How To Provision and Manage Remote Docker Hosts with Docker Machine on CentOS 7

Introduction

Docker Machine is a tool that makes it easy to provision and manage multiple Docker hosts remotely from your personal computer. Such servers are commonly referred to as Dockerized hosts, and as a matter of course, can be used to run Docker containers.

While Docker Machine can be installed on a local or a remote system, the most common approach is to install it on your local computer (native installation or virtual machine) and use it to provision Dockerized remote servers.

Though Docker Machine can be installed on most Linux distribution as well as on Mac OS X and Windows, in this tutorial, we’ll install it on your local machine running CentOS 7 and use it to provision Dockerized DigitalOcean Droplets.

Prerequisites

To follow this tutorial, you will need the following:

• A local machine running CentOS 7 with Docker installed. (see How To Install and Use Docker on CentOS 7 for instructions)
• A DigitalOcean API token. If you don’t have one, generate it using this guide. When you generate a token, be sure that it has read-write scope. That is the default, so if you do not change any option while generating it, it will have read-write capabilities. And to make it easier to use on the command line, be sure to assign the token to a variable as given in that article.

Step 1 — Installing Docker Machine on Your Local Computer

n this step, we’ll work through the process of installing Docker Machine on your local computer running CentOS 7.

To download and install the binary, type:

wget https://github.com/docker/machine/releases/download/v0.14.0/docker-machine-$(uname -s)-$(uname -m)

The name of the file should be docker-machine-Linux-x86_64. Rename it to docker-machine to make it easier to work with:

mv docker-machine-Linux-x86_64 docker-machine

Make it executable:

chmod +x docker-machine

Move or copy it to the usr/local/bin directory so that it will be available as a system command.

sudo mv docker-machine /usr/local/bin

Check the version, which will indicate that it’s properly installed:

docker-machine version

The output should be similar to

Output
docker-machine version 0.14.0, build 89b8332

Step 2 — Installing Additional Docker Machine Scripts

There are three bash scripts in the Docker Machine GitHub repository designed to facilitate the usage of the docker and docker-machine commands. They provide command completion and bash-prompt customization.

In this step, we’ll install the three scripts on your local machine. They will be downloaded and installed into the /etc/bash_completion.d directory.

The first script makes it possible to see the active machine in your bash prompt. This comes in handy when you working with and switching between multiple Dockerized machines. The script is called docker-machine-prompt.bash. To download it, type:

sudo wget https://raw.githubusercontent.com/docker/machine/master/contrib/completion/bash/docker-machine-prompt.bash -O /etc/bash_completion.d/docker-machine-prompt.bash

To complete the installation of the above file, you’ll have to set a custom value for the PS1 variable in your .bashrc file. So open it using nano (PS1 is a special shell variable used to modify the bash command prompt):

nano ~/.bashrc

Copy and paste the following line at the end of that file:

export PS1='[\u@\h \W$(__docker_machine_ps1 " [%s]")]\$ '

With this modification to your ~/.bashrc file, when you activate a machine, its name will be inserted into the shell prompt.

Save and close the file.

The second script is called docker-machine-wrapper.bash. It adds a use subcommand to the docker-machine command, making it easy to switch between Dockerized Machines. To download it, type

sudo wget https://raw.githubusercontent.com/docker/machine/master/contrib/completion/bash/docker-machine-wrapper.bash -O /etc/bash_completion.d/docker-machine-wrapper.bash

The third script is called docker-machine.bash. It adds bash completion for docker-machine commands. Download it using:

sudo wget https://raw.githubusercontent.com/docker/machine/master/contrib/completion/bash/docker-machine.bash -O /etc/bash_completion.d/docker-machine.bash

To apply the changes you’ve made so far, close, then reopen your terminal. If you’re logged into the machine via SSH, exit the session and log in again. Command completion for the docker and docker-machine commands should now be working. If it does not work, it’s likely that the bash-completion package is not installed. If that’s the case, install it using:

sudo yum install bash-completion

That should fix it.

Step 3 — Provisioning a Dockerized Host Using Docker Machine

Now that you have Docker and Docker Machine running on your local machine, you can now provision a Dockerized Droplet on your DigitalOcean account using Docker Machine’s docker-machine create command. If you’ve not done so already, assign your DigitalOcean API token to a bash variable using:

export shell-variable=your-api-token

To make the variable permanent, put it in your ~/.bashrc file. This step is optional, but it is necessary if you want to the value to persist across terminal sessions.

nano ~/.bashrc

Add a line similar to this anywhere:

~/.bashrc
export DOTOKEN=your-api-token

To activate the variable in the current terminal session, type:

source ~/.bashrc

To call the docker-machine create command successfully you must specify (at a minimum) the driver, the API token (or the variable that evaluates to it), and a unique name for the remote host. To create your first Dockerized host, type:

docker-machine create --driver digitalocean --digitalocean-access-token $DOTOKEN machine-name

Partial output as the host is being created follows. In this output, the name of the host is centos-docker:

Output
 ...

