i>content_filter</i> options to each of the Postfix services
receiving mail. We'll assume the MTA host has two IP addresses 192.0.2.1
and 192.0.2.2 assigned (IP aliases or separate physical interfaces),
which makes it easier to distinguish between internally originating mail
and the rest even if XFORWARD can not be used (older Postfix versions
or some other MTA):</p>
<pre>
# regular incoming mail, originating from anywhere (usually from outside)
# the MX record (or backup mailers) should point to this IP address
192.0.2.1:smtp inet n - n - - smtpd
-o content_filter=smtp-amavis:[127.0.0.1]:10040
# incoming mail from fetchmail, considered externally originating
# (add 'smtphost localhost/2345' to the poll section in .fetchmailrc)
127.0.0.1:2345 inet n - n - - smtpd
-o content_filter=smtp-amavis:[127.0.0.1]:10041
-o smtpd_client_restrictions=permit_mynetworks,reject
-o mynetworks=127.0.0.0/8
# IP address to be used by internal hosts for mail submission
192.0.2.2:smtp inet n - n - - smtpd
-o content_filter=smtp-amavis:[127.0.0.1]:10042
-o smtpd_client_restrictions=permit_mynetworks,reject
# or, tcp port 587 to be used by internal hosts for mail submission
submission inet n - n - - smtpd
-o content_filter=smtp-amavis:[127.0.0.1]:10042
-o smtpd_client_restrictions=permit_mynetworks,reject
# locally originating mail submitted on this host through a sendmail binary
pickup fifo n - n 60 1 pickup
-o content_filter=smtp-amavis:[127.0.0.1]:10043
</pre>
<p>A global option <i>content_filter</i> in file main.cf could provide
a convenient default, only services that need a different setting would
then need to override it.</p>
<p>Now let's make up names for policy banks which will cover all four cases.
We'll pick names EXT, EXT-FM, INT, INT-HOST for policy banks. The amavisd
needs to be told to load corresponding policy when a request comes in on each
of the listening ports:</p>
<pre>
$interface_policy{'10040'} = 'EXT';
$interface_policy{'10041'} = 'EXT-FM';
$interface_policy{'10042'} = 'INT';
$interface_policy{'10043'} = 'INT-HOST';
$interface_policy{'10044'} = 'AUTH'; # to be used in the next example
</pre>
<p>Next we'll prepare each policy and specify there the options which should
be different from global options. Note that the following policies serve
mostly as an example and to provide ideas -- they should not be considered
a recommendation. For example:</p>
<pre>
# regular incoming mail, originating from anywhere (usually from outside)
$policy_bank{'EXT'} = {
# just use global settings, no special overrides
};
# incoming mail from fetchmail, considered externally originating
$policy_bank{'EXT-FM'} = {
log_level => 2,
# no bounces for spam, not even for score below spam_dsn_cutoff_level_maps:
final_spam_destiny => D_DISCARD,
};
# locally originating mail guaranteed to be from inside
$policy_bank{'INT'} = {
# enable/redirect admin notifications for locally originating malware:
virus_admin_maps => ["virusalert\@$mydomain"],
spam_admin_maps => ["virusalert\@$mydomain"],
# be slightly more permissive on spam levels for mail from our hosts:
spam_kill_level_maps => [7.0],
spam_dsn_cutoff_level_maps => [15],
final_virus_destiny => D_BOUNCE, # (unless in viruses_that_fake_sender_maps)
final_spam_destiny => D_BOUNCE, # (unless above spam_dsn_cutoff_level_maps)
bypass_banned_checks_maps => [ 1 ], # allow sending any file type or name
# provide customized sender notifications for spam from our users:
notify_spam_sender_templ => read_text("$MYHOME/notify_spam_sender.txt"),
};
# mail locally submitted on the host on which MTA runs
$policy_bank{'INT-HOST'} = {
# NOTE: this is just an example; ignoring internally generated spam
# may not be such a good idea, consider zombified infected local PCs
bypass_spam_checks_maps => [ 1 ],
bypass_banned_checks_maps => [ 1 ],
final_spam_destiny => D_PASS,
final_banned_destiny => D_PASS,
};
# authenticated mail (used by the next example)
$policy_bank{'AUTH'} = {
# enable admin notifications for malware originating from our users:
virus_admin_maps => ["virusalert\@$mydomain"],
spam_admin_maps => ["virusalert\@$mydomain"],
# be slightly more permissive on spam levels for mail from our users:
spam_kill_level_maps => 7.0,
spam_dsn_cutoff_level_maps => 15,
bypass_banned_checks_maps => 1, # allow sending any file type or name
final_bad_header_destiny => D_BOUNCE; # block invalid headers
};
</pre>
<p>If not all four cases need to be distinguished, the same policy bank
name (or none at all) can be assigned to more than one port. Also the
MTA configuration can use the same amavisd port for more than one
of its incoming services if there is no need for different settings.</p>
<h3>Example 3</h3>
<p>Besides setting different <i>content_filter</i> options for different
