Racker is an experimental harness tool for provisioning and launching containers, with a focus on operating system containers.

By a "harness tool", we mean a combination of image bakery and payload launcher.

• The image bakery is based on modern and generic tools for creating machine images like mkosi and Packer, as well OCI-compliant container images. Container images can be acquired from both vendor- specific and standardized distribution channels like OCI-compliant image registries.
• A payload is any of an interactive command prompt (shell), a single program invocation, or a long-running daemon.

• A lightweight wrapper around systemd-nspawn to provide and launch container environments for/with systemd.
• A lightweight wrapper around vagrant to provide convenient access to all things needing a full VM, like running Windows on Linux or macOS.
• A tribute to the authors and contributors of GNU, Linux, systemd, Python, VirtualBox, Vagrant, Docker, Windows, Windows Docker Machine and countless others.

• Launch interactive command prompts or invoke programs non-interactively within a isolated and volatile Linux and Windows environments.
• Build upon the runtime harness framework to build solutions for running and testing software packages in different environments, mostly run headless and non-interactively.

Racker has two different subsystems / runner backends, one for Linux and another one for Windows.

• For running Linux operating system containers, Racker uses systemd and systemd-nspawn. Provisioning of additional software is performed using the native package manager of the corresponding Linux distribution.

On the host side, Racker can run on Linux, macOS, and Windows. On the container side, the following list of operating systems has been verified to work well.

• AmazonLinux 2022
• Arch Linux 20220501
• CentOS 7-9
• Debian 9-12 and unstable (stretch, buster, bullseye, bookworm, sid)
• Fedora 35-37
• openSUSE 15 and latest (leap, tumbleweed)
• Oracle Linux 8
• Red Hat RHEL 8 and 9
• Rocky Linux 8
• SUSE SLES 15 and BCI:latest
• Ubuntu LTS 20 and 22 (focal, jammy)

The aims of Racker are very similar to Docker, Podman, Distrobox and Toolbox. However, there are also some differences.

Most people running Linux probably want to use Podman these days. For more background, enjoy reading Container wars and Container Tools Guide.

• Racker is currently based on systemd-nspawn and Vagrant instead of Docker or Podman.
• Racker's focus is to provide easy provisioning and launching OS containers aka. OS-level virtualization, using systemd as init process.
• The acquisition and provisioning of operating system images does not need any special preparation steps, those are handled by Racker on the fly.
• Racker aims to provide concise usability by folding its usage into a single command.
• Racker is written in Python instead of Golang or Bash.

See also Comparison with similar tools - more details.

systemd-nspawn may be used to run a command or OS in a light-weight namespace container. In many ways it is similar to chroot, but more powerful since it fully virtualizes the file system hierarchy, as well as the process tree, the various IPC subsystems and the host and domain name.

It is primarily intended for use in development, experimenting, debugging, instrumentation, testing and building of software.

It can easily be used to start containers capable of booting up a complete and unmodified Linux distribution inside as normal system services.

For learning more details about systemd-nspawn, we strongly recommend to read the more extensive systemd-nspawn in a nutshell.

Install prerequisites:

apt-get update
apt-get install --yes systemd-container skopeo umoci python3-pip python3-venv

Install Racker:

python3 -m venv .venv
source .venv/bin/activate
pip install racker --upgrade

To install the latest development version, use this command instead:

pip install git+https://github.com/cicerops/racker --upgrade

The racker program aims to resemble the semantics of Docker by providing a command line interface compatible with the docker command.

# Invoke the vanilla Docker `hello-world` image.
# FIXME: Does not work yet.
# racker run -it --rm hello-world /hello
# racker run -it --rm quay.io/podman/hello

# Acquire rootfs images.
racker pull debian:bullseye-slim
racker pull fedora:37

# Launch an interactive shell.
racker run -it --rm debian:bullseye-slim bash
racker run -it --rm fedora:37 bash
racker run -it --rm docker://registry.access.redhat.com/ubi8/ubi-minimal /bin/bash
racker run -it --rm docker://quay.io/centos/centos:stream9 bash

# Launch a single command.
racker run -it --rm debian:11-slim hostnamectl
racker run -it --rm opensuse/tumbleweed hostnamectl
racker run -it --rm ubuntu:jammy /bin/cat /etc/os-release
racker run -it --rm registry.suse.com/suse/sle15 /bin/cat /etc/os-release
racker run -it --rm registry.suse.com/bci/bci-base:15.4 /bin/cat /etc/os-release
racker run -it --rm docker://ghcr.io/jpmens/mqttwarn-standard /usr/bin/hostnamectl

# Verbose mode.
racker --verbose run -it --rm fedora:37 hostnamectl

# Use stdin and stdout, with timing.
time echo "hello world" | racker run -it --rm fedora:37 cat /dev/stdin > hello
cat hello

The idea behind postroj is to provide an entrypoint to a command line interface implementing actions that don't fit into racker, mostly having a more high-level character.

Currently, postroj pkgprobe implements a flavor of full system integration/acceptance testing in order to test the soundness of actual installed binary distribution packages, in the spirit of autopkgtest.

