Config Discovery

Reference documentation for EtchaOS configuration discovery process.

EtchaOS is configured via Etcha, and the service starts as soon as the network is online (or after a 20 second timeout). When the service starts, Etcha will parse a config file under /etc/etcha.jsonnet.

The service will keep restarting until a valid config is discovered:

Config Discovery

EtchaOS’s config imports JSON or Jsonnet from various sources, merging them onto the previous configuration. This process is meant to support bare-metal, virtual machines, and cloud provider sources–if something is missing, please Contact Us.

The current configuration looks like this:

local dmi = std.split(std.native('getFile')('/sys/firmware/dmi/tables/DMI', '', false), 'etchaos=');
local dmiconfig = std.native('render')(if std.length(dmi) > 1 then std.split(dmi[1], '\n')[0] else '{fallthrough:true}');
local fallthrough(config) = config == {} || std.get(config, 'fallthrough', false,);

// Assume a match means stop fallthrough.  If the file path doesn't resolve, keep falling through.
local render(merge, path) = std.mergePatch(
  merge,
  std.mergePatch(
    {
      fallthrough: false,
    },
    std.native('render')(
      std.native('getFile')(
        path,
        '{fallthrough:true}',
        false,
      )
    )
  )
);

std.mergePatch(
  {
    run: {
      stateDir: '/var/lib/etcha',
    },
    vars: {
      etchaosArch: '%(etchaosArch)s',
      etchaosBuild: '$(date +%%Y%%m%%d)',
      etchaosLike: '%(idLike)s',
      etchaosURL: '%(etchaosURL)s',
      etchaosVariant: '%(etchaosVariant)s',
    },
  },
  std.mergePatch(
    dmiconfig,
    if fallthrough(dmiconfig) then (
      local cidata = render(dmiconfig, '/mnt/cidata/user-data');

      if fallthrough(cidata) then (
        local aws = render(cidata, 'http://169.254.169.254/latest/user-data');

        if fallthrough(aws) then (
          render(aws, 'http://metadata.google.internal/computeMetadata/v1/instance/attributes/startup-script#Metadata-Flavor:Google')
        ) else aws
      ) else cidata
    ) else dmiconfig
  )
)

Lets break down the sources individually:

1. Base Configuration

Etcha is configured with a base configuration defining a Config > run_stateDir and a Config > vars for future usage by Patterns.

2. SMBIOS (QEMU)

Etcha then attempts to render a Jsonnet or JSON value from SMBIOS data located under /sys/firmware/dmi/tables/DMI. It looks for a string starting with etchaos= and attempts to parse the remaining text. If no text is found, or the config value fallthrough is true, Etcha tries to resolve configs using the next step.

The SMBIOS data is typically used by virtual machines, such as QEMU. The value can be specified using the QEMU command line argument -smbios type=11. It’s recommended to put the SMBIOS data as a single line Jsonnet value and store it in a file.

Given a file named /tmp/bios with this content:

etchaos={fallthrough:true,sources:{push:{allowPush:true}}}

And using QEMU with this argument:

-smbios type=11,path=/tmp/smbios

Etcha would render and apply this config to the configuration at boot:

{
  "fallthrough": true,
  "sources": {
    "push": {
      "allowPush": true
    }
  }
}

3. cidata (any)

If SMBIOS fails to resolve or allows fallthrough, Etcha then attempts to render a Jsonnet or JSON value from cidata:

  • EtchaOS’s /etc/fstab is configured to mount any disk with the label cidata to /mnt/cidata.
  • Etcha will attempt to read /mnt/cidata/user-data and parse it as Jsonnet or JSON.
  • If no text is found, or the config value fallthrough is true, Etcha tries to resolve configs using the next step.

You can create a cidata volume using almost any filesystem supported by Linux out of the box. Most commonly, this volume is an ext4, vfat, or a CD-ROM, ISO, or floppy disk.

Given a file named user-data with these values:

{
  cli: {
    logLevel: 'debug',
  },
  fallthrough: true,
}

An ISO named cidata.iso can be created with the ID of cidata using genisoimage:

genisoimage -output cidata.iso -joliet -rock user-data

The ISO can be provided to QEMU with this argument:

-drive file=cidata.sio,media=cdrom

Etcha would render and apply this config to the configuration at boot:

{
  "cli": {
    "logLevel": "debug"
  },
  "fallthrough": true,
}

4. Instance User Data (AWS, OpenStack)

If cidata fails to resolve or allows fallthrough, Etcha then attempts to render a Jsonnet or JSON value from Instance User Data:

  • Etcha will attempt to read http://169.254.169.254/latest/user-data and parse it as Jsonnet or JSON.
  • If no text is found, or the config value fallthrough is true, Etcha tries to resolve configs using the next step.

This is primarily used on Amazon Web Services (AWS) and OpenStack, however the same functionality can be replicated by routing the IP address 169.254.169.254 to a web server and serving a config file with the path /latest/user-data.

5. Instance Metadata (GCP)

If Instance User Data fails to resolve or allows fallthrough, Etcha then attempts to render a Jsonnet or JSON value from Instance Metadata on Google Cloud Platform (GCP):

  • Etcha will attempt to read http://169.254.169.254/latest/user-data and parse it as Jsonnet or JSON.
  • If no text is found, or the config value fallthrough is true, Etcha tries to resolve configs using the next step.

EtchaOS Source

EtchaOS disk images come with an EtchaOS Config > sources configuration under /boot/efi/etcha.libsonnet. The source periodically pulls the latest EtchaOS JWT from https://etcha.dev/releases for your current EtchaOS arch and variant, checks it against the EtchaOS signing key, and updates EtchaOS if it is changed.

Config

This configuration is not enabled by default. You must merge the source into one of your configs above to enable it, i.e.:

std.mergePatch((import '/boot/efi/etchaos.libsonnet'), {
  ...your config...
})

You can modify/override this source in your config to change various aspects:

Variables

Additionally, you can set the following Config > vars in your config to control the behavior of the upgrade process:

  • etchaosArch controls the Variant architecture that will be installed. Changing this is not recommended.
  • etchaosReboot is a timeframe provided to the reboot command after EtchaOS is upgraded. The current reboot command is systemd-run --on-active=%s reboot where %s is replaced with the value of this variable. If this variable is set to never, EtchaOS will never reboot after an upgrade. The syntax is described in systemd.time.
  • etchaosURL sets the base URL to download the EtchaOS files (like initrd and vmlinuz). This can be repointed to an internal server to distribute EtchaOS files from.
  • etchaosVariant sets the Variant type to be installed. This can be changed to switch between variants.