Postfix services, one may use the option FILTER in Postfix lookup tables,
as described in Postfix man pages access(5) and header_checks(5), to specify
different <i>content_filter</i> settings based on various conditions,
such as sender domain name or IP address, mail header fields, etc.</p>
<p>Consider the next example which uses the FILTER settings to distinguish
from internally originating, authenticated external mail and the rest.</p>
<pre>
# global default:
content_filter=smtp-amavis:[127.0.0.1]:10044
# note that permit_mynetworks only checks for key presence and ignores rhs
mynetworks = cidr:/etc/postfix/mynetworks-filter.cidr
smtpd_recipient_restrictions =
reject_unauth_pipelining, reject_non_fqdn_recipient, reject_non_fqdn_sender,
reject_unknown_recipient_domain, reject_unknown_sender_domain,
check_client_access cidr:/etc/postfix/mynetworks-filter.cidr,
permit_mynetworks,
permit_sasl_authenticated, permit_tls_clientcerts,
reject_unauth_destination,
check_sender_access regexp:/etc/postfix/filter-catchall.regexp
</pre>
<p>The <tt>check_client_access cidr:/etc/postfix/mynetworks-filter.cidr</tt>
preceeds the <i>permit_mynetworks</i> (which uses the same cidr table,
but ignores the righthand side), and it serves to override the global
<i>content_filter</i> setting by the use of FILTER for each of the
networks (presumably internal) listed in mynetworks-filter.cidr.
The final effect is that mail matching networks listed in
mynetworks-filter.cidr will be sent for content filtering to tcp port 10042
(the FILTER setting in access map), authenticated non-local mail will be
sent for content filtering to port 10044 (the global setting), while all
the rest will be sent to port 10040 (as specified in catchall filter).
If there are any other overrides in master.cf like in the previous example,
they take precedence over the global settings, but the FILTER rules take
the ultimate precedence.</p>
<p>/etc/postfix/mynetworks-filter.cidr :</p>
<pre>
127.0.0.0/8 FILTER smtp-amavis:[127.0.0.1]:10042
10.0.0.0/8 FILTER smtp-amavis:[127.0.0.1]:10042
172.16.0.0/12 FILTER smtp-amavis:[127.0.0.1]:10042
192.168.0.0/16 FILTER smtp-amavis:[127.0.0.1]:10042
</pre>
<p>/etc/postfix/filter-catchall.regexp:</p>
<pre>
/^/ FILTER smtp-amavis:[127.0.0.1]:10040
</pre>
<p>Note that in place of the last catchall entry:
<tt>check_sender_access regexp:/etc/postfix/filter-catchall.regexp</tt>
one would be tempted to do:
<tt>check_sender_access static:FILTER smtp-amavis:[127.0.0.1]:10040</tt>,
but unfortunately spaces are not allowed within an option value
in master.cf, so we have to resort to a lookup table.</p>
<h2><a name="max_requests">$max_requests</a></h2>
<p>Amavisd-new runs under process control of Net::Server. This is a pre-forked
environment where $max_servers child processes are constantly kept alive and
ready to accept new tasks (mail messages to be checked). Each amavisd child
process is able to handle several tasks in a row, which helps to reduce
startup (fork) costs. In case of SMTP or LMTP protocol, each session may
consist of several SMTP/LMTP transactions. Each SMTP/LMTP transaction is
counted a one task, regardless of whether it came in from the same SMTP/LMTP
client in a multi-transaction session, or as separate sessions, possibly
from different SMTP/LMTP clients.</p>
<p>A configuration variable $max_requests (default value 20) controls the
approximate number of tasks each child process is willing to handle. After
that the child process terminates and Net::Server provides a new child process
to take its place.</p>
<p>The exact value of $max_requests is not critical. There are two
opposing needs, and some in-between value should be chosen.</p>
<p>On the low side, the number should not be too small in order for the
startup cost to be averaged out / sufficiently diluted over an entire
child lifetime. A value above 5 or 10 meets this goal in most amavisd-new
configurations.</p>
<p>On the high side, the value depends on the amavisd-new configuration.
The amavisd daemon itself is conservative in its use of dynamically
allocated memory and does not load mail into memory, but keeps mail
being processed and its components on files. Similarly, most of the
called external virus scanners and decoders are rational in their use
of memory (a notable exception was Archive::Tar which was used if a
pax or cpio command was not available, but is no longer supported).
Unfortunately this is not true for Perl module Mail::SpamAssassin,
which expects to have an entire decoded mail in memory in order
to be able to run its large set of rules on it in reasonable time.
This is a design decision of SpamAssassin.</p>
<p>When amavisd-new is not configured to use SpamAssassin, the value of
$max_requests can be quite high without any known or expected problems.
