Archives April 2016

Accessing Docker containers over the network

Run the container so that it can be reached on port 80

  1. The ports a container can be accessed on are determined when the container is initially ran with the -p flag.
    • The following command will run our image created in the previous lab and forward traffic on port 8080 of our host to the docker container on port 80. You can get your image name with docker images. If you followed our example prior, the image name will be dopensource/nginx.
    docker run -dit -p 8080:80 <image name>
  2. Your container should now be bound to port 8080 on your host system.
    • Try opening up a browser and typing in http://:8080
    • You should see the default nginx page, which is being served by the nginx container we created earlier.


Creating a Docker image based on another image

In this tutorial we will go over how to make changes to a running container while attached, then saving our progress. This will result in a new deployable image.

Install the nginx server to your container and commit the changes.

  1. First attach to the running CentOS container, like we did in the last tutorial.
    docker attach <container name>

    (You can use either container name or container id number. Look at the running container and find this information with docker ps)

  2. Install nginx via yum. You will also need to install the epel-release package to get access to nginx.

    yum install epel-release
    yum update
    yum install nginx
  3. Now that nginx is installed, exit out of the container with the following key combination:
    CTRL` + `P` and `CTRL` + `Q`
  4. Commit your changes to a new image with the following command:
    docker commit -m 'added nginx' --change='CMD ["nginx", "-g daemon off;"]' <container name> dopensource/nginx
  • Notice the similarities in syntax to git.
  • -m 'added nginx' creates a comment for this commit.
  • --change='CMD ... is changing the CMD command, which is what the image will run when it is first started up. In this example, we are telling the image to run nginx in the forground. Most base os images have CMD set to bash so you can interact with the os when attaching.
  • <container name> is the name of the container you want to commit from. You can again, get this from docker ps
  • dopensource/nginx is our name for the new image. dopensource is our organization and nginx is the name of the container.
  1. You can now view your new image with the following command:
    docker images


  • Notice there are two images here. One for the base CoreOS image, which we pulled from the docker hub, and the image we just created.

Run the new image

docker run -dit dopensource/nginx

In the next tutorial we will go over how to allow access to the container over the network.

Running and Connecting to a Docker Container

Connect to your running container

Now that we have a list of running containers, we can use this information to connect. When connecting to a container, you will have different types of interfaces depending on what software the container is running. Most base OS containers, such as Debian or CentOS, will give you a bash shell. Projects such as Asterisk or FreeSWITCH will give you access to the applications CLI, such as fs_cli or asterisk -rv.

  1. You can connect to the container previously listed with the docker ps command using the follwing:

    docker attach <container id>

  • You can either use the container id or container name.
  • In my previous image, the container id would be 60968049d6b3 and the container name would be pedantic_varahamihira
  • If you do not specify a name, docker will randomly assign one.
    • You can assign a container name when running with the --name flag.
  • Once you have ran the docker attach command, you will probably need to hit enter a couple times to see a prompt.


  1. To detach your terminal from the running container, use the follwoing key combination:

    CTRL + P and CTRL + Q

  2. If you type exit while attached to the container, the container will stop running. You can also use the following command, which will send a sigterm message to the container:

    docker stop <container name>

Creating a Docker container from an existing image

In this tutorial we will go over how to search and pull down an existing docker image from the docker hub to run it on your server or local system.


  • You will need to have docker running and installed on your system.

Searching the Docker Hub

  • You can search the Docker Hub for pre made images. These images are either made by the community, or official builds of different projects.
    docker search centos
  • You will get the following response. Notice what it shows you. This will include the following:

  1. A description
  2. Number of stars voted by the community
  3. Indication if the image is official and built by the organization in charge of the project
  4. Indication if the build is automated or not
  • Below is an example of what you will see when searching the Docker Hub.


  • Notice the first image is marked as the official CentOS docker image.

Creating a container from a remote image

  • Let’s create a container based on the official CenOS build.
    docker run -dit


  • Note, Docker will automatically download the image from the Docker Hub if the image is not installed on the host. Once completed, the image will be ran.

  • Here are the what the flags used in the previous command do:

Flag Function
-d Run container in background
-i Keep STDIN open, even if not attached
-t Allocate a pseudo-TTY

View running containers

  • Validate the container is running by getting a list of all running containers:
    docker ps


In our next tutorial we will go over how to attach to a container.