Updated: 2022/Sep/29
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POSTFIX-WRAPPER(5) File Formats Manual POSTFIX-WRAPPER(5)
NAME
postfix-wrapper - Postfix multi-instance API
DESCRIPTION
Support for managing multiple Postfix instances is available as of
version 2.6. Instances share executable files and documentation, but
have their own directories for configuration, queue and data files.
This document describes how the familiar "postfix start" etc. user
interface can be used to manage one or multiple Postfix instances, and
gives details of an API to coordinate activities between the postfix(1)
command and a multi-instance manager program.
With multi-instance support, the default Postfix instance is always
required. This instance is identified by the config_directory
parameter's default value.
GENERAL OPERATION
Multi-instance support is backwards compatible: when you run only one
Postfix instance, commands such as "postfix start" will not change
behavior at all.
Even with multiple Postfix instances, you can keep using the same
postfix commands in boot scripts, upgrade procedures, and other places.
The commands do more work, but humans are not forced to learn new
tricks.
For example, to start all Postfix instances, use:
# postfix start
Other postfix(1) commands also work as expected. For example, to find
out what Postfix instances exist in a multi-instance configuration,
use:
# postfix status
This enumerates the status of all Postfix instances within a
multi-instance configuration.
MANAGING AN INDIVIDUAL POSTFIX INSTANCE
To manage a specific Postfix instance, specify its configuration
directory on the postfix(1) command line:
# postfix -c /path/to/config_directory command
Alternatively, the postfix(1) command accepts the instance's
configuration directory via the MAIL_CONFIG environment variable (the
-c command-line option has higher precedence).
Otherwise, the postfix(1) command will operate on all Postfix
instances.
ENABLING POSTFIX(1) MULTI-INSTANCE MODE
By default, the postfix(1) command operates in single-instance mode. In
this mode the command invokes the postfix-script file directly
(currently installed in the daemon directory). This file contains the
commands that start or stop one Postfix instance, that upgrade the
configuration of one Postfix instance, and so on.
When the postfix(1) command operates in multi-instance mode as
discussed below, the command needs to execute start, stop, etc.
commands for each Postfix instance. This multiplication of commands is
handled by a multi-instance manager program.
Turning on postfix(1) multi-instance mode goes as follows: in the
default Postfix instance's main.cf file, 1) specify the pathname of a
multi-instance manager program with the multi_instance_wrapper
parameter; 2) populate the multi_instance_directories parameter with
the configuration directory pathnames of additional Postfix instances.
For example:
/etc/postfix/main.cf:
multi_instance_wrapper = $daemon_directory/postfix-wrapper
multi_instance_directories = /etc/postfix-test
The $daemon_directory/postfix-wrapper file implements a simple manager
and contains instructions for creating Postfix instances by hand. The
postmulti(1) command provides a more extensive implementation including
support for life-cycle management.
The multi_instance_directories and other main.cf parameters are listed
below in the CONFIGURATION PARAMETERS section.
In multi-instance mode, the postfix(1) command invokes the
$multi_instance_wrapper command instead of the postfix-script file.
This multi-instance manager in turn executes the postfix(1) command in
single-instance mode for each Postfix instance.
To illustrate the main ideas behind multi-instance operation, below is
an example of a simple but useful multi-instance manager
implementation:
#!/bin/sh
: ${command_directory?"do not invoke this command directly"}
POSTCONF=$command_directory/postconf
POSTFIX=$command_directory/postfix
instance_dirs=`$POSTCONF -h multi_instance_directories |
sed 's/,/ /'` || exit 1
err=0
for dir in $config_directory $instance_dirs
do
case "$1" in
stop|abort|flush|reload|drain)
test "`$POSTCONF -c $dir -h multi_instance_enable`" \
= yes || continue;;
start)
test "`$POSTCONF -c $dir -h multi_instance_enable`" \
= yes || {
$POSTFIX -c $dir check || err=$?
continue
};;
esac
$POSTFIX -c $dir "$@" || err=$?
done
exit $err
PER-INSTANCE MULTI-INSTANCE MANAGER CONTROLS
Each Postfix instance has its own main.cf file with parameters that
control how the multi-instance manager operates on that instance. This
section discusses the most important settings.
The setting "multi_instance_enable = yes" allows the multi-instance
manager to start (stop, etc.) the corresponding Postfix instance. For
safety reasons, this setting is not the default.
