Introduction

MQTT is a machine-to-machine messaging protocol, designed to provide lightweight publish/subscribe communication to "Internet of Things" devices. It is commonly used for geo-tracking fleets of vehicles, home automation, environmental sensor networks, and utility-scale data collection.

Mosquitto is a popular MQTT server (or broker, in MQTT parlance) that has great community support and is easy to install and configure.

In this tutorial, we'll install Mosquitto and set up our broker to use SSL to secure our password-protected MQTT communications.

Prerequisites

Before starting this tutorial, you will need:

• An Ubuntu 18.04 server with a non-root, sudo-enabled user and basic firewall set up, as detailed in this Ubuntu 18.04 server setup tutorial.
• A domain name pointed at your server, as per How to Set Up a Host Name with DigitalOcean. This tutorial will use mqtt.example.com throughout.
• An auto-renewable Let’s Encrypt SSL certificate for use with your domain and Mosquitto, generated using the Certbot tool. You can learn how to set this up in How To Use Certbot Standalone Mode to Retrieve Let's Encrypt SSL Certificates on Ubuntu 18.04 . You can add systemctl restart mosquitto as a renew_hook in Step 4. Be sure to use the same domain configured in the previous prerequisite step.

Step 1 — Installing Mosquitto

Ubuntu 18.04 has a fairly recent version of Mosquitto in its default software repository, so we can install it from there.

First, log in using your non-root user and update the package lists using apt update:

sudo apt update

Now, install Mosquitto using apt install:

sudo apt install mosquitto mosquitto-clients

By default, Ubuntu will start the Mosquitto service after install. Let's test the default configuration. We'll use one of the Mosquitto clients we just installed to subscribe to a topic on our broker.

Topics are labels that you publish messages to and subscribe to. They are arranged as a hierarchy, so you could have sensors/outside/temp and sensors/outside/humidity, for example. How you arrange topics is up to you and your needs. Throughout this tutorial we will use a simple test topic to test our configuration changes.

Log in to your server a second time, so you have two terminals side-by-side. In the new terminal, use mosquitto_sub to subscribe to the test topic:

mosquitto_sub -h localhost -t test

-h is used to specify the hostname of the MQTT server, and -t is the topic name. You'll see no output after hitting ENTER because mosquitto_sub is waiting for messages to arrive. Switch back to your other terminal and publish a message:

mosquitto_pub -h localhost -t test -m "hello world"

The options for mosquitto_pub are the same as mosquitto_sub, though this time we use the additional -m option to specify our message. Hit ENTER, and you should see hello world pop up in the other terminal. You've sent your first MQTT message!

Enter CTRL+C in the second terminal to exit out of mosquitto_sub, but keep the connection to the server open. We'll use it again for another test in Step 5.

Next, we'll secure our installation using password-based authentication.

Step 2 — Configuring MQTT Passwords

Let's configure Mosquitto to use passwords. Mosquitto includes a utility to generate a special password file called mosquitto_passwd. This command will prompt you to enter a password for the specified username, and place the results in /etc/mosquitto/passwd.

sudo mosquitto_passwd -c /etc/mosquitto/passwd sammy

Now we'll open up a new configuration file for Mosquitto and tell it to use this password file to require logins for all connections:

sudo nano /etc/mosquitto/conf.d/default.conf

This should open an empty file. Paste in the following:

allow_anonymous false
password_file /etc/mosquitto/passwd

Be sure to leave a trailing newline at the end of the file.

allow_anonymous false will disable all non-authenticated connections, and the password_file line tells Mosquitto where to look for user and password information. Save and exit the file.

Now we need to restart Mosquitto and test our changes.

sudo systemctl restart mosquitto

Try to publish a message without a password:

mosquitto_pub -h localhost -t "test" -m "hello world"

The message should be rejected:

Output
Connection Refused: not authorised.
Error: The connection was refused.

Before we try again with the password, switch to your second terminal window again, and subscribe to the 'test' topic, using the username and password this time:

mosquitto_sub -h localhost -t test -u "sammy" -P "password"

It should connect and sit, waiting for messages. You can leave this terminal open and connected for the rest of the tutorial, as we'll periodically send it test messages.

Now publish a message with your other terminal, again using the username and password:

mosquitto_pub -h localhost -t "test" -m "hello world" -u "sammy" -P "password"

The message should go through as in Step 1. We've successfully added password protection to Mosquitto. Unfortunately, we're sending passwords unencrypted over the internet. We'll fix that next by adding SSL encryption to Mosquitto.

Step 3 — Configuring MQTT SSL

To enable SSL encryption, we need to tell Mosquitto where our Let's Encrypt certificates are stored. Open up the configuration file we previously started:

sudo nano /etc/mosquitto/conf.d/default.conf

Paste in the following at the end of the file, leaving the two lines we already added:

. . .
listener 1883 localhost

listener 8883
certfile /etc/letsencrypt/live/mqtt.example.com/cert.pem
cafile /etc/letsencrypt/live/mqtt.example.com/chain.pem
keyfile /etc/letsencrypt/live/mqtt.example.com/privkey.pem

Again, be sure to leave a trailing newline at the end of the file.

