Lesson 007 · Linux MTA Operations Learning Path

Postfix master.cf: SMTP, Submission and Service Isolation

Anil Jalela · Published Sep 3, 2026 · 7 min read

Labelled Linux mail flow showing Postfix master.cf service isolation configuration decision verification failure evidence and safe recovery

The master daemon creates Postfix listeners and delivery processes; service overrides keep Internet SMTP separate from authenticated user submission. This lesson treats Postfix master.cf service isolation as one decision point in a longer message path. The configuration is useful only when an operator can show which message entered it, which identity and rule matched, what the next daemon returned, and how a failure is retried or contained.

Prerequisites and inherited lab checkpoint

Lesson 6 routing must pass intended relay and loop tests. Keep submission disabled until the Dovecot authentication socket is built.

For the Postfix master.cf service isolation lab, use reserved domain example.test, documentation addresses and disposable messages. Preserve postconf -n, postconf -M, package versions, DNS answers and a queue baseline before changing the lab. Keep console access and do not copy credentials, private keys or customer messages into the evidence record.

Install and prepare the required components

sudo dnf info postfix dovecot
sudo dnf install -y postfix dovecot
sudo cp -a /etc/postfix/master.cf /etc/postfix/master.cf.before-nitwings
rpm -q postfix dovecot

Confirm package availability and ownership in the enabled RHEL repositories before adding a third-party source. Record the repository, signing key fingerprint, version and support lifecycle.
A package install creates files and service identities; it does not establish safe relay, authentication, delivery or filtering behavior.
Back up only the files this lesson changes and record modes, owners and SELinux labels so rollback restores more than text.
Use systemctl cat, package file lists and local manual pages to identify paths on the installed build instead of assuming a path from another distribution.

Build the Postfix master.cf service isolation configuration and understand every boundary

Keep port 25 destination-based and define port 587 with encryption, authentication and sender ownership.

submission inet n - n - - smtpd
  -o syslog_name=postfix/submission
  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_sasl_type=dovecot
  -o smtpd_sasl_path=private/auth
  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
  -o smtpd_sender_login_maps=hash:/etc/postfix/sender_login
  -o smtpd_sender_restrictions=reject_authenticated_sender_login_mismatch

Indented -o fields affect only the submission service. Port 465, when genuinely required, uses wrapper-mode TLS and a separate master service rather than copying port 587 blindly.

Verify the working path

sudo postfix check
postconf -M submission/inet
postconf -P | grep '^submission/'
sudo postfix reload
sudo ss -lntp | grep -E ':(25|587) '
openssl s_client -starttls smtp -connect mail.example.test:587 -servername mail.example.test -crlf

Run the syntax or lookup test before reload. A reload must never be the first parser of a production configuration.
The positive test proves the intended path. The negative test proves an unauthorized sender, recipient or client is not accidentally accepted.
Stop the named dependency in the disposable lab and verify temporary failure or controlled bypass matches the documented policy.
Repeat the accepted path after restart and reboot, then compare effective configuration rather than only source files.

Production decisions before continuing

Decision	Choose deliberately	Evidence to retain
Failure policy	Authentication outage may temporarily stop submission, but it must not stop unauthenticated inbound delivery for hosted recipients on port 25.	SMTP transcript, queue state and dependency alert
Trust boundary	Port 25 trusts destination ownership; port 587 trusts a SASL identity and restricts the envelope sender to identities assigned to it.	Matching client, sender, recipient or daemon identity
Secrets and data	Restrict credentials, message samples and keys to the minimum service identity.	Owner, mode, label and secret rotation record
Activation	Validate, reload, run positive and negative tests, then watch one complete message.	Syntax output, queue ID and linked log events
Rollback	Restore the exact files and map/database state changed by this lesson.	Rollback command and repeated acceptance result

Place Postfix master.cf service isolation in the message path

Each master.cf row defines service name, socket type, privilege, chroot, process limit and command; indented overrides create service-specific policy.

Understand the component before configuring it

Layer	Question to answer	Evidence
Input	Destination port, remote client, TLS state and optional SASL identity	SMTP transcript, lookup input or message header
Decision	Selected master service plus expanded overrides	Effective configuration and exact matched rule
Output	Port-specific SMTP session and queue acceptance	Queue state, delivery status or downstream response
Dependency	TLS files, Dovecot auth socket, sender map and process capacity	Socket, timeout, journal and controlled outage test
Recovery	Can processing resume without duplicate, loss or unauthorized delivery?	Retained queue ID, backup and repeated acceptance

Build it step by step

Draw the path. Mark the connection, envelope, content and authenticated identities available at this stage.
Inventory the effective state. Capture package version, active service, sockets, Postfix parameters, master services and lookup results.
Prepare one coherent configuration. Substitute documented lab values and verify ownership, mode and SELinux context.
Validate before activation. Run component syntax checks, map queries and a non-delivering test where supported.
Exercise three outcomes. Send an intended message, an intended denial and a message while the named dependency is unavailable.
Trace one queue ID. Join ingress, policy, filtering, routing and final delivery events without relying on subject text.
Close the change. Restart or reboot where relevant, repeat tests, check queue age and document rollback.