For general sanity reasons, an upper limit could be a 100 for example,
although anything above 20 or so would not bring measurable benefit to
the maximum sustained mail throughput.</p>
<p>When amavisd-new <em>is</em> configured to use SpamAssassin however,
the slurping of entire mail in memory and decoding it may have implications,
depending on the $sa_mail_body_size_limit value, on the maximum mail size
allowed at the MTA (e.g. Postfix setting for <i>message_size_limit</i>)
and on the mail compression factor. Even though the allocated memory is
reclaimed by Perl after mail processing, and is reused for subsequent
processing, the process virtual memory footprint never shrinks, it can
only expand as needed.</p>
<p>The $sa_mail_body_size_limit sets a limit on a mail size beyond which
SpamAssassin is not called, so it can not contribute to memory usage
much beyond this limit, times a small factor (2-5?, due to multiple
internal representations of a message). If the $sa_mail_body_size_limit
is large, and MTA mail size is not limited, or if mail has a huge
mail header, the memory footprint can become noticable. For the rest
of a lifetime the child process that processed the mail stays at its
high virtual memory size. If this happens frequently, host resources
may become scarce. Limiting the number of tasks is very much desirable
in this case.</p>
<p>The default value of 20 for $max_servers was chosen as a good
compromise between averaging-out the startup costs and not wasting too
much resources on hosts with high message size limit and SpamAssassin
enabled.</p>
<p>In the setup with Postfix where its lmtp client is chosen to
feed amavisd-new, this client tries to keep LMTP session open and
submit several mail messages in multiple transactions. With recent
Postfix versions its SMTP client is capable and willing of using
multiple transaction sessions as well, although it seems to be
less persistent than the LMTP client.</p>
<p>According to SMTP and LMTP protocol specifications, dropping the
session on the server side is considered rude and should be used
only as a last resort. In order to respect the $max_requests setting
(which is not strictly enforced by amavisd, and is considered an
advisory value), the client side should preferably be configured with
a comparable limit. Starting with amavisd-new-2.2.0 the amavisd daemon
is more strict in enforcing the limit and drops the SMTP or LMTP session
after $max_servers is exceeded by one. This was a recommendation from
the Postfix community, as the option of reducing Postfix max_use setting
is considered less appropriate.</p>
<p>Nevertheless, Postfix doesn't take session dropping lightly, it backs
off a while after content filter forcibly drops the session, which is
undesired. Better behaviour is achieved when Postfix voluntarily terminates
a SMTP session before amavisd would reach its $max_requests limit.
This can be achieved by applying max_use to the Postfix smtp service
feeding a content filter (typically this entry in master.cf is named
'amavisfeed').</p>
<h2><a name="dkim">Setting up DKIM mail signing and verification</a></h2>
<p>A DKIM standard (RFC 4871) states the following, which applies
to its predecessor DomainKeys (historical: RFC 4870) as well:</p>
<blockquote>
<p><i>
DomainKeys Identified Mail (DKIM)</i> defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain.</p>
</blockquote>
<p>With added support in Postfix 2.3 for a milter protocol, it became
possible to use with Postfix many of existing milters (mail filters)
that were originally developed with sendmail in mind. It was hoped that
a widespread use of milters with sendmail offered a fertile ground for
software development, producing software of sufficient quality to be
able to use it with Postfix. It remains to be seen whether quality of
freely available milters comes anywhere close to high standards we
are accustomed to with Postfix, but with a bit of luck and reasonable
expectations, some of it can be put to good use.</p>
<p>Two of such milters are
<a href="http://sourceforge.net/projects/dkim-milter/">dkim-milter</a>
offering support for <i>DomainKeys Identified Mail (DKIM) Signatures</i>,
and <a href="http://sourceforge.net/projects/dk-milter/">dk-milter</a>,
offering support for <i>Domain-based Email Authentication (DomainKeys)</i>.
The DomainKeys (DK) is a predecessor of DKIM, as recognized by RFC 4870:</p>
<blockquote>
<p>The <i>DomainKeys</i> specification was a primary source from which the
<i>DomainKeys Identified Mail [DKIM]</i> specification has been derived.
