Adding upstream version 3.4.23.upstream/3.4.23upstream

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

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+PPoossttffiixx EETTRRNN HHoowwttoo
+
+-------------------------------------------------------------------------------
+
+PPuurrppoossee ooff tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee
+
+The SMTP ETRN command was designed for sites that have intermittent Internet
+connectivity. With ETRN, a site can tell the mail server of its provider to
+"Please deliver all my mail now". The SMTP server searches the queue for mail
+to the customer, and delivers that mail bbyy ccoonnnneeccttiinngg ttoo tthhee ccuussttoommeerr''ss SSMMTTPP
+sseerrvveerr. The mail is not delivered via the connection that was used for sending
+ETRN.
+
+As of version 1.0, Postfix has a fast ETRN implementation that does not require
+Postfix to examine every queue file. Instead, Postfix maintains a record of
+what queue files contain mail for destinations that are configured for ETRN
+service. ETRN service is no longer available for domains that aren't configured
+for the service.
+
+This document provides information on the following topics:
+
+  * Using the Postfix fast ETRN service
+  * How Postfix fast ETRN works
+  * Postfix fast ETRN service limitations
+  * Configuring the Postfix fast ETRN service
+  * Configuring a domain for ETRN service only
+  * Testing the Postfix fast ETRN service
+
+Other documents with information on this subject:
+
+  * flush(8), flush service implementation
+
+UUssiinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee
+
+The following is an example SMTP session that shows how an SMTP client requests
+the ETRN service. Client commands are shown in bold font.
+
+    220 my.server.tld ESMTP Postfix
+    HHEELLOO mmyy..cclliieenntt..ttlldd
+    250 Ok
+    EETTRRNN ssoommee..ccuussttoommeerr..ddoommaaiinn
+    250 Queuing started
+    QQUUIITT
+    221 Bye
+
+As mentioned in the introduction, the mail is delivered by connecting to the
+customer's SMTP server; it is not sent over the connection that was used to
+send the ETRN command.
+
+The Postfix operator can request delivery for a specific customer by using the
+command "sendmail -qRdestination" and, with Postfix version 1.1 and later,
+"postqueue -sdestination". Access to this feature is controlled with the
+authorized_flush_users configuration parameter (Postfix version 2.2 and later).
+
+HHooww PPoossttffiixx ffaasstt EETTRRNN wwoorrkkss
+
+When a Postfix delivery agent decides that mail must be delivered later, it
+sends the destination domain name and the queue file name to the flush(8)
+daemon which maintains per-destination logfiles with file names of queued mail.
+These logfiles are kept below $queue_directory/flush. Per-destination logfiles
+are maintained only for destinations that are listed with the
+$fast_flush_domains parameter and that have syntactically valid domain names.
+
+    Postfix                        Postfix               One logfile
+    delivery -(domain, queue ID)->  flush  -(queue ID)-> per eligible
+     agent                         daemon                   domain
+
+When Postfix receives a request to "deliver mail for a domain now", the flush
+(8) daemon moves all deferred queue files that are listed for that domain to
+the incoming queue, and requests that the queue manager deliver them. In order
+to force delivery, the queue manager temporarily ignores the lists of
+undeliverable destinations: the volatile in-memory list of dead domains, and
+the list of message delivery transports specified with the defer_transports
+configuration parameter.
+
+PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee lliimmiittaattiioonnss
+
+The design of the flush(8) server and of the flush queue introduce a few
+limitations that should not be an issue unless you want to turn on fast ETRN
+service for every possible destination.
+
+  * The flush(8) daemon maintains per-destination logfiles with queue file
+    names. When a request to "deliver mail now" arrives, Postfix will attempt
+    to deliver all recipients in the queue files that have mail for the
+    destination in question. This does not perform well with queue files that
+    have recipients in many different domains, such as queue files with
+    outbound mailing list traffic.
+
+  * The flush(8) daemon maintains per-destination logfiles only for
+    destinations listed with $fast_flush_domains. With other destinations you
+    cannot request delivery with "sendmail -qRdestination" or, with Postfix
+    version 1.1 and later, "postqueue -sdestination".
+
+  * Up to and including early versions of Postfix version 2.1, the "fast flush"
+    service may not deliver some messages if the request to "deliver mail now"
+    is received while a deferred queue scan is already in progress. The reason
+    is that the queue manager does not ignore the volatile in-memory list of
+    dead domains, and the list of message delivery transports specified with
+    the defer_transports configuration parameter.
+
+  * Up to and including Postfix version 2.3, the "fast flush" service may not
+    deliver some messages if the request to "deliver mail now" arrives while an
+    incoming queue scan is already in progress.
+
+CCoonnffiigguurriinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee
+
+The behavior of the flush(8) daemon is controlled by parameters in the main.cf
+configuration file.
+
+By default, Postfix "fast ETRN" service is available only for destinations that
+Postfix is willing to relay mail to:
+
+    /etc/postfix/main.cf:
+        fast_flush_domains = $relay_domains
+        smtpd_etrn_restrictions = permit_mynetworks, reject
+
+Notes:
+
+  * The relay_domains parameter specifies what destinations Postfix will relay
+    to. For destinations that are not eligible for the "fast ETRN" service,
+    Postfix replies with an error message.
+
+  * The smtpd_etrn_restrictions parameter limits what clients may execute the
+    ETRN command. By default, any client has permission.
