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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-06 01:46:30 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-06 01:46:30 +0000
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Adding upstream version 3.4.23.upstream/3.4.23upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
+ "http://www.w3.org/TR/html4/loose.dtd">
+
+<html>
+
+<head>
+
+<title>Postfix Per-Client/User/etc. Access Control</title>
+
+<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
+
+</head>
+
+<body>
+
+<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
+Per-Client/User/etc. Access Control</h1>
+
+<hr>
+
+<h2>Postfix restriction classes</h2>
+
+<p> The Postfix SMTP server supports access restrictions such as
+reject_rbl_client or reject_unknown_client_hostname on the right-hand side
+of SMTP server access(5) tables. This allows you to implement
+different junk mail restrictions for different clients or users.
+</p>
+
+<p> Having to specify lists of access restrictions for every
+recipient becomes tedious quickly. Postfix restriction classes
+allow you to give easy-to-remember names to groups of UCE restrictions
+(such as "permissive", "restrictive", and so on). </p>
+
+<p> The real reason for the existence of Postfix restriction classes
+is more mundane: you can't specify a lookup table on the right-hand
+side of a Postfix access table. This is because Postfix needs to
+open lookup tables ahead of time, but the reader probably does not
+care about these low-level details. </p>
+
+<p> Example: </p>
+
+<blockquote>
+<pre>
+/etc/postfix/main.cf:
+ smtpd_restriction_classes = restrictive, permissive
+ # With Postfix &lt; 2.3 specify reject_unknown_client.
+ restrictive = reject_unknown_sender_domain reject_unknown_client_hostname ...
+ permissive = permit
+
+ smtpd_recipient_restrictions =
+ permit_mynetworks
+ # reject_unauth_destination is not needed here if the mail
+ # relay policy is specified with smtpd_relay_restrictions
+ # (available with Postfix 2.10 and later).
+ reject_unauth_destination
+ check_recipient_access hash:/etc/postfix/recipient_access
+ ...
+
+/etc/postfix/recipient_access:
+ joe@my.domain permissive
+ jane@my.domain restrictive
+</pre>
+</blockquote>
+
+<p> With this in place, you can use "restrictive" or "permissive"
+on the right-hand side of your per-client, helo, sender, or recipient
+SMTPD access tables. </p>
+
+<p> The remainder of this document gives examples of how Postfix
+access restriction classes can be used to: </p>
+
+<ul>
+
+<li> <a href="#internal"> Shield an internal mailing list from
+outside posters</a>,
+
+<li> <a href="#external"> Prevent external access by internal
+senders</a>.
+
+</ul>
+
+<p> These questions come up frequently, and the examples hopefully
+make clear that Postfix restriction classes aren't really the right
+solution. They should be used for what they were designed to do,
+different junk mail restrictions for different clients or users.
+</p>
+
+<h2><a name="internal">Protecting internal email distribution
+lists</a></h2>
+
+<blockquote>
+
+<p> We want to implement an internal email distribution list.
+Something like all@our.domain.com, which aliases to all employees.
+My first thought was to use the aliases map, but that would lead
+to "all" being accessible from the "outside", and this is not
+desired... :-) </p>
+
+</blockquote>
+
+<p> Postfix can implement per-address access controls. What follows
+is based on the SMTP client IP address, and therefore is subject
+to IP spoofing. </p>
+
+<blockquote>
+<pre>
+/etc/postfix/main.cf:
+ smtpd_recipient_restrictions =
+ ...
