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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-10 19:59:03 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-10 19:59:03 +0000 |
| commit | a848231ae0f346dc7cc000973fbeb65b0894ee92 (patch) | |
| tree | 44b60b367c86723cc78383ef247885d72b388afe /html/OVERVIEW.html | |
| parent | Initial commit. (diff) | |
| download | postfix-a848231ae0f346dc7cc000973fbeb65b0894ee92.tar.xz postfix-a848231ae0f346dc7cc000973fbeb65b0894ee92.zip | |
Adding upstream version 3.8.5.upstream/3.8.5
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
| -rw-r--r-- | html/OVERVIEW.html | 937 |
1 files changed, 937 insertions, 0 deletions
diff --git a/html/OVERVIEW.html b/html/OVERVIEW.html new file mode 100644 index 0000000..767e2f0 --- /dev/null +++ b/html/OVERVIEW.html @@ -0,0 +1,937 @@ +<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> + +<html> + +<head> + +<title>Postfix Architecture Overview </title> + +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link rel='stylesheet' type='text/css' href='postfix-doc.css'> + +</head> + +<body> + +<h1> <img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix +Architecture Overview </h1> + +<hr> + +<h2> Introduction </h2> + +<p> This document presents an overview of the Postfix architecture, +and provides pointers to descriptions of every Postfix command +or server program. The text gives the general context in which +each command or server program is used, and provides pointers to +documents with specific usage examples and background information. +</p> + +<p> Topics covered by this document: </p> + +<ul> + +<li> <a href="#receiving"> How Postfix receives mail </a> + +<li> <a href="#delivering"> How Postfix delivers mail </a> + +<li> <a href="#behind"> Postfix behind the scenes </a> + +<li> <a href="#commands"> Postfix support commands </a> + +</ul> + +<h2><a name="receiving"> How Postfix receives mail </a> </h2> + +<p> When a message enters the Postfix mail system, the first stop +on the inside is the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>. The figure below shows the +main processes that are involved with new mail. Names followed by +a number are Postfix commands or server programs, while unnumbered +names inside shaded areas represent Postfix queues. </p> + +<blockquote> + +<table> + +<tr> + +<td colspan="4"> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a> </td> + +</tr> + +<tr> + +<td> Network </td> <td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="smtpd.8.html">smtpd(8)</a> +</td> + +<td> </td> + +<td rowspan="2" align="center"> <table> <tr> <td align="center"> +^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> +</td> </tr> </table> </td> + +</tr> + +<tr> + +<td colspan="3"> </td> <td> <tt> \ </tt> </td> + +</tr> + +<tr> + +<td> Network </td> <td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="qmqpd.8.html">qmqpd(8)</a> +</td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="cleanup.8.html">cleanup(8)</a> +</td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a +href="QSHAPE_README.html#incoming_queue"> incoming </a> </td> + +</tr> + +<tr> + +<td colspan="3"> </td> <td> <tt> / </tt> </td> + +</tr> + +<tr> + +<td colspan="2"> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="pickup.8.html">pickup(8)</a> +</td> + +<td> <tt> <- </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a +href="QSHAPE_README.html#maildrop_queue"> maildrop </a> </td> + +</tr> + +<tr> + +<td colspan="4" align="center"> </td> + +<td align="center"> ^<br> <tt> | </tt> </td> + +</tr> + +<tr> + +<td> Local </td> <td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="sendmail.1.html">sendmail(1)</a> +</td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> <a href="postdrop.1.html">postdrop(1)</a> +</td> + +</tr> + +</table> + +</blockquote> + +<ul> + +<li> <p> Network mail enters Postfix via the <a href="smtpd.8.html">smtpd(8)</a> or <a href="qmqpd.8.html">qmqpd(8)</a> +servers. These servers remove the SMTP or QMQP protocol encapsulation, +enforce some sanity checks to protect Postfix, and give the sender, +recipients and message content to the <a href="cleanup.8.html">cleanup(8)</a> server. The +<a href="smtpd.8.html">smtpd(8)</a> server can be configured to block unwanted mail, as +described in the <a href="SMTPD_ACCESS_README.html">SMTPD_ACCESS_README</a> document. </p> + +<li> <p> Local submissions are received with the Postfix <a href="sendmail.1.html">sendmail(1)</a> +compatibility command, and are queued in the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> by +the privileged <a href="postdrop.1.html">postdrop(1)</a> command. This arrangement even works +while the Postfix mail system is not running. The local <a href="pickup.8.html">pickup(8)</a> +server picks up local