Adding upstream version 3.5.24.upstream/3.5.24

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

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+PPoossttffiixx CCoonnnneeccttiioonn CCaacchhee
+
+-------------------------------------------------------------------------------
+
+IInnttrroodduuccttiioonn
+
+This document describes the Postfix connection cache implementation, which is
+available with Postfix version 2.2 and later.
+
+See Client-side TLS connection reuse for how this connection cache is used to
+implement multiple deliveries per TLS-encrypted connection.
+
+Topics covered in this document:
+
+  * What SMTP connection caching can do for you
+  * Connection cache implementation
+  * Connection cache configuration
+  * Connection cache safety mechanisms
+  * Connection cache limitations
+  * Connection cache statistics
+
+WWhhaatt SSMMTTPP ccoonnnneeccttiioonn ccaacchhiinngg ccaann ddoo ffoorr yyoouu
+
+With SMTP connection caching, Postfix can deliver multiple messages over the
+same SMTP connection. By default, Postfix 2.2 reuses an SMTP connection
+automatically when a destination has high volume of mail in the active queue.
+
+SMTP Connection caching is a performance feature. Whether or not it actually
+improves performance depends on the conditions:
+
+  * SMTP Connection caching can greatly improve performance when delivering
+    mail to a destination with multiple mail servers, because it can help
+    Postfix to skip over a non-responding server.
+
+  * Otherwise, the benefits of SMTP connection caching are minor: it eliminates
+    the latency of the TCP handshake (SYN, SYN+ACK, ACK), plus the latency of
+    the SMTP initial handshake (220 greeting, EHLO command, EHLO response).
+
+  * SMTP Connection caching gives no gains with respect to SMTP session tear-
+    down. The Postfix smtp(8) client normally does not wait for the server's
+    reply to the QUIT command, and it never waits for the TCP final handshake
+    to complete.
+
+  * SMTP Connection caching introduces some overhead: the client needs to send
+    an RSET command to find out if a connection is still usable, before it can
+    send the next MAIL FROM command. This introduces one additional round-trip
+    delay.
+
+For other potential issues with SMTP connection caching, see the discussion of
+limitations at the end of this document.
+
+CCoonnnneeccttiioonn ccaacchhee iimmpplleemmeennttaattiioonn
+
+For an overview of how Postfix delivers mail, see the Postfix architecture
+OVERVIEW document.
+
+The Postfix connection cache is shared among Postfix mail delivering processes.
+This maximizes the opportunity to reuse an open connection. Other MTAs such as
+Sendmail or exim have a non-shared connection cache. Here, a connection can be
+reused only by the mail delivering process that creates the connection. To get
+the same performance improvement as with a shared connection cache, non-shared
+connections need to be kept open for a longer time.
+
+The scache(8) server, introduced with Postfix version 2.2, maintains the shared
+connection cache. With Postfix version 2.2, only the smtp(8) client has support
+to access this cache.
+
+            /-- smtp(8) --> Internet
+
+    qmgr(8)
+                 |
+            \--  | smtp(8) --> Internet
+                 |
+                 ^
+                 |
+
+                 scache(8)
+
+When SMTP connection caching is enabled (see next section), the smtp(8) client
+does not disconnect after a mail transaction, but gives the connection to the
+scache(8) server which keeps the connection open for a limited amount of time.
+
+After handing over the open connection to the scache(8) server, the smtp(8)
+client continues with some other mail delivery request. Meanwhile, any smtp(8)
+client process can ask the scache(8) server for that cached connection and
+reuse it for mail delivery.
+
+The connection cache can be searched by destination domain name (the right-hand
+side of the recipient address) and by the IP address of the host at the other
+end of the connection. This allows Postfix to reuse a connection even when the
+remote host is mail server for domains with different names.
+
+CCoonnnneeccttiioonn ccaacchhee ccoonnffiigguurraattiioonn
+
+The Postfix smtp(8) client supports two connection caching strategies:
+
+  * On-demand connection caching. This is enabled by default, and is controlled
+    with the smtp_connection_cache_on_demand configuration parameter. When this
+    feature is enabled, the Postfix smtp(8) client automatically saves a
+    connection to the connection cache when a destination has a high volume of
+    mail in the active queue.
