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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

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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.

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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.

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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)

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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.

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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.

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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.

