In this article by Shrikrishna Holla, author of the book Orchestrating Docker, in this article, you will learn how to install Docker on various systems, both in development and in production. For Linux-based systems, since a kernel is already available, installation is as simple as the apt-get install or yum install commands. However, to run Docker on non-Linux operating systems such as OSX and Windows, you will need to install a helper application developed by Docker Inc., called Boot2Docker. This will install a lightweight Linux VM on VirtualBox, which will make Docker available through port 2375, assigned by the Internet Assigned Numbers Authority (IANA).

You will have installed Docker on your system, be it in development or production, and verified it.

This article explains:

• Introducing Docker
• Installing Docker
• Ubuntu (14.04 and 12.04)
• Mac OSX and Windows

(For more resources related to this topic, see here.)

Docker was developed by DotCloud Inc. (Currently Docker Inc.), as the framework they built their Platform as a Service (PaaS) upon. When they found increasing developer interest in the technology, they released it as open source and have since announced that they will completely focus on the Docker technology’s development, which is good news as it means continual support and improvement for the platform.

There have been many tools and technologies aimed at making distributed applications possible, even easy to set up, but none of them have as wide an appeal as Docker does, which is primarily because of its cross-platform nature and friendliness towards both system administrators and developers. It is possible to set up Docker in any OS, be it Windows, OSX, or Linux, and Docker containers work the same way everywhere. This is extremely powerful, as it enables a write-once-run-anywhere workflow. Docker containers are guaranteed to run the same way, be it on your development desktop, a bare-metal server, virtual machine, data center, or cloud. No longer do you have the situation where a program runs on the developer’s laptop but not on the server.

The nature of the workflow that comes with Docker is such that developers can completely concentrate on building applications and getting them running inside the containers, whereas sysadmins can work on running the containers in deployment. This separation of roles and the presence of a single underlying tool to enable it simplifies the management of code and the deployment process.

But don’t virtual machines already provide all of these features?

Virtual Machines (VMs) are fully virtualized. This means that they share minimal resources amongst themselves and each VM has its own set of resources allocated to it. While this allows fine-grained configuration of the individual VMs, minimal sharing also translates into greater resource usage, redundant running processes (an entire operating system needs to run!), and hence a performance overhead.

Docker, on the other hand, builds on a container technology that isolates a process and makes it believe that it is running on a standalone operating system. The process still runs in the same operating system as its host, sharing its kernel. It uses a layered copy-on-write filesystem called Another Unionfs (UFS), which shares common portions of the operating system between containers. Greater sharing, of course, can only mean less isolation, but vast improvements in Linux process’s resource management solutions such as namespaces and cgroups have allowed Docker to achieve VM-like sandboxing of processes and yet maintain a very small resource footprint.

Installing Docker

Docker is available in the standard repositories of most major Linux distributions. We will be looking at the installation procedures for Docker in Ubuntu 14.04 and 12.04 (Trusty and Precise), Mac OSX, and Windows. If you are currently using an operating system not listed above, you can look up the instructions for your operating system at https://docs.docker.com/installation/#installation.

Installing Docker in Ubuntu

Docker is supported by Ubuntu from Ubuntu 12.04 onwards. Remember that you still need a 64-bit operating system to run Docker. Let’s take a look at the installation instructions for Ubuntu 14.04.

Installing Docker in Ubuntu Trusty 14.04 LTS

Docker is available as a package in the Ubuntu Trusty release’s software repositories under the name of docker.io:

$ sudo apt-get update

$ sudo apt-get -y install docker.io

That’s it! You have now installed Docker onto your system. However, since the command has been renamed docker.io, you will have to run all Docker commands with docker.io instead of docker.

The package is named docker.io because it conflicts with another KDE3/GNOME2 package called docker. If you rather want to run commands as docker, you can create a symbolic link to the /usr/local/bin directory. The second command adds autocomplete rules to bash:

$ sudo ln -s /usr/bin/docker.io /usr/local/bin/docker

$ sudo sed -i '$acomplete -F _docker docker' > /etc/bash_completion.d/docker.io

Installing Docker in Ubuntu Precise 12.04 LTS

Ubuntu 12.04 comes with an older kernel (3.2), which is incompatible with some of the dependencies of Docker. So we will have to upgrade it:

$ sudo apt-get update

$ sudo apt-get -y install linux-image-generic-lts-raring linux-headers-generic-lts-raring

$ sudo reboot

The kernel that we just installed comes with AUFS built in, which is also a Docker requirement.

Now let’s wrap up the installation:

$ curl -s https://get.docker.io/ubuntu/ | sudo sh

This is a curl script for easy installation. Looking at the individual pieces of this script will allow us to understand the process better:

1. First, the script checks whether our Advanced Package Tool (APT) system can deal with https URLs, and installs apt-transport-https if it cannot:

   # Check that HTTPS transport is available to APT

   if [ ! -e /usr/lib/apt/methods/https ]; then apt-get update apt-get install -y apt-transport-https
    fi

2. Then it will add the Docker repository to our local key chain:

   $ sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 36A1D7869245C8950F966E92D8576A8BA88D21E9

   You may receive a warning that the package isn’t trusted. Answer yes to continue the installation.

