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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-06 01:46:30 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-06 01:46:30 +0000 |
| commit | b5896ba9f6047e7031e2bdee0622d543e11a6734 (patch) | |
| tree | fd7b460593a2fee1be579bec5697e6d887ea3421 /proto/OVERVIEW.html | |
| parent | Initial commit. (diff) | |
| download | postfix-upstream.tar.xz postfix-upstream.zip | |
Adding upstream version 3.4.23.upstream/3.4.23upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'proto/OVERVIEW.html')
| -rw-r--r-- | proto/OVERVIEW.html | 854 |
1 files changed, 854 insertions, 0 deletions
diff --git a/proto/OVERVIEW.html b/proto/OVERVIEW.html new file mode 100644 index 0000000..fc62ce1 --- /dev/null +++ b/proto/OVERVIEW.html @@ -0,0 +1,854 @@ +<!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=us-ascii"> + +</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 incoming queue. 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"> trivial-<br>rewrite(8) </td> + +</tr> + +<tr> + +<td> Network </td> <td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> smtpd(8) +</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"> qmqpd(8) +</td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> cleanup(8) +</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"> pickup(8) +</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"> sendmail(1) +</td> + +<td> <tt> -> </tt> </td> + +<td bgcolor="#f0f0ff" align="center" valign="middle"> postdrop(1) +</td> + +</tr> + +</table> + +</blockquote> + +<ul> + +<li> <p> Network mail enters Postfix via the smtpd(8) or qmqpd(8) +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 cleanup(8) server. The +smtpd(8) server can be configured to block unwanted mail, as +described in the SMTPD_ACCESS_README document. </p> + +<li> <p> Local submissions are received with the Postfix sendmail(1) +compatibility command, and are queued in the maildrop queue by +the privileged postdrop(1) command. This arrangement even works +while the Postfix mail system is not running. The local pickup(8) +server picks up local submissions, enforces some sanity checks to +protect Postfix, and gives the sender, recipients and message +content to the cleanup(8) server. </p> + +<li> <p> Mail from internal sources is given directly to the +cleanup(8) server. These sources are not shown in the figure, and +include: mail that is forwarded by the local(8) delivery agent (see +next section), messages that are returned to the sender by the +bounce(8) server (see second-next section), and postmaster +notifications about problems with Postfix. </p> + +<li> <p> The cleanup(8) 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 +ADDRESS_REWRITING_README +document. Optionally, the cleanup(8) server can be configured to +do light-weight content inspection with regular expressions as +described in the BUILTIN_FILTER_README document. The cleanup(8) +server places the result as a single file into the incoming queue, +and notifies the queue manager (see next section) of the arrival +of new mail. </p> + +<li> <p> The trivial-rewrite(8) server rewrites addresses to the +standard "user@fully.qualified.domain" form, as described in the +ADDRESS_REWRITING_README 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 incoming queue 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"> trivial-<br>rewrite(8) +</td> + +<td> </td> + +<td bgcolor="#f0f0ff" align="center"> smtp(8) </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"> lmtp(8) </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"> qmgr(8) </td> + +<td align="right"> <tt> --- </tt> </td> + +<td bgcolor="#f0f0ff" align="center"> local(8) </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"> virtual(8) </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"> pipe(8) </td> + +<td> <tt> -> </tt> </td> <td> Command </td> + +</tr> + +</table> + +</blockquote> + +<ul> + +<li> <p> The queue manager (the qmgr(8) server process in the +figure) is the heart of Postfix mail delivery. It contacts the +smtp(8), lmtp(8), local(8), virtual(8), pipe(8), discard(8) or +error(8) delivery agents, and sends a delivery request for one +or more recipient addresses. The discard(8) and error(8) 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 active queue with the +messages that it has opened for delivery. The active queue acts as +a limited window on potentially large incoming or deferred queues. +The limited active queue prevents the queue manager from running +out of memory under heavy load. </p> + +<p> The queue manager maintains a separate deferred queue 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 QSHAPE_README +and TUNING_README documents. </p> + +<li> <p> The trivial-rewrite(8) server resolves each recipient +address according to its local or remote address class, as defined +in the ADDRESS_CLASS_README document. Additional routing information +can be specified with the optional transport(5) table. The +trivial-rewrite(8) server optionally