Install Mosquitto from your Ubuntu or Debian repository, expose only an intentional listener, require authentication, restrict every user with ACLs, and put internet traffic behind TLS. A default Mosquitto process is not a production-ready public broker: without a configured listener it binds to loopback, and remote clients cannot connect. The procedure below builds a broker that can be tested from a real client and maintained safely.
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What you are building
Mosquitto is an MQTT broker: clients publish messages to topics and subscribe to topics to receive them. On a VPS, the broker is normally a system service managed by systemd (or by the service manager supplied by your distribution). The secure target for an internet-facing deployment is:
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- A patched Debian or Ubuntu VPS with a separate administrative account using
sudo. - One deliberately configured listener, normally TLS-protected on port 8883.
- Anonymous access disabled.
- Username/password authentication stored in a protected password file.
- ACL rules that limit each account to the topics it actually needs.
- A host firewall that exposes SSH only to administrators and MQTT only to expected networks where practical.
- Client-side certificate and hostname verification, followed by tests for both permitted and denied operations.
Mosquitto packages, service names, OpenSSL defaults and configuration paths vary by release. The project lists Debian and Ubuntu packages, source builds and a snap option on its download page. Before copying a production configuration, read the man page installed with your exact package; the Debian references cited here are for unstable/testing and may differ from an older stable release.
Prepare the Linux VPS before installing MQTT
Patch the operating system and create an administrator
Provision a currently supported Debian or Ubuntu image. Apply security updates before opening an MQTT port, create an individual administrative account, grant it sudo, and use SSH keys rather than a shared root password. Keep a second recovery path through your VPS provider in case a firewall change blocks SSH.
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Set the host firewall policy before exposing the broker. Permit SSH from your administration network, permit the chosen MQTT port only from client networks when you can identify them, and deny unused inbound ports. The exact commands depend on the distribution and provider, so verify the rules from a second session before disconnecting your current SSH session.
Confirm time, DNS and the broker name
Use a stable DNS name such as broker.example.com. The name clients use must match the certificate subject or, preferably, a subject-alternative name (SAN). Correct system time is also essential: clients reject certificates that are not yet valid or have expired.
Install Mosquitto and its client utilities
Install from the distribution repository
On Debian or Ubuntu, package names are normally mosquitto for the broker and mosquitto-clients for the command-line publisher and subscriber. Confirm the names for your release, then run:
sudo apt update
sudo apt install mosquitto mosquitto-clientsRepository packages are the simplest route because they provide a service unit, a service account and distribution-integrated paths. If you choose a source build or snap instead, follow the project instructions and adjust every path and service command to that installation.
Inspect the installed version and layout
Record the broker version and inspect the service definition before changing it:
mosquitto -h
mosquitto -c /etc/mosquitto/mosquitto.conf -v
systemctl cat mosquitto
systemctl status mosquitto --no-pagerThe foreground command is useful for understanding configuration errors, but do not run a second broker against the production listener while the system service is active. Debian-style packages commonly use /etc/mosquitto/mosquitto.conf plus an included directory such as /etc/mosquitto/conf.d/. Confirm the actual include_dir in your file and keep local overrides there so package upgrades do not overwrite them.
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Enable and start the service
sudo systemctl enable --now mosquitto
sudo systemctl status mosquitto --no-pager
sudo journalctl -u mosquitto -n 100 --no-pagerThe package service name can vary. Snap deployments use snap-specific systemd names and store configuration and related files under /var/snap/mosquitto/common/; the project documents that layout in Using the Mosquitto snap.
Choose and configure the listener
Why an unconfigured broker is unreachable
Mosquitto started without a listener configuration uses port 1883 on loopback. That permits local testing but is not remotely reachable. The broker must have an explicit listener for remote clients, and you should bind and expose only what you intend. The listener behavior is described in the mosquitto manual page.
Port 1883 versus port 8883
| Port | Typical use | Protection | When to expose it |
|---|---|---|---|
| 1883 | Plain MQTT | No transport encryption; credentials and messages can be observed on the network. | Keep disabled for public traffic, or bind it only to a private interface/network where an encrypted tunnel already exists. |
| 8883 | MQTT over TLS | Encrypted transport with server certificate validation; username/password can then be sent inside the protected session. | Common choice for an internet-facing listener. The number is a convention, not a security guarantee. |
The security property is encryption plus restricted reachability, not the port number itself. You may select another port, but every firewall rule and client command must match it.
