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Trying out rkt

This guide provides a short introduction to trying out rkt. For a more in-depth guide of a full end-to-end workflow of building an application and running it using rkt, check out the getting-started-guide

Using rkt on Linux

rkt consists of a single CLI tool, and is currently supported on amd64 Linux. A modern kernel is required but there should be no other system dependencies. We recommend booting up a fresh virtual machine to test out rkt.

To download the rkt binary, simply grab the latest release directly from GitHub:

wget https://github.com/coreos/rkt/releases/download/v1.7.0/rkt-v1.7.0.tar.gz
tar xzvf rkt-v1.7.0.tar.gz
cd rkt-v1.7.0
./rkt help

SELinux Note: rkt can use SELinux but the policy needs to be tailored to your distribution. We suggest that new users disable SELinux to get started. If you can help package rkt for your distro please help!

Optional: Set up Privilege Separation

To facilitate privilege separation between different commands, rkt uses a rkt group that has read-write access to the rkt data directory. This allows rkt fetch, which downloads and verifies images, to run as a non-root user. If you skip this section, you can run sudo rkt fetch instead but we recommend taking the following steps for production use cases.

rkt ships with a simple script that can help set up the appropriate permissions to facilitate non-root use:

sudo groupadd rkt
export WHOAMI=$(whoami); sudo gpasswd -a $WHOAMI rkt
sudo ./dist/scripts/setup-data-dir.sh

Trust the signing key for etcd images. This must be run as root as access to the keystore is restricted:

sudo ./rkt trust --prefix coreos.com/etcd

Now it's time to test this out by retrieving an etcd image using a non-root user in the rkt group. Make sure your shell is restarted to enable the rkt group for your user, or just run newgrp rkt to enable it and continue on the same session.

Now fetch the etcd image as an unprivileged user:

./rkt fetch coreos.com/etcd,version=v2.2.5

Success! Now rkt can fetch and download images as a non-root user.

rkt using Vagrant

For Mac (and other Vagrant) users we have set up a Vagrantfile. Make sure you have Vagrant 1.5.x or greater installed.

First, download the Vagrantfile and start a Linux machine with rkt installed by running vagrant up.

git clone https://github.com/coreos/rkt
cd rkt
vagrant up

If you want to run Vagrant on a Linux host machine, you may want to use libvirt as a hypervisor, instead of VirtualBox. In order to do this, you need to install the necessary plugins, convert the box and start the machine using libvirt provider.

vagrant plugin install vagrant-libvirt
vagrant plugin install vagrant-mutate
vagrant mutate ubuntu/vivid64 libvirt
vagrant up --provider=libvirt

With a subsequent vagrant ssh you will have access to run rkt:

vagrant ssh
rkt --help

Success! The rest of the guide can now be followed normally.

rkt basics

Building App Container Images (ACIs)

rkt's native image format is ACI, defined in the App Container spec. To build ACIs, a simple way to get started is by using acbuild. Another good resource is the appc build repository which has resources for building ACIs from a number of popular projects and languages. There are also tools for converting Docker images to ACIs (although note that rkt can also run Docker images natively directly from Docker repositories by using this library internally).

The example below uses a pre-built ACI for etcd (this was built by the build-aci script).

Downloading an App Container Image (ACI)

rkt uses content addressable storage (CAS) for storing an ACI on disk. In this example, the image is downloaded and added to the CAS. Downloading an image before running it is not strictly necessary (if it is not present, rkt will automatically retrieve it), but useful to illustrate how rkt works.

Since rkt verifies signatures by default, you will need to first trust the CoreOS public key used to sign the image, using rkt trust:

$ sudo rkt trust --prefix=coreos.com/etcd
Prefix: "coreos.com/etcd"
Key: "https://coreos.com/dist/pubkeys/aci-pubkeys.gpg"
GPG key fingerprint is: 8B86 DE38 890D DB72 9186  7B02 5210 BD88 8818 2190
  CoreOS ACI Builder <[email protected]>
Are you sure you want to trust this key (yes/no)? yes
Trusting "https://coreos.com/dist/pubkeys/aci-pubkeys.gpg" for prefix "coreos.com/etcd".
Added key for prefix "coreos.com/etcd" at "/etc/rkt/trustedkeys/prefix.d/coreos.com/etcd/8b86de38890ddb7291867b025210bd8888182190"

For more information, see the detailed, step-by-step guide for the signing procedure.

Now that we've trusted the CoreOS public key, we can fetch the ACI using rkt fetch:

$ sudo rkt fetch coreos.com/etcd:v2.0.4
rkt: searching for app image coreos.com/etcd:v2.0.4
rkt: fetching image from https://github.com/coreos/etcd/releases/download/v2.0.4/etcd-v2.0.4-linux-amd64.aci
Downloading aci: [==========================================   ] 3.47 MB/3.7 MB
Downloading signature from https://github.com/coreos/etcd/releases/download/v2.0.0/etcd-v2.0.4-linux-amd64.aci.asc
rkt: signature verified:
  CoreOS ACI Builder <[email protected]>
sha512-1eba37d9b344b33d272181e176da111e

Sometimes you will want to download an image from a private repository. This usually involves passing usernames and passwords or other kinds of credentials to the server. rkt currently supports authentication via configuration files. You can find configuration file format description (with examples!) in configuration documentation.

For the curious, we can see the files written to disk in rkt's CAS:

$ find /var/lib/rkt/cas/blob/
/var/lib/rkt/cas/blob/
/var/lib/rkt/cas/blob/sha512
/var/lib/rkt/cas/blob/sha512/1e
/var/lib/rkt/cas/blob/sha512/1e/sha512-1eba37d9b344b33d272181e176da111ef2fdd4958b88ba4071e56db9ac07cf62

Per the App Container Specification, the SHA-512 hash is of the tarball and can be reproduced with other tools:

$ wget https://github.com/coreos/etcd/releases/download/v2.0.4/etcd-v2.0.4-linux-amd64.aci
...
$ gzip -dc etcd-v2.0.4-linux-amd64.aci > etcd-v2.0.4-linux-amd64.tar
$ sha512sum etcd-v2.0.4-linux-amd64.tar
1eba37d9b344b33d272181e176da111ef2fdd4958b88ba4071e56db9ac07cf62cce3daaee03ebd92dfbb596fe7879938374c671ae768cd927bab7b16c5e432e8  etcd-v2.0.4-linux-amd64.tar

Launching an ACI

After it has been retrieved and stored locally, an ACI can be run by pointing rkt run at either the original image reference (in this case, "coreos.com/etcd:v2.0.4"), the full URL of the ACI, or the ACI hash. Hence, the following three examples are equivalent:

# Example of running via ACI name:version
$ sudo rkt run coreos.com/etcd:v2.0.4
...
Press ^] three times to kill container

# Example of running via ACI hash
$ sudo rkt run sha512-1eba37d9b344b33d272181e176da111e
...
Press ^] three times to kill container

# Example of running via ACI URL
$ sudo rkt run https://github.com/coreos/etcd/releases/download/v2.0.4/etcd-v2.0.4-linux-amd64.aci
...
Press ^] three times to kill container

In the latter case, rkt will do the appropriate ETag checking on the URL to make sure it has the most up to date version of the image.

Note that the escape character ^] is generated by Ctrl-] on a US keyboard. The required key combination will differ on other keyboard layouts. For example, the Swedish keyboard layout uses Ctrl-å on OS X and Ctrl-^ on Windows to generate the ^] escape character.