Postfix SMTP Access Policy Delegation

-------------------------------------------------------------------------------

Purpose of Postfix SMTP access policy delegation

The Postfix SMTP server has a number of built-in mechanisms to block or accept
mail at specific SMTP protocol stages. As of version 2.1, Postfix can delegate
policy decisions to an external server that runs outside Postfix.

With this policy delegation mechanism, a simple greylist policy can be
implemented with only a dozen lines of Perl, as is shown at the end of this
document. A complete example can be found in the Postfix source code, in the
directory examples/smtpd-policy.

Another example of policy delegation is the SPF policy server at http://
www.openspf.org/Software.

Policy delegation is now the preferred method for adding policies to Postfix.
It's much easier to develop a new feature in few lines of Perl, Python, Ruby,
or TCL, than trying to do the same in C code. The difference in performance
will be unnoticeable except in the most demanding environments. On active
systems a policy daemon process is used multiple times, for up to $max_use
incoming SMTP connections.

This document covers the following topics:

  * Policy protocol description
  * Policy client/server configuration
  * Example: greylist policy server
  * Greylisting mail from frequently forged domains
  * Greylisting all your mail
  * Routine greylist maintenance
  * Example Perl greylist server

Protocol description

The Postfix policy delegation protocol is really simple. The client request is
a sequence of name=value attributes separated by newline, and is terminated by
an empty line. The server reply is one name=value attribute and it, too, is
terminated by an empty line.

Here is an example of all the attributes that the Postfix SMTP server sends in
a delegated SMTPD access policy request:

    Postfix version 2.1 and later:
    request=smtpd_access_policy
    protocol_state=RCPT
    protocol_name=SMTP
    helo_name=some.domain.tld
    queue_id=8045F2AB23
    sender=foo@bar.tld
    recipient=bar@foo.tld
    recipient_count=0
    client_address=1.2.3.4
    client_name=another.domain.tld
    reverse_client_name=another.domain.tld
    instance=123.456.7
    Postfix version 2.2 and later:
    sasl_method=plain
    sasl_username=you
    sasl_sender=
    size=12345
    ccert_subject=solaris9.porcupine.org
    ccert_issuer=Wietse+20Venema
    ccert_fingerprint=C2:9D:F4:87:71:73:73:D9:18:E7:C2:F3:C1:DA:6E:04
    Postfix version 2.3 and later:
    encryption_protocol=TLSv1/SSLv3
    encryption_cipher=DHE-RSA-AES256-SHA
    encryption_keysize=256
    etrn_domain=
    Postfix version 2.5 and later:
    stress=
    [empty line]

Notes:

  * The "request" attribute is required. In this example the request type is
    "smtpd_access_policy".

  * The order of the attributes does not matter. The policy server should
    ignore any attributes that it does not care about.

  * When the same attribute name is sent more than once, the server may keep
    the first value or the last attribute value.

  * When an attribute value is unavailable, the client either does not send the
    attribute, sends the attribute with an empty value ("name="), or sends a
    zero value ("name=0") in the case of a numerical attribute.

  * The "recipient" attribute is available in the "RCPT TO" stage. It is also
    available in the "DATA" and "END-OF-MESSAGE" stages if Postfix accepted
    only one recipient for the current message.

  * The "recipient_count" attribute (Postfix 2.3 and later) is non-zero only in
    the "DATA" and "END-OF-MESSAGE" stages. It specifies the number of
    recipients that Postfix accepted for the current message.

  * The client address is an IPv4 dotted quad in the form 1.2.3.4 or it is an
    IPv6 address in the form 1:2:3::4:5:6.

  * For a discussion of the differences between reverse and verified
    client_name information, see the reject_unknown_client_hostname discussion
    in the postconf(5) document.

  * An attribute name must not contain "=", null or newline, and an attribute
    value must not contain null or newline.

  * The "instance" attribute value can be used to correlate different requests
    regarding the same message delivery. These requests are sent over the same
    policy connection (unless the policy daemon terminates the connection).
    Once Postfix sends a query with a different instance attribute over that
    same policy connection, the previous message delivery is either completed
    or aborted.

  * The "size" attribute value specifies the message size that the client
    specified in the MAIL FROM command (zero if none was specified). With
    Postfix 2.2 and later, it specifies the actual message size when the client
    sends the END-OF-DATA command.

