ostree native containers

rpm-ostree inherits work in ostree-rs-ext to create “container native ostree” functionality. This elevates OCI/docker containers to be natively supported as a transport mechanism for bootable operating systems.

Rebasing a client system

Use this to switch to booting from a container image:

$ rpm-ostree rebase ostree-unverified-registry:quay.io/fedora/fedora-coreos:stable

However, this model would just be using Docker/OCI transport “on the wire” for content that already exists today. This would aid things like mirroring the OS alongside other container images, but for many users the next step is more interesting:


After a rebase, all further rpm-ostree operations work as you’d expect. For example, rpm-ostree upgrade will look for a new container version. You can also rpm-ostree apply-live, etc. It also does still work to do “client side” rpm-ostree install etc.

URL format for ostree native containers

Ostree understand the following URL formats to retrieve and optionally verify the integrity of a container image or its content:

  • ostree-unverified-image:registry:<oci image> or ostree-unverified-image:docker://<oci image>: Fetch a container image without verify either the integrity of the container itself not its content. The container image is usually fetched over HTTPS which still provides integrity and confidentiality but not authenticity.

  • ostree-unverified-registry:<oci image>: Shortcut for the above use case.

  • ostree-remote-image:<ostree remote>:registry:<oci image> & ostree-remote-image:<ostree remote>:docker://<oci image>: Fetch a container image and verify that the included ostree commit is correctly signed by a key as configured locally in the specified ostree remote (/etc/ostree/remotes.d/<ostree remote>.conf).

  • ostree-remote-registry:<ostree remote>:<oci image>: Shortcut for the above use case.

  • ostree-image-signed:registry:<oci image> & ostree-image-signed:docker://<oci image>: Fetch a container image and verify that the container image is signed according to the policy set in /etc/containers/policy.json (see containers-policy.json(5)).

ostree (ostree-rs-ext) uses skopeo to fetch container images and thus supports the transports as documented in containers-transports(5).

Registry authentication

Today, the ostree stack will read /etc/ostree/auth.json and /run/ostree/auth.json which are in the same format as documented by containers-auth.json(5).

Using custom builds

The ostree container functionality supports layered container images; you can use any container buildsystem you like to add additional layers. See coreos-layering-examples many examples. Note: The functionality here is not specific to (Fedora) CoreOS, but it happens to be the farthest along in productizing this at the time of this writing.

This functionality is currently classified as experimental, but it is rapidly heading to stabiliziation.

Filesystem layout model

The ostree model defines effectively 3 partitions:

  • /usr: Read-only (at runtime, by default) binaries and data files
  • /etc: Mutable machine-local configuration files
  • /var: All other state

This means that it will not currently work to install e.g. RPM packages that add files in /opt by default.

Installing packages

You can use e.g. rpm-ostree install to install packages. This functions the same as with e.g. dnf or microdnf. It’s also possible to use rpm directly, e.g. rpm -Uvh https://mirror.example.com/iptables-1.2.3.rpm.

Installing config files

You can use any tooling you want to generate config files in /etc. When a booted system pulls an updated container images, the changes will also be applied.

Installing non-RPM content

A major change compared to previous rpm-ostree is that it is now clearly supported to install non-RPM binaries into /usr - these are equally “first-class” as binaries from the base image.

Adapting software

The way ostree works may require some changes in software.

Dealing with /opt

Some RPMs install files in /opt, which in the ostree model is /var/opt. In the case where the files in /opt are just binaries, one approach is to move them at build time:

FROM quay.io/fedora/fedora-coreos:testing-devel
RUN mkdir /var/opt && \
    rpm -Uvh https://downloads.linux.hpe.com/repo/stk/rhel/7/x86_64/current/hp-scripting-tools-11.60-20.rhel7.x86_64.rpm && \
    mv /var/opt/hp/ /usr/lib/hp && \
    echo 'L /opt/hp - - - - ../../usr/lib/hp' > /usr/lib/tmpfiles.d/hp.conf && \
    ostree container commit

Users and groups

At the current time, rpm-ostree will auto-synthesize systemd-sysusers snippets when useradd or groupadd are invoked during the process of e.g. rpm-ostree install.

This means that user and group IDs are allocated per machine.

Using “ostree container commit”

In a container build, it’s a current best practice to invoke this at the end of each RUN instruction (or equivalent). This will verify compatibility of /var, and also clean up extraneous files in e.g. /tmp.

In the future, this command may perform more operations.

Creating base images

There is now an rpm-ostree compose image command which generates a new base image using a treefile:

$ rpm-ostree compose image --initialize-mode=if-not-exists --format=ociarchive workstation-ostree-config/fedora-silverblue.yaml fedora-silverblue.ociarchive

The --initialize-mode=if-not-exists command here is what you almost always want: to create the image if it doesn’t exist, but to otherwise check for changes. It isn’t the default for historical reasons.

$ rpm-ostree compose image  --initialize-mode=if-not-exists --format=registry workstation-ostree-config/fedora-silverblue.yaml quay.io/example/exampleos:latest

Adding container image configuration

By default, the rpm-ostree compose image command creates container images with a minimal config. It notably does not include a default command or entrypoint.

To add more configuration to the created OCI images, you can pass an image configuration JSON document via the --image-config= argument.

Example image configuration JSON (config.json):

    "Env": [
    "Cmd": [
    "Labels": {
        "license": "MIT"

Example rpm-ostree compose image command:

rpm-ostree compose image --initialize --format=ociarchive --image-config=config.json manifest.yaml image.ociarchive

You can find the reference for the image configuration JSON format in the OCI Image Format Specification.

Converting OSTree commits to new base images

The ostree-container model creates a bidirectional bridge between ostree and OCI formatted containers. rpm-ostree compose tree today is a tool which natively accepts RPMs (and other content) and outputs an OSTree commit.

In ostree upstream, there is a simplistic CLI (and API) that “encapsulates” a commit into a container image with a single layer:

$ ostree container encapsulate --repo=/path/to/repo fedora/35/x86_64/silverblue docker://quay.io/myuser/fedora-silverblue:35

The encapsulate command accepts all the same “transport prefixes” as the skopeo CLI. For more information, see man skopeo.

However, this “single layer” is not an efficient way to deliver content. It means that any time anything in the ostree commit changes, clients need to download a full new tarball.

The ostree shared library has low level APIs that support creating reproducible “chunked” images. A key adavantage of this is that if e.g. just the kernel changes, one only downloads the layer containing the kernel/initramfs (plus a metadata layer) instead of everything.

Use a command like this to generate chunked images:

$ rpm-ostree compose container-encapsulate --repo=/path/to/repo fedora/35/x86_64/silverblue docker://quay.io/myuser/fedora-silverblue:35

This “chunked” format is used by default by rpm-ostree compose image.