From b5896ba9f6047e7031e2bdee0622d543e11a6734 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Mon, 6 May 2024 03:46:30 +0200 Subject: Adding upstream version 3.4.23. Signed-off-by: Daniel Baumann --- proto/CONNECTION_CACHE_README.html | 308 +++++++++++++++++++++++++++++++++++++ 1 file changed, 308 insertions(+) create mode 100644 proto/CONNECTION_CACHE_README.html (limited to 'proto/CONNECTION_CACHE_README.html') diff --git a/proto/CONNECTION_CACHE_README.html b/proto/CONNECTION_CACHE_README.html new file mode 100644 index 0000000..7ffd7bb --- /dev/null +++ b/proto/CONNECTION_CACHE_README.html @@ -0,0 +1,308 @@ + + + + + + +Postfix Connection Cache + + + + + + + +

Postfix Connection Cache

+ +

Introduction

+ +

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 + +

+ +

What SMTP connection caching can do for +you

+ +

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.

+ +

Connection cache implementation

+ +

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) | | | | v
\-- 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.

+ +

Connection cache configuration

+ +

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:
+ +
- 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),
  + +
- 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,
  + +
- a /file/name with domain names and/or relay host names as +defined above,
  + +
- 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)
+
```
+ +
+ +

+ +

Connection cache safety mechanisms

+ +

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], conn_use=2, 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.
+ +

+ +

Connection cache limitations

+ +

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.
+ +

+ + +

Connection cache statistics

+ +

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.
+ +

+ + + + + -- cgit v1.2.3

	`/--`	smtp(8)	`+-->` Internet
qmgr(8)			`\| \| \| \| v`
	`\--`		smtp(8)	`+-->` Internet
			`^ \|`
			scache(8)