  * The "sasl_*" attributes (Postfix 2.2 and later) specify information about
    how the client was authenticated via SASL. These attributes are empty in
    case of no SASL authentication.

  * The "ccert_*" attributes (Postfix 2.2 and later) specify information about
    how the client was authenticated via TLS. These attributes are empty in
    case of no certificate authentication. As of Postfix 2.2.11 these attribute
    values are encoded as xtext: some characters are represented by +XX, where
    XX is the two-digit hexadecimal representation of the character value. With
    Postfix 2.6 and later, the decoded string is an UTF-8 string without non-
    printable ASCII characters.

  * The "encryption_*" attributes (Postfix 2.3 and later) specify information
    about how the connection is encrypted. With plaintext connections the
    protocol and cipher attributes are empty and the keysize is zero.

  * The "etrn_domain" attribute is defined only in the context of the ETRN
    command, and specifies the ETRN command parameter.

  * The "stress" attribute is either empty or "yes". See the STRESS_README
    document for further information.

The following is specific to SMTPD delegated policy requests:

  * Protocol names are ESMTP or SMTP.

  * Protocol states are CONNECT, EHLO, HELO, MAIL, RCPT, DATA, END-OF-MESSAGE,
    VRFY or ETRN; these are the SMTP protocol states where the Postfix SMTP
    server makes an OK/REJECT/HOLD/etc. decision.

The policy server replies with any action that is allowed in a Postfix SMTPD
access(5) table. Example:

    action=defer_if_permit Service temporarily unavailable
    [empty line]

This causes the Postfix SMTP server to reject the request with a 450 temporary
error code and with text "Service temporarily unavailable", if the Postfix SMTP
server finds no reason to reject the request permanently.

In case of trouble the policy server must not send a reply. Instead the server
must log a warning and disconnect. Postfix will retry the request at some later
time.

Policy client/server configuration

The Postfix delegated policy client can connect to a TCP socket or to a UNIX-
domain socket. Examples:

    inet:127.0.0.1:9998
    unix:/some/where/policy
    unix:private/policy

The first example specifies that the policy server listens on a TCP socket at
127.0.0.1 port 9998. The second example specifies an absolute pathname of a
UNIX-domain socket. The third example specifies a pathname relative to the
Postfix queue directory; use this for policy servers that are spawned by the
Postfix master daemon.

To create a policy service that listens on a UNIX-domain socket called
"policy", and that runs under control of the Postfix spawn(8) daemon, you would
use something like this:

     1 /etc/postfix/master.cf:
     2     policy  unix  -       n       n       -       0       spawn
     3       user=nobody argv=/some/where/policy-server
     4
     5 /etc/postfix/main.cf:
     6     smtpd_recipient_restrictions =
     7         ...
     8         reject_unauth_destination
     9         check_policy_service unix:private/policy
    10         ...
    11     policy_time_limit = 3600

NOTES:

  * Lines 2, 11: the Postfix spawn(8) daemon by default kills its child process
    after 1000 seconds. This is too short for a policy daemon that may need to
    run for as long as the SMTP server process that talks to it. The default
    time limit is overruled in main.cf with an explicit "policy_time_limit"
    setting. The name of the parameter is the name of the master.cf entry
    ("policy") concatenated with the "_time_limit" suffix. See spawn(8) for
    more information about the time limit parameter.

  * Line 2: specify a "0" process limit instead of the default "-", to avoid
    "connection refused" and other problems when the smtpd process limit
    exceeds the default_process_limit setting.

  * Lines 8, 9: always specify "check_policy_service" AFTER
    "reject_unauth_destination" or else your system could become an open relay.

  * Solaris UNIX-domain sockets do not work reliably. Use TCP sockets instead:

     1 /etc/postfix/master.cf:
     2     127.0.0.1:9998  inet  n       n       n       -       0       spawn
     3       user=nobody argv=/some/where/policy-server
     4
     5 /etc/postfix/main.cf:
     6     smtpd_recipient_restrictions =
     7         ...
     8         reject_unauth_destination
     9         check_policy_service inet:127.0.0.1:9998
    10         ...
    11     127.0.0.1:9998_time_limit = 3600

Other configuration parameters that control the client side of the policy
delegation protocol:

  * smtpd_policy_service_max_idle (default: 300s): The amount of time before
    the Postfix SMTP server closes an unused policy client connection.