The purpose in submitting the RFC 4870 document is as an historical reference
for deployed implementations written prior to the DKIM specification.</p>
</blockquote>
<p>The <i>dkim-milter</i> is more reliable and better maintained than
<i>dk-milter</i>, which is slowly fading into oblivion and is no longer
actively maintained. Google.com has already switched to DKIM, and Yahoo
is following shortly. Similar holds true in the world of Perl modules:
there are modules <i>Mail::DomainKeys</i> and <i>Mail::DKIM</i>, both of
which can be used by SpamAssassin plugins. Again the <i>Mail::DKIM</i>
(by Jason Long, based on initial work by Anthony D. Urso) is actively
maintained, compared to its predecessor <i>Mail::DomainKeys</i>, whose
active maintenance has been discontinued. SpamAssassin makes it easy
to use each or both of them (for verification / whitelisting), just by
enabling the already provided plugins. Note that recent versions of
<i>Mail::DKIM</i> support also DomainKeys signatures.</p>
<p>Mail signing and verification is a two-part job: signing of originating
mail (or mail being redistributed) from our domain, and verifying signatures
of incoming mail. Both tasks can be done by the same program, or they can
be performed by separate entities. Traditionally with sendmail, both tasks
are performed by the same milter, which may be easier to maintain, but
has certain disadvantages.</p>
<p>Verifying signatures should be performed early, before any local mail
transformations get a chance of invalidating a signature, e.g. by performing
MIME conversions to quote-printable, by fixing syntactically invalid mail
header section, by frivolously reformatting or reordering some header
fields, by modifying/inserting/removing certain header fields, or by a
local mailing list modifying mail text, e.g. by appending footnotes.</p>
<p>Signing outgoing mail should be performed late, after mail sanitation,
after conversion to 7-bit characters (to avoid later uncontrollable changes
by a relaying or receiving MTA), and after adding header fields by a content
filter. Similar applies to local mailing lists, which may be rewriting
messages, requiring them to be re-signed by the domain hosting a mailing
list, just before being sent out.</p>
<p>Since SpamAssassin only provides signature verification but not signing,
one obvious choice for signing is to use dkim-milter in signing-only mode,
invoked by a Postfix smtpd service which is receiving content-checked mail
from a content filter such as amavisd-new. As this second-stage smtpd
service does not reliably know how a given message came into a mail system
and whether it is supposed to be signed or not, a clean solution is to
provide two parallel paths through a content filter, one used for mail
to be signed (originating mail), the other for all the rest:</p>
<pre>
+------+ (verify)
|verify| SpamAssassin
+--+---+ ^|
^^^ |v
incoming: ||| +----++-----+
MX ----> 25 smtpd ---> 10024 > >---> 10025 smtpd -->
submission: || | |
SASL --> 25 smtpd \ | amavisd | (notifications)
| +-> | >--->_
mynets-> 25 smtpd ---> 10026 >ORIGINATING>---> 10027 smtpd -->
+-> +-----------+ |
--> 587 smtpd / : v
(convert to 7-bit) +------+
| sign |
+------+
</pre>
<p>There are other benefits to providing two parallal paths: a content
filter may be configured to apply different rules and settings to mail
that is known to be originating from our users, compared to the rest.
Some suggestions: apply less strict banning rules, enable spam
administrator notifications for internally originating spam and
viruses, letting SpamAssassin rules be conditionalized based on
amavisd-new policy banks loaded, etc.</p>
<p>For verification there are two choices: either a SpamAssassin plugin
can do it by calling a Perl module, or a milter in verification-only
mode can be invoked by the incoming Postfix smtpd service. In the
setup described, SpamAssassin sees almost the same message as a milter
on the incoming smtpd would (just few header fields such as a
<i>Received</i> are prepended, no other changes), so is unlikely
that a signature would become invalid.</p>
<p>Invoking signature verification by SpamAssassin has an advantage that
DKIM-based or DomainKeys-based whitelisting or scoring can be used,
but has a disadvantage that possibly not all mail is checked (e.g. large
mail may be exempt from spam checks).</p>
<p>On the other hand, invoking signature verification by calling a milter
from incoming smtpd service can insert a header field reporting a result
of verification, which may be used by MUA or by SpamAssassin rules,
although currently no such rules are provided ready-made.</p>
<p>For the purpose of gathering experience, both methods for signature
verification are described. It turns out that one or the other
implementation (in C or in Perl) may be buggy, and being able to compare
both results of signature verification can help with troubleshooting.</p>
<p>Let's begin by starting a dkim milter in two instances, one dedicated
to signing, the other to verification. For security reasons all milters
should run under a dedicated username, certainly not as <i>root</i>,
not as user <i>amavis</i> and not as user <i>postfix</i> or <i>mail</i>:
</p>
<p><i>verifying:</i></p>
<pre>
dkim-filter -u dkfilter -b v \
-l -p inet:4443@127.0.0.1 -P /var/run/dkim-filter-v.pid
</pre>
<p><i>signing:</i></p>
<pre>
dkim-filter -u dkimfilter -b s -m ORIGINATING \
-c relaxed/simple -S rsa-sha1 \
-d example.com -s myselector -k /var/db/dkim/mykey.pem \
-l -p inet:4445@127.0.0.1 -P /var/run/dkim-filter-s.pid
</pre>
<p>Generating a public and a private pair of keys and publishing
a public key in DNS is described in the dkim milter documentation
and also in the DKIM RFC document.</p>
<p>We are not specifying option -i to milters, the default of
-i 127.0.0.1 suits our setup just fine, as mail to be signed is
coming from a content filter, usually on a loopback interface
from the IP address 127.0.0.1.</p>
<p>Now we can tie the verifying milter to a Postfix smtpd service
listening for incoming mail:</p>
<p><i>master.cf:</i></p>
<pre>
smtp inet n - n - 300 smtpd
-o milter_default_action=accept
-o milter_macro_daemon_name=MTA
-o smtpd_milters=inet:127.0.0.1:4443
</pre>
<p>and tie the signing milter to a Postfix smtpd service that is
receiving checked mail from amavisd, intended to be signed:</p>
<p><i>master.cf:</i></p>
<pre>
# mail return from a content filter (non-signing)
10025 inet n - n - - smtpd
-o content_filter=
... (other options, mail not to be signed) ...
