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<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN"
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<title>Postfix logging to file or stdout</title>
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<link rel='stylesheet' type='text/css' href='postfix-doc.css'>
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<body>
<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix
logging to file or stdout</h1>
<hr>
<h2>Overview </h2>
<p> Postfix supports its own logging system as an alternative to
syslog (which remains the default). This is available with Postfix
version 3.4 or later. </p>
<p> Topics covered in this document: </p>
<ul>
<li><a href="#log-to-file">Configuring logging to file</a>
<li><a href="#log-to-stdout">Configuring logging to stdout</a>
<li><a href="#logrotate">Rotating logs </a>
<li><a href="#limitations">Limitations</a>
</ul>
<h2> <a name="log-to-file"> Configuring logging to file </a> </h2>
<p> Logging to file solves a usability problem for MacOS, and
eliminates multiple problems for systemd-based systems. </p>
<ol>
<li> <p> Add the following line to master.cf if not already present
(note: there must be no whitespace at the start of the line): </p>
<blockquote>
<pre>
postlog unix-dgram n - n - 1 postlogd
</pre>
</blockquote>
<p> Note: the service type "<b>unix-dgram</b>" was introduced with
Postfix 3.4. Remove the above line before backing out to an older
Postfix version. </p>
<li> <p> Configure Postfix to write logging, to, for example,
/var/log/postfix.log. See also the "<a href="#logrotate">Logfile
rotation</a>" section below for logfile management. </p>
<p> In the example below, specifying maillog_file_permissions is
optional (Postfix 3.9 and later). The default value is 0600, i.e.,
only the super-user can access the file; the value 0644 also
adds 'group' and 'other' read access. </p>
<blockquote>
<pre>
# postfix stop
# postconf maillog_file=/var/log/postfix.log
# postconf maillog_file_permissions=0644 # (Postfix 3.9 and later)
# postfix start
</pre>
</blockquote>
<p> By default, the logfile name must start with "/var" or "/dev/stdout"
(the list of allowed prefixes is configured with the maillog_file_prefixes
parameter). This safety mechanism limits the damage from a single
configuration mistake. </p>
</ol>
<h2> <a name="log-to-stdout"> Configuring logging to stdout </a> </h2>
<p> Logging to stdout is useful when Postfix runs in a container,
as it eliminates a syslogd dependency. </p>
<ol>
<li> <p> Add the following line to master.cf if not already present (note:
there must be no whitespace at the start of the line): </p>
<blockquote>
<pre>
postlog unix-dgram n - n - 1 postlogd
</pre>
</blockquote>
<p> Note: the service type "<b>unix-dgram</b>" was introduced with
Postfix 3.4. Remove the above line before backing out to an older
Postfix version. </p>
<li> <p> Configure main.cf with "maillog_file = /dev/stdout". </p>
<li> <p> Start Postfix with "<b>postfix start-fg</b>". </p>
</ol>
<h2> <a name="logrotate"> Rotating logs </a> </h2>
<p> The command "<b>postfix logrotate</b>" may be run by hand or
by a cronjob. It logs all errors, and reports errors to stderr if
run from a terminal. This command implements the following steps:
</p>
<ul>
<li> <p> Rename the current logfile by appending a suffix that
contains the date and time. This suffix is configured with the
maillog_file_rotate_suffix parameter (default: %Y%m%d-%H%M%S). </p>
<li> <p> Reload Postfix so that postlogd(8) immediately closes the
old logfile. </p>
<li> <p> After a brief pause, compress the old logfile. The compression
program is configured with the maillog_file_compressor parameter
(default: gzip). </p>
<li> <p> The next time it logs an event, postlogd(8) will create a
new logfile, with permissions specified with the maillog_file_permissions
parameter (default: 0600). </p>
</ul>
<p> Notes: </p>
<ul>
<li> <p> This command will not rotate a logfile with a pathname under
the /dev directory, such as /dev/stdout. </p>
<li> <p> This command does not (yet) remove old logfiles. </p>
</ul>
<h2> <a name="limitations">Limitations</a> </h2>
<p> Background: </p>
<ul>
<li> <p> Postfix consists of a number of daemon programs that run
in the background, as well as non-daemon programs for local mail
submission or Postfix management.
<li> <p> Logging to the Postfix logfile or stdout requires the Postfix
postlogd(8) service. This ensures that simultaneous logging from
different programs will not get mixed up. </p>
<li> <p> All Postfix programs can log to syslog, but not all programs
have sufficient privileges to use the Postfix logging service, and
many non-daemon programs must not log to stdout as that would corrupt
their output. </p>
</ul>
<p> Limitations: </p>
<ul>
<li> <p> Non-daemon Postfix programs will log errors to syslogd(8)
before they have processed command-line options and main.cf parameters.
<li> <p> If Postfix is down, the non-daemon programs postfix(1),
postsuper(1), postmulti(1), and postlog(1), will log directly to
$maillog_file. These programs expect to run with root privileges,
for example during Postfix start-up, reload, or shutdown.
<li> <p> Other non-daemon Postfix programs will never write directly
to $maillog_file (also, logging to stdout would interfere with the
operation of some of these programs). These programs can log to
postlogd(8) if they are run by the super-user, or if their executable
file has set-gid permission. Do not set this permission on programs
other than postdrop(1), postqueue(1), and (Postfix ≥ 3.7)
postlog(1).
</ul>
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