  * smtpd_policy_service_max_ttl (default: 1000s): The amount of time before
    the Postfix SMTP server closes an active policy client connection.

  * smtpd_policy_service_timeout (default: 100s): The time limit to connect to,
    send to or receive from a policy server.

Example: greylist policy server

Greylisting is a defense against junk email that is described at http://
www.greylisting.org/. The idea was discussed on the postfix-users mailing list
one year before it was popularized.

The file examples/smtpd-policy/greylist.pl in the Postfix source tree
implements a simplified greylist policy server. This server stores a time stamp
for every (client, sender, recipient) triple. By default, mail is not accepted
until a time stamp is more than 60 seconds old. This stops junk mail with
randomly selected sender addresses, and mail that is sent through randomly
selected open proxies. It also stops junk mail from spammers that change their
IP address frequently.

Copy examples/smtpd-policy/greylist.pl to /usr/libexec/postfix or whatever
location is appropriate for your system.

In the greylist.pl Perl script you need to specify the location of the greylist
database file, and how long mail will be delayed before it is accepted. The
default settings are:

    $database_name="/var/mta/greylist.db";
    $greylist_delay=60;

The /var/mta directory (or whatever you choose) should be writable by "nobody",
or by whatever username you configure below in master.cf for the policy
service.

Example:

    # mkdir /var/mta
    # chown nobody /var/mta

Note: DO NOT create the greylist database in a world-writable directory such as
/tmp or /var/tmp, and DO NOT create the greylist database in a file system that
may run out of space. Postfix can survive "out of space" conditions with the
mail queue and with the mailbox store, but it cannot survive a corrupted
greylist database. If the file becomes corrupted you may not be able to receive
mail at all until you delete the file by hand.

The greylist.pl Perl script can be run under control by the Postfix master
daemon. For example, to run the script as user "nobody", using a UNIX-domain
socket that is accessible by Postfix processes only:

    1 /etc/postfix/master.cf:
    2     policy  unix  -       n       n       -       0       spawn
    3       user=nobody argv=/usr/bin/perl /usr/libexec/postfix/greylist.pl
    4
    5 /etc/postfix/main.cf:
    6      policy_time_limit = 3600

Notes:

  * Line 3: Specify "greylist.pl -v" for verbose logging of each request and
    reply.

  * Lines 2, 6: the Postfix spawn(8) daemon by default kills its child process
    after 1000 seconds. This is too short for a policy daemon that may run for
    as long as an SMTP client is connected to an SMTP server process. The
    default time limit is overruled in main.cf with an explicit
    "policy_time_limit" setting. The name of the parameter is the name of the
    master.cf entry ("policy") concatenated with the "_time_limit" suffix.

  * Line 2: specify a "0" process limit instead of the default "-", to avoid
    "connection refused" and other problems when the smtpd process limit
    exceeds the default_process_limit setting.

On Solaris you must use inet: style sockets instead of unix: style, as detailed
in the "Policy client/server configuration" section above.

    1 /etc/postfix/master.cf:
    2     127.0.0.1:9998  inet  n       n       n       -       0       spawn
    3       user=nobody argv=/usr/bin/perl /usr/libexec/postfix/greylist.pl
    4
    5 /etc/postfix/main.cf:
    6      127.0.0.1:9998_time_limit = 3600

To invoke this service you would specify "check_policy_service inet:127.0.0.1:
9998".

Greylisting mail from frequently forged domains

It is relatively safe to turn on greylisting for specific domains that often
appear in forged email. At some point in cyberspace/time a list of frequently
forged MAIL FROM domains could be found at http://www.monkeys.com/anti-spam/
filtering/sender-domain-validate.in.

     1 /etc/postfix/main.cf:
     2     smtpd_recipient_restrictions =
     3         reject_unlisted_recipient
     4         ...
     5         reject_unauth_destination
     6         check_sender_access hash:/etc/postfix/sender_access
     7         ...
     8     smtpd_restriction_classes = greylist
     9     greylist = check_policy_service unix:private/policy
    10
    11 /etc/postfix/sender_access:
    12     aol.com     greylist
    13     hotmail.com greylist
    14     bigfoot.com greylist
    15     ... etcetera ...

NOTES:

  * Line 9: On Solaris you must use inet: style sockets instead of unix: style,
    as detailed in the "Example: greylist policy server" section above.

