Lab 6: Configure daemon.json and observe the effect

Objective

Apply a production-grade /etc/docker/daemon.json and verify each setting actually takes effect. Misconfigured daemon.json is a common cause of subtle production incidents.

Tasks

Task 1: Write the daemon.json

sudo tee /etc/docker/daemon.json > /dev/null <<'EOF'
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "5"
  },
  "userland-proxy": false,
  "live-restore": true,
  "default-runtime": "runc"
}
EOF

Task 2: Validate the JSON

python3 -c 'import json,sys; json.load(open("/etc/docker/daemon.json"))'

A non-zero exit means the JSON is invalid and dockerd will not start.

Task 3: Restart the daemon

sudo systemctl restart docker
sudo systemctl status docker

A failed restart means the daemon.json is broken; journalctl -u docker --since "1 minute ago" will show the parse error.

Task 4: Verify each setting

docker info | grep -E '(Storage Driver|Logging Driver|Runtimes|Default Runtime)'
docker info | grep -i 'live restore'
# Live Restore Enabled: true

# Confirm userland-proxy is off
ps -ef | grep -c docker-proxy
# Should be 0 once you restart any containers started after the change

Task 5: Test log rotation

docker run -d --name logtest --log-opt max-size=1m --log-opt max-file=2 \
  busybox sh -c 'while true; do echo "padding $(date) $(head -c 100 /dev/urandom | base64)"; sleep 0.1; done'

sleep 90

ls -la $(docker inspect logtest --format '{{.LogPath}}' | xargs dirname)
# You should see at most 2 rotated files plus the active one

docker rm -f logtest

Expected outcome

• docker info reports the logging driver and live-restore.
• The userland proxy process is not running.
• Log files rotate at 1 MB.

Cleanup

docker rm -f logtest 2>/dev/null