# mail from our users returning from a content filter (DKIM signing)
10027 inet n - n - - smtpd
-o content_filter=
... (other options, mail intended to be signed) ...
-o milter_default_action=accept
-o milter_macro_daemon_name=ORIGINATING
-o smtpd_milters=inet:127.0.0.1:4445
</pre>
<p>As a sidenote, attaching milters to sendmail would use the same order
of invocations: signature verifying milter first, content filters next,
and signing milter last, for example:</p>
<pre>
dnl Verifiers:
INPUT_MAIL_FILTER(`dkim-filter-v', `S=inet:4443@127.0.0.1, T=R:2m')
dnl Content filter:
INPUT_MAIL_FILTER(`amavisd-milter',
`S=unix:/var/amavis/amavisd-milter.sock, F=T, T=S:10m;R:10m;E:10m')
dnl Signers:
INPUT_MAIL_FILTER(`dkim-filter-s', `S=inet:4445@127.0.0.1, T=R:2m')
</pre>
<p>Now back to a Postfix setup (or any dual-MTA setup). Default content
filter is to be amavisd, listening on port 10026 (intended for signing).
Locally submited or SASL-authenticated mail will go to a content filter
on this default port 10026 and will be signed on its way out. All other
mail (incoming) will be diverted to port 10024 for content filtering
by a final catchall FILTER, and will never hit the signing milter:</p>
<p><i>main.cf:</i></p>
<pre>
content_filter = smtp-amavis:[127.0.0.1]:10026
smtpd_recipient_restrictions =
...
permit_mynetworks
permit_sasl_authenticated
reject_unauth_destination
...
check_sender_access regexp:/etc/postfix/filter_10024_catchall
permit
</pre>
<p><i>/etc/postfix/filter_10024_catchall:</i></p>
<pre>
/^/ FILTER smtp-amavis:[127.0.0.1]:10024
</pre>
<p>In SpamAssassin all that is necessary is to add (or uncomment) a line in
any of the .pre files (e.g. in local.pre, or in init.pre and v320.pre):</p>
<pre>
loadplugin Mail::SpamAssassin::Plugin::DKIM
</pre>
<p>Perl module Mail::DKIM needs to be installed. Note that Mail::DKIM
starting with version 0.20 also recognizes DomainKeys signatures, so that
Plugin::DomainKeys is not needed any longer, and in fact its underlying
module is not supported any longer. It is advisable to stick to the most
recent version of Mail::DKIM.</p>
<p>The following SpamAssassin rules (in local.cf) work fairly well,
giving verified mail a little bit of advantage and slightly favourize
verified mail from some popular domains, encouraging people to start
signing their mail. Signed spam can be counterbalanced by other
measures.</p>
<pre>
score DKIM_VERIFIED -0.3
score DKIM_SIGNED 0
score DKIM_POLICY_SIGNALL 0
score DKIM_POLICY_SIGNSOME 0
score DKIM_POLICY_TESTING 0
# DKIM-based whitelisting of domains with good reputation:
score USER_IN_DKIM_WHITELIST -8.0
whitelist_from_dkim *@intl.paypal.com paypal.com
whitelist_from_dkim *@*.paypal.com
whitelist_from_dkim *@paypal.com
whitelist_from_dkim *@*.paypal.be
whitelist_from_dkim *@*.ebay.com
whitelist_from_dkim *@ebay.com
whitelist_from_dkim *@*.ebay.co.uk
whitelist_from_dkim *@ebay.co.uk
whitelist_from_dkim *@ebay.at
whitelist_from_dkim *@ebay.ca
whitelist_from_dkim *@ebay.de
whitelist_from_dkim *@ebay.fr
whitelist_from_dkim *@amazon.com
whitelist_from_dkim *@cisco.com
whitelist_from_dkim *@cern.ch
whitelist_from_dkim *@welcome.skype.com
whitelist_from_dkim *@skype.net
whitelist_from_dkim *@alert.bankofamerica.com
whitelist_from_dkim *@bankofamerica.com
whitelist_from_dkim *@*.cnn.com
whitelist_from_dkim *@cnn.com
# DKIM-based whitelisting of domains with less then perfect
# reputation can be given fewer negative score points:
score USER_IN_DEF_DKIM_WL -1.5
score ENV_AND_HDR_DKIM_MATCH 0
def_whitelist_from_dkim *@google.com
def_whitelist_from_dkim *@googlemail.com
def_whitelist_from_dkim *@gmail.com
def_whitelist_from_dkim *@yahoo.com
def_whitelist_from_dkim *@yahoo.com.au
def_whitelist_from_dkim *@yahoo.se
def_whitelist_from_dkim *@* googlegroups.com
def_whitelist_from_dkim *@* yahoogroups.com
def_whitelist_from_dkim *@* yahoogroups.co.uk
def_whitelist_from_dkim *@* yahoogroupes.fr
def_whitelist_from_dkim *@rogers.com
</pre>
<p>Another suggestions - penalize mail claiming to be from PayPal,
