EC
cannot connect to Docker daemon
DockerCRITICALNotableStartupHIGH confidence

Docker CLI cannot reach the Docker service

What this means

This is a fundamental connectivity error indicating that the Docker command-line interface (CLI) cannot communicate with the Docker daemon (the background service). This prevents all Docker operations, from building images to running containers.

Why it happens
  1. 1The Docker daemon service is not running on the host machine.
  2. 2A non-root user is trying to run Docker commands without the correct permissions (i.e., not being in the 'docker' group).
  3. 3The Docker daemon is configured to listen on a non-standard socket or TCP address, and the DOCKER_HOST environment variable is not set correctly.
  4. 4A system firewall is blocking communication to the Docker socket or TCP port.
How to reproduce

On a fresh Linux installation, after installing Docker, a user tries to run their first Docker command before starting the service.

trigger — this will error
trigger — this will error
# (Assuming Docker daemon is not running)
docker ps

expected output

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

Fix 1

Start the Docker Service

WHEN The Docker daemon is stopped.

Start the Docker Service
# On Linux with systemd
sudo systemctl start docker
sudo systemctl enable docker

# On Windows/macOS
# Launch the Docker Desktop application.

Why this works

This starts the core Docker engine, which listens for commands from the CLI.

Fix 2

Add User to the 'docker' Group

WHEN The error message includes 'permission denied' when accessing '/var/run/docker.sock'.

Add User to the 'docker' Group
# Add the current user to the docker group
sudo usermod -aG docker $USER
# IMPORTANT: You must log out and log back in for this change to take effect.
newgrp docker

Why this works

This grants the user's account the necessary file system permissions to communicate with the Docker daemon's Unix socket, avoiding the need for 'sudo' on every command.

What not to do

✕ Change permissions of the docker socket file ('/var/run/docker.sock') to 777.

This is a security risk as it allows any user on the system to control the Docker daemon, effectively giving them root access to the host. Always manage access via group membership.

Sources
Official documentation ↗

Docker post-installation steps

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