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Extensions are additional packages which client machines can install using package layering. While rpm-ostree itself is indifferent on the subject, most rpm-ostree-based distros encourage a containerized workflow for better separation of host and application layers. But sometimes, containerization is not ideal for some software, and yet it may not be desirable to bake them into the OSTree commit by default.

Package layering normally fetches such extensions from remote repos. However in some architectures there may be a better way to transfer them, or one may simply want tighter control over them and stronger binding between OSTree commit and extension versions (e.g. for reproducibility, guaranteed depsolve, QE, releng, etc..).

rpm-ostree compose extensions takes an extensions.yaml file describing OS extensions (packages) and a base OSTree commit. After performing a depsolve, it downloads the extension packages and places them in an output directory.


The format of the extensions.yaml file is as follow:

# The top-level object is a dict. The only supported key
# right now is `extensions`, which is a dict of extension
# names to extension objects.
  # This can be whatever name you'd like. The name itself
  # isn't used by rpm-ostree.
    # Optional; defaults to `os-extension`. An OS extension
    # is an extension intended to be `rpm-ostree install`ed.
    kind: os-extension
    # List of packages for this extension
        - strace
        - ltrace
    # Optional list of architectures on which this extension
    # is valid. These are RPM basearches. If omitted,
    # defaults to all architectures.
        - x86_64
        - aarch64
    # A development extension lists packages useful for
    # developing for the target OSTree, but won't be layered
    # on top. A common example is kernel modules. No
    # depsolving happens, packages listed are downloaded.
    kind: development
      - kernel-devel
      - kernel-headers
    # Optional name of a base package used to constrain the
    # EVR of all the packages in this extension.
    match-base-evr: kernel