Create a minimal TLS listener
Place a local override in the package-supported include directory. Adapt paths to your distribution and certificate layout:
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per_listener_settings true
listener 8883
protocol mqtt
allow_anonymous false
password_file /etc/mosquitto/passwd
cafile /etc/ssl/certs/ca-certificates.crt
certfile /etc/mosquitto/certs/server.crt
keyfile /etc/mosquitto/certs/server.key
tls_version tlsv1.2listener, password_file, cafile, certfile, keyfile and tls_version are documented configuration options in the Debian unstable and testing man pages. per_listener_settings true makes authentication and authorization settings apply independently to each listener, which prevents a later listener from accidentally inheriting a policy you intended for another one.
The example requires at least TLS 1.2. Choose TLS 1.3 when every client in your fleet supports it; otherwise keep a TLS 1.2 minimum and remove obsolete protocol versions. Test the installed Mosquitto/OpenSSL combination because available protocol labels and defaults are release-dependent.
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Create users and disable anonymous access
Build the password file safely
Mosquitto 2.0 and later require an explicit authentication choice. For a small, static user set, create the password file with mosquitto_passwd:
sudo install -d -m 750 -o mosquitto -g mosquitto /etc/mosquitto
sudo mosquitto_passwd -c /etc/mosquitto/passwd device01
sudo chown mosquitto:mosquitto /etc/mosquitto/passwd
sudo chmod 640 /etc/mosquitto/passwdThe -c option creates or overwrites the file, so use it only for the first user or when you intentionally want to replace the file. Add or change another account without -c:
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sudo mosquitto_passwd -D /etc/mosquitto/passwd retired-deviceLet the command prompt for the password. Supplying a password directly on the command line can expose it through shell history or process inspection, as the official authentication documentation warns. The Mosquitto service account must be able to read the file; ownership and mode above provide that while excluding ordinary users.
Require authentication on the listener
Keep allow_anonymous false on every public listener and point password_file at the protected file. The Mosquitto project states: “It is important to configure authentication on your Mosquitto instance, so unauthorised clients cannot connect.” Authentication alone does not decide which topics a user may read or write, so add ACLs before opening the firewall.
Restrict topics with an ACL
Give each device its own namespace
Create an ACL file readable by the broker service account, for example /etc/mosquitto/acl:
user device01
topic readwrite devices/device01/#
user device02
topic readwrite devices/device02/#Reference it from the listener configuration:
acl_file /etc/mosquitto/aclThis pattern lets device01 publish and subscribe below devices/device01/ while preventing it from using device02 topics. Replace the namespaces with your application’s actual layout. If a client only reports telemetry, use a write-only rule for its telemetry branch and a read-only rule for commands; do not grant readwrite merely because it is convenient.
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Test both directions. A client that can authenticate but receives “not authorized” is reaching the broker; inspect the exact topic spelling, wildcard level and ACL file path before loosening permissions.
When a static file is no longer enough
For a handful of accounts, a password file and ACL file are easy to audit. Larger fleets with changing roles and groups can evaluate Mosquitto’s Dynamic Security plugin, documented alongside the project’s authentication methods. It provides a more flexible username/password and access-control system, but it introduces another policy layer that must be backed up, monitored and tested.
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Use a name-matching server certificate
Obtain a certificate whose SAN contains the hostname clients will use, such as broker.example.com. Store the certificate chain and private key in a directory that is not world-readable. The broker needs to read the key; ordinary users and unrelated services do not.
sudo install -d -m 750 -o root -g mosquitto /etc/mosquitto/certs
sudo chown root:mosquitto /etc/mosquitto/certs/server.key
sudo chmod 640 /etc/mosquitto/certs/server.key
sudo chmod 644 /etc/mosquitto/certs/server.crtAdjust group ownership and modes to the package’s service account model. Never make a private key world-readable just to fix a startup error.
Decide whether clients also need certificates
Server-side TLS plus MQTT username/password is sufficient for many deployments. If every client must authenticate with a certificate, configure a trusted CA and set require_certificate true on the TLS listener. Certificate-based client authentication changes provisioning and revocation procedures, so use it when the operational benefit justifies that complexity.
Renew and verify certificates
Renew certificates before expiry, reload or restart the broker as supported by your package, and verify the complete chain from a client network after every renewal. A file existing on disk does not prove that a handshake will succeed: the hostname, chain, key permissions, firewall and client trust store all have to agree.
Apply firewall and VPS hardening
- Allow SSH only from an administration network or a VPN; avoid exposing password-based SSH to the entire internet.
