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Sextant - A tool for working with northstar NPK containers

Northstar containers are distributed in the NPK file format.

An NPK file contains both the container's application logic and the data files necessary to mount and run the container. To facilitate the creation, inspection and modification of NPKs, northstar provides the northstar-sextant CLI tool.

Packing an unsigned NPK

NPKs are created (packed) using the pack command of northstar-sextant. It requires the following input:

  1. A manifest file describing the NPK.
  2. A root folder containing the files required at runtime.

During packing, the contents of the folder will be copied to a squashfs image that will be mounted by the northstar runtime when the container is run.

For example, the following command packs the hello-world example container:

$ target/debug/northstar-sextant pack \
--manifest examples/container/hello-world/manifest.yaml \
--root target/release/hello-world \
--out target/northstar/repository

The output of northstar-sextant pack is single NPK file:

$ ls target/northstar/repository
hello-world-0.0.1.npk

Packing a signed NPK

NPKs can be signed using Ed25519 signatures. If the runtime is configured to check NPK signatures, containers with missing or invalid signatures will be rejected. To pack a signed version of the hello-world example container, a private key has to be provided:

$ target/debug/northstar-sextant pack \
--manifest examples/container/hello-world/manifest.yaml \
--root target/release/hello-world \
--key ./examples/keys/northstar.key \
--out target/northstar/repository

Generating repository keys

To sign NPKs using northstar-sextant a suitable key pair is needed. It can be generated using the northstar-sextant gen-key command. The following call creates a new key pair (repokey.key and repokey.pub) in the current directory:

target/debug/northstar-sextant gen-key --name repokey --out .

The private key repokey.key can be used for signing of NPKs while the public key repokey.pub is used by the northstar runtime to verify NPKs.

Unpacking an NPK

NPKs are ZIP files that contain among other things a squashfs image that will be mounted at runtime. To extract both the outer ZIP and the inner image, the unpack command of northstar-sextant can be used.

To unpack the hello-world example container, the northstar-sextant unpack can be used:

$ target/debug/northstar-sextant unpack \
--npk ./target/northstar/repository/hello-world-0.0.1.npk \
--out ./hello-world-container

The extracted container can be found in the output directory:

$ ls hello-world-container
fs.img  manifest.yaml  signature.yaml  squashfs-root

The squashfs-root directory holds the extracted contents of the fs.img squashfs image:

$ ls hello-world-container/squashfs-root/
dev  hello-world  lib  lib64  proc  system

We can see the hello-world binary as well as the empty mount points mentioned in the manifest.yaml.

Inspecting an NPK

To get information about an already packed NPK northstar-sextant provides the inspect command.

Inspecting an NPK with default settings

Inspecting an NPK without any additional parameters will show the following information:

  • List of files contained in the NPK
  • Content of manifest.yaml
  • Content of signature.yaml
  • List of files contained in the compressed squashfs image (fs.img) stored in the NPK

The hello-world example container can be inspected with the following command:

$ northstar-sextant inspect target/northstar/repository/hello-0.0.1.npk
...

Inspecting an NPK with the --short parameter

To facilitate the inspection of many containers as part of scripts, the inspect command features the --short parameter. It condenses the inspection output into a single line with the following information:

  • Container name
  • Container version
  • NPK format version
  • Whether this is a resource container

Inspecting the hello-world example container with the --short flag gives the following output:

$ northstar-sextant inspect --short hello-world-0.0.1.npk 
name: hello-world, version: 0.0.1, NPK version: 0.0.2, resource container: no