Managing multiple Postfix instances on a single host

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Overview

This document is a guide to managing multiple Postfix instances on a single
host using the postmulti(1) instance manager. Multi-instance support is
available with Postfix version 2.6 and later. See the postfix-wrapper(5) manual
page for background on the instance management framework, and on how to deploy
a custom instance manager.

Topics covered in this document:

  * Why multiple Postfix instances
  * Null-client instances versus service instances
  * Multi-instance walk-through
  * Components of a Postfix system
  * The default Postfix instance
  * Instance groups
  * Multi-instance configuration parameters
  * Using the postmulti(1) command
  * Credits

Why multiple Postfix instances

Postfix is a general-purpose mail system that can be configured to serve a
variety of needs. Examples of Postfix applications are:

  * Local mail submission for shell users and system processes.

  * Incoming (MX host) email from the Internet.

  * Outbound mail relay for a corporate network.

  * Authenticated submission for roaming users.

  * Before/after content-filter mail.

A single Postfix configuration can provide many or all of these services, but a
complex interplay of settings may be required, for example with master.cf
options overriding main.cf settings. In this document we take the view that
multiple Postfix instances may be a simpler way to configure a multi-function
Postfix system. With multiple Postfix instances, each instance has its own
directories for configuration, queue and data files, but it shares all Postfix
program and documentation files with other instances.

Since there is no single right way to configure your system, we recommend that
you choose what makes you most comfortable. If different Postfix services don't
involve incompatible main.cf or master.cf settings, and if they can be combined
together without complex tricks, then a single monolithic configuration may be
the simplest approach.

The purpose of multi-instance support in Postfix is not to force you to create
multiple Postfix instances, but rather to give you a choice. Multiple instances
give you the freedom to tune each Postfix instance to a single task that it
does well and to combine instances into complete systems.

With the introduction of the postmulti(1) utility and the reduction of the per-
instance configuration footprint of a secondary Postfix instance to just a
main.cf and master.cf file (other files are now in shared locations), we hope
that multiple instances will be easier to use than ever before.

Null-client instances versus service instances

In the multi-instance approach to configuring Postfix, the first simplification
is with the default local-submission Postfix instance.

Most UNIX systems require support for email submission with the sendmail(1)
command so that system processes such as cron jobs can send status reports, and
so that system users can send email with command-line utilities. Such email can
be handled with a null-client Postfix configuration that forwards all mail to a
central mail hub. The null client will typically either not run an SMTP
listener at all (master_service_disable = inet), or it will listen only on the
loopback interface (inet_interfaces = loopback-only).

When implementing specialized servers for inbound Internet email, outbound
MTAs, internal mail hubs, and so on, we recommend using a null client for local
submission and creating single-function secondary Postfix instances to serve
the specialized needs.

    Note: usually, you need to use different "myhostname" settings when you run
    multiple instances on the same host. Otherwise, there will be false "mail
    loops back to myself" alarms when one instance tries to send mail into
    another instance. Typically, the null-client instance will use the system's
    hostname, and other instances will use their own dedicated "myhostname"
    settings. Different names are not needed when instances send mail to each
    other with a protocol other than SMTP, or with SMTP over a TCP port other
    than 25 as is usual with SMTP-based content filters.

Multi-instance walk-through

Before discussing the fine details of multi-instance operation we first show
the steps for creating a border mail server. This server has with a null-client
Postfix instance for local submission, an input Postfix instance to receive
mail from the Internet, plus an advanced SMTP content-filter and an output
Postfix instance to deliver filtered email to its internal destination.

Setting up the null-client Postfix instance

On a border mail hub, while mail from the Internet requires a great deal of
scrutiny, locally submitted messages are typically limited to mail from cron
jobs and other system services. In this regard the border MTA is not different
from other Unix hosts in your environment. For this reason, it will submit
locally-generated email to the internal mail hub. We start the construction of
the border mail server with the default instance, which will be a local-
submission null client:

    /etc/postfix/main.cf:
        # We are mta1.example.com
        #
        myhostname = mta1.example.com
        mydomain = example.com

        # Flat user-account namespace in example.com:
        #
        #   user@example.com not user@host.example.com
        #
        myorigin = $mydomain

