Troubleshooting Docker Daemon

When Docker commands fail to communicate with the Docker daemon, follow these steps to diagnose and resolve the issue.

1. “Cannot connect to the Docker daemon” Error

If you encounter:

$ docker ps
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

the Docker Engine service (daemon) isn’t reachable. Determine if you’re targeting a local socket or a remote host.

a. Remote Access via DOCKER_HOST

Set the DOCKER_HOST environment variable to point at your remote Docker endpoint:

export DOCKER_HOST="tcp://192.168.1.10:2376"
docker ps

If the error persists, SSH into the remote host and check the Docker service status.

2. Checking the Docker Service Status

On most Linux distributions with systemd, Docker runs as a service. Verify its state:

sudo systemctl status docker

A healthy daemon appears as:

● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; preset: enabled)
   Active: active (running) since Wed 2020-10-21 04:21:01 UTC; 3 days ago
     Docs: https://docs.docker.com
 Main PID: 4197 (dockerd)
    Tasks: 13
   Memory: 130M
     CPU: 9min 6.980s
 CGroup: /system.slice/docker.service
         └─4197 /usr/bin/dockerd -H fd:// -H tcp://0.0.0.0 --containerd=/run/containerd/containerd.sock

If you see inactive or dead:

● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; preset: enabled)
   Active: inactive (dead) since Sat 2020-10-24 07:42:08 UTC; 21s ago
     Docs: https://docs.docker.com
  Process: 4197 ExecStart=/usr/bin/dockerd -H fd:// -H tcp://0.0.0.0 --containerd=/run/containerd/containerd.sock (code=exited, status=0/SUCCESS)

Start or restart the service:

sudo systemctl start docker

3. Inspecting Service Logs

Use journalctl to pinpoint errors and warnings:

sudo journalctl -u docker.service --since "1 hour ago"

Example log excerpt:

Oct 21 04:05:42 ubuntu-xenial systemd[1]: Starting Docker Application Container Engine...
Oct 21 04:05:42 time="2020-10-21T04:05:42.565Z" level=info msg="parsed scheme: \"unix\""
Oct 21 04:05:42 time="2020-10-21T04:05:42.847Z" level=warning msg="Your kernel does not support cgroup cfs"
Oct 21 04:05:43 time="2020-10-21T04:05:43.873Z" level=error msg="Error (Unable to complete operation)"

4. Verifying Daemon Configuration

Inspect /etc/docker/daemon.json for JSON syntax errors or conflicting settings:

{
  "debug": true,
  "hosts": ["tcp://192.168.1.10:2376"],
  "tls": true,
  "tlscert": "/var/docker/server.pem",
  "tlskey": "/var/docker/serverkey.pem"
}

After any change, reload and restart:

sudo systemctl daemon-reload
sudo systemctl restart docker

5. Ensuring Sufficient Disk Space

Docker stores images, containers, and volumes under /var/lib/docker. A full filesystem can crash the daemon.

1. Check disk usage:

   df -h
   

   Example:

   Filesystem     Size  Used Avail Use% Mounted on
   /dev/sda1       19G   14.7G   15M  99% /
   tmpfs          369M     0  369M   0% /dev/shm
   

2. Clean up unused resources:

   docker container prune   # remove all stopped containers
   docker image prune       # remove dangling images
   

6. Examining System Information and Events

Once the daemon is running, validate your environment:

docker system info

Sample output:

Client:
 Debug Mode: false

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 0
 Server Version: 19.03.5
 Storage Driver: overlay2
 Backing Filesystem: xfs
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

To view real-time Docker events (container lifecycle, network changes, etc.):

docker system events

Further Reading

• Docker Engine overview
• Docker daemon.json reference
• Docker CLI commands
• Managing Docker storage

With these steps—verifying connection methods, service status, logs, configuration, disk space, and system information—you can reliably troubleshoot Docker daemon issues.