How to SSH into a Docker Container

Docker has become an important tool for developers and system administrators. One common task is accessing the shell of a running Docker container, often referred to as SSHing into a container. It gives you direct access to its environment to manage and optimize your containerized applications. In addition, this process allows you to troubleshoot issues, run commands, inspect files, install additional software, and monitor the container’s performance. 

In this article, we will walk you through the possible methods of SSHing into a Docker container.

How to SSH into a Docker Container?

SSHing into a Docker container involves using the Secure Shell (SSH) protocol to access and interact with a running container. It provides a powerful way to interact with your Docker environment.

Prerequisites

To SSH into a Docker Container, make sure you meet the following pre-requisites:

• You must have Docker installed on your Ubuntu machine.
• A running Docker container is required before you SSH into a Docker container.
• You must have a basic knowledge of the command-line interface (CLI).

Method 1: Using docker exec

The most simple way to access a Docker container’s shell is by using the docker exec command. Moreover, this method lets us ssh docker container from host, which allows accessing the container’s shell from the host machine. This method does not require SSH and is recommended for most use cases.

Step 1: Find Your Container ID

First, you need to identify the container you want to SSH into. Now, list all running containers and note the ID of the one you want to access such as 4d279546e295:

docker ps

Step 2: Access the Container

Docker does not include SSH by default, but you can use the docker exec command to get a shell inside the container. Let’s open the shell inside the container:

docker exec -it 4d279546e295 /bin/bash

Replace the container ID with the actual ID of your container.

Alternatively, you can SSH into the container using its IP address. To do so, use the ip a command to find out the IP address of the machine:

ssh [email protected]

Method 2: Using docker attach

Another method to SSH into the Docker Container is possible via the docker attach command. This command attaches your terminal to a running container’s main process. This allows you to interact with the container’s console in real-time.

To SSH into a Docker Container, follow the below steps:

Step 1: Create Attach Container

To enable “docker attach” on a container, you need to start it in detached and interactive mode. It is possible through the -dit option along with the docker run command.

Let’s create a new container named “attach-test” based on the “ubuntu” image. Then, configure it for docker attach:

docker run --name attach-test -dit ubuntu

Step 2: Attach to Container

To connect to the container, use the docker attach command by specifying the container name. In our case, the attach-test is already running container:

docker attach attach-test

This opens a terminal session directly inside the container, allowing you to execute commands and monitor its activity:

This method attaches to the container’s main process, which might not be a shell. If the main process exits, the container will stop.

Method 3: Using Docker Compose

If you are using Docker Compose, you can configure your docker-compose.yml file to include an SSH server. Let’s SSH into a Docker Container via the configuration file:

Step 1: Edit docker-compose.yml

First, access the docker-compose.yml file in the nano editor: 

sudo nano docker-compose.yml

After that, add the SSH server installation and start commands to your service definition:

services:
myservice:
image: nginx
command: /bin/bash -c "apt-get update && apt-get install -y openssh-server && service ssh start && /bin/bash"
ports:
- "2222:22"

Step 2: Deploy the Service

Now, start your services with Docker Compose:

sudo docker-compose up -d

Step 3: SSH into the Container

Finally, use the mapped port such as 22 to SSH into the container:

ssh username@localhost -p 22

Method 4: Using docker run

The docker run command is the primary tool for creating and starting new Docker containers. To immediately access a container’s shell once it’s created, you can use the -it flags.

To create a container named run-test based on the Nginx image and directly access its Bash shell:

docker run --name run-test -it nginx /bin/bash

That is all from how to SSH Docker Container with possible methods.

Conclusion

While Docker doesn’t support SSH by default, there are several methods to access a container’s shell. The docker exec command is the most straightforward and recommended approach. However, if you need to use SSH specifically, you can run an SSH daemon inside the container or use Docker Compose. In this article, we discussed different methods to SSH into a Docker Container.

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