        # Postfix 2.6+, disable inet services, specifically disable smtpd(8)
        #
        master_service_disable = inet

        # No local delivery:
        #
        mydestination =
        local_transport = error:5.1.1 Mailbox unavailable
        alias_database =
        alias_maps =
        local_recipient_maps =

        # Send everything to the internal mailhub
        #
        relayhost = [mailhub.example.com]

        # Indexed table macro:
        # (use "hash", ... when cdb is not available)
        #
        default_database_type = cdb
        indexed = ${default_database_type}:${config_directory}/

        # Expose origin host of mail from "root", ...
        #
        smtp_generic_maps = ${indexed}generic

        # Send messages addressed to "root", ... to the MTA support team
        #
        virtual_alias_maps = ${indexed}virtual

    /etc/postfix/generic:
        # The smarthost supports "+" addressing (recipient_delimiter = +).
        # Mail from "root" exposes the origin host, without replies
        # and bounces going back to the same host.
        #
        # On clustered MTAs this file is typically machine-built from
        # a template file. The build process expands the template into
        # "mtaadmin+root=mta1"
        #
        root    	mtaadmin+root=mta1

    /etc/postfix/virtual:
        # Caretaker aliases:
        #
        root    	mtaadmin
        postmaster	root

You would typically also add a Makefile, to automatically run postmap(1)
commands when source files change. This Makefile also creates a "generic"
database when none exists.

    /etc/postfix/Makefile:
        MTAADMIN=mtaadmin

        all: virtual.cdb generic.cdb

        generic: Makefile
    	    @echo Creating $@
    	    @rm -f $@.tmp
    	    @printf '%s\t%s+root=%s\n' root $MTAADMIN `uname -n` > $@.tmp
    	    @mv $@.tmp generic

        %.cdb: %
    	    postmap cdb:$<

Construct the "virtual" and "generic" databases (the latter is created by
running "make"), then start and test the null-client:

    # cd /etc/postfix; make
    # postfix start
    # sendmail -i -f root -t <<EOF
    From: root
    To: root
    Subject: test

    testing
    EOF

The test message should be delivered the members of the "mtaadmin" address
group (or whatever address group you choose) with the following headers:

    From: mtaadmin+root=mta1@example.com
    To: mtadmin+root=mta1@example.com
    Subject: test

Setting up the "output" Postfix instance

With the null-client instance out of the way, we can create the MTA "output"
instance that will deliver filtered mail to the inside network. We add the
"output" instance first, because the output instance needs to be up and running
before the input instance can be fully tested, and when the system boots, the
"output" instance must start before the input instance. We will put the output
and input instances into a single instance group named "mta".

Just once, when adding the first secondary instance, enable multi-instance
support in the default (null-client) instance:

    # postmulti -e init

Then create the output instance:

    # postmulti -I postfix-out -G mta -e create

The instance configuration directory defaults to /etc/postfix-out, more
precisely, the "postfix-out" subdirectory of the parent directory of the
default-instance configuration directory. The new instance will be created in a
"disabled" state:

    /etc/postfix-out/main.cf
        #
        # ... "stock" main.cf settings ...
        #
        multi_instance_name = postfix-out
        queue_directory = /var/spool/postfix-out
        data_directory = /var/lib/postfix-out
        #
        multi_instance_enable = no
        master_service_disable = inet
        authorized_submit_users =

This instance has a "stock" master.cf file, and its queue and data directories,
also named "postfix-out", will be located in the same parent directories as the
corresponding directories of the default instance (e.g., /var/spool/postfix-out
and /var/lib/postfix-out).

While this instance is immediately safe to start, it is not yet usefully
configured. It needs to be customized to fit the role of a post-filter re-
injection SMTP service. Typical additions include:

    /etc/postfix-out/master.cf:
        # Replace default "smtp inet" entry with one listening on port 10026.
        127.0.0.1:10026     inet  n       -       n       -       -       smtpd

    /etc/postfix-out/main.cf
        # ...