Installing Docker...
Copying certs to the local machine directory...
Copying certs to the remote machine...
Setting Docker configuration on the remote daemon...
Checking connection to Docker...
Docker i up and running!
To see how to connect your Docker Client to the Docker Engine running on this virtual machine, run: docker-machine env centos-docker

An SSH key pair is created for the new host so that docker-machine can access it remotely. The Droplet is provisioned with the desired operating system, and Docker is installed on the system. When the command is complete, your Docker Droplet is up and running.

To see the newly create host from the command line, type:

docker-machine ls

The output should be similar to this:

Output
NAME                ACTIVE   DRIVER         STATE     URL                         SWARM   DOCKER    ERRORS
centos-docker   -        digitalocean   Running   tcp://203.0.113.71:2376            v18.05.0-ce

Step 4 — Specify the Base OS When Creating a Dockerized Host

This step shows you how to specify a base OS and version for the Dockerized Host being created with the --digitalocean-image flag.

For example, to create a Dockerized host using CentOS 7, type:

docker-machine create --driver digitalocean --digitalocean-image centos-7-x64 --digitalocean-access-token $DOTOKEN machine-name

What if you would like to run Debian 8 on your server? You would type the following command:

docker-machine create --driver digitalocean --digitalocean-image debian-8-x64 --digitalocean-access-token $DOTOKEN machine-name

By default, the base operating system used when creating a Dockerized host with Docker Machine is supposed to be the latest version (or the latest LTS version for Ubuntu). However, at the time of this publication, the docker-machine create command is still using Ubuntu 16.04 LTS as the base operating system, even though Ubuntu 18.04 is the latest LTS edition. So if you need to run Ubuntu 18.04, you’ll have to specify Ubuntu along with the desired version by passing the --digitalocean-image flag to the docker-machine create command:

docker-machine create --driver digitalocean --digitalocean-image ubuntu-18-04-x64 --digitalocean-access-token $DOTOKEN machine-name

The base operating system is not the only choice you have. You can also specify the size of the Droplet. By default, it is the smallest Droplet, which has 1 GB of RAM, a single CPU, and a 25 GB SSD.

Find the size of the Droplet you want to use by looking up the corresponding slug in the DigitalOcean API documentation.

For example, to provision a machine with 2 GB of RAM, two CPUs, and a 60 GB SSD, use the slug s-2vcpu-2gb:

docker-machine create --driver digitalocean --digitalocean-size s-2vcpu-2gb --digitalocean-access-token $DOTOKEN machine-name

To see all the flags specific to creating a Docker Machine using the DigitalOcean driver, type:

docker-machine create --driver digitalocean -h

Step 5 — Executing Other Docker Machine Commands

You’ve seen how to provision a Dockerized host using the create subcommand. You also seen how to list the hosts available to Docker Machine using the ls subcommand. In this step, you’ll learn about a few more of the other docker-machine subcommands.

To obtain detailed information about a Dockerized host, use the inspect subcommand, like so:

docker-machine inspect machine-name

The output should include lines like these. The Image line reveals the version of the Linux distribution used:

Output
{
    "ConfigVersion": 3,
    "Driver": {
        "IPAddress": "203.0.113.71",
        "MachineName": "centos-docker",
        "SSHUser": "root",
        "SSHPort": 22,
        ...
        "Image": "centos-7-0-x64",
        "Size": "s-1vcpu-1gb",
        ...
    },

---

To print the connection configuration for a host, type:

docker-machine config machine-name

The output should be similar to this:

Output
--tlsverify
--tlscacert="/home/kamit/.docker/machine/certs/ca.pem"
--tlscert="/home/kamit/.docker/machine/certs/cert.pem"
--tlskey="/home/kamit/.docker/machine/certs/key.pem"
-H=tcp://203.0.113.71:2376

The last line in the output of the docker-machine config command reveals the IP address of the host, but you can also get that piece of information by typing:

docker-machine ip machine-name

If you need to power down a remote host, you can use docker-machine to stop it:

docker-machine stop machine-name

Verify that it is stopped.

docker-machine ls

The status of the host has changed:

Ouput
NAME                 ACTIVE   DRIVER         STATE     URL                        SWARM   DOCKER    ERRORS
centos-docker                  digitalocean   Timeout