3. Finally, it adds the Docker repository to the APT sources list, and updates and installs the lxc-docker package:

   $ sudo sh -c "echo deb https://get.docker.io/ubuntu docker main
    /etc/apt/sources.list.d/docker.list"
    $ sudo apt-get update
    $ sudo apt-get install lxc-docker 

Docker versions before 0.9 had a hard dependency on LXC (Linux Containers) and hence couldn’t be installed on VMs hosted on OpenVZ. But since 0.9, the execution driver has been decoupled from the Docker core, which allows us to use one of numerous isolation tools such as LXC, OpenVZ, systemd-nspawn, libvirt-lxc, libvirt-sandbox, qemu/kvm, BSD Jails, Solaris Zones, and even chroot! However, it comes by default with an execution driver for Docker’s own containerization engine, called libcontainer, which is a pure Go library that can access the kernel’s container APIs directly, without any other dependencies.

To use any other containerization engine, say LXC, you can use the-e flag, like so: $ docker -d -e lxc.

Now that we have Docker installed, we can get going at full steam! There is one problem though: software repositories like APT are usually behind times and often have older versions. Docker is a fast-moving project and a lot has changed in the last few versions. So it is always recommended to have the latest version installed.

Upgrading Docker

You can upgrade Docker as and when it is updated in the APT repositories. An alternative (and better) method is to build from source. It is recommended to upgrade to the newest stable version as the newer versions might contain critical security updates and bug fixes. Also, the examples in this book assume a Docker version greater than 1.0, whereas Ubuntu’s standard repositories package a much older version.

Mac OSX and Windows

Docker depends on the Linux kernel, so we need to run Linux in a VM and install and use Docker through it. Boot2Docker is a helper application built by Docker Inc. that installs a VM containing a lightweight Linux distribution made specifically to run Docker containers. It also comes with a client that provides the same Application Program Interface (API) as that of Docker, but interfaces with the docker daemon running in the VM, allowing us to run commands from within the OSX/Windows terminal. To install Boot2Docker, carry out the following steps:

1. Download the latest release of Boot2Docker for your operating system from http://boot2docker.io/.
2. The installation image is shown as follows:

3. Run the installer, which will install VirtualBox and the Boot2Docker management tool.

Run Boot2docker. The first run will ask you for a Secure Shell (SSH) key passphrase. Subsequent runs of the script will connect you to a shell session in the virtual machine. If needed, the subsequent runs will initialize a new VM and start it.

Alternately, to run Boot2Docker, you can also use the terminal command boot2docker:

$ boot2docker init # First run

$ boot2docker start

$ export DOCKER_HOST=tcp://$(boot2docker ip 2>/dev/null):2375

You will have to run boot2docker init only once. It will ask you for an SSH key passphrase. This passphrase is subsequently used by boot2docker ssh to authenticate SSH access.

Once you have initialized Boot2Docker, you can subsequently use it with the boot2docker start and boot2docker stop commands.

DOCKER_HOST is an environment variable that, when set, indicates to the Docker client the location of the docker daemon. A port forwarding rule is set to the boot2Docker VM’s port 2375 (where the docker daemon runs). You will have to set this variable in every terminal shell you want to use Docker in.

Bash allows you to insert commands by enclosing subcommands within “ or $(). These will be evaluated first and the result will be substituted in the outer commands.

If you are the kind that loves to poke around, the Boot2Docker default user is docker and the password is tcuser.

The boot2Docker management tool provides several commands:

$ boot2docker

Usage: boot2docker [<options>] {help|init|up|ssh|save|down|poweroff|reset|

restart|config|status|info|ip|delete|download|version} [<args>]

When using boot2Docker, the DOCKER_HOST environment variable has to be available in the terminal session for Docker commands to work. So, if you are getting the Post http:///var/run/docker.sock/v1.12/containers/create: dial unix /var/run/docker.sock: no such file or directory error, it means that the environment variable is not assigned. It is easy to forget to set this environment variable when you open a new terminal. For OSX users, to make things easy, add the following line to your .bashrc or .bash_profile shells:

alias setdockerhost='export DOCKER_HOST=tcp://$(boot2docker ip 2>/dev/null):2375'

Now, whenever you open a new terminal or get the above error, just run the following command:

$ setdockerhost

This image shows how the terminal screen will look like when you have logged into the Boot2Docker VM.

Summary

I hope you got hooked to Docker. Docker technology will take you into the Docker world and try to dazzle you with its awesomeness.

In this article, you learned some history and some basics on Docker and how it works. We saw how it is different from and advantageous over VM.

Then we proceeded to install Docker on our development setup, be it Ubuntu, Mac, or Windows. Now you can pat your self on the back and proceed with Docker technology.

Resources for Article:

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Further resources on this subject:

• Managing Heroku from the Command Line [article]
• Target Exploitation [article]
• Wireless and Mobile Hacks [article]

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