        # Comment out if you don't use IPv6 internally
        # inet_protocols = ipv4
        inet_interfaces = loopback-only
        mynetworks_style = host
        smtpd_authorized_xforward_hosts = $mynetworks

        # Don't anvil(8) control the re-injection port.
        #
        smtpd_client_connection_count_limit = 0
        smtpd_client_event_limit_exceptions = $mynetworks

        # Best practice when inet_interfaces is set, as this is not a
        # "secondary IP personality" configuration.
        #
        smtp_bind_address = 0.0.0.0

        # All header rewriting happens upstream
        #
        local_header_rewrite_clients =

        # No local delivery on border gateway
        #
        mydestination =
        alias_maps =
        alias_database =
        local_recipient_maps =
        local_transport = error:5.1.1 Mailbox unavailable

        # May need a recipient_delimiter for per-user transport lookups:
        #
        recipient_delimiter = +

        # Only one (unrestricted client)
        # With multiple instances, rarely need "-o param=value" overrides
        # in master.cf, each instance gets its own main.cf file.
        #
        smtpd_recipient_restrictions = permit_mynetworks, reject

        # Tolerate occasional high latency in the  content filter.
        #
        smtpd_timeout = 1200s

        # Best when empty, with all parent domain matches explicit.
        #
        parent_domain_matches_subdomains =

        # Use the "relay" transport for inbound mail, and the default
        # "smtp" transport for outbound mail (bounces, ...). The latter
        # won't starve the former of delivery agent slots.
        #
        relay_domains = example.com, .example.com

        # With xforward, match the input instance setting, if you
        # want "yes", set both to "yes".
        #
        smtpd_client_port_logging = no

        # Transport settings ...
        # Message size limit
        # Concurrency tuning for "relay" and "smtp" transport
        # ...

With the "output" configuration in place, enable and start the instance:

    1 # postmulti -i postfix-out -x postconf -e \
    2     "master_service_disable =" "authorized_submit_users = root"
    3 # postmulti -i postfix-out -e enable
    4 # postmulti -i postfix-out -p start

This uses the postmulti(1) command to invoke postconf(1) in the context
(MAIL_CONFIG=/etc/postfix-out) of the output instance.

  * Lines 1-2: With "authorized_submit_users = root", the superuser can test
    the postfix-out instance with "postmulti -i postfix-out -x sendmail -bv
    recipient...", but otherwise local submission remains disabled.

  * Lines 1-2: With "master_service_disable =", the "inet" listeners are re-
    enabled.

  * Line 3: The output instance is enabled for multi-instance start/stop.

  * Line 4: The output instance is started.

Test the output instance by submitting probe messages via "sendmail -bv" and
"telnet". For production systems, in-depth configuration tests should be done
on a lab system. The simple tests just suggested will only confirm successful
deployment of a configuration that should already be known good.

Setting up the content-filter proxy

With the output instance ready, deploy your content-filter proxy. Most proxies
will need their own /etc/rc* start/stop script. Some proxies, however, are
started on demand by the Postfix spawn(8) service, in which case you need to
add the relevant spawn(8) entry to the output instance master.cf file.

Configure the proxy to listen on 127.0.0.1:10025 and to re-inject filtered
email to 127.0.0.1:10026. Start the proxy service if necessary, then test the
proxy via "telnet" or automated SMTP injectors. The proxy should support the
following ESMTP features: DSN, 8BITMIME, and XFORWARD. In addition, the proxy
should support multiple mail deliveries within an SMTP session.

Setting up the input Postfix instance

The input Postfix instance receives mail from the network and sends it through
the content filter. Now we create the input instance, also part of the "mta"
instance group:

    # postmulti -I postfix-in -G mta -e create

The new instance configuration directory defaults to /etc/postfix-in, more
precisely, the "postfix-in" subdirectory of the parent directory of the
default-instance configuration directory. The new instance will be created in a
"disabled" state:

    /etc/postfix-in/main.cf
        #
        # ... "stock" main.cf settings ...
        #
        multi_instance_name = postfix-in
        queue_directory = /var/spool/postfix-in
        data_directory = /var/lib/postfix-in
        #
        multi_instance_enable = no
        master_service_disable = inet
        authorized_submit_users =

As before, make appropriate changes to main.cf and master.cf to make the
instance production ready. Consider setting "soft_bounce = yes" during the
first few hours of deployment, so you can iron-out any unexpected "kinks".

