How to connect to docker container

Networking with standalone containers

Estimated reading time: 16 minutes

This series of tutorials deals with networking for standalone Docker containers. For networking with swarm services, see Networking with swarm services. If you need to learn more about Docker networking in general, see the overview.

This topic includes three different tutorials. You can run each of them on Linux, Windows, or a Mac, but for the last two, you need a second Docker host running elsewhere.

Use the default bridge network demonstrates how to use the default bridge network that Docker sets up for you automatically. This network is not the best choice for production systems.

Use user-defined bridge networks shows how to create and use your own custom bridge networks, to connect containers running on the same Docker host. This is recommended for standalone containers running in production.

Although overlay networks are generally used for swarm services, you can also use an overlay network for standalone containers. That’s covered as part of the tutorial on using overlay networks.

Use the default bridge network

In this example, you start two different alpine containers on the same Docker host and do some tests to understand how they communicate with each other. You need to have Docker installed and running.

Open a terminal window. List current networks before you do anything else. Here’s what you should see if you’ve never added a network or initialized a swarm on this Docker daemon. You may see different networks, but you should at least see these (the network IDs will be different):

Check that both containers are actually started:

Inspect the bridge network to see what containers are connected to it.

Near the top, information about the bridge network is listed, including the IP address of the gateway between the Docker host and the bridge network ( 172.17.0.1 ). Under the Containers key, each connected container is listed, along with information about its IP address ( 172.17.0.2 for alpine1 and 172.17.0.3 for alpine2 ).

The prompt changes to # to indicate that you are the root user within the container. Use the ip addr show command to show the network interfaces for alpine1 as they look from within the container:

Now try to ping the second container. First, ping it by its IP address, 172.17.0.3 :

This succeeds. Next, try pinging the alpine2 container by container name. This will fail.

Stop and remove both containers.

Remember, the default bridge network is not recommended for production. To learn about user-defined bridge networks, continue to the next tutorial.

Use user-defined bridge networks

List Docker’s networks:

Inspect the alpine-net network. This shows you its IP address and the fact that no containers are connected to it:

Verify that all containers are running:

Inspect the bridge network and the alpine-net network again:

Containers alpine3 and alpine4 are connected to the bridge network.

Not only that, but you can’t connect to alpine3 from alpine1 by its IP address either. Look back at the docker network inspect output for the bridge network and find alpine3 ’s IP address: 172.17.0.2 Try to ping it.

Detach from alpine1 using detach sequence, CTRL + p CTRL + q (hold down CTRL and type p followed by q ).

Stop and remove all containers and the alpine-net network.

Other networking tutorials

Now that you have completed the networking tutorials for standalone containers, you might want to run through these other networking tutorials:

docker attach

Estimated reading time: 6 minutes

Attach local standard input, output, and error streams to a running container

Usage

Refer to the options section for an overview of available OPTIONS for this command.

Description

Use docker attach to attach your terminal’s standard input, output, and error (or any combination of the three) to a running container using the container’s ID or name. This allows you to view its ongoing output or to control it interactively, as though the commands were running directly in your terminal.

Note: The attach command will display the output of the ENTRYPOINT/CMD process. This can appear as if the attach command is hung when in fact the process may simply not be interacting with the terminal at that time.

You can attach to the same contained process multiple times simultaneously, from different sessions on the Docker host.

Note: A process running as PID 1 inside a container is treated specially by Linux: it ignores any signal with the default action. So, the process will not terminate on SIGINT or SIGTERM unless it is coded to do so.

1MB memory buffer to maximize the throughput of the application. If this buffer is filled, the speed of the API connection will start to have an effect on the process output writing speed. This is similar to other applications like SSH. Because of this, it is not recommended to run performance critical applications that generate a lot of output in the foreground over a slow client connection. Instead, users should use the docker logs command to get access to the logs.

Override the detach sequence

If you want, you can configure an override the Docker key sequence for detach. This is useful if the Docker default sequence conflicts with key sequence you use for other applications. There are two ways to define your own detach key sequence, as a per-container override or as a configuration property on your entire configuration.

For example uses of this command, refer to the examples section below.

Options

Examples

Attach to and detach from a running container

Get the exit code of the container’s command

And in this second example, you can see the exit code returned by the bash process is returned by the docker attach command to its caller too:

docker network connect

Estimated reading time: 4 minutes

Connect a container to a network

Usage

Refer to the options section for an overview of available OPTIONS for this command.

Description

Connects a container to a network. You can connect a container by name or by ID. Once connected, the container can communicate with other containers in the same network.

For example uses of this command, refer to the examples section below.

Options

Examples

Connect a running container to a network

Connect a container to a network when it starts

Specify the IP address a container will use on a given network

You can specify the IP address you want to be assigned to the container’s interface.