We're adding two separate listener blocks to the config. The first, listener 1883 localhost, updates the default MQTT listener on port 1883, which is what we've been connecting to so far. 1883 is the standard unencrypted MQTT port. The localhost portion of the line instructs Mosquitto to only bind this port to the localhost interface, so it's not accessible externally. External requests would have been blocked by our firewall anyway, but it's good to be explicit.

listener 8883 sets up an encrypted listener on port 8883. This is the standard port for MQTT + SSL, often referred to as MQTTS. The next three lines, certfile, cafile, and keyfile, all point Mosquitto to the appropriate Let's Encrypt files to set up the encrypted connections.

Save and exit the file, then restart Mosquitto to update the settings:

sudo systemctl restart mosquitto

Update the firewall to allow connections to port 8883.

sudo ufw allow 8883

Output
Rule added
Rule added (v6)

Now we test again using mosquitto_pub, with a few different options for SSL:

mosquitto_pub -h mqtt.example.com -t test -m "hello again" -p 8883 --capath /etc/ssl/certs/ -u "sammy" -P "password"

Note that we're using the full hostname instead of localhost. Because our SSL certificate is issued for mqtt.example.com, if we attempt a secure connection to localhost we'll get an error saying the hostname does not match the certificate hostname (even though they both point to the same Mosquitto server).

--capath /etc/ssl/certs/ enables SSL for mosquitto_pub, and tells it where to look for root certificates. These are typically installed by your operating system, so the path is different for Mac OS, Windows, etc. mosquitto_pub uses the root certificate to verify that the Mosquitto server's certificate was properly signed by the Let's Encrypt certificate authority. It's important to note that mosquitto_pub and mosquitto_sub will not attempt an SSL connection without this option (or the similar --cafile option), even if you're connecting to the standard secure port of 8883.

If all goes well with the test, we'll see hello again show up in the other mosquitto_sub terminal. This means your server is fully set up! If you'd like to extend the MQTT protocol to work with websockets, you can follow the final step.

Step 4 — Configuring MQTT Over Websockets (Optional)

In order to speak MQTT using JavaScript from within web browsers, the protocol was adapted to work over standard websockets. If you don't need this functionality, you may skip this step.

We need to add one more listener block to our Mosquitto config:

sudo nano /etc/mosquitto/conf.d/default.conf

At the end of the file, add the following:

. . .
listener 8083
protocol websockets
certfile /etc/letsencrypt/live/mqtt.example.com/cert.pem
cafile /etc/letsencrypt/live/mqtt.example.com/chain.pem
keyfile /etc/letsencrypt/live/mqtt.example.com/privkey.pem

Again, be sure to leave a trailing newline at the end of the file.

This is mostly the same as the previous block, except for the port number and the protocol websockets line. There is no official standardized port for MQTT over websockets, but 8083 is the most common.

Save and exit the file, then restart Mosquitto.

sudo systemctl restart mosquitto

Now, open up port 8083 in the firewall.

sudo ufw allow 8083

To test this functionality, we'll use a public, browser-based MQTT client. There are a few out there, but the Eclipse Paho JavaScript Client is simple and straightforward to use. Open the Paho client in your browser. You'll see the following:

Fill out the connection information as follows:

• Host should be the domain for your Mosquitto server, mqtt.example.com.
• Port should be 8083.
• ClientId can be left to the default value, js-utility-DI1m6.
• Path can be left to the default value, /ws.
• Username should be your Mosquitto username; here, we used sammy.
• Password should be the password you chose.

The remaining fields can be left to their default values.

After pressing Connect, the Paho browser-based client will connect to your Mosquitto server.

To publish a message, navigate to the Publish Message pane, fill out Topic as test, and enter any message in the Message section. Next, press Publish. The message will show up in your mosquitto_sub terminal.

Conclusion

We've now set up a secure, password-protected and SSL-secured MQTT server. This can serve as a robust and secure messaging platform for whatever projects you dream up. Some popular software and hardware that work well with the MQTT protocol include:

• OwnTracks, an open-source geo-tracking app you can install on your phone. OwnTracks will periodically report position information to your MQTT server, which you could then store and display on a map, or create alerts and activate IoT hardware based on your location.
• Node-RED is a browser-based graphical interface for 'wiring' together the Internet of Things. You drag the output of one node to the input of another, and can route information through filters, between various protocols, into databases, and so on. MQTT is very well supported by Node-RED.
• The ESP8266 is an inexpensive wifi microcontroller with MQTT capabilities. You could wire one up to publish temperature data to a topic, or perhaps subscribe to a barometric pressure topic and sound a buzzer when a storm is coming!

These are just a few popular examples from the MQTT ecosystem. There is much more hardware and software out there that speaks the protocol. If you already have a favorite hardware platform, or software language, it probably has MQTT capabilities. Have fun getting your "things" talking to each other!