submissions, enforces some sanity checks to +protect Postfix, and gives the sender, recipients and message +content to the <a href="cleanup.8.html">cleanup(8)</a> server. </p> + +<li> <p> Mail from internal sources is given directly to the +<a href="cleanup.8.html">cleanup(8)</a> server. These sources are not shown in the figure, and +include: mail that is forwarded by the <a href="local.8.html">local(8)</a> delivery agent (see +next section), messages that are returned to the sender by the +<a href="bounce.8.html">bounce(8)</a> server (see second-next section), and postmaster +notifications about problems with Postfix. </p> + +<li> <p> The <a href="cleanup.8.html">cleanup(8)</a> server implements the final processing +stage before mail is queued. It adds missing From: and other message +headers, and transforms addresses as described in the +<a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a> +document. Optionally, the <a href="cleanup.8.html">cleanup(8)</a> server can be configured to +do light-weight content inspection with regular expressions as +described in the <a href="BUILTIN_FILTER_README.html">BUILTIN_FILTER_README</a> document. The <a href="cleanup.8.html">cleanup(8)</a> +server places the result as a single file into the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a>, +and notifies the queue manager (see next section) of the arrival +of new mail. </p> + +<li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server rewrites addresses to the +standard "user@fully.qualified.domain" form, as described in the +<a href="ADDRESS_REWRITING_README.html">ADDRESS_REWRITING_README</a> document. Postfix currently does not +implement a rewriting language, but a lot can be done via table +lookups and, if need be, regular expressions. </p> + +</ul> + +<h2> <a name="delivering"> How Postfix delivers mail </a> </h2> + +<p> Once a message has reached the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> the next step is +to deliver it. The figure shows the main components of the Postfix +mail delivery apparatus. Names followed by a number are Postfix +commands or server programs, while unnumbered names inside shaded +areas represent Postfix queues. </p> + +<blockquote> + +<table> + +<tr> + +<td rowspan="2" colspan="4"> </td> + +<td rowspan="2" bgcolor="#f0f0ff" align="center"> <a href="trivial-rewrite.8.html">trivial-<br>rewrite(8)</a> +</td> + +<td> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="smtp.8.html">smtp(8)</a> </td> + +<td> <tt> -> </tt> </td> <td> Network </td> + +</tr> + +<tr> + +<td align="right"> <tt> / </tt> </td> + +</tr> + +<tr> + +<td rowspan="2" colspan="4"> </td> + +<td rowspan="2" align="center"> <table> <tr> <td align="center"> +^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> +</td> </tr> </table> </td> + +<td align="right"> <tt> - </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="lmtp.8.html">lmtp(8)</a> </td> + +<td> <tt> -> </tt> </td> <td> Network </td> + +</tr> + +<tr> + +<td align="left"> <tt> / </tt> </td> + +</tr> + +<tr> + +<td bgcolor="#f0f0ff" align="center"> <a +href="QSHAPE_README.html#incoming_queue"> incoming </a> </td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> <a +href="QSHAPE_README.html#active_queue"> active </a> </td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="qmgr.8.html">qmgr(8)</a> </td> + +<td align="right"> <tt> --- </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="local.8.html">local(8)</a> </td> + +<td> <tt> -> </tt> </td> <td> File, command </td> + +</tr> + +<tr> + +<td rowspan="2" colspan="2"> </td> + +<td rowspan="2" align="center"> <table> <tr> <td align="center"> +^<br> <tt> | </tt> </td> <td align="center"> <tt> |<br> v </tt> +</td> </tr> </table> </td> + +<td rowspan="2" colspan="2"> </td> + +<td align="left"> <tt> \ </tt> </td> + +</tr> + +<tr> + +<td align="right"> <tt> - </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> <a href="virtual.8.html">virtual(8)</a> </td> + +<td> <tt> -> </tt> </td> <td> File </td> + +</tr> + +<tr> + +<td colspan="2"> </td> + +<td bgcolor="#f0f0ff" align="center"> <a +href="QSHAPE_README.html#deferred_queue"> deferred </a> </td> + +<td colspan="2"> </td> + +<td align="right"> <tt> \ </tt> </td> + +</tr> + +<tr> + +<td colspan="6"> + +<td bgcolor="#f0f0ff" align="center"> <a href="pipe.8.html">pipe(8)</a> </td> + +<td> <tt> -> </tt> </td> <td> Command </td> + +</tr> + +</table> + +</blockquote> + +<ul> + +<li> <p> The queue manager (the <a href="qmgr.8.html">qmgr(8)</a> server process in the +figure) is the heart of Postfix mail delivery. It contacts the +<a href="smtp.8.html">smtp(8)</a>, <a href="lmtp.8.html">lmtp(8)</a>, <a href="local.8.html">local(8)</a>, <a href="virtual.8.html">virtual(8)</a>, <a href="pipe.8.html">pipe(8)</a>, <a href="discard.8.html">discard(8)</a> or +<a href="error.8.html">error(8)</a> delivery agents, and sends a delivery request for one +or more recipient addresses. The <a href="discard.8.html">discard(8)</a> and <a href="error.8.html">error(8)</a> delivery +agents are special: they discard or bounce all mail, and are not +shown in the figure above. </p> + +<p> The queue manager maintains a small <a href="QSHAPE_README.html#active_queue">active queue</a> with the +messages that it has opened for delivery. The <a href="QSHAPE_README.html#active_queue">active queue</a> acts as +a limited window on potentially large <a href="QSHAPE_README.html#incoming_queue">incoming</a> or <a href="QSHAPE_README.html#deferred_queue">deferred queues</a>. +The limited <a href="QSHAPE_README.html#active_queue">active queue</a> prevents the queue manager from running +out of memory under heavy load. </p> + +<p> The queue manager maintains a separate <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> for mail +that cannot be delivered, so that a large mail backlog will not +slow down normal queue accesses. The queue manager's strategy for +delayed mail delivery attempts is described in the <a href="QSHAPE_README.html">QSHAPE_README</a> +and <a href="TUNING_README.html">TUNING_README</a> documents. </p> + +<li> <p> The <a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server resolves each recipient +address according to its local or remote address class, as defined +in the <a href="ADDRESS_CLASS_README.html">ADDRESS_CLASS_README</a> document. Additional routing information +can be specified with the optional <a href="transport.5.html">transport(5)</a> table. The +<a href="trivial-rewrite.8.html">trivial-rewrite(8)</a> server optionally queries the <a href="relocated.5.html">relocated(5)</a> table +for recipients whose address has changed; mail for such recipients is +returned to the sender with an explanation. </p> + +<li> <p> The <a href="smtp.8.html">smtp(8)</a> client looks up a list of mail exchangers for +the destination host, sorts the list by preference, and tries each +server in turn until it finds a server that responds. It then +encapsulates the sender, recipient and message content as required +by the SMTP protocol; this includes conversion of 8-bit MIME to +7-bit encoding. </p> + +<li> <p> The <a href="lmtp.8.html">lmtp(8)</a> client speaks a protocol similar to SMTP that +is optimized for delivery to mailbox servers such as Cyrus. The +advantage of this setup is that one Postfix machine can feed multiple +mailbox servers over LMTP. The opposite is true as well: one +mailbox server can be fed over LMTP by multiple Postfix machines. +</p> + +<li> <p> The <a href="local.8.html">local(8)</a> delivery agent understands UNIX-style mailboxes, +qmail-compatible maildir files, Sendmail-style system-wide <a href="aliases.5.html">aliases(5)</a> +databases, and Sendmail-style per-user .forward files. Multiple +local delivery agents can be run in parallel, but parallel delivery +to the same user is usually limited. </p> + +<p> The <a href="local.8.html">local(8)</a> delivery agent has hooks for alternative forms of +local delivery: you can configure it to deliver to mailbox files +in user home directories, you can configure it to delegate mailbox +delivery to an external command such as procmail, or you can delegate +delivery to a different Postfix delivery agent. </p> + +<li> <p> The <a href="virtual.8.html">virtual(8)</a> delivery agent is a bare-bones delivery +agent that delivers to UNIX-style mailbox or qmail-style maildir +files only. This delivery agent can deliver mail for multiple +domains, which makes it especially suitable for hosting lots of +small domains on a single machine. This is described in the +<a href="VIRTUAL_README.html">VIRTUAL_README</a> document. </p> + +<li> <p> The <a href="pipe.8.html">pipe(8)</a> mailer is the outbound interface to other mail +processing systems (the Postfix <a href="sendmail.1.html">sendmail(1)</a> command being the +inbound interface). The interface is UNIX compatible: it provides +information on the command line and on the standard input stream, +and expects a process exit status code as defined in <sysexits.h>. +Examples of delivery via the <a href="pipe.8.html">pipe(8)</a> mailer are in the <a href="MAILDROP_README.html">MAILDROP_README</a> +and <a href="UUCP_README.html">UUCP_README</a> documents. + +</ul> + +<h2> <a name="behind"> Postfix behind the scenes </a> </h2> + +<p> The previous sections gave an overview of how Postfix server +processes send and receive mail. These server processes rely on +other server processes that do things behind the scenes. The text +below attempts to visualize each service in its own context. As +before, names followed by a number are Postfix commands or server +programs, while unnumbered names inside shaded areas represent +Postfix queues. </p> + +<ul> + +<li> <p> The resident <a href="master.8.html">master(8)</a> server is the supervisor that keeps +an eye on the well-being of the Postfix mail system. It is typically +started at system boot time with the "postfix start" command, and +keeps running until the system goes down. The <a href="master.8.html">master(8)</a> server is +responsible for starting Postfix server processes to receive and +deliver mail, and for restarting servers that terminate prematurely +because of some problem. The <a href="master.8.html">master(8)</a> server is also responsible +for enforcing the server process count limits as specified in the +<a href="master.5.html"><b>master.cf</b></a> configuration file. The picture below gives the +program hierarchy when Postfix is started up. Only some of the mail +handling daemon processes are shown. </p> + +<table> + +<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> +<a href="postfix.1.html">postfix(1)</a> </td> </tr> + +<tr> <td colspan="2"> </td> <td align="center"> |<br> |</td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> +postfix-script(1) </td> </tr> + +<tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr> +<tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center"> +|<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr> +<td> </td> <td> \ </td> </tr> </table> </td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="postsuper.1.html">postsuper(1)</a> </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> <a href="master.8.html">master(8)</a> </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> <a href="postlog.1.html">postlog(1)</a> </td> </tr> + +<tr> <td> </td> <td> <table> <tr> <td> </td> <td> / </td> </tr> +<tr> <td> / </td> <td> </td> </tr> </table> </td> <td align="center"> +|<br> |</td> <td> <table> <tr> <td> \ </td> <td> </td> </tr> <tr> +<td> </td> <td> \ </td> </tr> </table> </td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> <a href="local.8.html">local(8)</a> </td> </tr> + +</table> + +<li> <p> The <a href="anvil.8.html">anvil(8)</a> server implements client connection and +request rate +limiting for all <a href="smtpd.8.html">smtpd(8)</a> servers. The <a href="TUNING_README.html">TUNING_README</a> document +provides guidance for dealing with mis-behaving SMTP clients. The +<a href="anvil.8.html">anvil(8)</a> service is available in Postfix version 2.2 and later. +</p> + +<table> + +<tr> <td> Network </td> <td> <tt> -> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a><br><br> </td> <td> <tt> <-> +</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="anvil.8.html">anvil(8)</a><br><br> +</td> </tr> + +</table> + +<li> <p> The <a href="bounce.8.html">bounce(8)</a>, <a href="defer.8.html">defer(8)</a> and <a href="trace.8.html">trace(8)</a> services each maintain +their own queue directory trees with per-message logfiles. Postfix +uses this information when sending "failed", "delayed" or "success" +delivery status notifications to the sender. </p> + +<p> The <a href="trace.8.html">trace(8)</a> service also implements support for the Postfix +"sendmail +-bv" and "sendmail -v" commands which produce reports about how +Postfix delivers mail, and is available with Postfix version 2.1 +and later. See <a href="DEBUG_README.html#trace_mail"> DEBUG_README +</a> for examples. </p> + +<table> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="cleanup.8.html">cleanup(8)</a> </td> <td +valign="middle"> <tt> -> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a><br> Postfix<br> queue </td> <td +valign="middle"> <tt> -> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> Delivery<br> agents</td> </tr> + +<tr> <td align="center"> ^<br> <tt> | </tt> </td> <td> </td> <td +align="center"> <tt> |<br> v </tt> </td> <td> </td> <td align="center"> +<tt> |<br> v </tt> </td> </tr> + +<tr> <td align="center"> (Non-)<br> delivery<br> notice </td> <td +valign="middle"> <tt> <- </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <a href="bounce.8.html">bounce(8)</a><br> <a href="defer.8.html">defer(8)</a><br> <a href="trace.8.html">trace(8)</a> </td> <td +valign="middle"> <tt> <- </tt> </td> <td align="center"> Queue +id,<br> recipient,<br> status</td> </tr> + +<tr> <td colspan="2"> </td> <td align="center"> <table> <tr> <td +align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> +|<br> v </tt> </td> </tr> </table> </td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> +Per- <br> message<br> logfiles </td> </tr> + +</table> + +<li> <p> The <a href="flush.8.html">flush(8)</a> servers maintain per-destination logs and +implement both ETRN and "sendmail -qRdestination", as described +in the <a href="ETRN_README.html">ETRN_README</a> document. This