Demo Docker Debug Mode

In this demo, you’ll learn how to turn on Docker daemon debug mode, verify that it’s active, and inspect verbose logs during container operations.

1. Check Current Debug Status

Start by viewing your Docker daemon’s current settings:

docker system info

Look for the Debug Mode line in the output:

Debug Mode: false

2. Create a Test Container & Inspect Default Logs

Launch a simple HTTPD container:

docker run -d --name test httpd:latest

Then review your system log (e.g., /var/log/messages or journalctl -u docker.service). You should only see high-level entries about container creation:

tail -n 20 /var/log/messages

3. Enable Docker Debug Mode

To capture detailed debug output, edit the Docker daemon configuration:

1. Open /etc/docker/daemon.json (create it if missing) and add:

   {
     "debug": true
   }
   

   Note

   If the file doesn’t exist, you can create it. Make sure the JSON remains valid—use a JSON linter if needed.

2. Reload the Docker daemon:

sudo systemctl reload docker

4. Confirm Debug Mode Is Active

Run the inspect command again:

docker system info

Now you should see Debug Mode: true along with extra metrics:

Debug Mode: true
File Descriptors: 23
Goroutines: 36
System Time: 2020-05-21T11:38:33.79317432Z
EventsListeners: 0

5. Generate and View Verbose Logs

Create a new container called test_debug:

docker run -d --name test_debug httpd:latest

Then tail your logs to see debug-level details:

tail -n 20 /var/log/messages

You’ll notice granular messages describing each step of the container lifecycle.

6. Reload Docker with SIGHUP & Disable Debug

If you prefer a manual reload instead of systemctl:

1. Update /etc/docker/daemon.json to disable debug mode:

   {
     "debug": false
   }
   

2. Identify the Docker daemon PID and send SIGHUP:

   pid=$(pgrep dockerd)
   sudo kill -SIGHUP $pid
   

   Warning

   Always verify you have the correct PID before sending signals. Killing the wrong process can disrupt your system.

3. Check that debug is now off:

docker system info | grep "Debug Mode"

Command Reference

Links and References

• Docker Daemon Configuration
• Understanding the Docker Logging Driver
• Docker System Commands