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diff --git a/proto/RESTRICTION_CLASS_README.html b/proto/RESTRICTION_CLASS_README.html new file mode 100644 index 0000000..4988a33 --- /dev/null +++ b/proto/RESTRICTION_CLASS_README.html @@ -0,0 +1,239 @@ +<!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=utf-8"> + +</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 < 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 <user@remote>: 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> |