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coreos / rkt


rkt is an App Container runtime for Linux


rkt - App Container runtime

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

Some of rkt's key features and goals include:

  • First-class integration with init systems (systemd, upstart) and cluster orchestration tools (fleet, Kubernetes)
  • Compatibility with other container software (e.g. rkt can run Docker images)
  • Modular and extensible architecture (network configuration plugins, swappable execution engines based on systemd or QEMU/KVM)

For more on the background and motivation behind rkt, read the original launch announcement.

App Container

rkt is an implementation of the App Container spec. rkt's native image format (ACI) and runtime/execution environment (pods) are defined in the specification.

Project status

rkt is maturing rapidly while under active development, and its interfaces are not yet considered stable. We do not recommend its use in production, but we encourage you to try out rkt and provide feedback via issues and pull requests.

Check out the roadmap for more details on the future of rkt.

Trying out rkt

Using rkt on Linux

rkt consists of a single self-contained CLI, 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:

tar xzvf rkt-v0.10.0.tar.gz
cd rkt-v0.10.0
./rkt help

Trying out rkt using Vagrant

For Mac (and other Vagrant) users we have set up a Vagrantfile: clone this repository and make sure you have Vagrant 1.5.x or greater installed. vagrant up starts up a Linux box and installs via some scripts rkt and actool. With a subsequent vagrant ssh you are ready to go:

git clone
cd rkt
vagrant up
vagrant ssh

Keep in mind while running through the examples that right now rkt needs to be run as root for most operations.

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: ""
Key: ""
GPG key fingerprint is: 8B86 DE38 890D DB72 9186  7B02 5210 BD88 8818 2190
  CoreOS ACI Builder <>
Are you sure you want to trust this key (yes/no)? yes
Trusting "" for prefix "".
Added key for prefix "" at "/etc/rkt/trustedkeys/prefix.d/"

In Documentation, you can find a 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
rkt: searching for app image
rkt: fetching image from
Downloading aci: [==========================================   ] 3.47 MB/3.7 MB
Downloading signature from
rkt: signature verified:
  CoreOS ACI Builder <>

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/

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

$ wget
$ 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, ""), 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
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
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.

Known issues

Due to a bug in the Linux kernel, using rkt's overlay support on top of an overlay filesystem requires Linux 4.3+.

Contributing to rkt

rkt is an open source project under the Apache 2.0 license, and contributions are gladly welcomed! See the Hacking Guide for more information on how to build and work on rkt. See CONTRIBUTING for details on submitting patches and the contribution workflow.