queries the relocated(5) table +for recipients whose address has changed; mail for such recipients is +returned to the sender with an explanation. </p> + +<li> <p> The smtp(8) 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 lmtp(8) 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 local(8) delivery agent understands UNIX-style mailboxes, +qmail-compatible maildir files, Sendmail-style system-wide aliases(5) +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 local(8) 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 virtual(8) 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 +VIRTUAL_README document. </p> + +<li> <p> The pipe(8) mailer is the outbound interface to other mail +processing systems (the Postfix sendmail(1) 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 pipe(8) mailer are in the MAILDROP_README +and UUCP_README 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 master(8) 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 master(8) 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 master(8) server is also responsible +for enforcing the server process count limits as specified in the +<b>master.cf</b> 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"> +postfix(1) </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"> postsuper(1) </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> master(8) </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> postlog(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"> smtpd(8) </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> qmgr(8) </td> <td> +</td> <td align="center" bgcolor="#f0f0ff"> local(8) </td> </tr> + +</table> + +<li> <p> The anvil(8) server implements client connection and +request rate +limiting for all smtpd(8) servers. The TUNING_README document +provides guidance for dealing with mis-behaving SMTP clients. The +anvil(8) 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> smtpd(8)<br><br> </td> <td> <tt> <-> +</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> anvil(8)<br><br> +</td> </tr> + +</table> + +<li> <p> The bounce(8), defer(8) and trace(8) 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 trace(8) 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"> cleanup(8) </td> <td +valign="middle"> <tt> -> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> qmgr(8)<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"> bounce(8)<br> defer(8)<br> trace(8) </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 flush(8) servers maintain per-destination logs and +implement both ETRN and "sendmail -qRdestination", as described +in the ETRN_README document. This moves selected queue files from +the deferred queue back to the incoming queue and requests their +delivery. The flush(8) 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"> smtpd(8)<br> sendmail(1)<br> +postqueue(1) </td> <td> <tt> - </tt> </td> <td align="center"> +Destination<br> to flush</td> <td> <tt> -> </tt> </td> <td +align="center" bgcolor="#f0f0ff"> flush(8) </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> qmgr(8) </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 proxymap(8) 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 scache(8) server maintains the connection cache for +the Postfix smtp(8) client. When connection caching is enabled for +selected destinations, the smtp(8) 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 smtp(8) client continues with some +other mail delivery request. Meanwhile, any smtp(8) process can +ask the scache(8) 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 CONNECTION_CACHE_README document. </p> + +<table> + +<tr> <td> </td> <td> <tt> /-- </tt> </td> <td align="center" +colspan="3" bgcolor="#f0f0ff"> smtp(8) </td> <td colspan="2"> <tt> +--> </tt> Internet </td> </tr> + +<tr> <td align="center" bgcolor="#f0f0ff"> qmgr(8) </td> <td> </td> +<td align="center" rowspan="3"> </td> <td align="center" +rowspan="3"><tt>|<br>|<br>|<br>|<br>v</tt></td> <td> </td> +</tr> + +<tr> <td> </td> <td> <tt> \-- </tt> </td> <td align="center" +colspan="2" bgcolor="#f0f0ff"> smtp(8) </td> <td align="left"> <tt> +--> </tt> Internet </td> </tr> + +<tr> <td colspan="3"> </td> <td align="center"><tt>^<br>|</tt></td> +<td> </td> </tr> + +<tr> <td colspan="3"> </td> <td align="center" colspan="3" +bgcolor="#f0f0ff"> scache(8) </td> </tr> + +</table> + +<li> <p> The showq(8) servers list the Postfix queue status. This +is the queue listing service that does the work for the mailq(1) +and postqueue(1) commands. </p> + +<table> + +<tr> <td> Output </td> <td> <tt> <- </tt> </td> <td align="center" +bgcolor="#f0f0ff"> mailq(1)<br> + +<a href="postqueue.1.html"> post-<br>queue(1) </a> <br> </td> <td> +<tt> <- </tt> </td> <td align="center" valign="middle" +bgcolor="#f0f0ff"> showq(8) </td> <td> <tt> <- </tt></td> <td +align="center" valign="middle" bgcolor="#f0f0ff"> Postfix<br> queue +</td> </tr> + +</table> + +<li> <p> The spawn(8) 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 SMTPD_POLICY_README