To start it again:

docker-machine start machine-name

Verify that it is started:

docker-machine ls

You will see that the STATE is now set Running for the host:

Ouput
NAME                 ACTIVE   DRIVER         STATE     URL                        SWARM   DOCKER    ERRORS

centos-docker         -        digitalocean   Running    tcp://203.0.113.71:2376            v18.05.0-ce

Finally, if you want to remove a host:

docker-machine rm machine-name

Step 6 — Executing Commands on a Dockerized Host via SSH

At this point, you’ve been getting information about your hosts, but you can do more than that. For example, you can execute native Linux commands on a Docker host by using the ssh subcommand of docker-machine from your local system. This section explains how to perform ssh commands via docker-machine as well as how to open an SSH session to a Dockerized host.

Assuming that you’ve provisioned a host with CentOS as the operating system, execute the following command from your local system to update the packages on the Docker host:

docker-machine ssh machine-name yum update

Not sure what kernel your remote Docker host is using? Type the following:

docker-machine ssh machine-name uname -r

Besides using the ssh subcommand to execute commands on the remote Docker host, you can also use it to log into the Dockerized host itself. That’s as easy as typing:

docker-machine ssh machine-name

Your command prompt will change to reflect the fact that you’re logged into the remote host:

root@machine-name#

To exit from the remote host, type:

exit

Step 7 — Activating a Dockerized Host

Activating a Docker host connects your local Docker client to that system, which makes it possible to run normal docker commands on the remote system. To activate a Docker host, type the following command:

eval $(docker-machine env machine-name)

Alternatively, you can activate it by using this command:

docker-machine use machine-name

After typing any of the above commands, your bash prompt should change to indicate that your Docker client is pointing to the remote Docker host. It will take this form. The name of the host will be at the end of the prompt:

username@localmachine:~ [machine-name]$

Now any docker command you type at this command prompt will be executed on that remote host.

If a host is active on the terminal that the docker-machine ls command is run, the asterisk under the ACTIVE column shows that it is the active one.

Output
 NAME         ACTIVE            DRIVER         STATE     URL                        SWARM   DOCKER    ERRORS
centos-docker   *        digitalocean   Running    tcp://203.0.113.71:2376            v18.05.0-ce

To exit from the remote Docker host, type the following:

docker-machine use -u

You will be returned to the prompt for your local system.

Now let’s create containers on the remote machine.

Step 8 — Creating Docker Containers on a Remote Dockerized Host

So far, you have provisioned a Dockerized Droplet on your DigitalOcean account and you’ve activated it — that is, your Docker client is pointing to it. The next logical step is to spin up containers on it. As an example, let’s try running the official Nginx container.

Use docker-machine use to select your remote machine:

docker-machine use machine-name

Now execute this command to run an Nginx container on that machine:

docker run -d -p 8080:80 --name httpserver nginx

In this command, we’re mapping port 80 in the Nginx container to port 8080 on the Dockerized host so that we can access the default Nginx page from anywhere.

If the command executed successfully, you will be able to access the default Nginx page by pointing your Web browser to http://docker_machine_ip:8080.

While the Docker host is still activated (as seen by its name in the prompt), you should be able to list the images on that host:

docker images

The output should include the Nginx image you just used, plus others you downloaded before:

Output
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
nginx               latest              ae513a47849c        3 weeks ago         109MB

You can also list the active or running containers on the host:

docker ps

If the Nginx container you ran in this step is the only active container, the output should look like this:

Output
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                  NAMES
4284f9d25548        nginx               "nginx -g 'daemon of…"   20 minutes ago      Up 20 minutes       0.0.0.0:8080->80/tcp   httpserver

To exit the prompt for the remote host, type exit. This will close the terminal as well:

exit

Step 9 — Disabling Crash Reporting (Optional)

By default, whenever an attempt to provision a Dockerized host using Docker Machine fails, or Docker Machine crashes, some diagnostic information is sent automatically to a Docker account on Bugsnag. If you’re not comfortable with this, you can disable the reporting by creating an empty file called no-error-report under your installations .docker/machine directory.

To accomplish that, simply type:

touch ~/.docker/machine/no-error-report

Check the file for error messages if provisioning fails or Docker Machine crashes.

Conclusion

This has been an introduction to installing and using Docker Machine to provision multiple Docker Droplets remotely from one local system. Now you should be able to quickly provision as many Dockerized hosts on your DigitalOcean account as you need.

For more on Docker Machines, visit the official documentation page. The three bash scripts downloaded in this tutorial are hosted on this GitHub page.