moves selected queue files from +the <a href="QSHAPE_README.html#deferred_queue">deferred queue</a> back to the <a href="QSHAPE_README.html#incoming_queue">incoming queue</a> and requests their +delivery. The <a href="flush.8.html">flush(8)</a> service is available with Postfix version +1.0 and later. </p> + +<table> + +<tr> <td colspan="4"> </td> <td align="center" bgcolor="#f0f0ff"> +<a href="QSHAPE_README.html#incoming_queue"> incoming </a><br>^ +<br><a href="QSHAPE_README.html#deferred_queue"> deferred </a> +</td> </tr> + +<tr> <td colspan="4"> </td> <td align="center"> ^<br> |</td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="smtpd.8.html">smtpd(8)</a><br> <a href="sendmail.1.html">sendmail(1)</a><br> +<a href="postqueue.1.html">postqueue(1)</a> </td> <td> <tt> - </tt> </td> <td align="center"> +Destination<br> to flush</td> <td> <tt> -> </tt> </td> <td +align="center" bgcolor="#f0f0ff"> <a href="flush.8.html">flush(8)</a> </td> <td> <tt> <- +</tt> </td> <td align="center"> Deferred<br> destination,<br> queue +id </td> <td> <tt> - </tt> </td> <td align="center" bgcolor="#f0f0ff"> +Delivery<br> agents,<br> <a href="qmgr.8.html">qmgr(8)</a> </td> </tr> + +<tr> <td colspan="4"> </td> <td align="center"> <table> <tr> <td +align="center"> ^<br> <tt> | </tt> </td> <td align="center"> <tt> +|<br> v </tt> </td> </tr> </table> </td> </tr> + +<tr> <td colspan="4"> </td> <td align="center"> Per-dest-<br> +ination<br> logs </td> </tr> + +</table> + +<li> <p> The <a href="proxymap.8.html">proxymap(8)</a> servers provide read-only and read-write +table lookup +service to Postfix processes. This overcomes chroot restrictions, +reduces the number of open lookup tables by sharing one open +table among multiple processes, and implements single-updater +tables. </p> + +<li> <p> The <a href="scache.8.html">scache(8)</a> server maintains the connection cache for +the Postfix <a href="smtp.8.html">smtp(8)</a> client. When connection caching is enabled for +selected destinations, the <a href="smtp.8.html">smtp(8)</a> client does not disconnect +immediately after a mail transaction, but gives the connection to +the connection cache server which keeps the connection open for a +limited amount of time. The <a href="smtp.8.html">smtp(8)</a> client continues with some +other mail delivery request. Meanwhile, any <a href="smtp.8.html">smtp(8)</a> process can +ask the <a href="scache.8.html">scache(8)</a> server for that cached connection and reuse it +for mail delivery. As a safety measure, Postfix limits the number +of times that a connection may be reused. </p> + +<p> When delivering mail to a destination with multiple mail servers, +connection caching can help to skip over a non-responding server, +and thus dramatically speed up delivery. SMTP connection caching +is available in Postfix version 2.2 and later. More information +about this feature is in the <a href="CONNECTION_CACHE_README.html">CONNECTION_CACHE_README</a> document. </p> + +<table> + +<tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center" +colspan="3" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td colspan="2"> <tt> +--> </tt> </td> <td> Internet </td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> </td> +<td align="center" rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> +</tr> + +<tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center" +colspan="4" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td align="left"> + </td> </tr> + +<tr> <td colspan="2"> </td> <td> </td> <td +align="center"><tt>^<br>|</tt></td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" colspan="3" +bgcolor="#f0f0ff"> <a href="scache.8.html">scache(8)</a> </td> </tr> + +</table> + +<p> A Postfix <a href="smtp.8.html">smtp(8)</a> client can reuse a TLS-encrypted connection +(with "<a href="postconf.5.html#smtp_tls_connection_reuse">smtp_tls_connection_reuse</a> = yes"). This can greatly reduce +the overhead of connection setup and improves message delivery +rates. After a Postfix <a href="smtp.8.html">smtp(8)</a> client connects to a remote SMTP +server and sends plaintext EHLO and STARTTLS commands, the <a href="smtp.8.html">smtp(8)</a> +client inserts a <a href="tlsproxy.8.html">tlsproxy(8)</a> process into the connection as shown +below. </p> + +<p> After the mail transaction completes, the Postfix <a href="smtp.8.html">smtp(8)</a> client +gives the <a href="smtp.8.html">smtp(8)</a>-to-<a href="tlsproxy.8.html">tlsproxy(8)</a> connection to the <a href="scache.8.html">scache(8)</a> +server, which keeps the connection open for a limited amount of +time. The <a href="smtp.8.html">smtp(8)</a> client continues with some other mail delivery +request. Meanwhile, any Postfix <a href="smtp.8.html">smtp(8)</a> client can ask the <a href="scache.8.html">scache(8)</a> +server