From 5e61585d76ae77fd5e9e96ebabb57afa4d74880d Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 27 Apr 2024 14:06:34 +0200 Subject: Adding upstream version 3.5.24. Signed-off-by: Daniel Baumann --- html/SMTPD_POLICY_README.html | 811 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 811 insertions(+) create mode 100644 html/SMTPD_POLICY_README.html (limited to 'html/SMTPD_POLICY_README.html') diff --git a/html/SMTPD_POLICY_README.html b/html/SMTPD_POLICY_README.html new file mode 100644 index 0000000..da57bc5 --- /dev/null +++ b/html/SMTPD_POLICY_README.html @@ -0,0 +1,811 @@ + + + + + + +Postfix SMTP Access Policy Delegation + + + + + + + +

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. 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 https://web.archive.org/web/20190221142057/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:

+ + + +

Protocol description

+ +

The Postfix policy delegation protocol is really simple. The client +sends a request and the server sends a response. Unless there was an +error, the server must not close the connection, so that the same +connection can be used multiple times.

+ +

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=
+Postfix version 2.9 and later:
+ccert_pubkey_fingerprint=68:B3:29:DA:98:93:E3:40:99:C7:D8:AD:5C:B9:C9:40
+Postfix version 3.0 and later:
+client_port=1234
+Postfix version 3.1 and later:
+policy_context=submission
+Postfix version 3.2 and later:
+server_address=10.3.2.1
+server_port=54321
+[empty line]
+
+
+ +

Notes:

+ + + +

The following is specific to SMTPD delegated policy requests: +

+ + + +

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.

+ +

Simple 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
+12     # smtpd_policy_service_request_limit = 1
+
+
+ +

NOTES:

+ + + +
+
+ 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
+12     # smtpd_policy_service_request_limit = 1
+
+
+ +

Configuration parameters that control the client side of the +policy delegation protocol:

+ + + +

Configuration parameters that control the server side of the +policy delegation protocol:

+ + + +

Advanced policy client configuration

+ +

The previous section lists a number of Postfix main.cf parameters +that control time limits and other settings for all policy clients. +This is sufficient for simple configurations. With more complex +configurations it becomes desirable to have different settings per +policy client. This is supported with Postfix 3.0 and later.

+ +

The following example shows a "non-critical" policy service +with a short timeout, and with "DUNNO" as default action when the +service is unvailable. The "DUNNO" action causes Postfix to ignore +the result.

+ +
+
+1 /etc/postfix/main.cf:
+2     mua_recipient_restrictions = 
+3         ...
+4         reject_unauth_destination
+5         check_policy_service { inet:host:port, 
+6             timeout=10s, default_action=DUNNO
+7             policy_context=submission }
+8         ...
+
+
+ +

Instead of a server endpoint, we now have a list enclosed in {}.

+ + + +

Inside the list, syntax is similar to what we already know from +main.cf: items separated by space or comma. There is one difference: +you must enclose a setting in parentheses, as in "{ name = value +}", if you want to have space or comma within a value or around +"=". This comes in handy when different policy servers require +different default actions with different SMTP status codes or text: +

+ +
+
+1 /etc/postfix/main.cf:
+2     smtpd_recipient_restrictions = 
+3         ...
+4         reject_unauth_destination
+5         check_policy_service {
+6           inet:host:port1, 
+7           { default_action = 451 4.3.5 See http://www.example.com/support1 }
+8         } 
+9         ...
+
+
+ +

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     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         ...
+12     # smtpd_policy_service_request_limit = 1
+
+
+ +

Notes:

+ + + +

With Solaris < 9, or Postfix < 2.10 on any Solaris +version, 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         ...
+12     # smtpd_policy_service_request_limit = 1
+
+
+ +

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/greylist
+10 
+11 /etc/postfix/sender_access:
+12     aol.com     greylist
+13     hotmail.com greylist
+14     bigfoot.com greylist
+15     ... etcetera ...
+
+
+ +

NOTES:

+ + + +

Greylisting all your mail

+ +

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:

+ + + +

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";
+    }
+}
+
+ + + + -- cgit v1.2.3