Docker daemon configuration conflict prevents startup

What this failure means

The Docker daemon failed to start because conflicting options were supplied — the same setting was configured both in daemon.json and on the daemon command line. The daemon exits instead of starting.

Symptoms

Faultline looks for one or more of these log fragments:

unable to configure the Docker daemon with file
conflicting options: hosts
merged json
daemon.json
failed to start daemon
could not change group /var/run/docker.sock
You cannot specify a host flag in daemon.json together with the -H
Flag --hosts cannot be used together with 'hosts' key in daemon configuration file

Diagnosis

Docker does not allow the same option to be specified in both daemon.json and via CLI flags. When it detects a conflict during startup, the daemon aborts with a configuration error. This is distinct from the daemon being unreachable after a successful start.

Common conflicts:

• hosts set in daemon.json and via -H or --host flags at the same time.
• data-root or log-driver specified both in the config file and as command-line arguments.
• Remote-access settings (tcp://) added to daemon.json on a system that already passes --host via systemd unit override.
• Stale or malformed daemon.json left from a prior Docker install.

Fix steps

1. Check the daemon error message to identify which option is conflicting:

   journalctl -u docker --no-pager -n 30
   # or on systems without journald:
   cat /var/log/docker.log | tail -30
   

2. Open /etc/docker/daemon.json and compare its contents against the active dockerd command or systemd unit:

   cat /etc/docker/daemon.json
   systemctl cat docker | grep ExecStart
   

3. Remove the conflicting key from daemon.json if the same option is already on the command line, or move all configuration into daemon.json and remove it from the command line.

4. Validate the JSON syntax before restarting:

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

5. Restart the daemon:

   sudo systemctl restart docker
   docker info
   

Validation

• sudo systemctl status docker shows active (running).
• docker info succeeds without error.

Try it locally

journalctl -u docker --no-pager -n 30
cat /etc/docker/daemon.json
systemctl cat docker
docker info

How Faultline detects it

Use faultline explain docker-daemon-config-conflict to see the full playbook.

faultline analyze build.log
faultline explain docker-daemon-config-conflict

Generated from playbooks/bundled/log/runtime/docker-daemon-config-conflict.yaml. Do not edit directly.