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master.cf is Postfix’s service map: it tells the master(8) process which daemons exist, how they are started, what endpoint each uses, and which service-specific limits or security settings apply. Edit it when you need to expose a listener such as SMTP submission on port 587, change a daemon’s privilege or chroot mode, or tune process and wakeup behavior. After every edit, run postfix reload; otherwise the running master process continues using the old definitions.
What master.cf controls
Postfix separates global mail parameters from service startup definitions. The main.cf file contains global parameter values, while master.cf describes the services that the resident master(8) process starts on demand. The official master(8) manual describes master as the process that runs Postfix daemons for network receipt and delivery, local delivery, and related work.
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Postfix resolves duplicate definitions of the same service name and type by using the last definition. The order of otherwise distinct service definitions does not determine mail-flow behavior. These rules matter when a package template contains a disabled example and you add an enabled definition later: the effective entry is the final matching one.
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The eight fields in every service entry
A service entry has eight whitespace-separated fields. The final field contains the daemon command and any arguments; the first seven describe how that command is exposed and managed.
| Field | Meaning | Typical values or use |
|---|---|---|
| service | The service name used by Postfix. | smtp, submission, smtpd-backed listeners, pickup, cleanup, or qmgr. |
| type | The endpoint or activation mechanism. | inet for a TCP listener, unix for a Unix-domain socket, and fifo for a FIFO endpoint. |
| private | Whether access is restricted to Postfix-private clients. | y or n, depending on the service’s intended callers. |
| unprivileged | Whether the daemon runs without elevated privilege. | y is common for daemons that do not need privileged access. |
| chroot | Whether the service uses the Postfix chroot setting. | y or n; this must match the files and sockets available inside the chroot. |
| wakeup | Automatic wakeup interval, or the default behavior. | A number of seconds such as 60 or 300, or - when no periodic wakeup is configured for that entry. |
| process_limit | Maximum number of concurrent processes for the service. | 1 for a single-process internal service, another finite value for a listener, or 0 where the service is not meant to be started concurrently. |
| command + args | The daemon executable and optional command-line arguments. | smtpd, pickup, cleanup, qmgr, or another Postfix daemon, followed by supported arguments. |
Do not treat a value copied from one distribution as a universal requirement. Package templates and Postfix versions can choose different defaults. Read the comments and active entries installed on the machine before changing a field, and verify the corresponding daemon’s manual page when you alter a security-sensitive value.
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Thinking in terms of a pipeline makes an unfamiliar file understandable. A network message is accepted by an SMTP server process, normalized and queued by internal services, then handed to a transport that delivers it locally or to another host.
SMTP client -> smtpd -> cleanup -> qmgr -> smtp/local/virtual/lmtp
local mail -> pickup -> cleanup -> qmgr -> delivery transportNetwork receipt with smtpd
An inet entry that runs smtpd accepts TCP connections. A conventional public SMTP listener is named smtp and normally serves port 25. The listener’s restrictions, TLS policy, and authentication behavior come from global parameters unless service-specific overrides are added below the entry.
Local injection with pickup
Messages submitted by local processes enter through pickup, usually using a Unix endpoint. Pickup reads the locally submitted message and passes it to cleanup. It is not a replacement for an Internet-facing SMTP listener.
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Normalization and queueing with cleanup and qmgr
cleanup performs message cleanup and canonicalization before the message is placed in the queue. qmgr schedules queued deliveries and selects an appropriate transport. These services are internal plumbing: changing their process limits or wakeup behavior can affect every delivery path, so make such changes only with a clear operational reason.
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Delivery transports
The smtp transport sends mail to remote SMTP servers. local delivers to local system users, virtual handles virtual mailbox delivery, and lmtp hands messages to an LMTP server. The selected transport depends on Postfix routing and domain configuration; master.cf makes the daemon available but does not by itself decide which destination uses it.
Supporting daemons
anvil provides connection and rate accounting support. scache caches SMTP-client connections, and tlsmgr manages TLS session-cache data and randomness. Their entries are often present even when an administrator never invokes them directly. Disabling a supporting service without understanding which features depend on it can produce indirect failures.
A readable baseline configuration
The following excerpt illustrates the documented field layout and common internal services. It is an explanatory example, not a promise that every operating-system package 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 qmgrIn this sample, smtp is a TCP service, while pickup and cleanup use Unix-domain endpoints and qmgr uses a FIFO. The dashes in the sample mean that the corresponding field uses the service’s default behavior rather than an explicit numeric setting. Treat the process and wakeup numbers as illustrative: a vendor template can select different values, and a site may need different limits.
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Enabling authenticated submission on port 587
Port 587 is normally a separate submission service rather than a change to the port-25 listener. A typical definition starts with an inet service and then places security overrides on indented continuation lines. The exact TLS and SASL parameter names and values must agree with the authentication stack and certificates configured in main.cf.
submission inet n - y - - smtpd
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_recipient_restrictions=permit_sasl_authenticated,rejectThis snippet demonstrates the structure, not a drop-in policy for every installation. The first line defines the service; each indented -o name=value line overrides one parameter only for that service. Keep the overrides narrow so that port 25 and port 587 do not accidentally inherit the same client policy.
