Packaging unikernels

Learn how to access pre-built unikernels, Unikraft library components, or distribute newly constructed unikernels.

Package management with kraft is handled through the pkg subcommand which offers convenient mechanisms for retrieving, packaging and distributing components used during the formation of unikernels based on Unikraft. kraft pkg can also help retrieve or distribute completely pre-built unikernels ready for use.

To get started building unikernels or accessing pre-built unikernels, we must first learn how we can retrieve one or many of these components using kraft pkg.

Checking for updates#

By default and when left unconfigured, kraft will retrieve a list of official Unikraft library components that can be used later on when building a unikernel. The first step is to retrieve the latest information about these components. Simply run:

kraft pkg update

This will locally store information locally about packages and library components. This location on-disk and the remote location of the component locations can be configured to yours needs.

It is recommended to regularly run kraft pkg update as Unikraft regularly updates its components with new features, bug fixes, newly supported applications, libraries, etc. See Unikraft's release schedule.

In some cases, such as with kraft build, updating is done implicitly before the command's main functionality is executed. Because of this, in some subcommands, a --no-update flag is offered to prevent this operation from occurring.

Package component types#

Unikraft ships (and kraft allows you to ship) packages of different component types. A component is a building block, and ultimately a code base, repository or distributable archive or binary, which is used within the Unikraft ecosystem. Components are one of 5 unique types:

coreA repository containing the Unikraft core codebase which consists of a Make and KConfig-based build system. The Unikraft core provides the glue for stiching other components as well as your application together.
archThe component that defines behavior and hardware interactions specific to the target architecture.
platA repository containing code that defines the interaction with the underlying hardware, depending on whether a hypervisor is present or not, and which hypervisor is present.
libA repository containing a Unikraft external library, that defines an user-space functionality.
appThe final component type is that of a fully functioning unikernel. The app component defines the use of the core, a arch and plat combination, as well as a list of libs.

Package Managers#

Internal to kraft are programmable package managers that allows you to fetch resources that either comprise of library components (distributed through the KraftKit manifest specification) of a unikernel or fully pre-built unikernels (distributed using the OCI Image Specification).

Manifest-based packages#

A manifest-based package is a simple representation of a component that is one of three distributable types:

  • A Git repository;
  • A directory; or,
  • A tarball.

OCI-based packages#

It is possible to access pre-built and to distribute yourself unikernel images using the OCI Image Specification. This feature enables you to use existing registries to store and distribute unikernel images quickly and easily.

Built in to kraft is a reference to our public-access OCI registry which is hosted simply at To access unikernel images in another registry, simply add it as a source.

You can configure different backends for OCI images, the default is directory-based and will store the artifacts locally on disk. Alternatively, you can use containerd as a backend by setting the containerd_addr to the daemon socket in the global settings.

Listing and searching for packages#

You can list all packages and components with:

kraft pkg list

To view the latest applications distributed as OCI images that are provided by the Unikraft OSS Project's registry, simply run the following:

kraft pkg ls -u --apps

The above command lists components by --apps and requests an update (-u|--update) such that it queries the remote registry.

Sourcing additional packages#

By default, you do not need to source any additional repositories or manifests if you have a freshly installed version of kraft and simply wish to get started using existing applications, libraries or the Unikraft core based on the publicly available and official channels.

To manage components with kraft, we must first know where we can find them. This is done by "sourcing" them from a remote location with:

kraft pkg source SOURCE

Where SOURCE is, for example:

# Source a component represented as a public Git repository
kraft pkg source
# Sourcing a private Git repository
kraft pkg source
# Sourcing a manifest (index) of packages
kraft pkg source
# Sourcing a manifest of a single package
kraft pkg source
# Source an OCI registry
kraft pkg source localhost:3000

In the above example, there are three types of sources:

  1. Git repositories representing a component: When sourcing repositories directly, e.g. when you wish to use a git repository representing a microlibrary, kraft pkg source will attempt to determine the component type with the naming convention $type-$name (e.g. lib-lwip, app-helloworld).

    See kraft pkg source --help for more information about how to correctly annotate sources with unconventional names.

  2. Manifests: It is possible to pass kraft pkg source a local path on disk or a remote URL to a Manifest file which represents either a list of components or a single component.

    The officially maintained manifest for all Unikraft components (and default manifest if KraftKit is left unconfigured) is:
  3. OCI Registries: It is possible to add a reference to a remote OCI registry to view pre-built unikernel images.

Removing a source of packages#

To remove a source, you can unsource it by invoking:

kraft pkg unsource [SOURCE]

Invoking kraft pkg unsource without the optional [SOURCE] positional argument (which must be an exact match) you will be prompted to interactively select which source(s) you would like to remove from KraftKit.

Retrieving a package#

To cache a package locally on your machine either for multiple uses or within the context of a project's build, you can use:

kraft pkg pull [PACKAGE]

Without any flags, the above command will store a copy in the manifest cache directory. When used with -w, it will save it in the context of a project.

Packaging your application#

Once you have successfully built a Unikraft unikernel and you wish to distribute your application, you can use the top-level kraft pkg subcommand. Left without any arguments, the command will use information supplied in the Kraftfile to determine how to package the artifact(s):

# Simply invoke kraft pkg within the application directory
kraft pkg
# Or pass a path to a an application directory
kraft pkg path/to/my/app

To package your image as an OCI image, you can force the type and OCI Image Reference name (or tag) to be used:

kraft pkg --as oci --name

In the above example, a "Hello, world!" project with a single target (qemu/x86_64) is packaged as an OCI image with the name

If your project has multiple targets, each built target will be packaged together to produce a multi-platform image.

Distributing your application#

Once packaged, it is possible to push the resulting image to the remote registry. Simply invoke:

kraft pkg push
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