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master.cf is Postfix’s service map. It tells the resident master(8) process which daemons to start, where they listen, what privileges and limits they use, and which service-specific parameter overrides apply. main.cf supplies global parameters; master.cf assembles those parameters into runnable services. After any edit, run postfix reload or the new definitions will not take effect.
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What master.cf controls
The Postfix master(8) daemon is the resident process manager. It starts mail daemons on demand for network reception, local injection, queue management, and delivery. The master.cf file describes those services and the conditions under which they run.
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It is not a general-purpose list of every Postfix setting. Global behavior—such as the server’s hostname, queue directories, relay policy, and default TLS parameters—normally belongs in main.cf. A master.cf entry can, however, override selected main.cf values for one service. This is how an authenticated submission listener can have stricter TLS and SASL requirements than a public SMTP listener.
Package maintainers can ship different templates, and Postfix versions can add or retire services. Treat the file installed on the host as the starting point rather than copying a template from another distribution unchanged.
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How a message moves through the services
The most useful way to read master.cf is as a mail-flow map:
SMTP client -> smtpd -> cleanup -> qmgr -> smtp/local/virtual/lmtp
local mail -> pickup -> cleanup -> qmgr -> delivery transportAn SMTP connection is accepted by smtpd. Messages created by local programs enter through pickup. Both paths hand the message to cleanup, which performs canonicalization and places the result in the queue. The qmgr queue manager schedules delivery, then a transport such as smtp, local, virtual, or lmtp performs onward or local delivery.
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Several supporting services do not represent a final delivery hop. anvil maintains connection and rate accounting, scache caches SMTP-client connections, and tlsmgr manages TLS-session cache data and randomness. Whether each is enabled depends on the installed Postfix configuration and the features in use.
The eight fields in a service entry
Each logical entry has eight whitespace-separated fields. A line whose first character is not whitespace starts a service. Lines beginning with whitespace continue that service, which is why indented -o name=value settings belong below the service they modify.
| Field | Meaning | Typical values or effect |
|---|---|---|
service |
The service name used by Postfix and, for network services, the endpoint name. | smtp, submission, pickup, cleanup, qmgr |
type |
The endpoint mechanism used to contact the service. | inet for a TCP listener, unix for a Unix-domain socket, or fifo for a FIFO endpoint |
private |
Whether access is restricted to Postfix-private clients. | y restricts access; n permits the normal service access model |
unprivileged |
Whether the service runs without elevated privilege. | y requests an unprivileged process; the appropriate value depends on the daemon |
chroot |
Whether the service uses the Postfix chroot setting. | y enables chroot operation where configured; - uses the default behavior |
wakeup |
An automatic wake-up interval for services that need periodic activation. | A number of seconds, or - for the default/no periodic wake-up behavior |
process_limit |
The maximum number of concurrent processes for that service. | An integer such as 1; 0 has a service-specific meaning and must be interpreted with the daemon documentation |
command + args |
The daemon executable followed by optional arguments. | smtpd, pickup, cleanup, or another Postfix service command |
Whitespace, rather than visual column alignment, separates fields. Keep columns aligned for human review, but do not insert tabs or spaces inside a value. An indented continuation line is not a new service; it is an override or argument attached to the preceding entry.
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If the same service name and type are defined more than once, the last definition takes effect. Apart from that duplicate-resolution rule, the order of service definitions does not determine mail-flow behavior.
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The following excerpt is illustrative. It shows the structure of a network listener and core internal services; it is not a claim that every distribution ships these exact values.
smtp inet n - y - - smtpd
pickup unix n - y 60 1 pickup
cleanup unix n - y - 0 cleanup
qmgr fifo n - y 300 1 qmgrThe first line creates an inet SMTP listener handled by smtpd. The remaining lines expose internal endpoints for local injection, message cleanup, and queue management. Their process and wake-up values are examples for explaining the syntax; compare them with the template installed by your operating system before changing production settings.
How to enable client submission on port 587
Client submission should be a separate service definition so its authentication and encryption policy can be reviewed independently from the general SMTP listener. The service name submission is normally associated with TCP port 587 through the system service database. Confirm that mapping on the host and confirm that the firewall and any upstream network policy allow the port.
1. Inspect the installed template
Open the existing master.cf and look for a commented or active submission inet entry. Do not assume that a line copied from another Postfix release has the same defaults, available parameters, or package-specific comments.
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2. Add one service entry
A typical structure looks like this, with the policy values selected for your deployment:
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 indented lines are service-local overrides. They do not change the corresponding global values for the ordinary smtp listener. The example requires TLS and enables SASL authentication, then permits authenticated recipients while rejecting unauthenticated attempts. Existing site policy may use a different restriction chain; preserve required relay, access, and client checks rather than replacing them blindly.
3. Decide between explicit and implicit TLS
Submission commonly starts as a plain connection that is upgraded with STARTTLS. An implicit-TLS service instead begins inside TLS and needs the appropriate wrapper-mode setting. The Postfix TLS guidance documents the special-service pattern and the override -o smtpd_tls_wrappermode=yes for implicit TLS where that mode is appropriate. Do not add wrapper mode to a listener intended for STARTTLS; clients and port policy must agree with the chosen mode.
4. Check authentication prerequisites
The listener alone does not create user accounts or an authentication backend. Ensure that the SASL mechanism, credential source, TLS certificates, and relay restrictions required by your environment are already configured. A service that advertises authentication but cannot reach its SASL provider will accept connections and then fail at the authentication step.