+
+To enable "fast ETRN" for some other destination, specify:
+
+    /etc/postfix/main.cf:
+        fast_flush_domains = $relay_domains, some.other.domain
+
+To disable "fast ETRN", so that Postfix rejects all ETRN requests and so that
+it maintains no per-destination logfiles, specify:
+
+    /etc/postfix/main.cf:
+        fast_flush_domains =
+
+CCoonnffiigguurriinngg aa ddoommaaiinn ffoorr EETTRRNN sseerrvviiccee oonnllyy
+
+While an "ETRN" customer is off-line, Postfix will make spontaneous attempts to
+deliver mail to it. These attempts are separated in time by increasing time
+intervals, ranging from $minimal_backoff_time to $maximal_backoff_time, and
+should not be a problem unless a lot of mail is queued.
+
+To prevent Postfix from making spontaneous delivery attempts you can configure
+Postfix to always defer mail for the "ETRN" customer. Mail is delivered only
+after the ETRN command or with "sendmail -q", with "sendmail -qRdomain", or
+with "postqueue -sdomain"(Postfix version 1.1 and later only),
+
+In the example below we configure an "etrn-only" delivery transport which is
+simply a duplicate of the "smtp" and "relay" mail delivery transports. The only
+difference is that mail destined for this delivery transport is deferred as
+soon as it arrives.
+
+     1 /etc/postfix/master.cf:
+     2   # =============================================================
+     3   # service type  private unpriv  chroot  wakeup  maxproc command
+     4   #               (yes)   (yes)   (yes)   (never) (100)
+     5   # =============================================================
+     6   smtp      unix  -       -       n       -       -       smtp
+     7   relay     unix  -       -       n       -       -       smtp
+     8   etrn-only unix  -       -       n       -       -       smtp
+     9
+    10 /etc/postfix/main.cf:
+    11   relay_domains = customer.tld ...other domains...
+    12   defer_transports = etrn-only
+    13   transport_maps = hash:/etc/postfix/transport
+    14
+    15 /etc/postfix/transport:
+    16   customer.tld     etrn-only:[mailhost.customer.tld]
+
+Translation:
+
+  * Line 8: The "etrn-only" mail delivery service is a copy of the "smtp" and
+    "relay" service.
+
+  * Line 11: Don't forget to authorize relaying for this customer, either via
+    relay_domains or with the permit_mx_backup feature.
+
+  * Line 12: The "etrn-only" mail delivery service is configured so that
+    spontaneous mail delivery is disabled.
+
+  * Lines 13-16: Mail for the customer is given to the "etrn-only" mail
+    delivery service.
+
+  * Line 16: The "[mailhost.customer.tld]" turns off MX record lookups; you
+    must specify this if your Postfix server is the primary MX host for the
+    customer's domain.
+
+TTeessttiinngg tthhee PPoossttffiixx ffaasstt EETTRRNN sseerrvviiccee
+
+By default, "fast ETRN" service is enabled for all domains that match
+$relay_domains. If you run Postfix with "fast ETRN" service for the very first
+time, you need to run "sendmail -q" once in order to populate the per-site
+deferred mail logfiles. If you omit this step, no harm is done. The logfiles
+will eventually become populated as Postfix routinely attempts to deliver
+delayed mail, but that will take a couple hours. After the "sendmail -q"
+command has completed all delivery attempts (this can take a while), you're
+ready to test the "fast ETRN" service.
+
+To test the "fast ETRN" service, telnet to the Postfix SMTP server from a
+client that is allowed to execute ETRN commands (by default, that's every
+client), and type the commands shown in boldface:
+
+    220 my.server.tld ESMTP Postfix
+    HHEELLOO mmyy..cclliieenntt..ttlldd
+    250 Ok
+    EETTRRNN ssoommee..ccuussttoommeerr..ddoommaaiinn
+    250 Queuing started
+
+where "some.customer.domain" is the name of a domain that has a non-empty
+logfile somewhere under $queue_directory/flush.
+
+In the maillog file, you should immediately see a couple of logfile records, as
+evidence that the queue manager has opened queue files:
+
+    Oct  2 10:51:19 myhostname postfix/qmgr[51999]: 682E8440A4:
+        from=<whatever>, size=12345, nrcpt=1 (queue active)
+    Oct  2 10:51:19 myhostname postfix/qmgr[51999]: 02249440B7:
+        from=<whatever>, size=4711, nrcpt=1 (queue active)
+
+What happens next depends on whether the destination is reachable. If it's not
+reachable, the mail queue IDs will be added back to the some.customer.domain
+logfile under $queue_directory/flush.
+
+Repeat the exercise with some other destination that your server is willing to
+relay to (any domain listed in $relay_domains), but that has no mail queued.
+The text in bold face stands for the commands that you type:
+
+    220 my.server.tld ESMTP Postfix
+    HHEELLOO mmyy..cclliieenntt..ttlldd
+    250 Ok
+    EETTRRNN ssoommee..ootthheerr..ccuussttoommeerr..ddoommaaiinn
+    250 Queuing started
+
+This time, the "ETRN"" command should trigger NO mail deliveries at all. If
+this triggers delivery of all mail, then you used the wrong domain name, or
+"fast ETRN" service is turned off.
+
+Finally, repeat the exercise with a destination that your mail server is not
+willing to relay to. It does not matter if your server has mail queued for that
+destination.
+
+    220 my.server.tld ESMTP Postfix
+    HHEELLOO mmyy..cclliieenntt..ttlldd
+    250 Ok
+    EETTRRNN nnoott..aa..ccuussttoommeerr..ddoommaaiinn
+    459 <not.a.customer.domain>: service unavailable
+
+In this case, Postfix should reject the request as shown above.
+