Create a network alias for a container

—alias option can be used to resolve the container by another name in the network being connected to.

Network implications of stopping, pausing, or restarting containers

You can pause, restart, and stop containers that are connected to a network. A container connects to its configured networks when it runs.

To verify the container is connected, use the docker network inspect command. Use docker network disconnect to remove a container from the network.

Once connected in network, containers can communicate using only another container’s IP address or name. For overlay networks or custom plugins that support multi-host connectivity, containers connected to the same multi-host network but launched from different Engines can also communicate in this way.

You can connect a container to one or more networks. The networks need not be the same type. For example, you can connect a single container bridge and overlay networks.

How to SSH into a Running Docker Container and Run Commands

Home » DevOps and Development » How to SSH into a Running Docker Container and Run Commands

Docker is a utility that lets you create a container for running applications. A Docker container is a fully-contained virtual machine.

This guide will show you three methods to SSH into a Docker container and run commands.

Method 1: Use docker exec to Run Commands in a Docker Container

The docker exec command runs a specified command within an already running container. You can use it to SSH into a Docker container by creating a bash shell (a shell where you can type commands).

The basic syntax for using docker exec to run a command in containers is:

Start by pulling a Docker image if you haven’t already. For example, you can load Nginx:

Then, run the image:

List all running containers to verify:

You should now see your nginx-test image loaded.

To get access and run commands in that Docker container, type the following:

Now, you are logged in to the nginx-test container. Therefore, any commands you enter will perform in that container. The –i option specifies interactive, and the –t enables a terminal typing interface.

Method 2: Use the docker attach Command to Connect to a Running Container

The docker attach command links a local input, output, and error stream to a container. By default, it launches in a bash shell. To connect to a running container, enter the following:

In the example below, the system will connect to the nginx-test container:

Once the command is executed, you will be working in the container. Any commands you run will affect the virtual Docker environment.

Method 3: Use SSH to Connect to a Docker Container

You can connect to a Docker container using SSH (Secure Shell). Normally, SSH is used to connect remotely over a network to a server. The technology works the same when connecting to a virtual Docker container on your system.

Important: We do not recommend this method, since it inflates the image beyond the normal scope. You will need to have an image with SSL already configured for this to work.

Step 1: Enable SSH on System

Start by installing and enabling the SSH service:

Step 2: Get IP Address of Container

Get the container’s IP address by using the docker inspect command and filtering out the results.

For modern Docker engines, use the command:

For older Docker engines, run:

The system will display the IP address as seen in the image above.

Step 3: SSH Into Docker Container

Ping the IP address to make sure it’s available:

Use the SSH tool to connect to the image:

The system should prompt for a password of the root user for that container. If it says Connection refused, likely the container is not provisioned for SSH. If the prompt changes, you are now connected via SSH, and can run commands in the container.

If you are provisioning multiple remote virtual machines, you could use the docker-machine ssh command to connect to a virtual machine through Docker. For most users, the first two command methods are recommended.

accessing a docker container from another container

I created two docker containers based on two different images. One of db and another for webserver. Both containers are running on my mac osx.

I can access db container from host machine and same way can access webserver from host machine.

However, how do I access db connection from webserver?

The way I started db container is

I started wls container as

I can access db on host by connecting to

I can access wls on host as

8 Answers 8

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It’s easy. If you have two or more running container, complete next steps:

Now you connect from web1 to web2 container or the other way round.

Use the internal network IP addresses which you can find by running:

Note that only internal IP addresses and ports are accessible to the containers connected by the network bridge.

The link below offers a nice how too, on connecting two containers. You can skip the attach portion, since that is just a useful how to on adding items to images.

The part you are interested in is the communication between two containers. The easiest way, is to refer to the DB container by name from the webserver container.

then when you start the web app. use:

I’d use docker compose instead, which will build a network for you. However; you will need to download docker compose for your system. https://docs.docker.com/compose/install/#prerequisites

an example setup is like this:

file name is base.yml

This will overwrite the db. with a different setup. When needing to connect to these services from each container, you use the name set under service, in this case, webserver and db.

I think this might actually be a more useful setup in your case. Since you can set all the variables you need in the yml files and just run the command for docker compose when you need them started. So a more start it and forget it setup.

UPDATE: After using features further and seeing how others have done it for CICD programs like Jenkins. Network is also a viable solution.

For a detailed example of using network in a cicd pipeline, you can refer to this link: https://git.in.moodle.com/integration/nightlyscripts/blob/master/runner/master/run.sh

Which is the script that is ran in Jenkins for a huge integration tests for Moodle, but the idea/example can be used anywhere. I hope this helps others.