document. </p> + +<li> <p> The tlsmgr(8) server runs when TLS (Transport Layer +Security, formerly known as SSL) is turned on in the Postfix smtp(8) +client or smtpd(8) 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 smtp(8) client or smtpd(8) +server processes. The state of this PRNG is periodically saved to +a file, and is read when tlsmgr(8) starts up. </p> + +<li> <p> Maintain the optional Postfix smtp(8) client or smtpd(8) +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 TLS_README +document. </p> + +<table> + +<tr> <td>Network<tt>-> </tt> </td> <td align="center" +bgcolor="#f0f0ff"> <br> smtpd(8) <br> </td> <td colspan="2"> +<tt> <---seed---<br><br><-session-> </tt> </td> <td +align="center" bgcolor="#f0f0ff"> <br> tlsmgr(8) <br> </td> +<td colspan="3"> <tt> ---seed---><br> <br><-session-> +</tt> </td> <td align="center" bgcolor="#f0f0ff"> <br> smtp(8) <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 verify(8) server verifies that a sender or recipient +address is deliverable before the smtpd(8) server accepts it. The +verify(8) server queries a cache with address verification results. +If a result is not found, the verify(8) 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 ADDRESS_VERIFICATION_README +document. The verify(8) 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"> smtpd(8) </td> <td rowspan="3" align="center" + valign="middle"> <tt> <-> </tt> </td> <td rowspan="3" + bgcolor="#f0f0ff" align="center" valign="middle"> verify(8) + </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 postscreen(8) server can be put "in front" of Postfix +smtpd(8) 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 postscreen(8) +keeps the zombies away, more smtpd(8) processes remain available +for legitimate clients. </p> + +<p> postscreen(8) maintains a temporary whitelist for clients that +pass its tests; by allowing whitelisted clients to skip tests, +postscreen(8) minimizes its impact on legitimate email traffic. +</p> + +<p> The postscreen(8) server is available with Postfix 2.8 and +later. To keep the implementation simple, postscreen(8) delegates +DNS white/blacklist lookups to dnsblog(8) server processes, and +delegates TLS encryption/decryption to tlsproxy(8) server processes. +This delegation is invisible to the remote SMTP client, and is not +shown in the diagram below. </p> + +<table> + +<tr> <td> zombie </td> </tr> + +<tr> <td> </td> <td align="left"> <tt> \ </tt> </td> </tr> + +<tr> <td> zombie </td> <td align="left"> <tt> - </tt> </td> <td> +</td> <td> </td> <td> </td> <td align="right"> <tt> - </tt> </td> +<td bgcolor="#f0f0ff" align="center"> smtpd(8) </td> </tr> + +<tr> <td> </td> <td align="right"> <tt> \ </tt> </td> <td> </td> +<td align="left"> <tt> / </tt> </td> </tr> + +<tr> <td bgcolor="#f0f0ff" align="center"> other </td> <td> <tt> +--- </tt> </td> <td bgcolor="#f0f0ff" align="center" valign="middle"> +postscreen(8) </td> </tr> + +<tr> <td> </td> <td align="right"> <tt> / </tt> </td> <td> </td> +<td align="right"> <tt> \ </tt> </td> </tr> + +<tr> <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"> smtpd(8) +</td> </tr> + +<tr> <td> </td> <td align="left"> <tt> / </tt> </td> </tr> + +<tr> <td> zombie </td> </tr> + + +</table> + +</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 sendmail(1), mailq(1), and +newaliases(1) 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 postfix(1) 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 postalias(1) command maintains Postfix aliases(5) type +databases. This is the program that does the work for the +newaliases(1) command. </p> + +<li> <p> The postcat(1) 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 postconf(1) command displays or updates Postfix main.cf +parameters and displays system dependent information about the +supported file locking methods, and the supported types of lookup +tables. </p> + +<li> <p> The postdrop(1) command is the mail posting utility that +is run by the Postfix sendmail(1) command in order to deposit mail +into the maildrop queue directory. </p> + +<li> <p> The postkick(1) command makes some Postfix internal +communication channels available for use in, for example, shell +scripts. </p> + +<li> <p> The postlock(1) command provides Postfix-compatible mailbox +locking for use in, for example, shell scripts. </p> + +<li> <p> The postlog(1) command provides Postfix-compatible logging +for shell scripts. </p> + +<li> <p> The postmap(1) command maintains Postfix lookup tables +such as canonical(5), virtual(5) and others. It is a cousin of the +UNIX makemap command. </p> + +<li> <p> The postmulti(1) command repeats the "postfix start" etc. +command for each Postfix instance, and supports creation, deletion +etc. of Postfix instances. For a tutorial, see MULTI_INSTANCE_README. +</p> + +<li> <p> The postqueue(1) command is the privileged command that +is run by Postfix sendmail(1) and mailq(1) in order to flush or +list the +mail queue. </p> + +<li> <p> The postsuper(1) 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> |