+ check_recipient_access hash:/etc/postfix/access
+ <i>...the usual stuff...</i>
+
+/etc/postfix/access:
+ all@my.domain permit_mynetworks,reject
+ all@my.hostname permit_mynetworks,reject
+</pre>
+</blockquote>
+
+<p> Specify <b>dbm</b> instead of <b>hash</b> if your system uses
+<b>dbm</b> files instead of <b>db</b> files. To find out what map
+types Postfix supports, use the command <b>postconf -m</b>. </p>
+
+<p> Now, that would be sufficient when your machine receives all
+Internet mail directly from the Internet. That's unlikely if your
+network is a bit larger than an office. For example, your backup
+MX hosts would "launder" the client IP address of mail from the
+outside so it would appear to come from a trusted machine. </p>
+
+<p> In the general case you need two lookup tables: one table that
+lists destinations that need to be protected, and one table that
+lists domains that are allowed to send to the protected destinations.
+</p>
+
+<p> What follows is based on the sender SMTP envelope address, and
+therefore is subject to SMTP sender spoofing. </p>
+
+<blockquote>
+<pre>
+/etc/postfix/main.cf:
+ smtpd_recipient_restrictions =
+ ...
+ check_recipient_access hash:/etc/postfix/protected_destinations
+ <i>...the usual stuff...</i>
+
+ smtpd_restriction_classes = insiders_only
+ insiders_only = check_sender_access hash:/etc/postfix/insiders, reject
+
+/etc/postfix/protected_destinations:
+ all@my.domain insiders_only
+ all@my.hostname insiders_only
+
+/etc/postfix/insiders:
+ my.domain OK <i>matches my.domain and subdomains</i>
+ another.domain OK <i>matches another.domain and subdomains</i>
+</pre>
+</blockquote>
+
+<p> Getting past this scheme is relatively easy, because all one
+has to do is to spoof the SMTP sender address. </p>
+
+<p> If the internal list is a low-volume one, perhaps it makes more
+sense to make it moderated. </p>
+
+<h2><a name="external">Restricting what users can send mail to
+off-site destinations</a></h2>
+
+<blockquote>
+
+<p> How can I configure Postfix in a way that some users can send
+mail to the internet and other users not. The users with no access
+should receive a generic bounce message. Please don't discuss
+whether such access restrictions are necessary, it was not my
+decision. </p>
+
+</blockquote>
+
+<p> Postfix has support for per-user restrictions. The restrictions
+are implemented by the SMTP server. Thus, users that violate the
+policy have their mail rejected by the SMTP server. Like this:
+</p>
+
+<blockquote>
+<pre>
+554 &lt;user@remote&gt;: Access denied
+</pre>
+</blockquote>
+
+<p> The implementation uses two lookup tables. One table defines
+what users are restricted in where they can send mail, and the
+other table defines what destinations are local. It is left as an
+exercise for the reader to change this into a scheme where only
+some users have permission to send mail to off-site destinations,
+and where most users are restricted. </p>
+
+<p> The example assumes DB/DBM files, but this could also be done
+with LDAP or SQL. </p>
+
+<blockquote>
+<pre>
+/etc/postfix/main.cf:
+ smtpd_recipient_restrictions =
+ ...
+ check_sender_access hash:/etc/postfix/restricted_senders
+ <i>...other stuff...</i>
+
+ smtpd_restriction_classes = local_only
+ local_only =
+ check_recipient_access hash:/etc/postfix/local_domains, reject
+
+/etc/postfix/restricted_senders:
+ foo@domain local_only
+ bar@domain local_only
+
+/etc/postfix/local_domains:
+ this.domain OK <i>matches this.domain and subdomains</i>
+ that.domain OK <i>matches that.domain and subdomains</i>
+</pre>
+</blockquote>
+
+<p> Specify <b>dbm</b> instead of <b>hash</b> if your system uses
+<b>dbm</b> files instead of <b>db</b> files. To find out what map
+types Postfix supports, use the command <b>postconf -m</b>. </p>
+
+<p> Note: this scheme does not authenticate the user, and therefore it can be
+bypassed in several ways: </p>
+
+<ul>
+
+<li> <p> By sending mail via a less restrictive mail
+relay host. </p>
+
+<li> <p> By sending mail as someone else who does have permission
+to send mail to off-site destinations. </p>
+
+</ul>
+
+</body>
+
+</html>