  * Line 6: Be sure to specify "check_sender_access" AFTER
    "reject_unauth_destination" or else your system could become an open mail
    relay.

  * Line 3: With Postfix 2.0 snapshot releases, "reject_unlisted_recipient" is
    called "check_recipient_maps". Postfix 2.1 understands both forms.

  * Line 3: The greylist database gets polluted quickly with bogus addresses.
    It helps if you protect greylist lookups with other restrictions that
    reject unknown senders and/or recipients.

Greylisting all your mail

If you turn on greylisting for all mail you will almost certainly want to make
exceptions for mailing lists that use one-time sender addresses, because such
mailing lists can pollute your greylist database relatively quickly.

     1 /etc/postfix/main.cf:
     2     smtpd_recipient_restrictions =
     3         reject_unlisted_recipient
     4         ...
     5         reject_unauth_destination
     6         check_sender_access hash:/etc/postfix/sender_access
     7         check_policy_service unix:private/policy
     8         ...
     9
    10 /etc/postfix/sender_access:
    11     securityfocus.com OK
    12     ...

NOTES:

  * Line 7: On Solaris you must use inet: style sockets instead of unix: style,
    as detailed in the "Example: greylist policy server" section above.

  * Lines 6-7: Be sure to specify check_sender_access and check_policy_service
    AFTER reject_unauth_destination or else your system could become an open
    mail relay.

  * Line 3: The greylist database gets polluted quickly with bogus addresses.
    It helps if you precede greylist lookups with restrictions that reject
    unknown senders and/or recipients.

Routine greylist maintenance

The greylist database grows over time, because the greylist server never
removes database entries. If left unattended, the greylist database will
eventually run your file system out of space.

When the status file size exceeds some threshold you can simply rename or
remove the file without adverse effects; Postfix automatically creates a new
file. In the worst case, new mail will be delayed by an hour or so. To lessen
the impact, rename or remove the file in the middle of the night at the
beginning of a weekend.

Example Perl greylist server

This is the Perl subroutine that implements the example greylist policy. It is
part of a general purpose sample policy server that is distributed with the
Postfix source as examples/smtpd-policy/greylist.pl.

#
# greylist status database and greylist time interval. DO NOT create the
# greylist status database in a world-writable directory such as /tmp
# or /var/tmp. DO NOT create the greylist database in a file system
# that can run out of space.
#
$database_name="/var/mta/greylist.db";
$greylist_delay=60;

#
# Auto-whitelist threshold. Specify 0 to disable, or the number of
# successful "come backs" after which a client is no longer subject
# to greylisting.
#
$auto_whitelist_threshold = 10;

#
# Demo SMTPD access policy routine. The result is an action just like
# it would be specified on the right-hand side of a Postfix access
# table.  Request attributes are available via the %attr hash.
#
sub smtpd_access_policy {
    my($key, $time_stamp, $now);

    # Open the database on the fly.
    open_database() unless $database_obj;

    # Search the auto-whitelist.
    if ($auto_whitelist_threshold > 0) {
        $count = read_database($attr{"client_address"});
        if ($count > $auto_whitelist_threshold) {
            return "dunno";
        }
    }

    # Lookup the time stamp for this client/sender/recipient.
    $key =
        lc $attr{"client_address"}."/".$attr{"sender"}."/".$attr{"recipient"};
    $time_stamp = read_database($key);
    $now = time();

    # If new request, add this client/sender/recipient to the database.
    if ($time_stamp == 0) {
        $time_stamp = $now;
        update_database($key, $time_stamp);
    }

    # The result can be any action that is allowed in a Postfix access(5) map.
    #
    # To label the mail, return ``PREPEND headername: headertext''
    #
    # In case of success, return ``DUNNO'' instead of ``OK'', so that the
    # check_policy_service restriction can be followed by other restrictions.
    #
    # In case of failure, return ``DEFER_IF_PERMIT optional text...'',
    # so that mail can still be blocked by other access restrictions.
    #
    syslog $syslog_priority, "request age %d", $now - $time_stamp if $verbose;
    if ($now - $time_stamp > $greylist_delay) {
        # Update the auto-whitelist.
        if ($auto_whitelist_threshold > 0) {
            update_database($attr{"client_address"}, $count + 1);
        }
        return "dunno";
    } else {
        return "defer_if_permit Service temporarily unavailable";
    }
}

