Adding upstream version 3.4.23.upstream/3.4.23upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 166 insertions, 0 deletions

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+PPoossttffiixx PPeerr--CClliieenntt//UUsseerr//eettcc.. AAcccceessss CCoonnttrrooll
+
+-------------------------------------------------------------------------------
+
+PPoossttffiixx rreessttrriiccttiioonn ccllaasssseess
+
+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.
+
+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).
+
+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.
+
+Example:
+
+    /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
+
+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.
+
+The remainder of this document gives examples of how Postfix access restriction
+classes can be used to:
+
+  * Shield an internal mailing list from outside posters,
+  * Prevent external access by internal senders.
+
+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.
+
+PPrrootteeccttiinngg iinntteerrnnaall eemmaaiill ddiissttrriibbuuttiioonn lliissttss
+
+    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... :-)
+
+Postfix can implement per-address access controls. What follows is based on the
+SMTP client IP address, and therefore is subject to IP spoofing.
+
+    /etc/postfix/main.cf:
+        smtpd_recipient_restrictions =
+            ...
+            check_recipient_access hash:/etc/postfix/access
+            ...the usual stuff...
+
+    /etc/postfix/access:
+        all@my.domain   permit_mynetworks,reject
+        all@my.hostname permit_mynetworks,reject
+
+Specify ddbbmm instead of hhaasshh if your system uses ddbbmm files instead of ddbb files.
+To find out what map types Postfix supports, use the command ppoossttccoonnff --mm.
+
+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.
+
+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.
+
+What follows is based on the sender SMTP envelope address, and therefore is
+subject to SMTP sender spoofing.
+
+    /etc/postfix/main.cf:
+        smtpd_recipient_restrictions =
+            ...
+            check_recipient_access hash:/etc/postfix/protected_destinations
+            ...the usual stuff...
+
+        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  matches my.domain and subdomains
+        another.domain  OK  matches another.domain and subdomains
+
+Getting past this scheme is relatively easy, because all one has to do is to
+spoof the SMTP sender address.
+
+If the internal list is a low-volume one, perhaps it makes more sense to make
+it moderated.
+
+RReessttrriiccttiinngg wwhhaatt uusseerrss ccaann sseenndd mmaaiill ttoo ooffff--ssiittee ddeessttiinnaattiioonnss
+
+    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.
+
+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:
+
+    554 <user@remote>: Access denied
+
+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.
+
+The example assumes DB/DBM files, but this could also be done with LDAP or SQL.
+
+    /etc/postfix/main.cf:
+        smtpd_recipient_restrictions =
+            ...
+            check_sender_access hash:/etc/postfix/restricted_senders
+            ...other stuff...
+
+        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      matches this.domain and subdomains
+        that.domain     OK      matches that.domain and subdomains
+
+Specify ddbbmm instead of hhaasshh if your system uses ddbbmm files instead of ddbb files.
+To find out what map types Postfix supports, use the command ppoossttccoonnff --mm.
+
+Note: this scheme does not authenticate the user, and therefore it can be
+bypassed in several ways:
+
+  * By sending mail via a less restrictive mail relay host.
+
+  * By sending mail as someone else who does have permission to send mail to
+    off-site destinations.
+