Operate and inspect the component

postconf -M
postconf -P
postconf -M submission/inet
postconf -P submission/inet/smtpd_tls_security_level
sudo ss -lntp | grep master
journalctl -u postfix --since '-15 minutes'

Replace sample hostnames, addresses and queue IDs only after resolving them from the lab. Do not paste production identities into a public command transcript.
postconf -n shows non-default global parameters; postconf -M and postconf -P expose master service and field overrides.
A successful lookup proves only that input. Test present, absent, disabled and dependency-unavailable results separately.
Use the queue ID as the correlation key. Message subjects and recipient addresses are not unique and may contain sensitive information.

Evidence and acceptance criteria

Evidence	Healthy result	Failure meaning
Syntax	All component validators succeed before activation	The running service would parse an unreviewed or invalid state
Positive path	Port 25 accepts hosted-domain mail and port 587 accepts an allowed authenticated sender over TLS.	The intended message cannot complete this decision point
Negative path	Both ports reject unauthenticated relay and submission rejects authenticated sender impersonation.	The configuration may relay, authenticate, route or deliver too broadly
Dependency failure	Removing the auth socket breaks AUTH on 587 while port 25 remains available.	Messages may be lost, permanently rejected or silently bypass controls
Persistence	Effective state and tests agree after restart and reboot	Only transient state was changed

Global SMTP AUTH couples two unrelated services

SASL is enabled globally, so AUTH appears on the Internet receiver and a Dovecot maintenance window affects inbound sessions. Moving SASL and sender-login controls into the submission service restores the operational boundary.

Troubleshooting by symptom

Symptom	Inspect first	Defensible next action
AUTH absent	TLS state, selected service and socket	Check capability after STARTTLS and effective overrides
Port closed	master row, bind socket and firewall	Repair the first absent layer
Override ignored	postconf -P key and indentation	Correct service/field syntax
Sender spoof accepted	sender-login map and restriction order	Map and test every authorized identity

Unsafe operations and recovery boundaries

Unsafe: global AUTH exposes unnecessary attack surface on port 25.
Unsafe: permit_sasl_authenticated without sender ownership permits account-level impersonation.
Unsafe: opening submission before TLS and rate controls are ready exposes credentials.

Rewritten knowledge checks

What owns the listeners?

The Postfix master daemon reads master.cf and creates service processes.

What does an indented -o line do?

It overrides one parameter for one master service.

Why separate submission?

User authentication and sender policy differ from server-to-server SMTP.

Why test an absent value?

It proves the configuration denies or returns not-found instead of matching a broad default.

Why retain a queue ID?

It joins SMTP, policy, routing, filtering and delivery evidence without relying on non-unique subject text.

What must be captured before this change?

The effective configuration, package versions, owned sockets, a UTC test result and the exact files or database rows that rollback will restore.

Why is a service restart not an acceptance test?

A running process does not prove the intended message path, denial path, dependency failure behavior or persistence.

What makes a placeholder safe to replace?

Its source, format, owner and scope are explained and the substituted value is verified before activation.

When should rollout stop?

Stop when identity is ambiguous, syntax fails, the negative test becomes permissive, a dependency failure produces permanent loss or rollback cannot be executed.

What evidence belongs in handover?

The reviewed configuration, syntax output, positive and negative transcripts, logs, monitoring threshold, backup location and tested rollback result.

Cumulative lab checkpoint

Capture the inherited checkpoint and state the exact sender, recipient, client address and expected SMTP result.
Install the required package from a recorded source and save the package/file/service inventory.
Apply the complete lab configuration, including permissions, socket paths, map generation and service ownership.
Run syntax and lookup validation, then activate without closing the recovery session.
Complete positive, negative and dependency-outage tests while retaining queue IDs and UTC logs.
Restart the participating services, repeat the accepted path and confirm no unexplained deferred mail remains.
Execute rollback once, prove the previous behavior, then reapply the reviewed state as the checkpoint for the next lesson.

Primary references

View complete Linux MTA Operations learning path