mosquitto_pub and mosquitto_sub commands in IOT Protocols - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of mosquitto_pub with QoS 1

What is the output or result when running this command?

mosquitto_pub -h test.mosquitto.org -t "home/temperature" -m "22" -q 1

IOT Protocols

mosquitto_pub -h test.mosquitto.org -t "home/temperature" -m "22" -q 1

A The command fails with a syntax error due to missing -p port option

B The message "22" is published but with QoS 0, ignoring the -q flag

C The message "22" is published to topic "home/temperature" with QoS 1, and the command exits with status 0

D The command connects but does not send any message because -m is ignored

💡 Hint

The -q flag sets the Quality of Service level for message delivery.

The command publishes the message "22" to the topic "home/temperature" on the public broker with QoS 1, which means the message is delivered at least once. The command exits successfully.

💻 Command Output

intermediate

mosquitto_sub output for retained messages

If a retained message "ON" exists on topic "home/lights", what will this command output immediately after subscribing?

mosquitto_sub -h test.mosquitto.org -t "home/lights"

IOT Protocols

mosquitto_sub -h test.mosquitto.org -t "home/lights"

A It prints an error about missing -m flag

B It immediately prints "ON" and then waits for new messages

C It prints nothing until a new message is published

D It disconnects immediately without output

💡 Hint

Retained messages are sent to new subscribers immediately.

When subscribing to a topic with a retained message, mosquitto_sub receives and prints that retained message right away before waiting for new messages.

❓ Configuration

advanced

Correct mosquitto_pub command to use TLS with client certificate

Which mosquitto_pub command correctly publishes message "Hello" to topic "secure/topic" using TLS with client certificate and key files located at /certs/client.crt and /certs/client.key?

A mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cafile /certs/ca.crt --cert /certs/client.crt --key /certs/client.key -p 8883

B mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cert /certs/client.crt --key /certs/client.key

C mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --tls --cert /certs/client.crt --key /certs/client.key

D mosquitto_pub -h broker.example.com -t "secure/topic" -m "Hello" --cafile /certs/ca.crt -p 1883

💡 Hint

TLS requires specifying the CA file and client cert/key along with the secure port.

Option A correctly specifies the CA certificate file, client certificate, client key, and uses the standard TLS port 8883. This enables secure TLS connection with client authentication.

❓ Troubleshoot

advanced

Troubleshooting mosquitto_sub connection failure

You run this command:

mosquitto_sub -h 192.168.1.100 -t "sensor/data" -p 1884

But it fails to connect. What is the most likely cause?

A The topic name "sensor/data" is invalid

B The mosquitto_sub command requires -m flag to connect

C The IP address 192.168.1.100 is unreachable due to firewall

D The broker is not listening on port 1884, default is 1883

💡 Hint

Check the broker's listening port and firewall settings.

By default, mosquitto brokers listen on port 1883. If the broker is not configured to listen on 1884, the connection will fail. The topic name is valid, and -m is not required for subscribing.

🔀 Workflow

expert

Sequence to test message delivery with mosquitto_pub and mosquitto_sub

Arrange the steps in correct order to test publishing and subscribing to a topic using mosquitto_pub and mosquitto_sub on the same machine.

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 2,3,1,4

💡 Hint

Subscribe first to receive messages published after subscription.

You must start the subscriber first to listen for messages. Then publish the message. The subscriber will output the message, and finally you can stop the subscription.