2025-09-05 07:10:12 -07:00

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Fedora Ignition Approach for Bootc Installation

Overview

Fedora CoreOS uses Ignition instead of traditional kickstart for system configuration and bootc installation. Ignition is a modern, JSON-based configuration system designed for immutable infrastructure and container-first deployments.

Key Technical Concepts

1. Ignition vs Kickstart

Aspect	Ignition	Kickstart
Format	JSON	Shell-like syntax
Purpose	Immutable infrastructure	Package-based installation
Configuration	Declarative	Imperative
Container Support	Native	Via `ostreecontainer` directive
Modernity	Modern, cloud-native	Traditional, legacy

2. Ignition Configuration Structure

{
  "ignition": {
    "version": "3.4.0"
  },
  "storage": {
    "disks": [
      {
        "device": "/dev/sda",
        "partitions": [
          {
            "label": "BIOS-BOOT",
            "number": 1,
            "size": 1024
          },
          {
            "label": "EFI-SYSTEM",
            "number": 2,
            "size": 268435456
          },
          {
            "label": "ROOT",
            "number": 3,
            "size": 0
          }
        ]
      }
    ],
    "filesystems": [
      {
        "device": "/dev/disk/by-partlabel/ROOT",
        "format": "xfs",
        "path": "/"
      }
    ]
  },
  "systemd": {
    "units": [
      {
        "name": "bootc-install.service",
        "enabled": true,
        "contents": "[Unit]\nDescription=Install bootc container\n[Service]\nType=oneshot\nExecStart=/usr/bin/bootc install to-disk /dev/sda\n[Install]\nWantedBy=multi-user.target"
      }
    ]
  }
}

3. Container Installation via Ignition

Method 1: Systemd Service

{
  "systemd": {
    "units": [
      {
        "name": "bootc-install.service",
        "enabled": true,
        "contents": "[Unit]\nDescription=Install bootc container\n[Service]\nType=oneshot\nExecStart=/usr/bin/bootc install to-disk /dev/sda\n[Install]\nWantedBy=multi-user.target"
      }
    ]
  }
}

Method 2: Direct Container Pull

{
  "storage": {
    "files": [
      {
        "path": "/etc/containers/registries.conf.d/bootc.conf",
        "mode": 420,
        "contents": {
          "source": "data:text/plain;base64,W1JlZ2lzdHJ5XQpsb2NhdGlvbj0icXVheS5pbyIKc2VjdXJlPWZhbHNlCg=="
        }
      }
    ]
  }
}

Fedora CoreOS ISO Analysis

ISO Structure

fedora-coreos-42.20250803.3.0-live-iso.x86_64.iso
├── coreos/
│   ├── features.json          # Installer capabilities
│   ├── igninfo.json          # Points to ignition.img
│   ├── kargs.json            # Kernel arguments
│   └── miniso.dat            # Minimal ISO data
├── images/
│   ├── ignition.img          # Ignition configuration
│   ├── initrd.img            # Initial ramdisk
│   ├── rootfs.img            # Root filesystem
│   └── vmlinuz               # Kernel
├── EFI/fedora/grub.cfg       # EFI boot configuration
└── isolinux/isolinux.cfg     # BIOS boot configuration

Boot Configuration

Kernel arguments: coreos.liveiso=fedora-coreos-42.20250803.3.0 ignition.firstboot ignition.platform.id=metal
Ignition integration: ignition.img loaded as initrd
No kickstart references in boot configuration

Features Capabilities

{
  "installer-config": true,
  "installer-config-directives": {
    "append-karg": true,
    "architecture": true,
    "console": true,
    "copy-network": true,
    "delete-karg": true,
    "dest-device": true,
    "fetch-retries": true,
    "ignition-file": true,
    "ignition-hash": true,
    "ignition-url": true,
    "image-file": true,
    "image-url": true,
    "insecure": true,
    "insecure-ignition": true,
    "network-dir": true,
    "offline": true,
    "platform": true,
    "preserve-on-error": true,
    "save-partindex": true,
    "save-partlabel": true,
    "secure-ipl": true,
    "stream": true,
    "stream-base-url": true
  },
  "live-initrd-network": true
}

