Introduction

Note: This is a sharp knife feature - be careful with it. It was added to Juju in version 2.3.1.

Cloudinit-userdata

Allows the user to provide additional cloudinit data to be included in the cloudinit data created by Juju.

Specifying a key will overwrite what juju puts in the cloudinit file with the following caveats:

users and bootcmd keys will cause an error
The packages key will be appended to the packages listed by juju
The runcmds key will cause an error. You can specify preruncmd and postruncmd keys to prepend and append the runcmd created by Juju.

Use cases

setting a default locale for deployments that wish to use their own locale settings
adding custom CA certificates for models that are sitting behind an HTTPS proxy
adding a private apt mirror to enable private packages to be installed
blacklist SSH fingerprints from being printed to the console for security-focused deployments

Background

Juju uses cloud-init to customise instances once they have been provisioned by the cloud. The cloudinit-userdata model configuration setting (model config) allows you to tweak what happens to machines when they are created up via the "user data" feature.

From the website:

Cloud images are operating system templates and every instance starts out as an identical clone of every other instance. It is the user data that gives every cloud instance its personality and cloud-init is the tool that applies user data to your instances automatically.

Using

Providing custom user data to cloudinit

Create a file, cloudinit-userdata.yaml, which starts with the cloudinit-userdata key and data you wish to include in the cloudinit file. Note: juju reads the value as a string, though formatted as YAML.

Template cloudinit-userdata.yaml:

cloudinit-userdata: |
    <key>: <value>
    <key>: <value>

Provide the path your file to the model-config command:

juju model-config cloudinit-userdata.yaml

Reading the current setting

To read the current value, provide the cloudinit-userdata key to the model-config command as a command-line parameter. Adding the --format yaml option ensures that it is properly formatted.

juju model-config cloudinit-userdata --format yaml

Sample output:

cloudinit-userdata: |
  packages:
    - 'python-keystoneclient'
    - 'python-glanceclient'

Clearing the current custom user data

Use the --reset option to the model-config command to clear anything that has been previously set.

juju model-config --reset cloudinit-userdata

Known Issues

In cloud-init, the runcmd key accepts both lists of strings and strings. However, Juju only supports strings currently. This will be resolved when "cloudinit-userdata doesn't handle lists in rancid properly" is fixed.
custom cloudinit-userdata must be passed via file, not as options on the command line (like the config command)

manadart/doc.md