eBay, Yahoo or Gmail but was not signed by their official mailers:</p>
<pre>
header __L_ML1 Precedence =~ m{\b(list|bulk)\b}i
header __L_ML2 exists:List-Id
header __L_ML3 exists:List-Post
header __L_ML4 exists:Mailing-List
header __L_ML5 Return-Path:addr =~ m{^([^\@]+-(request|bounces|admin|owner)|owner-[^\@]+)(\@|\z)}mi
meta __L_VIA_ML __L_ML1 || __L_ML2 || __L_ML3 || __L_ML4 || __L_ML5
header __L_FROM_Y1 From:addr =~ m{[\@.]yahoo\.com$}mi
header __L_FROM_Y2 From:addr =~ m{\@yahoo\.com\.(ar|br|cn|hk|my|sg)$}mi
header __L_FROM_Y3 From:addr =~ m{\@yahoo\.co\.(id|in|jp|nz|uk)$}mi
header __L_FROM_Y4 From:addr =~ m{\@yahoo\.(ca|de|dk|es|fr|gr|ie|it|pl|se)$}mi
meta __L_FROM_YAHOO __L_FROM_Y1 || __L_FROM_Y2 || __L_FROM_Y3 || __L_FROM_Y4
header __L_FROM_GMAIL From:addr =~ m{\@gmail\.com$}mi
meta L_NOTVALID_YAHOO !DKIM_VERIFIED && __L_FROM_YAHOO && !__L_VIA_ML
priority L_NOTVALID_YAHOO 500
score L_NOTVALID_YAHOO 2.8
meta L_NOTVALID_GMAIL !DKIM_VERIFIED && __L_FROM_GMAIL && !__L_VIA_ML
priority L_NOTVALID_GMAIL 500
score L_NOTVALID_GMAIL 2.8
header __L_FROM_PAYPAL From:addr =~ /[\@.]paypal\.com$/mi
header __L_FROM_EBAY From:addr =~ /[\@.]ebay\.(com|at|be|ca|ch|de|ee|es|fr|hu|ie|in|it|nl|ph|pl|pt|se|co\.(kr|uk)|com\.(au|cn|hk|mx|my|sg))$/mi
meta L_NOTVALID_PAY !DKIM_VERIFIED && (__L_FROM_PAYPAL || __L_FROM_EBAY)
score L_NOTVALID_PAY 6
# accept replies from abuse@yahoo.com even if not dkim/dk-signed:
whitelist_from_rcvd abuse@yahoo.com yahoo.com
whitelist_from_rcvd MAILER-DAEMON@yahoo.com yahoo.com
</pre>
<p>In amavisd.conf two parallel paths need to be provided,
one receiving on port 10024 and forwarding to 10025,
the other receiving on port 10026 and forwarding to 10027.</p>
<pre>
$inet_socket_port = [10024,10026]; # listen on two ports
</pre>
<p>The 10024>10025 path will be controlled by a default policy bank,
the other (10026>10027), dedicated to mail intended to be signed,
will use a policy bank (arbitrarily) named ORIGINATING:</p>
<pre>
$forward_method = 'smtp:[127.0.0.1]:10025'; # MTA with non-signing service
$notify_method = 'smtp:[127.0.0.1]:10027'; # MTA with signing service
# switch policy bank to 'ORIGINATING' for mail received on port 10026:
$interface_policy{'10026'} = 'ORIGINATING';
$policy_bank{'ORIGINATING'} = { # mail originating from our users
originating => 1, # indicates our client, introduced in amavisd-new-2.5.0
#
# force MTA to convert mail to 7-bit before DKIM signing
# to avoid later conversions which could destroy signature:
smtpd_discard_ehlo_keywords => ['8BITMIME'],
#
# forward to a smtpd service providing DKIM signing service:
forward_method => 'smtp:[127.0.0.1]:10027',
#
# other special treatment of locally originating mail, e.g.:
spam_admin_maps => ["spamalert\@$mydomain"], # warn of spam from us
virus_admin_maps => ["virusalert\@$mydomain"],
banned_filename_maps => ['ALT-RULES'], # more relaxed rules
spam_quarantine_cutoff_level_maps => undef, # quarantine all spam
spam_dsn_cutoff_level_maps => undef,
spam_dsn_cutoff_level_bysender_maps => # bounce to local senders only
[ { lc(".$mydomain") => undef, '.' => 15 } ],
};
</pre>
<p>The <tt>smtpd_discard_ehlo_keywords=>['8BITMIME']</tt> serves to
persuade Postfix to convert mail to 7-bit quoted-printable before submitting
it to content filtering and signing (this configuration variable was
introduced with version 2.4.3 of amavisd-new). Avoiding 8-bit characters
in mail body makes signatures less susceptible to breaking by some
relaying or receiving MTA over which we have no control. Note that
the same effect (making Postfix convert outgoing mail to 7-bits
before DKIM signing) could be achieved by a Postfix setting
<tt>smtp_discard_ehlo_keywords=8bitmime</tt> on a smtp service
feeding mail to be signed to amavisd, but this would require setting
up two such services, one with the option and one without.</p>
<p>Note that 8-bit to 7-bit conversion may break a S/MIME or PGP signature,
so if mail signing is in use, it may not be desirable to let Postfix
do the conversion, and it may be acceptable to take a risk that a remote
