Containers are basically taking over the world; Docker, rkt, systemd-nspawn, LXC, etc. Today, we're going to talk about rkt (pronounced "rocket") and how to get started building rkt-runnable containers.

What is rkt?

rkt (pronounced "rock-it") is a CLI for running app containers on Linux. rkt is designed to be composable, secure, and fast.

Alright, so whats an App Container (appc)?

An application container is a way of packaging and executing processes on a computer system that isolates the application from the underlying host operating system.

In other words, rkt is a runtime implementation that uses the appc container spec. It leverages systemd to manage processes within the container, making it compatible with orchestration tools such as fleet and Kubernetes.

Containers can include basically anything from a single, static binary, to an entire root file system. Today, we're going to look at building a Fedora based container that can then be used as a foundation for building other containers on top of it. This will effectively give us the equivalent of using the Fedora docker image.

Boot up CoreOS

To follow along, you'll need to boot an instance of CoreOS of some kind (AWS, GCE, Azure, DigitalOcean, Vagrant, etc). I would use either the latest Beta or Alpha channel release to be sure you have the latest versions of rkt and actool.

Fetching Fedora

We're striving for a super minimal image to use as our base layer, and it just so happens that the folks over at Fedora build a Docker base image which is nothing more than a stripped down Fedora file system. So we're going to use that, but we're not going to use Docker at all to get it.

Here you will find all of the Fedora build images. Builds in green have passed and are probably safe to use. We're going to be using Fedora 23, so look for a build that says f23-candidate, Fedora-Docker-Base-23....

Once you've SSHd into your machine as the core user, fetch the Fedora image:

mkdir fedoraLayer

# fetch and unpack fedora build
curl | tar -xJ -C fedoraLayer
cd fedoraLayer

HASH=$(cat repositories | awk -F '"latest": "' '{ print $2 }' | awk '{ sub(/[^a-zA-Z0-9]+/, ""); print }')

mv $HASH/layer.tar .
rm -rf $HASH repositories
sudo tar -xf layer.tar --same-owner --preserve-permissions

sudo rm layer.tar
cd ../

The HASH variable represents the directory inside the tarball that contains the rootfs; we take the contents of said directory and move it up one level so that /home/core/fedoraLayer contains the rootfs.

Installing acbuild

acbuild is a nifty little interactive CLI for building your container manifest. If you want the most flexibility, you can feel free to write out the manifest by hand.

When this post was written, acbuild was still in early development, so we're going to build it from source. For those unfamiliar with CoreOS, CoreOS comes with basically nothing; no package manager and only a very small set of tools. It does however come with a tool called toolbox which is a container that we can use to actually do some work. We're going to use toolbox to fetch and build acbuild from source.

# Clone acbuild to a known directory inside /home/core.
# We're specifically going to clone it /home/core/builds/acbuild.
mkdir $HOME/builds && cd $HOME/builds
git clone


# now inside toolbox
yum install -y golang git

# the host filesystem is mounted to /media/root
cd /media/root/home/core/builds/acbuild

# exit toolbox by pressing ctrl+c

# now back on the host system, outside of toolbox

sudo mkdir -p /opt/bin || true
# /usr is readonly, but /opt/bin is in the PATH, so symlink our 
# acbuild binary to that directory
sudo ln -s /home/core/builds/acbuild/bin/acbuild /opt/bin

acbuild --help

If all goes well, you should see the acbuild help menu at the very end.

Building our container

acbuild works by declaring that you are beginning the build of a container (this creates a hidden directory in your CWD that will be used to hold the state of everything as we go), running subcommands, writing the aci (app container image), and then telling acbuild that we're done. Here's our build process:

sudo acbuild begin /home/core/fedoraLayer
sudo acbuild set-name <your domain>/fedora
sudo acbuild label add version "latest"
sudo acbuild write fedora.aci
sudo acbuild end
actool validate fedora.aci

What we're doing here is:

  • Telling acbuild to use our fedora rootfs that we extracted as the rootfs for the container
  • Setting the name to <your domain>/fedora. For example, I would use This is very similar to the naming convention you see in docker when hosting containers on something like and acts as a name that you will reference your container by.
  • We set the label "version" to "latest"
  • We write out everything to fedora.aci. This what we will actually run with rkt.
  • Tell acbuild we're done
  • Validate our container with actool.

Thats it, we're done! Well almost. We have a container, but it's pretty useless because we didnt tell it what to execute when we run it with rkt. Lets create it again, but this time we'll tell it to run /bin/date.

sudo acbuild begin /home/core/fedoraLayer
sudo acbuild set-name <your domain>/fedora
sudo acbuild label add version "latest"
sudo acbuild set-exec -- /bin/date
sudo acbuild write fedora.aci
sudo acbuild end
actool validate fedora.aci

Now we can actually run it:

sudo rkt run --insecure-options=all ./fedora.aci
rkt: using image from local store for image name
rkt: using image from file /home/core/fedora.aci
[123872.191605] date[4]: Tue Dec 15 00:14:06 UTC 2015

Advanced rkt containers

Stay tuned for more posts about building more advanced rkt containers, building your own container repository with appc discovery, and more!