for that cached connection and reuse it for mail delivery. +</p> + +<table> + +<tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center" +colspan="3" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td colspan="2"> <tt> +--> </tt> </td> <td align="center"bgcolor="#f0f0ff"> <a href="tlsproxy.8.html">tlsproxy(8)</a> +</td> <td> <tt> --> </tt> </td> <td> Internet </td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> <a href="qmgr.8.html">qmgr(8)</a> </td> <td> </td> +<td align="center" rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> +</tr> + +<tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center" +colspan="4" bgcolor="#f0f0ff"> <a href="smtp.8.html">smtp(8)</a> </td> <td align="left"> + </td> </tr> + +<tr> <td colspan="2"> </td> <td> </td> <td +align="center"><tt>^<br>|</tt></td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" colspan="3" +bgcolor="#f0f0ff"> <a href="scache.8.html">scache(8)</a> </td> </tr> + +</table> + +<li> <p> The <a href="showq.8.html">showq(8)</a> servers list the Postfix queue status. This +is the queue listing service that does the work for the <a href="mailq.1.html">mailq(1)</a> +and <a href="postqueue.1.html">postqueue(1)</a> commands. </p> + +<table> + +<tr> <td> Output </td> <td> <tt> <- </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <a href="mailq.1.html">mailq(1)</a><br> + +<a href="postqueue.1.html"> post-<br>queue(1) </a> <br> </td> <td> +<tt> <- </tt> </td> <td align="center" valign="middle" +bgcolor="#f0f0ff"> <a href="showq.8.html">showq(8)</a> </td> <td> <tt> <- </tt></td> <td +align="center" valign="middle" bgcolor="#f0f0ff"> Postfix<br> queue +</td> </tr> + +</table> + +<li> <p> The <a href="spawn.8.html">spawn(8)</a> servers run non-Postfix commands on request, +with the client connected via socket or FIFO to the command's +standard input, output and error streams. You can find examples of +its use in the <a href="SMTPD_POLICY_README.html">SMTPD_POLICY_README</a> document. </p> + +<li> <p> The <a href="tlsmgr.8.html">tlsmgr(8)</a> server runs when TLS (Transport Layer +Security, formerly known as SSL) is turned on in the Postfix <a href="smtp.8.html">smtp(8)</a> +client or <a href="smtpd.8.html">smtpd(8)</a> server. This process has two duties: </p> + +<ul> + +<li> <p> Maintain the pseudo-random number generator (PRNG) that +is used to seed the TLS engines in Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a> +server processes. The state of this PRNG is periodically saved to +a file, and is read when <a href="tlsmgr.8.html">tlsmgr(8)</a> starts up. </p> + +<li> <p> Maintain the optional Postfix <a href="smtp.8.html">smtp(8)</a> client or <a href="smtpd.8.html">smtpd(8)</a> +server caches with TLS session keys. Saved keys can improve +performance by reducing the amount of computation at the start of +a TLS session. </p> + +</ul> + +<p> TLS support is available in Postfix version 2.2 and later. +Information about the Postfix TLS implementation is in the <a href="TLS_README.html">TLS_README</a> +document. </p> + +<table> + +<tr> <td>Network<tt>-> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <br> <a href="smtpd.8.html">smtpd(8)</a> <br> </td> <td colspan="2"> +<tt> <---seed---<br><br><-session-> </tt> </td> <td +align="center" bgcolor="#f0f0ff"> <br> <a href="tlsmgr.8.html">tlsmgr(8)</a> <br> </td> +<td colspan="3"> <tt> ---seed---><br> <br><-session-> +</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> <a href="smtp.8.html">smtp(8)</a> <br> + </td> <td> <tt> -></tt>Network </td> </tr> + +<tr> <td colspan="3"> </td> <td align="right"> <table> <tr> <td> +</td> <td> / </td> </tr> <tr> <td> / </td> <td> </td> </tr> </table> +</td> <td align="center"> |<br> |</td> <td align="left"> <table> +<tr> <td> \ </td> <td> </td> </tr> <tr> <td> </td> <td> \ </td> +</tr> </table> </td> <td colspan="3"> </td> </tr> + +<tr> <td colspan="2"> </td> <td align="center" bgcolor="#f0f0ff"> +smtpd<br> session<br> cache </td> <td> </td> <td align="center" +bgcolor="#f0f0ff"> PRNG<br> state <br>file </td> <td> </td> <td +align="center" bgcolor="#f0f0ff"> smtp<br> session<br> cache </td> +<td colspan="2"> </td> </tr> + +</table> + + +<li> <p> The <a href="verify.8.html">verify(8)</a> server verifies that a sender or recipient +address is deliverable before the <a href="smtpd.8.html">smtpd(8)</a> server accepts it. The +<a href="verify.8.html">verify(8)</a> server queries a cache with address verification results. +If a result is not found, the <a href="verify.8.html">verify(8)</a> server injects a probe +message into the Postfix queue and processes the status update from +a delivery agent or queue manager. +This process is described in the <a href="ADDRESS_VERIFICATION_README.html">ADDRESS_VERIFICATION_README</a> +document. The <a href="verify.8.html">verify(8)</a> service is available with Postfix version +2.1 and later. </p> + +<table> + +<tr> + + <td rowspan="2" colspan="5" align="center" valign="middle"> + </td> <td rowspan="3" align="center" valign="bottom"> + <tt> -> </tt> </td> <td rowspan="3" align="center" + valign="middle"> probe<br> message </td> <td rowspan="3" + align="center" valign="middle"> <tt> -> </tt> </td> <td + rowspan="3" bgcolor="#f0f0ff" align="center" valign="middle"> + Postfix<br> mail<br> queue </td> + +</tr> + +<tr> <td> </td> </tr> + +<tr> + + <td rowspan="3" align="center" valign="middle"> Network </td> + <td rowspan="3" align="center" valign="middle"> <tt> -> </tt> + </td> <td rowspan="3" bgcolor="#f0f0ff" align="center" + valign="middle"> <a href="smtpd.8.html">smtpd(8)</a> </td> <td rowspan="3" align="center" + valign="middle"> <tt> <-> </tt> </td> <td rowspan="3" + bgcolor="#f0f0ff" align="center" valign="middle"> <a href="verify.8.html">verify(8)</a> + </td> + +</tr> + +<tr> + + <td rowspan="1" colspan="3"> </td> <td rowspan="1" align="center" + valign="middle"> <tt> |</tt><br> <tt> v</tt> </td> + +</tr> + +<tr> + + <td rowspan="3" align="center" valign="top"> <tt> <- </tt> + </td> <td rowspan="3" align="center" valign="middle"> probe<br> + status </td> <td rowspan="3" align="center" valign="middle"> + <tt> <- </tt> </td> <td rowspan="3" bgcolor="#f0f0ff" + align="center" valign="middle"> Postfix<br> delivery<br> agents + </td> <td rowspan="3" align="left" valign="middle"> <tt>-></tt> + Local<br> <tt>-></tt> Network</td> + +</tr> + +<tr> + + <td rowspan="3" colspan="4" align="center" valign="middle"> + </td> <td rowspan="3" align="center" valign="middle"> + <tt> ^</tt><br> <tt> |</tt><br> <tt> v</tt> </td> + +</tr> + +<tr> <td> </td> </tr> + +<tr> <td colspan="4"> </td> </tr> + +<tr> + + <td colspan="4" align="center" valign="middle"> </td> + <td bgcolor="#f0f0ff" align="center" valign="middle"> Address<br> + verification<br> cache </td> + +</tr> + +</table> + +<li> <p> The <a href="postscreen.8.html">postscreen(8)</a> server can be put "in front" of Postfix +<a href="smtpd.8.html">smtpd(8)</a> processes. Its purpose is to accept connections from the +network and to decide what SMTP clients are allowed to talk to +Postfix. According to the 2008 MessageLabs annual report, 81% of +all email was spam, and 90% of that was sent by botnets; by 2010, +those numbers were 92% and 95%, respectively. While <a href="postscreen.8.html">postscreen(8)</a> +keeps the zombies away, more <a href="smtpd.8.html">smtpd(8)</a> processes remain available +for legitimate clients. </p> + +<p> <a href="postscreen.8.html">postscreen(8)</a> maintains a temporary allowlist for clients that +pass its tests; by allowing allowlisted clients to skip tests, +<a href="postscreen.8.html">postscreen(8)</a> minimizes its impact on legitimate email traffic. +</p> + +<p> The <a href="postscreen.8.html">postscreen(8)</a> server is available with Postfix 2.8 and +later. To keep the implementation simple, <a href="postscreen.8.html">postscreen(8)</a> delegates +DNS allow/denylist lookups to <a href="dnsblog.8.html">dnsblog(8)</a> server processes, and +delegates TLS encryption/decryption to <a href="tlsproxy.8.html">tlsproxy(8)</a> server processes. +This delegation is invisible to the remote SMTP client. </p> + +<table> + +<tr> <td colspan="2"> </td> <td align="center"> zombie </td> </tr> + +<tr> <td colspan="3"> </td> <td align="left"> <tt> \ </tt> </td> </tr> + +<tr> <td> zombie </td> <td> <tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="tlsproxy.8.html">tlsproxy(8)</a> </td> <td align="left"> <tt> - </tt> </td> <td> +</td> <td> </td> <td> </td> <td align="right"> <tt> - </tt> </td> +<td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a> </td> </tr> + +<tr> <td colspan="3"> </td> <td align="right"> <tt> \ </tt> </td> <td> </td> +<td align="left"> <tt> / </tt> </td> </tr> + +<tr> <td colspan="2"> </td> <td bgcolor="#f0f0ff" align="center"> other </td> <td> <tt> +--- </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> +<a href="postscreen.8.html">postscreen(8)</a> </td> </tr> + +<tr> <td colspan="3"> </td> <td align="right"> <tt> / </tt> </td> <td> </td> +<td align="right"> <tt> \ </tt> </td> </tr> + +<tr> <td colspan="2"> </td> <td bgcolor="#f0f0ff" align="center"> other </td> <td align="left"> +<tt> - </tt> </td> <td> </td> <td> </td> <td> </td> <td align="right"> +<tt> - </tt> </td> <td bgcolor="#f0f0ff" align="center"> <a href="smtpd.8.html">smtpd(8)</a> +</td> </tr> + +<tr> <td colspan="3"> </td> <td align="left"> <tt> / </tt> </td> </tr> + +<tr> <td colspan="2"> </td> <td align="center"> zombie </td> </tr> + +</table> + +<li> <p>The <a href="postlogd.8.html">postlogd(8)</a> server provides an alternative to syslog +logging, which remains the default. This feature is available with +Postfix version 3.4 or later, and supports the following modes: +</p> + + +<ul> + +<li> <p>Logging to file, which addresses a usability problem with +MacOS, and eliminates information loss caused by systemd rate limits. +</p> + +<table> + +<tr> <td bgcolor="#f0f0ff" rowspan="3" valign="middle" align="center"> +commands<br>or daemons</td> <td colspan="4"> </td> </tr> + +<tr> <td colspan="2"> <td> <tt> -> </tt> </td> <td bgcolor="#f0f0ff"> +<a href="postlogd.8.html">postlogd(8)</a> </td> <td> <tt> -> </tt> </td> <td> /path/to/file +</td> </tr> + +<tr> <td colspan=6> </td> </tr> + +</table> + +<li> <p>Logging to stdout, which eliminates a syslog dependency +when Postfix runs inside a container. </p> + +<table> + +<tr> <td bgcolor="#f0f0ff" rowspan="3" valign="middle" align="center"> +commands<br>or daemons</td> <td colspan="4"> </td> <td +rowspan="3" align="center"> stdout inherited<br>from "postfix +start-fg" </td> </tr> + +<tr> <td colspan="2"> <tt> -> </tt> </td> <td bgcolor="#f0f0ff"> +<a href="postlogd.8.html">postlogd(8)</a> </td> <td> <tt> -> </tt> </td> </tr> + +<tr> <td colspan=5> </td> </tr> + +</table> + +</ul> + +<p> See <a href="MAILLOG_README.html">MAILLOG_README</a> for details and limitations. </p> + +</ul> + +<h2> <a name="commands"> Postfix support commands </a> </h2> + +<p> The Postfix architecture overview ends with a summary of +command-line utilities for day-to-day use of the Postfix mail +system. Besides the Sendmail-compatible <a href="sendmail.1.html">sendmail(1)</a>, <a href="mailq.1.html">mailq(1)</a>, and +<a href="newaliases.1.html">newaliases(1)</a> commands, the Postfix system comes with it own +collection of command-line utilities. For consistency, these are +all named post<i>something</i>. </p> + +<ul> + +<li> <p> The <a href="postfix.1.html">postfix(1)</a> command controls the operation of the mail +system. It is the interface for starting, stopping, and restarting +the mail system, as well as for some other administrative operations. +This command is reserved to the super-user. </p> + +<li> <p> The <a href="postalias.1.html">postalias(1)</a> command maintains Postfix <a href="aliases.5.html">aliases(5)</a> type +databases. This is the program that does the work for the +<a href="newaliases.1.html">newaliases(1)</a> command. </p> + +<li> <p> The <a href="postcat.1.html">postcat(1)</a> command displays the contents of Postfix +queue files. This is a limited, preliminary utility. This program +is likely to be superseded by something more powerful that can also +edit Postfix queue files. </p> + +<li> <p> The <a href="postconf.1.html">postconf(1)</a> command displays or updates Postfix <a href="postconf.5.html">main.cf</a> +parameters and displays system dependent information about the +supported file locking methods, and the supported types of lookup +tables. </p> + +<li> <p> The <a href="postdrop.1.html">postdrop(1)</a> command is the mail posting utility that +is run by the Postfix <a href="sendmail.1.html">sendmail(1)</a> command in order to deposit mail +into the <a href="QSHAPE_README.html#maildrop_queue">maildrop queue</a> directory. </p> + +<li> <p> The <a href="postkick.1.html">postkick(1)</a> command makes some Postfix internal +communication channels available for use in, for example, shell +scripts. </p> + +<li> <p> The <a href="postlock.1.html">postlock(1)</a> command provides Postfix-compatible mailbox +locking for use in, for example, shell scripts. </p> + +<li> <p> The <a href="postlog.1.html">postlog(1)</a> command provides Postfix-compatible logging +for shell scripts. </p> + +<li> <p> The <a href="postmap.1.html">postmap(1)</a> command maintains Postfix lookup tables +such as <a href="canonical.5.html">canonical(5)</a>, <a href="virtual.5.html">virtual(5)</a> and others. It is a cousin of the +UNIX makemap command. </p> + +<li> <p> The <a href="postmulti.1.html">postmulti(1)</a> command repeats the "postfix start" etc. +command for each Postfix instance, and supports creation, deletion +etc. of Postfix instances. For a tutorial, see <a href="MULTI_INSTANCE_README.html">MULTI_INSTANCE_README</a>. +</p> + +<li> <p> The <a href="postqueue.1.html">postqueue(1)</a> command is the privileged command that +is run by Postfix <a href="sendmail.1.html">sendmail(1)</a> and <a href="mailq.1.html">mailq(1)</a> in order to flush or +list the +mail queue. </p> + +<li> <p> The <a href="postsuper.1.html">postsuper(1)</a> command maintains the Postfix queue. It +removes old temporary files, and moves queue files into the right +directory after a change in the hashing depth of queue directories. +This command is run at mail system startup time and when Postfix +is restarted. </p> + +</ul> + +</body> + +</html> |