Manual testing can start with:

    /etc/postfix-in/main.cf
        # Accept only local traffic, but allow impersonation:
        inet_interfaces = 127.0.0.1
        smtpd_authorized_xclient_hosts = 127.0.0.1

This allows you to use the Postfix-specific XCLIENT SMTP command to safely
simulate connections from remote systems before any remote systems are able to
connect. If the test results look good, revert the above settings to the
required production values. Typical settings in the pre-filter input instance
include:

    /etc/postfix-in/main.cf
        #
        # ...
        #

        # No local delivery on border gateway
        #
        mydestination =
        alias_maps =
        alias_database =
        local_recipient_maps =
        local_transport = error:5.1.1 Mailbox unavailable

        # Don't rewrite remote headers
        #
        local_header_rewrite_clients =

        # All recipients of not yet filtered email go to the same filter
    together.
        #
        # With multiple instances, the content-filter is specified
        # via transport settings not the "content_filter" transport
        # switch override! Here the filter listens on local port 10025.
        #
        # If you need to route some users or recipient domains directly to the
        # output instance bypassing the filter, just define a transport table
        # with suitable entries.
        #
        default_transport = smtp:[127.0.0.1]:10025
        relay_transport = $default_transport
        virtual_transport = $default_transport
        transport_maps =

        # Pass original client log information through the filter.
        #
        smtp_send_xforward_command = yes

        # Avoid splitting the envelope and scanning messages multiple times.
        # Match the re-injection server's recipient limit.
        #
    	smtp_destination_recipient_limit = 1000

        # Tolerate occasional high latency in the content filter.
        #
        smtp_data_done_timeout = 1200s

        # With xforward, match the output instance setting, if you
        # want "yes", set both to "yes".
        #
        smtpd_client_port_logging = no

        # ... Lots of settings for inbound MX host ...

With the "input" instance configured, enable and start it:

    # postmulti -i postfix-in -x postconf -e \
        "master_service_disable =" "authorized_submit_users = root"
    # postmulti -i postfix-in -e enable
    # postmulti -i postfix-in -p start

That's it. You now have a 3-instance configuration. A null-client sending all
locally submitted mail to the internal mail hub and a pair of "mta" instances
that receive mail from the Internet, pass it through a content-filter, and then
deliver it to the internal destination.

Running "postfix start" or "postfix stop" will now start/stop all three Postfix
instances. You can use "postfix -c /config/path start" to start just one
instance, or use the instance name (or instance group name) via postmulti(1):

    # postmulti -i - -p stop
    # postmulti -g mta -p status
    # postmulti -i postfix-out -p flush
    # postmulti -i postfix-in -p reload
    # ...

This example ends the multi-instance "walk through". The remainder of this
document provides background information on Postfix multi-instance support
features and options.

Components of a Postfix system

A Postfix system consists of the following components:

Shared among all instances:

  * Command-line utilities for administrators and users installed in
    $command_directory, $sendmail_path, $mailq_path and $newaliases_path.

  * Daemon executables, and run-time support files installed in
    $daemon_directory.

  * Bundled documentation, installed in $html_directory, $manpage_directory and
    $readme_directory.

  * Entries in /etc/passwd and /etc/group for the $mail_owner user and
    $setgid_group group. The the $mail_owner user provides the mail system with
    a protected (non-root) execution context. The $setgid_group group is used
    exclusively to support the setgid postdrop(1) and postqueue(1) utilities
    (it must not be the primary group or secondary group of any users,
    including the $mail_owner user).

Private to each instance:

  * The main.cf, master.cf (and other optional) configuration files in
    $config_directory.

  * The maildrop, incoming, active, deferred and hold queues in
    $queue_directory (which contains additional directories needed by Postfix,
    and which optionally doubles as a chroot jail for Postfix daemon
    processes).

  * Various caches (TLS session, address verification, ...) in $data_directory.

The Postfix configuration parameters mentioned above are collectively referred
to as "installation parameters". Their default values are set when the Postfix
software is built from source, and all but one may be optionally set to a non-
default value via the main.cf file. The one parameter that (catch-22) cannot be
set in main.cf is $config_directory, as this defines the location of the
main.cf file itself.

