Postfix logging to file or stdout

多肉大叔

April 22, 2025

Excerpted from the Postfix logging to file or stdout on April 22, 2025.

Overview

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.

Topics covered in this document:

Configuring logging to file

Logging to file solves a usability problem for MacOS, and eliminates multiple problems for systemd-based systems.

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 "unix-dgram" was introduced with Postfix 3.4. Remove the above line before backing out to an older Postfix version.
Configure Postfix to write logging, to, for example, /var/log/postfix.log. See also the "Logfile rotation" section below for logfile management.
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.
```
# postfix stop
# postconf maillog_file=/var/log/postfix.log
# postconf maillog_file_permissions=0644 # (Postfix 3.9 and later)
# 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.

Configuring logging to stdout

Logging to stdout is useful when Postfix runs in a container, as it eliminates a syslogd dependency.

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 "unix-dgram" was introduced with Postfix 3.4. Remove the above line before backing out to an older Postfix version.
Configure main.cf with "maillog_file = /dev/stdout".
Start Postfix with "postfix start-fg".

Rotating logs

The command "postfix logrotate" 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).
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).

Notes:

This command will not rotate a logfile with a pathname under the /dev directory, such as /dev/stdout.
This command does not (yet) remove old logfiles.

Limitations

Background:

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.
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.
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), postqueue(1), and (Postfix ≥ 3.7) postlog(1).