To do so, it implements the concept of curated operating system images, whose labels have a different layout than labels of Docker filesystem images.

Getting started:

# List available images.
postroj list-images

# Acquire images for curated operating systems.
postroj pull debian-bullseye
postroj pull fedora-37

# Acquire rootfs images for all available distributions.
postroj pull --all

# Run a self test procedure, invoking `hostnamectl` on all containers.
postroj selftest hostnamectl

Package testing:

# Run a self test procedure, invoking example probes on all containers.
postroj selftest pkgprobe

# Run two basic probes on different operating systems.
postroj pkgprobe --image=debian-bullseye --check-unit=systemd-journald
postroj pkgprobe --image=fedora-37 --check-unit=systemd-journald
postroj pkgprobe --image=archlinux-20220501 --check-unit=systemd-journald

# Run probes that need to install a 3rd party package beforehand.

postroj pkgprobe \
    --image=debian-stretch \
    --package=http://ftp.debian.org/debian/pool/main/w/webfs/webfs_1.21+ds1-12_amd64.deb \
    --check-unit=webfs \
    --check-network=http://localhost:8000

postroj pkgprobe \
    --image=debian-bullseye \
    --package=https://dl.grafana.com/oss/release/grafana_8.5.1_amd64.deb \
    --check-unit=grafana-server \
    --check-network=http://localhost:3000

postroj pkgprobe \
    --image=centos-8 \
    --package=https://dl.grafana.com/oss/release/grafana-8.5.1-1.x86_64.rpm \
    --check-unit=grafana-server \
    --check-network=http://localhost:3000

A SuT which just uses a dummy probe /bin/systemctl is-active systemd-journald on Debian 10 "buster" cycles quite fast, essentially demonstrating that the overhead of environment setup/teardown is insignificant.

time postroj pkgprobe --image=debian-buster --check-unit=systemd-journald

real    0m0.589s
user    0m0.161s
sys     0m0.065s

On a cold system, where the filesystem image would need to be acquired before spawning the container, it's still fast enough:

time postroj pkgprobe --image=debian-bookworm --check-unit=systemd-journald

real    0m22.582s
user    0m8.572s
sys     0m3.136s

• Q: How does it work?

  A: Directly quoting the machinectl documentation here:

  Note that systemd-run with its --machine= switch may be used in place of the machinectl shell command, and allows non-interactive operation, more detailed and low-level configuration of the invoked unit, as well as access to runtime and exit code/status information of the invoked shell process.

  In particular, use systemd-run's --wait switch to propagate exit status information of the invoked process. Use systemd-run's --pty switch for acquiring an interactive shell, similar to machinectl shell. In general, systemd-run is preferable for scripting purposes.

• Q: How does it work, really?

  A: Roughly speaking...
  • skopeo and umoci are used to acquire root filesystem images from Docker image registries.
  • systemd-nspawn is used to run commands on root filesystems for provisioning them.
  • Containers are started with systemd-nspawn --boot.
  • systemd-run is used to interact with running containers.
  • machinectl is used to terminate containers.
• Q: How is this project related with Docker?

  A: The runtime is completely independent of Docker, it is solely based on systemd-nspawn containers instead. However, root filesystem images can be pulled from Docker image registries in the spirit of machinectl pull-dkr. Other than this, the racker command aims to be a drop-in replacement for its corresponding docker counterpart.
• Q: Do I need to have Docker installed on my machine?

  A: No, Racker works without Docker.
• Q: How are machine names assigned?

  A: Machine names for spawned containers are automatically assigned. The name will be assembled from the distribution's fullname attribute, prefixed with postroj-. Examples: postroj-debian-buster, postroj-centos-8.
• Q: Does the program need root privileges?

  A: Yes, the program currently must be invoked with root or corresponding sudo privileges. However, it would be sweet to enable unprivileged operations soon. systemd-nspawn should be able to do it, using --private-users or --user?
• Q: Where does the program store its data?

  A: Data is stored at /var/lib/postroj. In this manner, it completely gets out of the way of any other images, for example located at /var/lib/machines. Thus, any images created or managed by Racker will not be listed by machinectl list-images.

  A: The download cache is located at /var/cache/postroj/downloads.
• Q: Where are the filesystem images stored?

  A: Activated filesystem images are located at /var/lib/postroj/images.
• Q: How large are curated filesystem images?

  A: The preference for curated filesystem images is to use their corresponding "slim" variants where possible, aiming to only use artefacts with download sizes < 100 MB.
• Q: Are container disks ephemeral?

  A: Yes, by default, all container images will be ephemeral, i.e. all changes to them are volatile.

It's always the cable. ;]

1. If you see that your container might not have network access, make sure to provide a valid DNS configuration in your host's /etc/resolv.conf. When in doubt, please add nameserver 9.9.9.9 as the first entry.

2. Sometimes, configuring an arbitrary name server is not permitted. An indicator could be this configuration within /etc/resolv.conf:

   nameserver 127.0.0.53
   options edns0 trust-ad

   If that is the case, you can look up the trusted DNS servers by running resolvectl status or resolvectl dns. Then, use the DNS server listed there to announce per nameserver <dns host> entry to your container's /etc/resolv.conf.