Instance Customization

When provisioning bare-metal machines, it is usually required to customize the resulting instances. Common use cases include injecting SSH keys, adding users, installing software, starting services or configuring networking.

It is recommended to use UserData or NetworkData together with a first-boot configuration software such as cloud-init, Glean or Ignition. Most cloud images already come with one of these programs installed and configured.

Note: all customizations described in this document apply only to the final instance provisioned by Metal3 and do not apply during the inspection, preparing and provisioning phases.

Modified images

Rather than using an official cloud image, a user may build a custom image per cluster or even per host. There are numerous tools to achieve that, the one that the Metal3 community often employs is diskimage-builder.

This approach has two major downsides:

  1. Per-host images take a lot of disk space, especially since Ironic has a local image cache.
  2. diskimage-builder allows only basic customization out of box, code will need to be written for anything complex.

It is recommended to use UserData or NetworkData instead when possible.

NetworkData

Network data describes the desired networking configuration for the deployed operating system, and is typically applied on first-boot via tools such as cloud-init.

The data is specified in the OpenStack network_data.json format supported by cloud-init and Glean. The format is not very well documented, but you can consult the network_data JSON schema shipped with OpenStack.

Usually, one network data secret is created per host and should be linked to it. For example, given a local file host-0-network.json, you can create a secret:

kubectl create secret generic host-0-networkdata --from-file=networkData=host-0-network.json

Then you can attach it to the host during its enrollment or when starting provisioning:

apiVersion: metal3.io/v1alpha1
kind: BareMetalHost
metadata:
  name: host-0
  namespace: my-cluster
spec:
  online: true
  bootMACAddress: 80:c1:6e:7a:e8:10
  bmc:
    address: ipmi://192.168.1.13
    credentialsName: host-0-bmc
  image:
    checksum: http://192.168.0.150/SHA256SUMS
    url: http://192.168.0.150/jammy-server-cloudimg-amd64.img
    checksumType: auto
  networkData:
    name: host-0-networkdata

UserData

User data describes the desired configuration of the instance in a format specific to the first-boot software:

  • cloud-init supports two formats: cloud-config YAML and a shell script (distinguished by the header).
  • Ignition uses its own format.
  • Glean does not support user data at all.

For example, you can create a cloud-config file host-0.yaml:

#cloud-config
users:
- name: metal3
  ssh_authorized_keys:
  - ssh-ed25519 ABCD... metal3@example.com

kubectl create secret generic host-0-userdata --from-file=userData=host-0.json

Then you can attach it to the host during its enrollment or when starting provisioning:

apiVersion: metal3.io/v1alpha1
kind: BareMetalHost
metadata:
  name: host-0
  namespace: my-cluster
spec:
  online: true
  bootMACAddress: 80:c1:6e:7a:e8:10
  bmc:
    address: ipmi://192.168.1.13
    credentialsName: host-0-bmc
  image:
    checksum: http://192.168.0.150/SHA256SUMS
    url: http://192.168.0.150/jammy-server-cloudimg-amd64.img
  userData:
    name: host-0-userdata

Implementation notes

User and network data are passed to the instance via a so called config drive, which is a small additional disk partition created on the root device during provisioning. This partition contains user and network data, as well as meta data with a host name, as files.

Ironic is responsible for creating a partition image (usually, in the ISO 9660 format) and passing it to the IPA ramdisk together with the rest of the deployment information. Once the instance boots, the partition is mounted by the first boot software and the configuration loaded from it.

Both cloud-init and Ignition support various data sources, from which user and network data are fetched. Depending on the image type, different sources may be enabled by default:

  • In case of cloud-init, make sure that the config drive data source is enabled. This is not the same as the OpenStack data source, although both are used with OpenStack.

  • For Ignition to work, you must use an OpenStack Platform image (see supported platforms).