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+PPoossttffiixx llooggggiinngg ttoo ffiillee oorr ssttddoouutt
+
+-------------------------------------------------------------------------------
+
+OOvveerrvviieeww
+
+Postfix supports it own logging system as an alternative to syslog (which
+remains the default). This is available with Postfix version 3.4 or later.
+
+Topics covered in this document:
+
+ * Configuring logging to file
+ * Configuring logging to stdout
+ * Rotating logs
+ * Limitations
+
+CCoonnffiigguurriinngg llooggggiinngg ttoo ffiillee
+
+Logging to file solves a usability problem for MacOS, and eliminates multiple
+problems for systemd-based systems.
+
+ 1. Add the following line to master.cf if not already present (note: there
+ must be no whitespace at the start of the line):
+
+ postlog unix-dgram n - n - 1 postlogd
+
+ Note: the service type "uunniixx--ddggrraamm" was introduced with Postfix 3.4. Remove
+ the above line before backing out to an older Postfix version.
+
+ 2. Configure Postfix to write logging, to, for example, /var/log/postfix.log.
+ See also the "Logfile rotation" section below for logfile management.
+
+ # postfix stop
+ # postconf maillog_file=/var/log/postfix.log
+ # postfix start
+
+ 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.
+
+CCoonnffiigguurriinngg llooggggiinngg ttoo ssttddoouutt
+
+Logging to stdout is useful when Postfix runs in a container, as it eliminates
+a syslogd dependency.
+
+ 1. Add the following line to master.cf if not already present (note: there
+ must be no whitespace at the start of the line):
+
+ postlog unix-dgram n - n - 1 postlogd
+
+ Note: the service type "uunniixx--ddggrraamm" was introduced with Postfix 3.4. Remove
+ the above line before backing out to an older Postfix version.
+
+ 2. Configure main.cf with "maillog_file = /dev/stdout".
+
+ 3. Start Postfix with "ppoossttffiixx ssttaarrtt--ffgg".
+
+RRoottaattiinngg llooggss
+
+The command "ppoossttffiixx llooggrroottaattee" 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:
+
+ * 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).
+
+ * Reload Postfix so that postlogd(8) immediately closes the old logfile.
+
+ * After a brief pause, compress the old logfile. The compression program is
+ configured with the maillog_file_compressor parameter (default: gzip).
+
+Notes:
+
+ * This command will not rotate a logfile with pathname under the /dev
+ directory, such as /dev/stdout.
+
+ * This command does not (yet) remove old logfiles.
+
+LLiimmiittaattiioonnss
+
+Background:
+
+ * Postfix consists of a number of daemon programs, and non-daemon programs
+ some of which are used for local mail submission, and some for Postfix
+ management.
+
+ * Logging to Postfix logfile or stdout requires the Postfix postlogd(8)
+ service. This ensures that simultaneous logging from different programs
+ will not get mixed up.
+
+ * 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.
+
+Limitations:
+
+ * Non-daemon Postfix programs will log errors to syslogd(8) before they have
+ processed command-line options and main.cf parameters.
+
+ * 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.
+
+ * 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)
+ and postqueue(1).
+