Understanding and Configuring Logging – Mosquitto Broker Configuration

Mosquitto has two different log types

• System status logs
• Informational and debugging logs

Broker System Status logs

These are always logged to the $SYS topic and can be accessed using an MQTT client subscribed to the $SYS/# topic.

System status logs are always enabled by default but you can disable them or change the refresh time using the setting:

sys_interval seconds

Setting this to zero will disable publishing of the hierarchy. The default interval is 10 seconds.

Different brokers will provide different $SYS log topics. This Github page has a list and description of the $SYS log topics split into optional and required.

Information and Debugging logs

These provide detailed information about clients, and client broker commands.

Using them you can see which clients are connected, subscribed etc.

Logging is configured in the Logging section of the mosquitto.conf file.

You can choose which event types to log, by default error, information, notice and warning events are logged.

For debugging I use log_type all.

This is essentially the same as starting mosquitto with the verbose switch – mosquitto -v

Here is a screen shot showing a client connect, subscribe and disconnect sequence and the console log messages that are generated.

Note: On Windows logging is disabled when starting mosquitto as a service. This is because the broker is running as a background process and has no console attached.

Logging Destinations

You can log messages to multiple destinations.

Options are:

• topic – sent on topic tree
• stdout
• file
• syslog

For example you could send log messages to the console, to the $SYS topic, and to a log file.

The screenshot below shows my mosquitto.conf file configured to log to three destinations. (console, logfile and topic)

You can also disable logging using the log_dest none option.

Viewing Topic logs in a Client

Normally you will view these logs on the Mosquitto broker console but you can also view them using an MQTT client if you have logged them to a topic.

If you send the log messages to topic they are sent to the $SYS/broker/log topic , and you need to subscribe the topic tree $SYS/broker/log/#.

Two common question are:

• Can I see all connected clients?
• Can I list all topics?

The broker doesn’t let you do this directly but by enabling logging to a topic and monitoring the topic with an MQTT client you can get a good idea.

However using syslog and remote logging you can view connection details. See below.

Using Syslog

On Linux systems you can also use syslog to log the messages.

Messages are sent to the syslog file in the /var/log folder.

There are 2 settings

log_dest syslog
log_facility 5 #this is what I use

If you use syslog for logging it is possible to send all your logs to a central log server or to a log analyser.

Using the log_facility  allows you to only send logs from mosquitto.

Coming– Centralised mosquitto logging and analytics using node-red.

Historical Logs

On Linux if you log using to the file /var/log/mosquitto.log then the log file will automatically be rotated as an entry is made in the logrotate.d folder by the install script.

The file is shown below:

If you look in the /var/log/mosquitto folder you will see the mosquitto.log file and zipped archives.

The default configuration archives the log file when it exceeds 100k.

Log rotation is handled by cron and runs once per day.

You can manually rotate the mosquitto logs by using the command

sudo logrotate -v /etc/logrotate.d/mosquitto

Viewing Mosquitto.log on Running Broker

When testing I start mosquitto manually using the verbose option to see the console.

However if you have a running server started automatically you don’t have access to the console.

You can still debug by using the tail command to view the log file with the -f option which will effectively show new entries as they are logged.

The command is

sudo tail -n20 -f /var/log/mosquitto/mosquitto.log

Viewing The $SYS logs

To view the system status logs you need to subscribe to the $SYS topic using a MQTT client.

The topic path is $SYS/#

You can use the mosquiito_sub client that comes with the broker as shown below.

In the screenshot below ws4 is the broker name.

You can also use the Paho MQTT Python client see subscribing to topics.

If you use node-red I have created a flow that you can download that displays them in browser.

Log Timestamps

By default you will see the Unix times stamp format which isn’t really very useful when debugging from the console so you may want to make it more user friendly by adding the entry

log_timestamp_format %Y-%m-%dT%H:%M:%S

The logs should then appear as :

Recommended Settings

Default log-types are :

error,warning, notice
and information

and they are sufficient.

On all systems I would log to a file.

/var/log/mosquitto/mosquitto.log

The install script automatically assigns permissions to the mosquitto user.

If you start mosquitto from the command line and try logging to the folder you will probably get an error as the user will not have permissions.

You can either change the permissions on the folder or alternatively change the log folder to a folder that you already have permissions to.

You could also start mosquitto as the mosquitto user using:

sudo -u mosquitto mosquitto -c mos2.conf

where mos2.conf is your test configuration file.

see run command as another user

Common Questions and Answers

Q- Are $SYS logs saved to disk?

A- No

Q– Why Do I get the message:  error: unable to open log file /var/log/mosquitto/mosquitto.log for writing.

A- This is a permission problem and is caused when you start mosquitto manually. You can either start mosquitto as the mosquitto user using sudo (shown above) or when testing change the log file location to your home directory.

Q- How can I list All topics?

A_ Mosquitto doesn’t provide this functionality by default but you can use topic logging as discussed above.

Q- How can I list All connected clients?

A- Use topic logging.

Q- Can I use the $SYS logs for monitoring the broker status?

A- Yes but not probably the best method and some brokers don’t have them enabled. If the broker doesn’t have any other methods built in (mosquitto doesn’t) then you would probably be better using an MQTT client to send background data and monitor that. See this article- why you shouldn’t use the $SYS topic for monitoring.

See also monitoring MQTT brokers .

Mosquitto Configuration Tutorials

• Installing and Testing the Mosquitto MQTT broker
• Quick Guide to The Mosquitto.conf File With Examples
• Configuring and Testing MQTT Topic Restrictions
• Configuring Username and Password Authentication
• Configuring TLS (SSL) on Mosquitto
• Understanding and Configuring Bridging on Mosquitto

Other Related Articles and Resources:

• MQTT for Beginners

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