MASTER(5)

💖 MASTER(5) 💖

File Formats Manual · master · the iconic one, the blueprint, the moment

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✨ NAME ✨

master — the format of the Postfix master process configuration file, bestie. it's giving "the file that tells everything else what to do." 👑

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🌸 DESCRIPTION 🌸

okay so. the Postfix mail system is built out of a small number of (mostly) client commands that you, the human, invoke directly — and a larger number of services that just vibe in the background doing their thing. 🫶

those Postfix services? they're implemented by daemon processes. they run in the background, and they get started on-demand by the master(8) process — like, the master is the group chat admin who pings people when they're needed. 💅

the master.cf configuration file is the whole point of this page. it defines:

• how a client program connects to a service, and
• what daemon program runs when that service gets requested.

most of these daemon processes are short-lived — they show up, do their job, and terminate voluntarily after either:

• serving max_use clients, or
• being inactive for max_idle or more units of time.

like, they serve their purpose and then exit gracefully. main character energy, honestly. ✨

important: all daemons specified in here must speak a Postfix-internal protocol. no cap.

so if you wanna run non-Postfix software? you don't just shove it in here, bestie. instead use the local(8), pipe(8) or spawn(8) services, or run that software with inetd(8) or an equivalent. those are the designated doors for outsiders. 🚪

heads up! after you change master.cf you must run:

postfix reload

to reload the configuration. literally do not skip this step. it will not pick up your changes otherwise. 😭 no reload, no slay.

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✨ SYNTAX ✨

ok so the general format of master.cf? it's giving very structured, very demure, here's the tea 💅

• Empty lines and whitespace-only lines are ignored, and so are lines whose first non-whitespace character is a #. (yes bestie, comment away 🎀)
• A logical line starts with non-whitespace text. A line that starts with whitespace continues a logical line. (so indentation = "and also…", not "new thing")
• Each logical line defines a single Postfix service. Each service is identified by its name and type as described below. When multiple lines specify the same service name and type, only the last one is remembered. Otherwise, the order of master.cf service definitions does not matter. (last one wins, no cap 😤)

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Each logical line consists of eight fields separated by whitespace. They're described below in the order they appear in the master.cf file.

Where applicable, a field of - requests that the built-in default value be used. For boolean fields, specify y or n to override the default value.

🦋 Service name

The service name syntax depends on the service type, which is described next.

🌸 Service type

Specify one of the following service types:

• inet — The service listens on a TCP/IP socket and is accessible via the network. ✨

The service name is specified as host:port, denoting the host and port on which new connections should be accepted. The host part (and colon) may be omitted. Either host or port may be given in symbolic form (see hosts(5) or services(5)) or in numeric form (IP address or port number). Host information may be enclosed inside "[]"; this form is necessary only with IPv6 addresses.

Examples: a service named 127.0.0.1:smtp or ::1:smtp receives mail via the loopback interface only; and a service named 10025 accepts connections on TCP port 10025 via all interfaces configured with the inet_interfaces parameter.

Note: with Postfix version 2.2 and later, specify "inet_interfaces = loopback-only" in main.cf, instead of hard-coding loopback IP address information in master.cf or in main.cf. (bestie please stop hard-coding, it's 2.2+ era 🙈)

• unix — The service listens on a UNIX-domain stream socket and is accessible for local clients only. 🫶

The service name is a pathname relative to the Postfix queue directory (pathname controlled with the queue_directory configuration parameter in main.cf).

On Solaris 8 and earlier systems the unix type is implemented with streams sockets.

• unix-dgram — The service listens on a UNIX-domain datagram socket and is accessible for local clients only. 🦋

The service name is a pathname relative to the Postfix queue directory (pathname controlled with the queue_directory configuration parameter in main.cf).

• fifo (obsolete) — The service listens on a FIFO (named pipe) and is accessible for local clients only. 💀

The service name is a pathname relative to the Postfix queue directory (pathname controlled with the queue_directory configuration parameter in main.cf).

• pass — The service listens on a UNIX-domain stream socket, and is accessible to local clients only. It receives one open connection (file descriptor passing) per connection request. 👑

The service name is a pathname relative to the Postfix queue directory (pathname controlled with the queue_directory configuration parameter in main.cf).

On Solaris 8 and earlier systems the pass type is implemented with streams sockets.

This feature is available as of Postfix version 2.5. ✨

🎀 Private (default: y)

Whether a service is internal to Postfix (pathname starts with private/), or exposed through Postfix command-line tools (pathname starts with public/). Internet (type inet) services can't be private. (nope, not allowed, don't even try it 💅)

💖 Unprivileged (default: y)

Whether the service runs with root privileges or as the owner of the Postfix system (the owner name is controlled by the mail_owner configuration variable in the main.cf file).

