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It used to be tricky to get Docker working on OS X, which is why I wrote this here blog post. With the release of Docker 1.8, it just got way easier. Now there's a new thing, Docker Toolbox, that makes it super easy. You should install that instead of reading this post. Or you can read it if you want, I think it's a pretty good blog post and you'll learn some stuff. No pressure, though.

Have you heard of Docker? You probably have—everybody’s talking about it. It’s the new hotness. Even my dad’s like, “what’s Docker? I saw someone twitter about it on the Facebook. You should call your mom.”

Docker is a program that makes running and managing containers super easy. It has the potential to change all aspects of server-side applications, from development and testing to deployment and scaling. It’s pretty cool.

Recently, I’ve been working through The Docker Book. It’s a top notch book and I highly recommend it, but I’ve had some problems running the examples on OS X. After a certain point, the book assumes you’re using Linux and skips some of the extra configuration required to make the examples work on OS X. This isn’t the book’s fault; rather, it speaks to underlying issues with how Docker works on OS X.

This post is a walkthrough of the issues you’ll face running Docker on OS X and the workarounds to deal with them. It’s not meant to be a tutorial on Docker itself, but I encourage you to follow along and type in all the commands. You’ll get a better understanding of how Docker works in general and on OS X specifically. Plus, if you decide to dig deeper into Docker on your Mac, you’ll be saved hours of troubleshooting. Don’t say I never gave you nothing.

First, let’s talk about how Docker works and why running it on OS X no work so good.

How Docker Works

Docker is a client-server application. The Docker server is a daemon that does all the heavy lifting: building and downloading images, starting and stopping containers, and the like. It exposes a REST API for remote management.

The Docker client is a command line program that communicates with the Docker server using the REST API. You will interact with Docker by using the client to send commands to the server.

The machine running the Docker server is called the Docker host. The host can be any machine—your laptop, a server in the Cloud™, etc—but, because Docker uses features only available to Linux, that machine must be running Linux (more specifically, the Linux kernel).

Docker on Linux

Suppose we want to run containers directly on our Linux laptop. Here’s how it looks:

Docking on Linux

The laptop is running both the client and the server, thus making it the Docker host. Easy.

Docker on OS X

Here’s the thing about OS X: it’s not Linux. It doesn’t have the kernel features required to run Docker containers natively. We still need to have Linux running somewhere.

Enter boot2docker. boot2docker is a “lightweight Linux distribution made specifically to run Docker containers.” Spoiler alert: you’re going to run it in a VM on your Mac.

Here’s a diagram of how we’ll use boot2docker:

Docking on OS X

We’ll run the Docker client natively on OS X, but the Docker server will run inside our boot2docker VM. This also means boot2docker, not OS X, is the Docker host, not OS X.

Make sense? Let’s install dat software.

Installation

Step 1: Install VirtualBox

Go here and do it. You don’t need my help with that.

Step 2: Install Docker and boot2docker

You have two choices: the offical package from the Docker site or homebrew. I prefer homebrew because I like to manage my environment from the command line. The choice is yours.

Step 3: Initialize and start boot2docker

First, we need to initialize boot2docker (we only have to do this once):

Next, we can start up the VM. Do like it says:

Step 4: Set the DOCKER_HOST environment variable

The Docker client assumes the Docker host is the current machine. We need to tell it to use our boot2docker VM by setting the DOCKER_HOST environment variable:

☞Your VM might have a different IP address—use whatever boot2docker up told you to use. You probably want to add that environment variable to your shell config.

Step 5: Profit

Let’s test it out:

Great success. To recap: we’ve set up a VirtualBox VM running boot2docker. The VM runs the Docker server, and we’re communicating with it using the Docker client on OS X.

Bueno. Let’s do some containers.

Common Problems

We have a “working” Docker installation. Let’s see where it falls apart and how we can fix it.

Problem #1: Port Forwarding

The Problem: Docker forwards ports from the container to the host, which is boot2docker, not OS X.

Let’s start a container running nginx:

This command starts a new container as a daemon (-d), automatically forwards the ports specified in the image (-P), gives it the name ‘web’ (--name web), and uses the nginx image. Our new container has the unique identifier 0092c03e1eba....

Verify the container is running:

Under the PORTS heading, we can see our container exposes port 80, and Docker has forwarded this port from the container to a random port, 49153, on the host.

Let’s curl our new site:

It didn’t work. Why?

Remember, Docker is mapping port 80 to port 49153 on the Docker host. If we were on Linux, our Docker host would be localhost, but we aren’t, so it’s not. It’s our VM.

The Solution: Use the VM’s IP address.

boot2docker comes with a command to get the IP address of the VM:

Let’s plug that into our curl command:

Success! Sort of. We got the web page, but we got The VM’s Host only interface IP address is:, too. What’s the deal with that nonsense.

Turns out, boot2docker ip outputs the IP address to standard output and The VM's Host only interface IP address is: to standard error. The $(boot2docker ip) subcommand captures standard output but not standard error, which still goes to the terminal. Scumbag boot2docker.

This is annoying. I am annoyed. Here’s a bash function to fix it:

Stick that in your shell config, then use it like so:

Groovy. This gives us a reference for the IP address in the terminal, but it would be nice to have something similar for other apps, like the browser. Let’s add a dockerhost entry to the /etc/hosts file:

Now we can use it everywhere:

Great success. Make sure to stop and remove the container before continuing:

☞VirtualBox assigns IP addresses using DHCP, meaning the IP address could change. I

Docker For Mac Nsenter