+
+    Example:
+
+        /etc/postfix/main.cf:
+            smtp_connection_cache_on_demand = yes
+
+  * Per-destination connection caching. This is enabled by explicitly listing
+    specific destinations with the smtp_connection_cache_destinations
+    configuration parameter. After completing delivery to a selected
+    destination, the Postfix smtp(8) client always saves the connection to the
+    connection cache.
+
+    Specify a comma or white space separated list of destinations or pseudo-
+    destinations:
+
+      o if mail is sent without a relay host: a domain name (the right-hand
+        side of an email address, without the [] around a numeric IP address),
+
+      o if mail is sent via a relay host: a relay host name (without the [] or
+        non-default TCP port), as specified in main.cf or in the transport map,
+
+      o a /file/name with domain names and/or relay host names as defined
+        above,
+
+      o a "type:table" with domain names and/or relay host names on the left-
+        hand side. The right-hand side result from "type:table" lookups is
+        ignored.
+
+    Examples:
+
+        /etc/postfix/main.cf:
+            smtp_connection_cache_destinations = $relayhost
+            smtp_connection_cache_destinations = hotmail.com, ...
+            smtp_connection_cache_destinations = static:all (not recommended)
+
+CCoonnnneeccttiioonn ccaacchhee ssaaffeettyy mmeecchhaanniissmmss
+
+Connection caching must be used wisely. It is anti-social to keep an unused
+SMTP connection open for a significant amount of time, and it is unwise to send
+huge numbers of messages through the same connection. In order to avoid
+problems with SMTP connection caching, Postfix implements the following safety
+mechanisms:
+
+  * The Postfix scache(8) server keeps a connection open for only a limited
+    time. The time limit is specified with the smtp_connection_cache_time_limit
+    and with the connection_cache_ttl_limit configuration parameters. This
+    prevents anti-social behavior.
+
+  * The Postfix smtp(8) client reuses a session for only a limited number of
+    times. This avoids triggering bugs in implementations that do not correctly
+    handle multiple deliveries per session.
+
+    As of Postfix 2.3 connection reuse is preferably limited with the
+    smtp_connection_reuse_time_limit parameter. In addition, Postfix 2.11
+    provides smtp_connection_reuse_count_limit to limit how many times a
+    connection may be reused, but this feature is unsafe as it introduces a
+    "fatal attractor" failure mode (when a destination has multiple inbound
+    MTAs, the slowest inbound MTA will attract most connections from Postfix to
+    that destination).
+
+    Postfix 2.3 logs the use count of multiply-used connections, as shown in
+    the following example:
+
+        Nov  3 16:04:31 myname postfix/smtp[30840]: 19B6B2900FE:
+        to=<wietse@test.example.com>, orig_to=<wietse@test>,
+        relay=mail.example.com[1.2.3.4], ccoonnnn__uussee==22, delay=0.22,
+        delays=0.04/0.01/0.05/0.1, dsn=2.0.0, status=sent (250 2.0.0 Ok)
+
+  * The connection cache explicitly labels each cached connection with
+    destination domain and IP address information. A connection cache lookup
+    succeeds only when the correct information is specified. This prevents mis-
+    delivery of mail.
+
+CCoonnnneeccttiioonn ccaacchhee lliimmiittaattiioonnss
+
+Postfix SMTP connection caching conflicts with certain applications:
+
+  * With Postfix versions < 3.4, the Postfix shared connection cache cannot be
+    used with TLS, because an open TLS connection can be reused only in the
+    process that creates it. For this reason, the Postfix smtp(8) client
+    historically always closed the connection after completing an attempt to
+    deliver mail over TLS.
+
+  * Postfix connection caching currently does not support multiple SASL
+    accounts per mail server. Specifically, Postfix connection caching assumes
+    that a SASL credential is valid for all hostnames or domain names that
+    deliver via the same mail server IP address and TCP port, and assumes that
+    the SASL credential does not depend on the message originator.
+
+CCoonnnneeccttiioonn ccaacchhee ssttaattiissttiiccss
+
+The scache(8) connection cache server logs statistics about the peak cache size
+and the cache hit rates. This information is logged every
+connection_cache_status_update_time seconds, when the process terminates after
+the maximal idle time is exceeded, or when Postfix is reloaded.
+
+  * Hit rates for connection cache lookups by domain will tell you how useful
+    connection caching is.
+
+  * Connection cache lookups by network address will always fail, unless you're
+    sending mail to different domains that share the same MX hosts.
+
+  * No statistics are logged when no attempts are made to access the connection
+    cache.
+