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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minutemaster.cf is Postfix’s service map: it tells the resident master(8) process which daemons to start, where they listen, what privileges they use, and how many processes may run. The file controls service-level behavior, while main.cf stores global parameter values. Understanding the eight fields, the mail-flow roles, and the reload procedure lets you enable services such as authenticated submission on port 587 without changing the wrong layer.
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What master.cf controls
Postfix is a collection of cooperating daemons rather than one monolithic server. The master(8) daemon is the resident process manager that starts Postfix daemons on demand—for network reception and delivery, local delivery, queue management, and other support work. Its service definitions are read from master.cf.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsEach logical entry describes one service. A non-whitespace line starts an entry; a line beginning with whitespace continues the preceding entry. That continuation rule is why service-specific -o name=value overrides appear on indented lines beneath a service. Whitespace separates fields, so columns may be aligned for readability but do not require a particular number of spaces.
The file is not a replacement for main.cf. Global settings belong in main.cf; master.cf determines which services exist and can override selected parameters for one service. If the same service name and service type are defined more than once, the last definition takes effect. The order of otherwise distinct service definitions does not determine mail-flow behavior.
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The Postfix mail-flow model
A useful way to read a configuration is to follow a message through the daemons:
SMTP client -> smtpd -> cleanup -> qmgr -> smtp/local/virtual/lmtp
local mail -> pickup -> cleanup -> qmgr -> delivery transportAn incoming network connection is accepted by an smtpd service. Messages generated by local applications enter through pickup. Both paths hand the message to cleanup, which performs canonicalization and queues it. The qmgr queue manager schedules delivery. A transport such as smtp, local, virtual, or lmtp then delivers the message onward or to a local destination.
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The eight fields in every service entry
The Postfix master(5) format has eight whitespace-separated fields. The final field contains the executable name and any arguments; an indented continuation can add service-local overrides.
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| Field | Meaning | Typical values or examples |
|---|---|---|
| service | The service name used by Postfix. | smtp, submission, pickup, cleanup |
| type | The endpoint or activation mechanism. | inet, unix, fifo, or another supported type |
| private | Whether access is restricted to Postfix-private clients. | y or n |
| unprivileged | Whether the service runs without elevated privilege. | y or n |
| chroot | Whether the service uses the Postfix chroot setting. | y or n |
| wakeup | Automatic wakeup interval, or the default/no periodic wakeup behavior. | A time value such as 60 or 300, or - |
| process_limit | Maximum number of concurrent processes for the service. | A number such as 1, or the configured limit |
| command and arguments | The daemon executable followed by optional arguments and overrides. | smtpd, pickup, cleanup, qmgr |
For example, in smtp inet n - y - - smtpd, smtp is the service name, inet makes it a network listener, the service is not private, it is unprivileged, it uses the configured chroot choice, it has no periodic wakeup interval, and it starts the smtpd daemon. The dashes are meaningful defaults in those positions; they are not decorative separators.
Reading a practical baseline
The following fragment is deliberately illustrative. Package templates and Postfix versions can ship different defaults, so compare it with the template installed on your host before making changes.
smtp inet n - y - - smtpd
pickup unix n - y 60 1 pickup
cleanup unix n - y - 0 cleanup
qmgr fifo n - y 300 1 qmgrNetwork SMTP listener
The smtp service with type inet accepts network SMTP connections and starts smtpd. Its address and port exposure are part of the listener configuration, while authentication, TLS, and recipient restrictions can be supplied globally or overridden on a dedicated service.
Local pickup
pickup is normally a local Unix service. It notices messages submitted through Postfix’s local maildrop and passes them into cleanup. The illustrative wakeup value of 60 means the service is checked on that interval; the process limit of 1 allows one pickup process at a time.
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Cleanup
cleanup receives messages from both network and local-entry paths. It normalizes addresses and other message data, then places the result in the queue. A process limit of 0 in a template has Postfix-specific semantics; do not substitute a different value without checking the manual for the installed version.
Queue manager
qmgr schedules queued mail for delivery transports. In the example it is activated through a fifo endpoint, has a 300-second wakeup interval, and is limited to one process. Actual distributions may choose another endpoint or values.
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Enable the services required by the mail roles your host actually performs, and keep unused listeners disabled. A typical installation needs a network smtpd listener, local pickup, cleanup, qmgr, and the delivery transports required by its domains. Additional services are justified by a specific function:
submission: a separate client-submission listener, normally used on port 587 with explicit TLS and authentication policy.smtpsor another TLS listener: an implicit-TLS service when that mode is required by clients; its TLS wrapper setting must be applied to that service.local,virtual, orlmtp: delivery transports selected by the domains and transport maps in your deployment.anvil,scache, andtlsmgr: support daemons for connection accounting, SMTP-client connection caching, and TLS cache/randomness management.
Do not infer that a service is safe merely because it appears in a vendor template. Review its listener type, privilege flags, process limit, and any indented overrides. A service that accepts network connections deserves a separate security review from a private Unix or FIFO service.
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How to enable authenticated submission on port 587
Port 587 is normally represented by a service named submission with type inet. The important design is to give submission its own service definition so that client TLS, SASL authentication, recipient restrictions, and logging policy do not accidentally alter the general SMTP listener.