Though config_directory cannot be set in main.cf, postfix(1) and most of the
other command-line Postfix utilities allow you to specify a non-default
configuration directory via a command line option (typically -c) or via the
MAIL_CONFIG environment variable. In this way, it is possible to have multiple
configuration directories on the same machine, and to have multiple running
master(8) daemons each with its own configuration files, queue directory and
data directory.

These multiple running copies of master(8) share the base Postfix software.
They do not (and cannot) share their configuration directories, queue
directories or data directories.

Each combination of configuration directory, together with the queue directory
and data directory (specified in the corresponding main.cf file) make up a
Postfix instance.

The default Postfix instance

One Postfix instance is special: this is the instance whose configuration
directory is the default one compiled into the Postfix utilities. The location
of the default configuration directory is typically /etc/postfix, and can be
queried via the "postconf -d config_directory" command. We call the instance
with this configuration directory the "default instance".

The default instance is responsible for local mail submission. The setgid
postdrop(1) utility is used by the sendmail(1) local submission program to
spool messages into the maildrop sub-directory of the queue directory of the
default instance.

Even in the rare case when "sendmail -C" is used to submit local mail into a
non-default Postfix instance, for security reasons, postdrop(1) will consult
the default main.cf file to check the validity of the requested non-default
configuration directory.

So, while in most other respects, all instances are equal, the default instance
is "more equal than others". You may choose to create additional instances, but
you must have at least the default instance, with its configuration directory
in the default compiled-in location.

Instance groups

The postmulti(1) multi-instance manager supports the notion of an instance
"group". Typically, the member instances of an instance group constitute a
logical service, and are expected to all be running or all be stopped.

In many cases a single Postfix instance will be a complete logical "service".
You should define such instances as stand-alone instances that are not members
of any instance "group". The null-client instance is an example of a non-group
instance.

When a logical service consists of multiple Postfix instances, often a pair of
pre-filter and post-filter instances with a content filter proxy between them,
the related instances should be members of a single instance group (however,
the content filter usually has its own start/stop procedure that is separate
from any Postfix instance).

The default instance main.cf file's $multi_instance_directories configuration
parameter lists the configuration directories of all secondary (non-default)
instances. Together with the default instance, these secondary instances are
managed by the multi-instance manager. Instances are started in the order
listed, and stopped in the opposite order. For instances that are members of a
service "group", you should arrange to start the service back-to-front, with
the output stages started and ready to receive mail before the input stages are
started.

Multi-instance configuration parameters

multi_instance_wrapper
    This default-instance configuration parameter must be set to a suitable
    multi-instance manager's "wrapper" program that controls the starting,
    stopping, etc. of a multi-instance Postfix system. To use the postmulti(1)
    manager described in this document, this parameter should be set with the
    "postmulti -e init" command.

multi_instance_directories
    This default-instance configuration parameter specifies an optional list of
    the secondary instances controlled via the multi-instance manager.
    Instances are listed in their "start" order, with the default instance
    always started first (if enabled). If $multi_instance_directories is left
    empty, the postfix(1) command runs with multi-instance support turned off,
    and none of the multi_instance_ configuration parameters will have any
    effect.

    Do not assign a non-empty list of secondary instance configuration
    directories to multi_instance_directories until you have configured a
    suitable multi_instance_wrapper setting! This is best accomplished via the
    "postmulti -e init" command.

multi_instance_name
    Each Postfix instance may be assigned a distinct name (with "postfix -
    e create/import/assign -I name..."). This name can be used with the
    postmulti(1) command-line utility to perform tasks on the instance by name
    (rather than the full pathname of its configuration directory). Choose a
    name that concisely captures the role of the instance (it must start with
    "postfix-"). It is an error for two instances to have the same
    $multi_instance_name. You can leave an instance "nameless" by leaving this
    parameter at the default empty setting.

    To avoid confusion in your logs, if you don't assign each secondary
    instance a non-empty (distinct) $multi_instance_name, you should make sure
    that the $syslog_name setting is different for each instance. The
    $syslog_name parameter defaults to $multi_instance_name when the latter is
    non-empty. If at all possible, the syslog_name should start with "postfix-
    ", this helps log parsers to identify log entries from secondary Postfix
    instances.

multi_instance_group
    Each Postfix instance may be assigned an "instance group" name (with
    "postfix -e create/import/assign -G name..."). The default (empty) value of
    multi_instance_group parameter indicates a stand-alone instance that is not
    part of any group. The group name can be used with the postmulti(1)
    command-line utility to perform a task on the members of a group by name.
    Choose a single-word group name that concisely captures the role of the
    group.

multi_instance_enable
    This parameter controls whether a Postfix instance will be started by a
    Postfix multi-instance manager. The default value is "no". The instance can
    be started explicitly with "postfix -c /path/to/config/directory"; this is
    useful for testing.

