SMTPD_POLICY_README [plain text]
PPoossttffiixx SSMMTTPP AAcccceessss PPoolliiccyy DDeelleeggaattiioonn
-------------------------------------------------------------------------------
PPuurrppoossee ooff PPoossttffiixx SSMMTTPP aacccceessss ppoolliiccyy ddeelleeggaattiioonn
The Postfix SMTP server has a number of built-in mechanisms to block or accept
mail at specific SMTP protocol stages. In addition, the Postfix SMTP server can
delegate decisions to an external policy server (Postfix 2.1 and later).
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
PPrroottooccooll ddeessccrriippttiioonn
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:
PPoossttffiixx vveerrssiioonn 22..11 aanndd llaatteerr::
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
PPoossttffiixx vveerrssiioonn 22..22 aanndd llaatteerr::
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
PPoossttffiixx vveerrssiioonn 22..33 aanndd llaatteerr::
encryption_protocol=TLSv1/SSLv3
encryption_cipher=DHE-RSA-AES256-SHA
encryption_keysize=256
etrn_domain=
PPoossttffiixx vveerrssiioonn 22..55 aanndd llaatteerr::
stress=
PPoossttffiixx vveerrssiioonn 22..99 aanndd llaatteerr::
ccert_pubkey_fingerprint=68:B3:29:DA:98:93:E3:40:99:C7:D8:AD:5C:B9:C9:40
[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.
PPoolliiccyy cclliieenntt//sseerrvveerr ccoonnffiigguurraattiioonn
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 2-3: this creates the service called "policy" that listens on a UNIX-
domain socket. The service is implemented by the Postfix spawn(8) daemon,
which executes the policy server program that is specified with the aarrggvv
attribute, using the privileges specified with the uusseerr attribute.
* Line 2: specify a "0" process limit instead of the default "-", to avoid
"connection refused" and other problems when you increase the smtpd process
limit.
* Lines 8, 9: always specify "check_policy_service" AFTER
"reject_unauth_destination" or else your system could become an open relay.
* Line 11: this increases the time that a policy server process may run to
3600 seconds. The default time limit of 1000 seconds is too short; the
policy daemon needs to run long as the SMTP server process that talks to
it. See the spawn(8) manpage for more information about the
transport_time_limit parameter.
Note: the "policy_time_limit" parameter will not show up in "postconf"
command output before Postfix version 2.9. This limitation applies to
many parameters whose name is a combination of a master.cf service name
(in the above example, "policy") and a built-in suffix (in the above
example: "_time_limit").
* 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.
EExxaammppllee:: ggrreeyylliisstt ppoolliiccyy sseerrvveerr
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 greylist unix - n n - 0 spawn
3 user=nobody argv=/usr/bin/perl /usr/libexec/postfix/greylist.pl
4
5 /etc/postfix/main.cf:
6 greylist_time_limit = 3600
7 smtpd_recipient_restrictions =
8 ...
9 reject_unauth_destination
10 check_policy_service unix:private/greylist
11 ...
Notes:
* Lines 2-3: this creates the service called "greylist" that listens on a
UNIX-domain socket. The service is implemented by the Postfix spawn(8)
daemon, which executes the greylist.pl script that is specified with the
aarrggvv attribute, using the privileges specified with the uusseerr attribute.
* Line 2: specify a "0" process limit instead of the default "-", to avoid
"connection refused" and other problems when you increase the smtpd process
limit.
* Line 3: Specify "greylist.pl -v" for verbose logging of each request and
reply.
* Line 6: this increases the time that a greylist server process may run to
3600 seconds. The default time limit of 1000 seconds is too short; the
greylist daemon needs to run long as the SMTP server process that talks to
it. See the spawn(8) manpage for more information about the
transport_time_limit parameter.
Note: the "greylist_time_limit" parameter will not show up in
"postconf" command output before Postfix version 2.9. This limitation
applies to many parameters whose name is a combination of a master.cf
service name (in the above example, "greylist") and a built-in suffix
(in the above example: "_time_limit").
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
7 smtpd_recipient_restrictions =
8 ...
9 reject_unauth_destination
10 check_policy_service inet:127.0.0.1:9998
11 ...
GGrreeyylliissttiinngg mmaaiill ffrroomm ffrreeqquueennttllyy ffoorrggeedd ddoommaaiinnss
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/greylist
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.
GGrreeyylliissttiinngg aallll yyoouurr mmaaiill
If you turn on greylisting for all mail you may want to make exceptions for
mailing lists that use one-time sender addresses, because each message will be
delayed due to greylisting, and the one-time sender addresses can pollute your
greylist database relatively quickly. Instead of making exceptions, you can
automatically whitelist clients that survive greylisting repeatedly; this
avoids most of the delays and most of the database pollution problem.
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.
RRoouuttiinnee ggrreeyylliisstt mmaaiinntteennaannccee
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.
EExxaammppllee PPeerrll ggrreeyylliisstt sseerrvveerr
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";
}
}