- Allow TCP 8883 only from known client ranges when the application permits it. If clients are distributed globally, keep the port public but enforce TLS, authentication and ACLs.
- Do not open 1883 on the public interface unless you have a documented private-network use case.
- Deny unused inbound ports and review provider-level security-group rules as well as the VPS firewall.
- Keep the operating system, Mosquitto package and certificate chain patched.
- Run the packaged service under its dedicated
mosquittoaccount; do not run the broker as root. - Collect and review service logs, authentication failures and repeated connection attempts.
Firewall syntax and log locations vary by distribution and provider. Make a change, verify it from an allowed client and an intentionally blocked network, and keep a rollback rule ready.
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Validate the complete security boundary
Reload only after checking configuration
Check the configuration in the foreground if your package supports that workflow, then restart or reload through systemd:
sudo systemctl restart mosquitto
sudo systemctl status mosquitto --no-pager
sudo journalctl -u mosquitto -n 100 --no-pagerResolve syntax errors, unreadable files and certificate-key mismatches shown in the journal before testing remotely.
Test an authorized TLS publish and subscription
Use the documented mosquitto_pub and mosquitto_sub clients from a network that should be allowed. The CA file must contain the issuer trusted by the broker hostname certificate:
mosquitto_sub -h broker.example.com -p 8883 --cafile /path/ca.crt
-u device01 -P 'REDACTED' -t devices/device01/state -v
mosquitto_pub -h broker.example.com -p 8883 --cafile /path/ca.crt
-u device01 -P 'REDACTED' -t devices/device01/state -m onlineThe subscriber should display the published topic and payload. Keep credentials out of shell history in real operations; use a safer credential-handling method in your automation.
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Do not declare the VPS secure after one successful publish. Exercise each boundary:
- Connect without
-uand-P; anonymous access should be rejected. - Use a deliberately wrong password; authentication should fail.
- Connect with an incorrect hostname or an untrusted CA; certificate verification should fail when verification is enabled.
- Authenticate as
device01and try to publish or subscribe belowdevices/device02/; the ACL should deny it. - Try port 1883 from an external network; it should be closed, filtered or limited to the intended private interface.
- Test from an unapproved source network and confirm the firewall blocks the connection.
These are checks to perform in your environment, not assumptions about the result. Record the client version, hostname, source network and broker log entry for each test.
Troubleshoot common connection failures
“Connection refused” or timeout
- Confirm the service is running with
systemctl status mosquittoand inspectjournalctl -u mosquitto. - Check that an explicit
listenerexists; a default loopback listener cannot accept remote traffic. - Verify the VPS firewall, provider security group, DNS address and selected port.
- Confirm the broker is listening on the expected address with your distribution’s socket inspection tool.
TLS handshake or certificate errors
- Use the exact DNS name in the certificate SAN, not an unrelated IP address.
- Give the client the correct CA chain and ensure the VPS clock is accurate.
- Check that
certfileandkeyfileform a matching pair and that the Mosquitto account can read them. - Compare the configured minimum TLS version with the client’s capabilities; requiring TLS 1.3 excludes clients that support only TLS 1.2.
- After renewal, reload the service and test from outside the VPS.
Authentication or authorization failures
- Verify the listener has
allow_anonymous falseand the correctpassword_file. - Check password-file ownership and mode; the broker service account must be able to read it.
- Remember that
mosquitto_passwd -coverwrites an existing file; recreate or add users intentionally. - Check the ACL path, username spelling, topic case and wildcard placement.
- If one listener is intended to be public and another private, keep
per_listener_settings trueand define policy for each listener explicitly.
Version, package and maintenance notes
Debian and Ubuntu releases ship different Mosquitto versions and defaults. Read the man page installed on the VPS instead of assuming that an option documented for Debian testing exists unchanged in an older stable release. Snap installations use /var/snap/mosquitto/common/ and snap-specific systemd service names, as described in the project’s snap documentation.
Back up the password and ACL files securely, restrict who can change them, and document which listener, certificate chain, firewall rule and client topic namespace belong together. Rotate credentials when devices are retired, remove stale ACL entries, and schedule certificate-expiry checks. After package upgrades, review the effective configuration, service status and a small set of allowed and denied client tests.
Quick Recap
Reference documentation
- Mosquitto downloads and installation options
- Mosquitto command and listener behavior
- Authentication and authorization methods
- Debian unstable mosquitto.conf reference
- Debian testing mosquitto.conf reference
- Mosquitto utilities and documentation
- Using the Mosquitto snap
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