The default setting "multi_instance_enable = no" is useful for manual
testing with "postfix -c /path/name start" etc. The multi-instance
manager will not start such an instance, and it will skip commands such
as "stop" or "flush" that require a running Postfix instance. The
multi-instance manager will execute commands such as "check",
"set-permissions" or "upgrade-configuration", and it will replace
"start" by "check" so that problems will be reported even when the
instance is disabled.
MAINTAINING SHARED AND NON-SHARED FILES
Some files are shared between Postfix instances, such as executables
and manpages, and some files are per-instance, such as configuration
files, mail queue files, and data files. See the NON-SHARED FILES
section below for a list of per-instance files.
Before Postfix multi-instance support was implemented, the executables,
manpages, etc., have always been maintained as part of the default
Postfix instance.
With multi-instance support, we simply continue to do this.
Specifically, a Postfix instance will not check or update shared files
when that instance's config_directory value is listed with the default
main.cf file's multi_instance_directories parameter.
The consequence of this approach is that the default Postfix instance
should be checked and updated before any other instances.
MULTI-INSTANCE API SUMMARY
Only the multi-instance manager implements support for the
multi_instance_enable configuration parameter. The multi-instance
manager will start only Postfix instances whose main.cf file has
"multi_instance_enable = yes". A setting of "no" allows a Postfix
instance to be tested by hand.
The postfix(1) command operates on only one Postfix instance when the
-c option is specified, or when MAIL_CONFIG is present in the process
environment. This is necessary to terminate recursion.
Otherwise, when the multi_instance_directories parameter value is
non-empty, the postfix(1) command executes the command specified with
the multi_instance_wrapper parameter, instead of executing the commands
in postfix-script.
The multi-instance manager skips commands such as "stop" or "reload"
that require a running Postfix instance, when an instance does not have
"multi_instance_enable = yes". This avoids false error messages.
The multi-instance manager replaces a "start" command by "check" when a
Postfix instance's main.cf file does not have "multi_instance_enable =
yes". This substitution ensures that problems will be reported even
when the instance is disabled.
No Postfix command or script will update or check shared files when its
config_directory value is listed in the default main.cf's
multi_instance_directories parameter value. Therefore, the default
instance should be checked and updated before any Postfix instances
that depend on it.
Set-gid commands such as postdrop(1) and postqueue(1) effectively
append the multi_instance_directories parameter value to the legacy
alternate_config_directories parameter value. The commands use this
information to determine whether a -c option or MAIL_CONFIG environment
setting specifies a legitimate value.
The legacy alternate_config_directories parameter remains necessary for
non-default Postfix instances that are running different versions of
Postfix, or that are not managed together with the default Postfix
instance.
ENVIRONMENT VARIABLES
MAIL_CONFIG
When present, this forces the postfix(1) command to operate only
on the specified Postfix instance. This environment variable is
exported by the postfix(1) -c option, so that postfix(1)
commands in descendant processes will work correctly.
CONFIGURATION PARAMETERS
The text below provides only a parameter summary. See postconf(5) for
more details.
multi_instance_directories (empty)
An optional list of non-default Postfix configuration
directories; these directories belong to additional Postfix
instances that share the Postfix executable files and
documentation with the default Postfix instance, and that are
started, stopped, etc., together with the default Postfix
instance.
multi_instance_wrapper (empty)
The pathname of a multi-instance manager command that the
postfix(1) command invokes when the multi_instance_directories
parameter value is non-empty.
multi_instance_name (empty)
The optional instance name of this Postfix instance.
multi_instance_group (empty)
The optional instance group name of this Postfix instance.
multi_instance_enable (no)
Allow this Postfix instance to be started, stopped, etc., by a
multi-instance manager.
NON-SHARED FILES
config_directory (see 'postconf -d' output)
The default location of the Postfix main.cf and master.cf
configuration files.
data_directory (see 'postconf -d' output)
The directory with Postfix-writable data files (for example:
caches, pseudo-random numbers).
queue_directory (see 'postconf -d' output)
The location of the Postfix top-level queue directory.
SEE ALSO
postfix(1) Postfix control program
postmulti(1) full-blown multi-instance manager
$daemon_directory/postfix-wrapper simple multi-instance manager
LICENSE
The Secure Mailer license must be distributed with this software.
AUTHOR(S)
Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA
Wietse Venema
Google, Inc.
111 8th Avenue
New York, NY 10011, USA
POSTFIX-WRAPPER(5)