MTA will clobber signatures if it decides the mail text is to be converted
to 7-bits QP. The only reliable solution in this case is to configure
MUA clients to stick to 7-bit characters/encodings before generating
S/MIME or PGP signatures.</p>
<p>The following text from the Postfix documentation file MILTER_README
<em>should be disregarded</em> -- amavisd <em>is</em> 8-bit clean,
and we want Postfix to do conversion to 7-bits on the signing
path but not on the other path:
<span style="text-decoration:line-through;font-style:italic">Content
filters may break domain key etc. signatures. If you use an SMTP-based
content filter, then you should add a line to master.cf with
"-o disable_mime_output_conversion=yes", as described in the
advanced content filter example.</span></p>
<p>While testing how the configured system plays with some mailing lists
(such as <i>postfix-users</i> or SpamAssassin <i>users</i> list), one has
to keep in mind that amavisd-new caches spam checking results of recently
seen message bodies: a mail going out to a mailing list is not yet signed
as it reaches a content filter, but the SpamAssassin verdict is remembered
at that point (claiming the message is not signed). When this message
with unchanged body comes back from a mailing list, this time signed
in the header section by our domain, the signature should prove correct,
yet the cached result from a minute ago still claims the message is not
signed. If this is of concern, one can turn off caching of spam checking
results for ham by setting: <tt>$spam_check_negative_ttl = 0;</tt></p>
<h3>Some experience with DKIM and DomainKeys</h3>
<p>Recent versions of software components must be used to avoid bugs and
interoperability problems with earlier versions:</p>
<ul>
<li>use Postfix 2.3.12 or later, or 2.4.5 or later, or 2.5 (or later);</li>
<li>amavisd-new 2.4.3 polished some corner issues on modifying mail header
section when releasing from a quarantine and defanging, and added some
goodies affecting DKIM and DomainKeys to facilitate integration;
version 2.5.2 (or later) is recommended;</li>
<li>Mail::DKIM is very solid; use the latest version, currently 0.29;</li>
<li>SpamAssassin 3.2.3 or later;</li>
<li>At the time of writing the current version of dkim-milter is 2.4.0.</li>
</ul>
<p>Instead of signing mail with <i>dkim-milter</i>, the same can be
achieved by using Jason Long's <i>DKIM Proxy</i>, which is a Perl program
calling a Perl module <i>Mail::DKIM</i>, i.e. the same modules as used
by a SpamAssassin DKIM plugin. As the <i>Mail::DKIM</i> turned out to be
a reliable and quite efficient module, this may be a good alternative
to <i>dkim-milter</i> (which is also quite good in recent versions).</p>
<p>The coming version of amavisd-new 2.6.0 will provide a built-in
signing and verification capability by using a module <i>Mail::DKIM</i>
directly, greatly simplifying the setup currently described here.</p>
<p>Under FreeBSD just install mail/dkim-milter from freshly updated ports,
and add the following to <tt>/etc/rc.conf</tt>:</p>
<pre>
milterdkim_enable="YES"
milterdkim_uid='dkimfilter'
milterdkim_domain='example.com'
milterdkim_profiles="verifier signer"
milterdkim_verifier_socket='inet:4443@127.0.0.1'
milterdkim_verifier_flags='-b v -C dns=a,int=a'
milterdkim_signer_socket='inet:4445@127.0.0.1'
milterdkim_signer_selector='myselector'
milterdkim_signer_key='/var/db/domainkeys/myselector.key.pem'
milterdkim_signer_alg='rsa-sha1'
milterdkim_signer_flags='-b s -c relaxed/simple -m ORIGINATING -o - -C int=a'
</pre>
<p>Recent startup script supplied by FreeBSD port already ensures that
deamon runs under its dedicated username, supplying a default value for
option -u.</p>
<p>Mail transformations as performed by some mailing lists are probably
the most challenging problem facing DKIM deployment (and to other schemes
as well). Nevertheless, mailing lists can be configured to either avoid
transformations which invalidate mail signatures, or can re-sign fan-out mail.
