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single: airshipctl single: bootstrap single: ISO single: image single: CLI single: ephemeral host

Airshipctl Bootstrap Image Generator

This spec defines the new isogen sub-command for airshipctl bootstrap and describes the interface for image builder. Airship CLI tool will be extended with an ability to generate an ISO image or image for USB stick. This image can be used to boot up an ephemeral node with Kubernetes cluster installed.

Jira tasks:

Problem Description

Common approach for spinning new Kubernetes cluster is Cluster API deployed on top of a small single node cluster based on kind or minikube. In order to create Kubernetes cluster on hardware nodes in Data Center user must deploy this single node cluster on a virtual machine attached to PXE network or to deploy operating system and Kubernetes cluster to one of the hardware servers.

In scope of Airship 2.0 user needs to be able to bootstrap ephemeral Kubernetes cluster with minimal required services (e.g. Cluster API, Metal3, etc). Ephemeral Cluster should be deployed remotely (if possible) and deployment process needs to be fully automated.

Impacted Components

  • airshipctl

Proposed Change

Airship 2.0 command line tool (i.e. airshipctl) will be able to perform full cycle of bootstrapping ephemeral Kubernetes node.

First bootstrap step is to generate ISO or flash drive image. Image generator is executed inside of a container and returns LiveCD or LiveUSB image.

Image generator must implement interface defined below (see Image Generator Container Interface section) since isogen command treats image generator container as a black box.

Airshipctl Subcommand

airshipctl bootstrap is extended with isogen subcommand. Subcommand is extendable by adding Container Runtime Interface drivers.

Command flags:

  • -c or --conf Configuration file (YAML-formatted) path for ISO builder container. If option is omitted airshipctl config is used to determine isogen configuration file path. This configuration file is used to identify container execution options (e.g. CRI, volume binds etc) and as a source of ISO builder parameters (e.g. cloud-init configuration file name). File format described in Command and ISO Builder Configuration File Format

Command arguments:

  • - can be used when rendered document model has been passed to STDIN.

Subcommand should implement following steps:

  • Utilize the airshipctl config to identify the location of YAML documents which contains site information.
  • Extract information for ephemeral node from the appropriate documents, such as IP, Name, MAC, etc.
  • Generate the appropriate user-data and network-config for Cloud-Init.
  • Execute container with ISO builder and put YAML-formatted builder config, user-data and network-config to a container volume.

YAML manipulations which are required for operations described above rely on functions and methods that have been implemented as a part of airshipctl document command.

Image Generator Container Interface

Image generator container input.

  • Volume (host directory) mounted to certain directory in container. Example: docker run -v /source/path/on/host:/dst/path/in/container ...
  • YAML-formatted configuration file saved on the mounted volume. Described in Command and ISO Builder Configuration File Format
  • Shell environment variable BUILDER_CONFIG which contains ISO builder configuration file path (e.g. if volume is bound to /data in the container then BUILDER_CONFIG=/data/isogen.yaml).
  • Cloud-init configuration file named according to userDataFileName parameter of builder section specified in ISO builder configuration file. User data file must be placed to the root of the volume which is bound to the container.
  • Network configuration for cloud init (i.e. network-config) named according to networkConfigFileName parameter of builder section specified in ISO builder configuration file. Network configuration file must be placed in the root of the volume which is bound to the container.

Image generator output.

  • YAML-formatted metadata file which describes output artifacts. File name for metadata is specified in builder section of ISO builder configuration file (see Command and ISO Builder Configuration File Format for details). Metadata file name is specified in aitshipctl configuration files and handeled by airshipctl config command. Metadata must satisfy following schema.

    $schema: ''
    type: 'object'
        type: 'string'
        description: >
          Image file path on host. Host path of the volume is extracted
          from ISO builder confgiration file passed by isogen command to
          container volume.
  • ISO or flash disk image placed according to bootImagePath parameter of output metadata file.

Command and ISO Builder Configuration File Format

YAML formatted configuration file is used for both isogen command and ISO builder container. Configuration file is copied to volume directory on the host. ISO builder uses shell environment variable BUILDER_CONFIG to read determine configuration file path inside container.

Configuration file format.

$schema: ''
type: 'object'
    type: 'object'
    description: 'Configuration parameters for container'
        type: 'string'
        description: >
          Container volume directory binding.
          Example: /source/path/on/host:/dst/path/in/container
        type: 'string'
        description: 'ISO generator container image URL'
        type: 'string'
        description: >
          (Optional) Container Runtime Interface driver (default: docker)
        type: 'bool'
        description: >
          (Optional)Defines if container should be started in privileged mode
          (default: false)
    type: 'object'
    description: 'Configuration parameters for ISO builder'
        type: 'string'
        description: >
          Cloud Init user-data file name placed to the container volume root
        type: 'string'
        description: >
          Cloud Init network-config file name placed to the container
          volume root
        type: 'string'
        description: 'File name for output matadata'

Security Impact

  • Kubernetes Certificates are saved on the ISO along with other Cloud Init configuration parameters.
  • Clound-init contains sensitive information (e.g. could contain ssh keys).

Performance impact



  • Modify existing LiveCD ISO image using Golang library.
    • Requires implementation of ISO modification module in Golang.
    • Each time user generated new image ISO content has to be copied to temporary build directory since ISO 9660 is read only file system.
    • Support multiple operating systems is challenging since there is no standard for ISO image directory structure and live booting.


  • Image Generator reference implementation based on Debian container from airship/images Git repository
    • Dockerfile with all packages required to build LiveCD ISO.
    • Builder script.
  • airshipctl bootstrap extension with new command (i.e. airshipctl bootstrap isogen)
    • Define interface for running container execution which enables following methods:
      • Pull image: download container image if it's not presented locally
      • Run container: start container, wait for builder script is finished, output builder log if CLI debug flag is enabled
      • Run container with output: executes run container method and prints its STDOUT
      • Remove container: removes container if command execution successful.
    • Implement interface for docker Container Runtime Environment


  • New version of hardware nodes definition format in Treasuremap since Metal3-IO will replace MAAS for Airship 2.0