Choose explicit TLS behavior
Submission commonly requires an encrypted connection before authentication. Confirm that the certificate, key, protocol policy, and SASL provider are valid in the global configuration, then set the submission service’s TLS requirement deliberately. The Postfix TLS guide also documents a special-service pattern using -o smtpd_tls_wrappermode=yes for implicit TLS where that mode is appropriate. Implicit TLS and STARTTLS are different protocols: select the one your clients and deployment require rather than enabling both by assumption.
Review authentication and recipient restrictions
Authentication should be enabled only on the submission service when that is the intended design. Restrict relay permission to authenticated clients and retain recipient checks that prevent unauthenticated open relay behavior. A syntactically valid master.cf entry cannot compensate for an unsafe restriction list.
Confirm the listener and firewall
An enabled service must be reachable at the network layer. Check that Postfix binds the intended address and port, that the host firewall permits TCP 587, and that no other daemon already owns the socket. If IPv4 and IPv6 bindings are configured separately, verify both rather than assuming one implies the other.
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Safe editing and reload workflow
- Back up the active file. Copy the current
master.cfto a root-readable backup and record the installed Postfix version. Inspect the package’s comments and existing disabled examples before editing. - Change one service at a time. Preserve the eight-field structure. Keep columns aligned for humans, but remember that one or more spaces or tabs are the actual separators.
- Indent continuation lines. Put every service-local
-o parameter=valueoverride on a new line beginning with whitespace. A missing indent turns an intended override into a malformed new service line. - Audit security and capacity choices. Check the endpoint type, privilege and chroot flags, process limit, wakeup value, TLS policy, SASL setting, and recipient restrictions together. A change that looks harmless in isolation can alter exposure or resource use.
- Reload Postfix. Run
postfix reload. The officialmaster(5)manual states: “After changing master.cf you must execute "postfix reload" to reload the configuration.” Editing the file without this step leaves the running master process with its previous service map. - Verify the result. Check listening sockets, service logs, queue behavior, and an authenticated submission test suited to the environment. Confirm that the test message follows the intended path and that no unexpected listener appeared.
A reload applies the new service definitions without treating a reboot as the normal configuration mechanism. If a reload reports an error, restore the backup or correct the malformed entry, then reload again and inspect the logs before testing mail flow.
How to compare two master.cf files
Do not compare files only by looking for enabled or commented lines. Use the following axes so that a small override is not missed:
- Enabled services and endpoints: list service names, endpoint types, addresses or ports, and whether each line is active.
- Service-local security: compare every indented TLS, SASL, relay, client-restriction, and logging override.
- Privilege and isolation: compare the private, unprivileged, and chroot fields, then verify that required files and sockets exist inside any chroot.
- Capacity and scheduling: compare wakeup intervals and process limits. These values influence how often a daemon is prompted and how many instances can run concurrently.
- Delivery path: trace pickup or smtpd through cleanup and qmgr to smtp, local, virtual, or lmtp. A different transport entry can change where mail is delivered even when the listener looks identical.
- Version and distribution context: identify the Postfix release and package template for each file. Defaults, comments, and available service names can differ between releases and operating systems.
Keep global parameters in main.cf separate from service-local overrides. If two files appear to disagree, first determine whether one value is global and the other is an indented override that applies only to a particular service.
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Editing the wrong entry
Administrators sometimes uncomment a sample submission line but leave a later duplicate definition disabled or differently configured. Because the last definition wins for the same service name and type, inspect the entire file for duplicates before deciding which line is effective.
Putting an override at column one
A line such as -o smtpd_sasl_auth_enable=yes must be indented. Without leading whitespace it is parsed as the start of a service entry and can trigger a syntax error or unexpected behavior. Re-indent it beneath the service it belongs to.
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Changing a field without checking dependencies
Enabling chroot while leaving certificates, resolver files, or authentication sockets outside the chroot can make connections fail after reload. Raising a process limit can increase resource consumption, while lowering it can create queueing under load. Treat these fields as operational controls, not cosmetic formatting.
Expecting a file edit to open a port immediately
The service does not use a new definition until postfix reload runs. Even after reload, a host firewall, an address-binding choice, or another process can prevent a socket from listening. Check the local socket table and Postfix logs before troubleshooting client credentials.
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Testing only a successful local message
A local test exercises pickup and may never touch smtpd or submission. For a submission change, test an authenticated client against the intended port and TLS mode, then verify the queue and recipient result. For a delivery change, follow the message through qmgr and the selected transport.
A practical review checklist
- The service name and endpoint type match the intended role.
- All eight fields are present and separated by whitespace.
- Continuation lines are indented and contain only valid service-local overrides.
- Public listeners have deliberate TLS, authentication, and relay restrictions.
- Private, unprivileged, and chroot settings match the daemon’s required files and sockets.
- Wakeup and process-limit values are appropriate for the workload rather than copied blindly.
- Duplicate service definitions have been checked, with the final matching definition understood.
main.cfglobal settings andmaster.cflocal overrides are not being confused.postfix reloadhas been run after the edit.- Listening sockets, logs, queue state, and an end-to-end test confirm the intended behavior.
What to remember
master.cf is the launch and policy map for Postfix services, not a second copy of main.cf. Learn the eight fields, read indented -o lines as service-specific overrides, and trace mail from smtpd or pickup through cleanup and qmgr to its delivery transport. Submission on port 587 is a separate service whose TLS, SASL, and relay rules deserve an explicit review. Finally, every edit requires postfix reload before the new definitions can take effect.
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