Examples of mailing lists which work very well with DKIM (and DomainKeys),
preserving existing signatures provided by posters, are the <i>postfix-users</i>
( postfix-users@postfix.org ) and the <i>SpamAssassin users</i> list
( users@spamassassin.apache.org ). Example of re-signing mailing lists
are Yahoo groups. A representative of another type of mailing lists is
Mailman, which often modifies mail body and strips out original signatures,
unless explicitly configured not to.</p>
<p>Several big players are already signing mail from their customers
or employees: Yahoo! (worldwide), Gmail, eBay, Earthlink, google.com,
Amazon, CNN, Skype, Cisco, etc. (not to mention porcupine.org :)</p>
<p>When signatures are missing on mail from domains which are known to be
signing all their mail (yahoo.com, gmail.com), the most common reason is
that a sender submitted his mail through some other provider, but supplied
his Yahoo or gmail e-mail address in the <i>From</i> header field.
Similar to other schemes designed to prevent faking of sending address,
the DKIM (and the DomainKeys) encourages mail submission only through
a domain which is used in the <i>From</i> address - although there are
other possibilities for roaming users in the DKIM system.</p>
<p>People will need to become aware that their best choice is to submit
mail through their native domain to prevent their messages from being
treated as second-class. With a widespread support for authorized mail
submission for roaming users (SASL, TLS) through a mail submission port
(tcp port 587, RFC 4409), supported by practically all modern clients
and mailers, there is no longer any good excuse for submitting mail
through foreign mail submission agents.</p>
<p>Note that some spam is also being signed by DomainKeys or DKIM lately,
which is a good thing -- it indicates the sender owns (or <i>ownz</i>)
a domain they are sending mail from. This either shows sender's sincere
desire of not hiding behind a faked sender mail address (in which
case such mail can be easily filtered if necessary), or they are using
a short-lived temporary domain (perhaps through <i>domain kiting</i>),
which can be counteracted by black lists of few-days old freshly
registered domains (such as <a href="http://support-intelligence.com/dob/"
>http://support-intelligence.com/dob/</a>) or other reputation schemes.
Signing and verifying mail is a good mechanism for companies to reliably
whitelist mail from their partner companies or frequent clients.</p>
<p>Some references:</p>
<ul>
<li><a href="http://www.dkim.org/info/dkim-faq.html">DKIM
Frequently Asked Questions</a> (at MIPA)</li>
<li><a href="http://www.rfc-editor.org/rfc/rfc4686.txt">RFC 4686</a>:
Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)</li>
<li><a href="http://www.rfc-editor.org/rfc/rfc4871.txt">RFC 4871</a>:
DomainKeys Identified Mail (DKIM) Signatures</li>
<li><a href="http://www.rfc-editor.org/rfc/rfc4870.txt">RFC 4870</a>:
(historical document) Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)</li>
<li><a href="http://www.rfc-editor.org/rfc/rfc5016.txt">RFC 5016</a>:
Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices
Protocol</li>
<li><a href="http://ietf.org/html.charters/dkim-charter.html">IETF
charter: Domain Keys Identified Mail (DKIM)</a></li>
<li><a href="http://www.postfix.org/MILTER_README.html">Postfix
before-queue Milter support</a> (original Postfix documentation)</li>
<li><a href="http://www.dkim.org/deploy/">DKIM Deployment
Reports</a> (at <i>Mutual Internet Practices Association</i> - MIPA)</li>
<li><a href="http://jason.long.name/dkimproxy/">Mail::DKIM and dkimproxy</a></li>
<li><a href="http://cpan.perl.org/authors/id/J/JA/JASLONG/"
>Mail::DKIM</a> module download page at CPAN</li>
<li><a href="http://sourceforge.net/projects/dkim-milter/">dkim-milter</a></li>
<li>See also:
<a href="http://www.arschkrebs.de/postfix/postfix_cisco_pix_bugs.shtml"
>Cisco PIX bugs</a> or this
<a href="http://www.heise-online.co.uk/security/Cisco-PIX-obstructs-anti-spam-protocol--/news/93725"
>heise-online article</a> on handling DKIM-signed mail
(the solution is to upgrade PIX to version 7.2(2.19) or 8.0(2.7),
or to disable a <i>smtp protocol fixup</i> (mis)feature</li>
</ul>
<hr />
<p>
<i><a href="http://www.ijs.si/people/mark/">mm</a></i>
<br />Last updated: 2008-02-22
</p>
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