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After saving the file, execute:
postfix reloadThen verify that the expected socket is listening, inspect Postfix logs for configuration or TLS errors, and perform an authenticated submission test using a real client configured for the selected TLS mode. Confirm both successful authenticated delivery and rejection of an unauthenticated relay attempt.
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A safe editing and rollout workflow
- Back up the current file. Preserve a timestamped copy so you can restore the last known-good service map.
- Read the local template first. Distribution packages may comment out optional services or use different privilege, chroot, and process-limit choices.
- Change one service at a time. Keep the original entry nearby while editing so that a failed change has a clear comparison point.
- Preserve the eight-field structure. Keep the service name, endpoint type, access flags, wake-up value, process limit, and command in their own whitespace-separated fields.
- Indent every service-specific override. A non-indented
-oline starts a new entry and will not modify the intended service. - Review security boundaries. Check listener addresses, TLS requirements, SASL enablement, relay restrictions, privilege flags, chroot choices, and process limits together.
- Reload Postfix. The
master(5)manual explicitly requirespostfix reloadafter changingmaster.cf. - Verify behavior, not just syntax. Check listening sockets, logs, queue movement, local delivery, outbound delivery, and an authenticated submission path relevant to the host.
A reload asks the resident master process to reread its service definitions. It does not replace the need to test the resulting network exposure or mail policy.
Keep main.cf and master.cf responsibilities separate
Use main.cf for |
Use master.cf for |
|---|---|
| Global parameter values and defaults | Which daemons exist as services and how they are started |
| Server-wide identity, queue, relay, and default TLS policy | Network versus Unix/FIFO endpoint type |
| Defaults shared by several listeners | Per-service privilege, chroot, wake-up, and process limits |
| Baseline authentication and restriction settings | Indented -o parameter=value overrides for one listener or daemon |
When a value appears in both places, the service-local override applies to the service that carries it. This is useful for a submission listener, but it can also create confusing differences if an override is left behind after a policy change. During reviews, search both files for the parameter and identify which service receives each effective value.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing which services should be enabled
Public SMTP reception
Enable an smtp inet listener only when the host is intended to receive SMTP connections. Review its address binding, TLS behavior, client restrictions, and relay policy independently of submission.
Client submission
Use a separate submission service when mail clients need authenticated sending. Its TLS and SASL requirements should be explicit in indented overrides, not inferred from the public SMTP listener.
Local injection and queue services
pickup, cleanup, and qmgr form the internal path from locally submitted mail to queued delivery. Their endpoint types and process limits are normally kept close to the package defaults unless a documented operational requirement justifies a change.
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Delivery transports
smtp, local, virtual, and lmtp represent different onward or local-delivery paths. Enable and tune the transports that your address classes and routing configuration actually use; an unused transport adds configuration surface without providing a mail path.
Support daemons
anvil, scache, and tlsmgr support accounting, connection reuse, and TLS state. Their presence should match the features and defaults of the installed Postfix version rather than a generic checklist.
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- The new listener is not present. The file may have been edited without a reload. Run
postfix reload, then inspect the listening sockets and logs. - A continuation override is ignored. Check that every
-oline starts with whitespace and immediately follows the service it modifies. - The wrong duplicate entry wins. Search for repeated service/type pairs. The last definition is effective, so an old later entry can silently override a new one.
- Clients connect but cannot authenticate. Confirm that
smtpd_sasl_auth_enableis enabled for the submission service and that the configured SASL provider and credentials are available. - TLS negotiation fails. Check certificate availability, the selected TLS mode, and whether wrapper mode was enabled accidentally on a STARTTLS listener.
- Mail is accepted but not delivered. Follow the path from
cleanuptoqmgrand then the selected delivery transport. Queue and transport logs identify where processing stops. - A service starts with unexpected privilege or isolation. Recheck the
private,unprivileged, andchrootcolumns against the daemon’s requirements and the package template. - Connection load causes resource pressure. Review the service’s
process_limitand wake-up settings together with the host’s capacity. A lower limit can protect the host but may reduce concurrency.
How to compare two master.cf files
Do not compare only the visible list of service names. A useful review records these differences for each service:
- Enabled or disabled status and the listening endpoint or socket type
- Service-local TLS, SASL, relay, and restriction overrides
private,unprivileged, andchrootflags- Wake-up intervals and process limits
- The command and arguments that actually run
- The resulting delivery path from injection through queue management to transport
- Postfix version and operating-system package differences that explain template changes
Two files can expose the same port yet have materially different security because one places authentication or TLS settings on an indented override and the other leaves the service inheriting global defaults.
Operational checklist
- Identify whether each non-indented line starts a new service.
- Count the eight fields and verify that the command is in the final field.
- Confirm that every continuation line is indented.
- Look for duplicate service/type definitions and check which one appears last.
- Separate global
main.cfpolicy from service-localmaster.cfoverrides. - Review TLS mode, SASL, relay restrictions, privilege, chroot, wake-up, and process limits for each network listener.
- Run
postfix reloadafter saving. - Verify sockets, logs, queue behavior, local and outbound delivery, and authenticated submission.
With those checks, master.cf becomes a readable map of Postfix’s running architecture rather than a block of unexplained columns: listeners feed cleanup, queue management schedules work, transports deliver it, and each service can carry the isolation and security policy appropriate to its role.
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