How to fix unable to configure the Docker daemon with file /etc/docker/daemon.json in Docker

First, get the exact error message to understand what's wrong:

On Linux (systemd):

sudo journalctl -u docker.service -n 50 --no-pager

Or check the service status:

sudo systemctl status docker.service

On Windows (Docker Desktop):

Check the Docker Desktop troubleshooting logs or Event Viewer.

Common error patterns to look for:

- EOF - Empty or truncated file
- json: cannot unmarshal - Wrong data type
- directives are specified both as a flag and in the configuration file: hosts - Configuration conflict
- configuration validation from file failed - Invalid configuration value
- permission denied - File access issue

The most common cause is invalid JSON. Validate your daemon.json file:

Using jq (recommended):

sudo cat /etc/docker/daemon.json | jq .

If valid, it will pretty-print the JSON. If invalid, it will show the error location.

Using Python:

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

Using an online validator:

Copy the contents and paste into https://jsonlint.com/

Common JSON mistakes:

// WRONG - trailing comma
{
  "storage-driver": "overlay2",
  "log-driver": "json-file",
}

// CORRECT - no trailing comma
{
  "storage-driver": "overlay2",
  "log-driver": "json-file"
}

// WRONG - missing comma between entries
{
  "storage-driver": "overlay2"
  "log-driver": "json-file"
}

// CORRECT - comma between entries
{
  "storage-driver": "overlay2",
  "log-driver": "json-file"
}

If the error mentions EOF or the file is empty/blank, the file must contain at least an empty JSON object:

Check if file is empty:

sudo cat /etc/docker/daemon.json

If empty or doesn't exist, create it with valid content:

echo '{}' | sudo tee /etc/docker/daemon.json

Or add some basic configuration:

cat << 'EOF' | sudo tee /etc/docker/daemon.json
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  }
}
EOF

Then restart Docker:

sudo systemctl restart docker

This is the most common conflict. On Debian/Ubuntu systems with systemd, Docker is started with a -H flag by default. If you also specify hosts in daemon.json, Docker fails.

Error message:

the following directives are specified both as a flag and in the configuration file: hosts

Solution 1: Create a systemd override file

sudo mkdir -p /etc/systemd/system/docker.service.d
sudo cat << 'EOF' > /etc/systemd/system/docker.service.d/override.conf
[Service]
ExecStart=
ExecStart=/usr/bin/dockerd
EOF

This clears the default -H flag, allowing daemon.json to control the hosts setting.

Reload and restart:

sudo systemctl daemon-reload
sudo systemctl restart docker

Solution 2: Use systemctl edit (interactive)

sudo systemctl edit docker.service

Add the same content as above, save, and restart.

Solution 3: Remove hosts from daemon.json

If you don't need remote access, simply remove the hosts key from daemon.json and use the default Unix socket.

JSON requires specific data types. Boolean values must not be quoted.

WRONG - strings instead of booleans:

{
  "iptables": "false",
  "live-restore": "true"
}

CORRECT - actual booleans:

{
  "iptables": false,
  "live-restore": true
}

WRONG - numbers as strings:

{
  "max-concurrent-downloads": "3"
}

CORRECT - actual numbers:

{
  "max-concurrent-downloads": 3
}

Refer to the Docker documentation for the correct type of each configuration option.

The Docker daemon runs as root and needs read access to daemon.json.

Check file permissions:

ls -la /etc/docker/daemon.json

Should show something like:

-rw-r--r-- 1 root root 123 Jan 1 12:00 /etc/docker/daemon.json

Fix permissions if needed:

sudo chown root:root /etc/docker/daemon.json
sudo chmod 644 /etc/docker/daemon.json

For Docker installed via snap:

The config file location is different:

/var/snap/docker/current/config/daemon.json

Check AppArmor isn't blocking access:

sudo dmesg | grep -i apparmor

If you're configuring custom runtimes and see an error about reserved names:

Error:

runtime name 'runc' is reserved

WRONG:

{
  "runtimes": {
    "runc": {
      "path": "/usr/bin/runc"
    }
  }
}

CORRECT - use a different name:

{
  "runtimes": {
    "custom-runc": {
      "path": "/usr/local/bin/runc"
    }
  }
}

The names "runc" and "io.containerd.runc.v2" are reserved by Docker.

Hidden characters or wrong encoding can cause parsing failures.

Check file encoding:

file /etc/docker/daemon.json

Should show: ASCII text or UTF-8 Unicode text

If it shows UTF-16 or mentions BOM, convert it:

sudo iconv -f UTF-16 -t UTF-8 /etc/docker/daemon.json > /tmp/daemon.json
sudo mv /tmp/daemon.json /etc/docker/daemon.json

Check for hidden characters:

cat -A /etc/docker/daemon.json

Look for unusual characters like ^M (carriage return) or non-printable characters.

Recreate the file from scratch:

If in doubt, delete and recreate:

sudo rm /etc/docker/daemon.json
sudo cat << 'EOF' > /etc/docker/daemon.json
{
  "log-driver": "json-file"
}
EOF

After making changes, validate and restart:

Validate JSON one more time:

sudo cat /etc/docker/daemon.json | jq .

Restart Docker:

sudo systemctl daemon-reload  # If you modified systemd files
sudo systemctl restart docker

Verify Docker is running:

sudo systemctl status docker
docker ps
docker info

Check that your configuration is applied:

docker info | grep -i "logging driver"
docker info | grep -i "storage driver"

The output should reflect the settings in your daemon.json.