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
Port Encryption Description 2375 None Unencrypted traffic 2376 TLS Secure, encrypted
When using port 2376, ensure you have valid certificates configured on both client and server.
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)"
Adjust the --since flag to narrow down log entries for faster troubleshooting.
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" }
A conflict between daemon flags (in daemon.json) and CLI or systemd overrides can prevent Docker from starting. Remove duplicate host or TLS settings.
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.
Check disk usage:
Example:
Filesystem Size Used Avail Use% Mounted on /dev/sda1 19G 14.7G 15M 99% / tmpfs 369M 0 369M 0% /dev/shm
Clean up unused resources:
docker container prune # remove all stopped containers docker image prune # remove dangling images
Command Description docker container pruneDelete stopped containers docker image pruneRemove dangling or unused images docker volume pruneClean up unused volumes
Pruning operations are irreversible. Use docker system df to preview reclaimable space.
Once the daemon is running, validate your environment:
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.):
Further Reading With these steps—verifying connection methods, service status, logs, configuration, disk space, and system information—you can reliably troubleshoot Docker daemon issues.