    When an instance is disabled, the postfix(1) "start" command is replaced by
    "check".

    Some postfix(1) commands (such as "stop", "flush", ...) require a running
    Postfix instance, and skip instances that are disabled.

    Other postfix(1) commands (such as "status", "set-permissions", "upgrade-
    configuration", ...) do not require a running Postfix system, and apply to
    all instances whether enabled or not.

The postmulti(1) utility can be used to create (or destroy) instances. It can
also be used to "import" or "deport" existing instances into or from the list
of managed instances. When using postmulti(1) to manage instances, the above
configuration parameters are managed for you automatically. See below.

Using the postmulti(1) command

  * Initializing the multi-instance manager
  * Listing managed instances
  * Starting or stopping a multi-instance system
  * Ad-hoc multi-instance operations
  * Creating a new Postfix instance
  * Destroying a Postfix instance
  * Importing an existing Postfix instance
  * Deporting a managed Postfix instance
  * Assigning a new name or group name
  * Enabling/disabling managed instances

Initializing the multi-instance manager

Before postmulti(1) is used for the first time, you must install it as the
multi_instance_wrapper for your Postfix system and enable multi-instance
operation of the default Postfix instance. You can then proceed to add new or
existing instances to the multi-instance configuration. This initial
installation is accomplished as follows:

        # postmulti -e init

This updates the default instance main.cf file as follows:

        # Use postmulti(1) as a postfix-wrapper(5)
        #
        multi_instance_wrapper = ${command_directory}/postmulti -p --

        # Configure the default instance to start when in multi-instance mode
        #
        multi_instance_enable = yes

If you prefer, you can make these changes by editing the default main.cf
directly, or by using "postconf -e".

Listing managed instances

The list of managed instances consists of the default instance and the
additional instances whose configuration directories are listed (in start
order) under the multi_instance_directories parameter of the default main.cf
configuration file.

You can list selected instances, groups of instances or all instances by
specifying only the instance matching options with the "-l" option. The "-a"
option is assumed if no other instance selection options are specified (this
behavior changes with the "-e" option). As a special case, even if it has an
explicit name, the default instance can always be selected via "-i -".

    # postmulti -l -a
    # postmulti -l -g a_group
    # postmulti -l -i an_instance

The output is one line per instance (in "postfix start" order):

                                               
    |name   |group|enabled|config_directory    |
    |       |     |       |                    |
    |-      |-    |yes    |/etc/postfix        |
    |       |     |       |                    |
    |mta-out|mta  |yes    |/etc/postfix/mta-out|
    |       |     |       |                    |
    |mta-in |mta  |yes    |/etc/postfix-mta-in |
    |       |     |       |                    |
    |msa-out|msa  |yes    |/etc/postfix-msa-out|
    |       |     |       |                    |
    |msa-in |msa  |yes    |/etc/postfix-msa-in |
    |       |     |       |                    |
    |test   |-    |no     |/etc/postfix-test   |
    |       |     |       |                    |

The first line showing the column headings is not part of the output. When
either the instance name or the instance group is not set, it is shown as a "-
".

When selecting an existing instance via the "-i" option, you can always use the
full pathname of its configuration directory instead of the instance (short)
name. This is the only way to select a non-default nameless instance. The
default instance can be selected via "-i -", whether it has a name or not.

To list instances in reverse start order, include the "-R" option together with
the instance selection options.

Starting or stopping a multi-instance system

To start, stop, reload, etc. the complete (already configured as above) multi-
instance system just use postfix(1) as you would with a single-instance system.
The Postfix multi-instance wrapper framework insulates Postfix init.d start and
package upgrade scripts from the details of multi-instance management!

The -p option of postmulti(1) turns on postfix(1) compatibility mode. With this
option the remaining arguments are exactly those supported by postfix(1), but
commands are applied to all instances or all enabled instances as appropriate.
As described above, this switch is required when using postmulti(1) as the
multi_instance_wrapper.

