Docker Daemon

The Docker Daemon is one of the most important components of Docker. It is the background service responsible for managing Docker objects such as images, containers, networks, and volumes.

When you execute commands such as:

docker run nginx
docker build -t myapp .
docker pull ubuntu
docker ps

the Docker CLI does not directly create or manage containers. Instead, it communicates with the Docker Daemon, which performs the requested operations.

1. What is the Docker Daemon?

The Docker Daemon is a background process that listens for Docker API requests and manages Docker resources.

On Linux, the daemon process is commonly:

> dockerd

Its primary responsibilities include:

Creating and managing containers
Building images
Pulling and pushing images
Managing networks
Managing volumes
Managing container lifecycle
Communicating with container runtimes
Providing the Docker API
Managing image and container metadata

2. Docker Client vs Docker Daemon

One of the most important concepts to understand is the difference between the Docker Client and the Docker Daemon.

Component	Purpose
Docker CLI	Provides the command-line interface
Docker Client	Sends API requests
Docker Daemon	Processes Docker API requests
Container Runtime	Responsible for running containers
Docker Registry	Stores and distributes images

The Docker CLI is primarily a client; the Docker Daemon performs the actual Docker management operations.

3. What is dockerd?

dockerd is the Docker daemon process.

You can check whether it is running on a Linux system using:

>systemctl status docker
● docker.service - Docker Application Container Engine
     Loaded: loaded
     Active: active (running)

4. Starting and Stopping Docker Daemon

On systems using systemd, Docker can generally be managed using:

sudo systemctl start docker
sudo systemctl stop docker
sudo systemctl restart docker
sudo systemctl status docker
sudo systemctl enable docker
sudo systemctl disable docker

5. How Docker CLI Communicates with the Daemon

The Docker CLI communicates with the Docker Daemon through the Docker Engine API.

On a typical Linux installation, communication commonly happens through the Unix socket:


/var/run/docker.sock

Often this is represented by:


/var/run/docker.sock

which may resolve to:


/run/docker.sock

You can check it with:


ls -l /var/run/docker.sock

Example:


srw-rw---- 1 root docker ... /var/run/docker.sock

The s at the beginning indicates that this is a Unix domain socket.