- Inspect the installed template. Make a backup of the current
master.cf, then locate any existingsubmission inetentry. Do not create a second active definition without understanding which one will win; for the same service name and type, the last definition is effective. - Add or enable the listener. Remove the comment marker from the appropriate
submission inetline or add one that matches your package’s syntax. The executable in the final field is normallysmtpd. - Add indented service overrides. Place each
-o parameter=valueline underneath the submission entry, beginning it with whitespace. A common pattern is to require TLS, enable SASL authentication, require authentication before accepting relayable recipients, and use submission-specific logging. The exact parameter values must match the authentication and certificate setup in yourmain.cf. - Choose the TLS mode deliberately. Submission commonly negotiates TLS after connecting. If you configure an implicit-TLS listener instead, the TLS guide documents the service-local override
-o smtpd_tls_wrappermode=yes; apply it only to the listener intended for implicit TLS, not indiscriminately to ordinary SMTP or submission. - Check exposure and policy. Confirm that the host firewall and any front-end proxy expose port 587, that the certificate and SASL mechanism are available, and that unauthenticated clients cannot use the listener as an open relay.
- Reload and test. Run
postfix reload, then verify the listening socket, inspect logs for service startup and authentication events, and perform an authenticated submission test appropriate to the environment.
A schematic service definition looks like this; the override names and values are examples, not universal defaults:
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submission inet n - y - - smtpd
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_recipient_restrictions=permit_sasl_authenticated,rejectThe indentation is functional: without it, Postfix treats the line as a new service entry rather than a continuation. Keep the general smtp service’s policy separate unless you intentionally want both listeners to behave identically.
A safe editing and reload workflow
- Back up first. Preserve the working file so you can restore it if a listener disappears or a daemon fails to start.
- Read the local template and version documentation. Distribution packages can enable different services, use different chroot choices, or place comments around optional listeners.
- Change one service at a time. This makes a new socket, authentication policy, or process-limit problem easier to identify.
- Preserve readable alignment. Alignment helps human review, but remember that whitespace—not a fixed column width—separates fields.
- Review all eight fields. Check endpoint type, private access, privilege, chroot, wakeup, process limit, executable, and arguments. Then review every indented
-ooverride. - Reload Postfix. Editing the file alone does not activate the new definitions. The official manual states: "After changing master.cf you must execute "postfix reload" to reload the configuration."
- Observe the result. Confirm listeners, inspect logs, submit a test message, and watch queue behavior. A successful reload does not prove that TLS certificates, SASL authentication, recipient restrictions, or downstream delivery are correct.
master.cf and main.cf: keep their jobs separate
| Question | Where to look |
|---|---|
| Which daemons exist and which ports or endpoints activate them? | master.cf |
| What global TLS, relay, queue, or domain parameters apply? | main.cf |
| Should one listener require TLS or authentication differently? | An indented -o name=value override under that service in master.cf |
| Why did a changed service not appear? | Check that postfix reload was run and that another definition of the same service and type is not later in the file. |
This separation prevents a common mistake: changing a global parameter when the requirement is limited to submission, or editing a service line when the value is meant to apply to every Postfix daemon.
How to compare two master.cf files
When reviewing a package upgrade, a migration, or a security change, compare configurations by function rather than by line order:
- Enabled services and endpoints: identify every
inet,unix,fifo, and other active definition, including the ports represented by network listeners. - Service-local security: compare TLS, SASL, relay, recipient, and logging overrides on indented lines.
- Privilege and isolation: compare private, unprivileged, and chroot flags.
- Capacity behavior: compare wakeup intervals and process limits; a smaller limit can serialize work, while a larger one can increase resource pressure.
- Delivery path: verify that
pickupandsmtpdstill feedcleanup, thatqmgrremains available, and that the expected delivery transports are present. - Version and distribution differences: treat comments and defaults as package-specific, and validate the effective behavior on the host rather than copying a file from another installation.
Troubleshooting common failures
The new listener is not accepting connections
Check that the service line is active rather than commented, uses the intended inet type, and is not shadowed by a later definition with the same service name and type. Run postfix reload, then inspect the host’s listening sockets and Postfix logs. Firewall or front-end proxy rules can also prevent clients from reaching a correctly configured listener.
Indented overrides seem ignored
Verify that every continuation line begins with whitespace and follows the intended service immediately. A missing indent turns the line into a malformed or separate entry. Also check for a later definition of the same service and type, which can replace the earlier one.
Clients connect but cannot authenticate or submit
Review the submission service’s TLS and SASL overrides, certificate availability, and recipient restrictions together. A listener can be reachable while still refusing clients because TLS is required, authentication is unavailable, or the restriction policy rejects unauthenticated recipients. Compare the logs from a controlled authenticated test with the effective settings in main.cf.
Mail is accepted but remains queued
Follow the flow after cleanup: confirm that qmgr is active and that the selected smtp, local, virtual, or lmtp transport is defined and able to deliver. Queue behavior is not fixed by changing a network listener alone.
A reload appears successful but behavior is unchanged
Remember that a file edit has no effect until postfix reload is executed. If behavior still does not change, check whether the edited entry is overridden later in the file and whether the client is reaching a different listener or proxy than the one you changed.
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Quick Recap
Practical rules to retain
master.cfdefines how Postfix services are started;main.cfcontains global parameter values.- Every logical entry follows the eight-field service format.
- Whitespace-leading lines are continuations, making them the place for service-specific
-ooverrides. - The normal path is
pickuporsmtpd, thencleanup,qmgr, and a delivery transport. - Submission and TLS listeners are separate service definitions whose security settings must be reviewed explicitly.
- After editing, run
postfix reloadand verify sockets, logs, authentication, and queue delivery.
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