The local(8), pipe(8), spawn(8), and virtual(8) daemons require privileges. (they're the high-maintenance ones, they need their root energy 👑)

🌈 Chroot (default: Postfix >= 3.0: n, Postfix < 3.0: y)

Whether or not the service runs chrooted to the mail queue directory (pathname is controlled by the queue_directory configuration variable in the main.cf file).

Chroot should not be used with the local(8), pipe(8), spawn(8), and virtual(8) daemons. Although the proxymap(8) server can run chrooted, doing so defeats most of the purpose of having that service in the first place. (like… why would you 😭)

The files in the examples/chroot-setup subdirectory of the Postfix source show how to set up a Postfix chroot environment on a variety of systems. See also BASIC_CONFIGURATION_README for issues related to running daemons chrooted.

⭐ Wake up time (default: 0)

Automatically wake up the named service after the specified number of seconds. The wake up is implemented by connecting to the service and sending a wake up request. A ? at the end of the wake-up time field requests that no wake up events be sent before the first time a service is used. Specify 0 for no automatic wake up. (wake up bestie, it's time to work ☀️)

The pickup(8), qmgr(8) and flush(8) daemons require a wake up timer.

🔥 Process limit (default: $default_process_limit)

The maximum number of processes that may execute this service simultaneously. Specify 0 for no process count limit.

NOTE: Some Postfix services must be configured as a single-process service (for example, qmgr(8)) and some services must be configured with no process limit (for example, cleanup(8)). These limits must not be changed. 🚨 like do NOT touch these, i am so serious

💜 Command name + arguments

The command to be executed. Characters that are special to the shell such as ">" or "|" have no special meaning here, and quotes cannot be used to protect arguments containing whitespace. To protect whitespace, use "{" and "}" as described below. (the shell is not invited to this party 🙅)

The command name is relative to the Postfix daemon directory (pathname is controlled by the daemon_directory configuration variable).

The command argument syntax for specific commands is specified in the respective daemon manual page.

The following command-line options have the same effect for all daemon programs:

• -D — Run the daemon under control by the command specified with the debugger_command variable in the main.cf configuration file. See DEBUG_README for hints and tips. 🐛
• -o { name = value } (long form, Postfix >= 3.0) — Override the named main.cf configuration parameter. The parameter value can refer to other parameters as $name etc., just like in main.cf. See postconf(5) for syntax.

The "long form" supports whitespace in parameter values. Whitespace after the outer "{", around "=", and before the outer "}" is ignored. Example:

/etc/postfix/master.cf:
    submission inet .... smtpd
        -o { smtpd_xxx_yyy = text with whitespace... }

NOTE: Over-zealous use of parameter overrides makes the Postfix configuration hard to understand and maintain. At a certain point, it might be easier to configure multiple instances of Postfix, instead of configuring multiple personalities via master.cf. (bestie, chill on the overrides, your future self will thank you 😭)

• -o name=value (short form) — Override the named main.cf configuration parameter. The parameter value can refer to other parameters as $name etc., just like in main.cf. See postconf(5) for syntax.

The "short form" does not support whitespace around the "=" or in parameter values. To specify a parameter value that contains whitespace, use the long form described above, or use commas instead of spaces if the parameter supports that, or specify the value in main.cf. Example:

/etc/postfix/master.cf:
    submission inet .... smtpd
        -o smtpd_xxx_yyy=$submission_xxx_yyy

/etc/postfix/main.cf
    submission_xxx_yyy = text with whitespace...

NOTE: Over-zealous use of parameter overrides makes the Postfix configuration hard to understand and maintain. At a certain point, it might be easier to configure multiple instances of Postfix, instead of configuring multiple personalities via master.cf. (yes i'm repeating myself, it's that important 💅)

• -v — Increase the verbose logging level. Specify multiple -v options to make a Postfix daemon process increasingly verbose. 🗣️ (-vvvv = main character energy, fr fr)
• Command-line arguments that start with {
• Command-line arguments that contain whitespace
• Command-line arguments that must be empty — Specify "{" and "}" around such arguments (Postfix 3.0 and later). The outer "{" and "}" will be removed, together with any leading or trailing whitespace in the remaining text. ✨

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📚 SEE ALSO

• master(8), process manager
• postconf(5), configuration parameters

🧚 README FILES

Use "postconf readme_directory" or "postconf html_directory" to locate this information.

• BASIC_CONFIGURATION_README, basic configuration
• DEBUG_README, Postfix debugging

📜 LICENSE

The Secure Mailer license must be distributed with this software.

👑 AUTHOR(S)

Initial version by Magnus Baeck Lund Institute of Technology Sweden

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

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and that's the whole page bestie 🫶 this one came from the MASTER(5) manpage in the Postfix source, and it's dated like the original footer said — same source, same version, same date, i'm just here to make it slay ✨