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

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:

df -h

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 prune`	Delete stopped containers
`docker image prune`	Remove dangling or unused images
`docker volume prune`	Clean up unused volumes

Pruning operations are irreversible. Use docker system df to preview reclaimable space.

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

Introduction

Docker Engine

Docker Image Management

Docker Engine - Security

Docker Engine - Networking

Docker Engine - Storage

Docker Compose

Docker Swarm

Kubernetes

Docker Engine Enterprise

Docker Trusted Registry

Disaster Recovery

Troubleshooting Docker Daemon

1. “Cannot connect to the Docker daemon” Error

a. Remote Access via `DOCKER_HOST`

2. Checking the Docker Service Status

3. Inspecting Service Logs

4. Verifying Daemon Configuration

5. Ensuring Sufficient Disk Space

6. Examining System Information and Events

Further Reading

Watch Video

​1. “Cannot connect to the Docker daemon” Error

​a. Remote Access via DOCKER_HOST

​2. Checking the Docker Service Status

​3. Inspecting Service Logs

​4. Verifying Daemon Configuration

​5. Ensuring Sufficient Disk Space

​6. Examining System Information and Events

​Further Reading

Watch Video

1. “Cannot connect to the Docker daemon” Error

a. Remote Access via `DOCKER_HOST`

2. Checking the Docker Service Status

3. Inspecting Service Logs

4. Verifying Daemon Configuration

5. Ensuring Sufficient Disk Space

6. Examining System Information and Events

Further Reading