If you want to specify a subset of instances by name, or group name, or run
arbitrary commands (not just "postfix stop/start/etc. in the context
(MAIL_CONFIG environment variable setting) of a particular instance or group of
instances, then you can use the instance-aware postmulti(1) utility directly.

Ad-hoc multi-instance operations

The postmulti(1) command can be used by the administrator to run arbitrary
commands in the context of one or more Postfix instances. The most common use-
case is stopping or starting a group of Postfix instances:

    # postmulti -g mygroup -p start
    # postmulti -g mygroup -p flush
    # postmulti -g mygroup -p reload
    # postmulti -g mygroup -p status
    # postmulti -g mygroup -p stop
    # postmulti -g mygroup -p upgrade-configuration

The -p option is essentially a short-hand for a leading postfix command
argument, but with appropriate additional options turned on depending on the
first argument. In the case of "start", disabled instances are "checked"
(postfix check) rather than simply skipped.

The resulting command is executed for each candidate instance with the
MAIL_CONFIG environment variable set to the configuration directory of the
corresponding Postfix instance.

The postmulti(1) utility is able to launch commands other than postfix(1), Use
the -x option to ask postmulti to execute an ad-hoc command for all instances,
a group of instances, or just one instance. With ad-hoc commands the
multi_instance_enable parameter is ignored: the command is unconditionally
executed for the instances selected via -a, -g or -i. In addition to
MAIL_CONFIG, the following instance parameters are exported into the command
environment:

    command_directory=$command_directory
    daemon_directory=$daemon_directory
    config_directory=$config_directory
    queue_directory=$queue_directory
    data_directory=$data_directory
    multi_instance_name=$multi_instance_name
    multi_instance_group=$multi_instance_group
    multi_instance_enable=$multi_instance_enable

The config_directory setting is of course the same as MAIL_CONFIG, and is
arguably redundant, but leaving it in is less surprising. If you want to skip
disabled instances, just check multi_instance_enable environment variable and
exit if it is set to "no".

The ability to run ad-hoc commands opens up a wealth of additional
possibilities:

  * Specify an instance by name rather than configuration directory when using
    sendmail(1) to send a verification probe:

        $ postmulti -i postfix-myinst -x sendmail -bv test@example.net

  * Display non-default main.cf settings of all Postfix instances. This uses an
    inline shell script to package together multiple shell commands to execute
    for each instance:

        $ postmulti -x sh -c 'echo "-- $MAIL_CONFIG"; postconf -n'

  * Put all mail in enabled member instances of a group on hold:

        # postmulti -g group_name -x \
            sh -c 'test $multi_instance_enable = yes && postsuper -h ALL'

  * Show top 10 domains in the deferred queue of all instances:

        # postmulti -x sh -c 'echo "-- $MAIL_CONFIG"; qshape deferred | head -
        12'

Creating a new Postfix instance

The postmulti(1) command can be used to create additional Postfix instances.
New instances are created with local submission and all "inet" services
disabled via the following non-default parameter settings in the main.cf file:

    authorized_submit_users =
    master_service_disable = inet

The above settings ensure that new instances are safe to start immediately:
they will not conflict with inet listeners in existing Postfix instances. They
will also not accept any mail until they are fully configured, at which point
you can do away with one or both of the above safety measures.

The postmulti(1) command encourages a preferred way of organizing the
configuration directories, queue directories and data directories of non-
default instances. If the default instance settings are:

    config_directory = /conf-path/postfix
    queue_directory = /queue-path/postfix
    data_directory = /data-path/postfix

A newly-created instance named postfix-myinst will by default have:

    multi_instance_enable = no
    multi_instance_name = postfix-myinst
    config_directory = /conf-path/postfix-myinst
    queue_directory = /queue-path/postfix-myinst
    data_directory = /data-path/postfix-myinst

You can override any of these defaults when creating the instance, but unless
you want to spread instance queue directories over multiple file-systems, use
the default naming strategy. It keeps the multiple instances organized in a
uniform, predictable fashion.

When specifying the instance name later, you can refer to it either as
"postfix-myinst", or via the full path of the configuration directory.