Ignition Configuration Examples

1. Basic Bootc Installation

{
  "ignition": {
    "version": "3.4.0"
  },
  "storage": {
    "disks": [
      {
        "device": "/dev/sda",
        "wipeTable": true,
        "partitions": [
          {
            "label": "BIOS-BOOT",
            "number": 1,
            "size": 1024,
            "typeGuid": "21686148-6449-6E6F-744E-656564454649"
          },
          {
            "label": "EFI-SYSTEM",
            "number": 2,
            "size": 268435456,
            "typeGuid": "C12A7328-F81F-11D2-BA4B-00A0C93EC93B"
          },
          {
            "label": "ROOT",
            "number": 3,
            "size": 0,
            "typeGuid": "4F68BCE3-E8CD-4DB1-96E7-FBCAF984B709"
          }
        ]
      }
    ],
    "filesystems": [
      {
        "device": "/dev/disk/by-partlabel/ROOT",
        "format": "xfs",
        "path": "/"
      }
    ]
  },
  "systemd": {
    "units": [
      {
        "name": "bootc-install.service",
        "enabled": true,
        "contents": "[Unit]\nDescription=Install bootc container\n[Service]\nType=oneshot\nExecStart=/usr/bin/bootc install to-disk /dev/sda\n[Install]\nWantedBy=multi-user.target"
      }
    ]
  }
}

2. Registry Authentication

{
  "ignition": {
    "version": "3.4.0"
  },
  "storage": {
    "files": [
      {
        "path": "/etc/ostree/auth.json",
        "mode": 420,
        "contents": {
          "source": "data:text/plain;base64,ewogICJhdXRocyI6IHsKICAgICJxdWF5LmlvIjogewogICAgICAiYXV0aCI6ICI8eW91ciBzZWNyZXQgaGVyZT4iCiAgICB9CiAgfQp9"
        }
      },
      {
        "path": "/etc/containers/registries.conf.d/bootc.conf",
        "mode": 420,
        "contents": {
          "source": "data:text/plain;base64,W1JlZ2lzdHJ5XQpsb2NhdGlvbj0icXVheS5pbyIKc2VjdXJlPWZhbHNlCg=="
        }
      }
    ]
  }
}

3. Network Configuration

{
  "ignition": {
    "version": "3.4.0"
  },
  "networkd": {
    "units": [
      {
        "name": "00-eth0.network",
        "contents": "[Match]\nName=eth0\n[Network]\nDHCP=yes"
      }
    ]
  }
}

Advantages of Ignition Approach

1. Modern Design

JSON-based - easy to generate programmatically
Declarative - describes desired state, not steps
Immutable - configuration is applied once
Cloud-native - designed for modern infrastructure

2. Container Integration

Native container support - no special directives needed
Registry authentication - built-in support
Container runtime - integrated by default
Bootc compatibility - works seamlessly with bootc

3. Flexibility

Modular - can combine different configuration sources
Extensible - easy to add new configuration types
Versioned - configuration format is versioned
Validated - JSON schema validation

Implementation for Debian

Calamares Integration

Generate Ignition JSON from Calamares configuration
Create custom module for container installation
Handle registry authentication via Ignition
Integrate with bootc installation process

debian-installer Integration

Replace preseed with Ignition configuration
Add container runtime to installer environment
Implement Ignition parser in installer
Handle container installation via systemd units

Key Implementation Steps

Study Ignition specification and examples
Design JSON generation from user input
Implement container installation logic
Handle registry authentication and configuration
Test with real bootc containers

Comparison: Ignition vs Kickstart vs Preseed

Feature	Ignition	Kickstart	Preseed
Format	JSON	Shell-like	Debian-specific
Container Support	Native	Via directive	None
Modernity	High	Medium	Low
Flexibility	High	Medium	Low
Validation	Schema-based	Runtime	Limited
Cloud Integration	Excellent	Good	Poor
Immutable Design	Yes	No	No

Tools and Resources

CoreOS Installer

coreos-installer - tool for creating custom ISOs
butane - YAML to Ignition converter
ignition-validate - configuration validator

Configuration Examples

Fedora CoreOS examples - official configuration samples
Butane configs - YAML-based Ignition generation
Container installation - bootc integration patterns

Documentation

Next Steps for Implementation

Study Ignition examples for container installation
Design JSON generation from user configuration
Implement Calamares module for Ignition support
Create debian-installer Ignition integration
Test with real bootc containers and registries

Conclusion

The Ignition approach used by Fedora CoreOS is more modern and suitable for our Debian bootc-image-builder than traditional kickstart or preseed. Its JSON-based configuration, native container support, and immutable design make it ideal for container-first deployments.

Key advantages:

Easier to generate programmatically
More flexible than kickstart/preseed
Container-native by design
Better suited for modern infrastructure
Future-proof approach

This makes Ignition the recommended approach for both Calamares and debian-installer integration in our Debian bootc-image-builder project.

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