To create a new instance just use the -e create option:

    # postmulti -I postfix-myinst -e create

If the new instance is to belong to a group of related instances that implement
a single logical service, assign it to a group:

    # postmulti -I postfix-myinst -G mygroup -e create

If you want to override the conventional values of the instance installation
parameters, specify their values on the command-line:

    # postmulti [-I postfix-myinst] [-G mygroup] -e create \
    	"config_directory = /path/to/config_directory" \
    	"queue_directory = /path/to/queue_directory" \
    	"data_directory = /path/to/data_directory"

A note on the -I and -G options above. These are always used to assign a name
or group name to an instance, while the -i and -g options always select
existing instances. By default, the configuration directories of newly managed
instances are appended to the instance list. You can use the "-i" or "-g" or "-
a" options to insert the new instance before the specified instance or group,
or at the beginning of the instance list (multi_instance_directories parameter
of the default instance).

If you do specify a name (use "-I" with a name that is not "-") for the new
instance, you may omit any of the 3 instance installation parameters whose
instance-name based value is acceptable. Otherwise, all three instance
installation parameters are required. You should set the "syslog_name"
explicitly in the main.cf file of a "nameless" instance, in order to avoid
confusion in the mail logs when multiple instances are in use.

Destroying a Postfix instance

If you no longer need an instance, you can destroy it via:

    # postmulti -i postfix-myinst -p stop
    # postmulti -i postfix-myinst -e disable
    # postmulti -i postfix-myinst -e destroy

The instance must be stopped, disabled and have no queued messages. This is
expected to fully delete a just created instance that has never been used. If
the instance is not freshly created, files added after the instance was created
will remain in the configuration, queue or data directories, in which case the
corresponding directory may not be fully removed and a warning to that effect
will be displayed. You can complete the destruction of the instance manually by
removing any unwanted remnants of the instance-specific "private" directories.

Importing an existing Postfix instance

If you already have an existing secondary Postfix instance that is not yet
managed via postmulti(1), you can "import" it into the list of managed
instances. If your instance is already using the default configuration
directory naming scheme, just specify the corresponding instance name (the
multi_instance_name parameter in its configuration file will be adjusted to
match this name if necessary):

    # postmulti -I postfix-myinst [-G mygroup] -e import

Otherwise, you must specify the location of its configuration directory:

    # postmulti [-I postfix-myinst] [-G mygroup] -e import \
    	"config_directory = /path/of/config_directory"

When the instance is imported, you can assign a name or a group. As with
"create", you can control the placement of the new instance in the start order
by using "-i", "-g" or "-a" to prepend before the selected instance or
instances.

An imported instance is usually not multi-instance "enabled", unless it was
part of a multi-instance configuration at an earlier time. If it is fully
configured and ready to run, don't forget to enable it and if necessary start
it. When other enabled instances are already running, new instances need to be
started individually when they are first created or imported.

To find out what instances are running, use:

    # postfix status

Deporting a managed Postfix instance

You can "deport" an existing instance from the list of managed instances. This
does not destroy the instance, rather the instance just becomes a stand-alone
Postfix instance not registered with the multi-instance manager. postmulti(1)
will refuse to "deport" an instance that is not stopped and disabled.

    # postmulti -i postfix-myinst -p stop
    # postmulti -i postfix-myinst -e disable
    # postmulti -i postfix-myinst -e deport

Assigning a new name or group name

You can assign a new name or new group to a managed instance. Use "-" as the
new value to assign the instance to no group or make it nameless. To specify a
nameless secondary instance use the configuration directory path instead of the
old name:

    # postmulti -i postfix-old [-I postfix-new] [-G newgroup] -e assign

Enabling/disabling managed instances

You can enable or disable a managed instance. As documented in postfix-wrapper
(5), disabled instances are skipped with actions that start, stop or control
running Postfix instances.

    # postmulti -i postfix-myinst -e enable
    # postmulti -i postfix-myinst -e disable

Credits

Wietse Venema created Postfix, designed and implemented the multi-instance
wrapper framework and provided design feedback that made the postmulti(1)
utility much more general and useful than originally envisioned.

The postmulti(1) utility was developed by Victor Duchovni of Morgan Stanley,
who also wrote the initial version of this document.

