Custom node setup

During node enlistment, deployment, commissioning and node installation, MAAS sends Tempita-derived configuration files to the cloud-init process running on the target node. MAAS refers to this process as preseeding.

These preseed files are used to configure a node’s ephemeral and installation environments and can be modified or augmented to c custom node configuration.

Customisation in MAAS happens in two ways:

  1. Curtin, a preseeding system similar to Kickstart or d-i (Debian Installer), applies customisations during operating system (OS) image installation. MAAS performs these changes on deployment, during OS installation, but before the machine reboots into the installed OS. Curtin customisations are perfect for administrators who want their deployments to have identical setups all the time, every time. This blog post contains an excellent high-level overview of custom MAAS installs using Curtin.
  2. Cloud-init, a system for setting up machines immediately after instantiation. Curtin applies customisations after the first boot, when MAAS changes a machine’s status to ‘Deployed.’ Customisations are per-instances, meaning that user-supplied scripts must be re-specified on redeployment. Cloud-init customisations are the best way for MAAS users to customise their deployments, similar to how the various cloud services prepare VMs when launching instances.

Curtin

Templates

The Tempita template files are found within the /etc/maas/preseeds/ directory on the region controller. Each template uses a filename prefix that corresponds to a particular phase of MAAS node deployment:

Phase Filename prefix
1. Enlistment enlist
2. Commissioning commissioning
3. Installation curtin (Curtin)

Additionally, the template for each phase typically consists of two files. The first is a higher-level file that often contains little more than a URL or a link to further credentials, while a second file contains the executable logic.

The enlist template, for example, contains only minimal variables, whereas enlist_userdata includes both user variables and initialisation logic.

Tempita’s inheritance mechanism is the reverse of what you might expect. Inherited files, such as enlist_userdata, become the new template which can then reference variables from the higher-level file, such as enlist.

Template naming

MAAS interprets templates in order of their filename. This order allows for base configuration options and parameters to be overridden based on a combination of operating system, architecture, sub-architecture, release, and node name.

Some earlier versions of MAAS only support Ubuntu. If the node operating system is Ubuntu, then filenames without {os} will also be tried, to maintain backward compatibility.

Consequently, template files are interpreted in the following order:

  1. {prefix}_{os}_{node_arch}_{node_subarch}_{release}_{node_name} or {prefix}_{node_arch}_{node_subarch}_{release}_{node_name}

  2. {prefix}_{os}_{node_arch}_{node_subarch}_{release} or {prefix}_{node_arch}_{node_subarch}_{release}

  3. {prefix}_{os}_{node_arch}_{node_subarch} or {prefix}_{node_arch}_{node_subarch}

  4. {prefix}_{os}_{node_arch} or {prefix}_{node_arch}

  5. {prefix}_{os}

  6. {prefix}

  7. generic

The node needs to be the node name, as shown in the web UI URL.

The prefix can be either enlist, enlist_userdata, commissioning, curtin, curtin_userdata or preseed_master. Alternatively, you can omit the prefix and the following underscore.

For example, to create a generic configuration template for Ubuntu 16.04 Xenial running on an x64 architecture, the file would need to be called ubuntu_amd64_generic_xenial_node.

To create the equivalent template for curtin_userdata, the file would be called curtin_userdata_ubuntu_amd64_generic_xenial_node.

Any file targetting a specific node will replace the values and configuration held within any generic files. If those values are needed, you will need to copy these generic template values into your new file.

Configuration

You can customise the Curtin installation by either editing the existing curtin_userdata template or by adding a custom file as described above.

Curtin provides hooks to execute custom code before and after installation takes place. These hooks are named early and late respectively, and they can both be overridden to execute the Curtin configuration in the ephemeral environment. Additionally, the late hook can be used to execute a configuration for a machine being installed, a state known as in-target.

Curtin commands look like this:

foo: ["command", "--command-arg", "command-arg-value"]

Each component of the given command makes up an item in an array. Note, however, that the following won’t work:

foo: ["sh", "-c", "/bin/echo", "foobar"]

This syntax won’t work because the value of sh's -c argument is itself an entire command. The correct way to express this is:

foo: ["sh", "-c", "/bin/echo foobar"]

The following is an example of an early command that will run before the installation takes place in the ephemeral environment. The command pings an external machine to signal that the installation is about to start:

early_commands:
  signal: [wget, "--no-proxy", "http://example.com/", "--post-data", "system_id=&signal=starting_install", "-O", "/dev/null"]

The following is an example of two late commands that run after installation is complete. Both run in-target, on the machine being installed.

The first command adds a PPA to the machine. The second command creates a file containing the node’s system ID:

late_commands:
  add_repo: ["curtin", "in-target", "--", "add-apt-repository", "-y", "ppa:my/ppa"]
  custom: ["curtin", "in-target", "--", "sh", "-c", "/bin/echo -en 'Installed ' > /tmp/maas_system_id"]

Cloud-init

Using cloud-init to customise a node after deployment is relatively easy. If you’re not familiar with the MAAS command-line interface (CLI), start by reviewing the MAAS CLI page.

After you’re logged in, use the following command to deploy a machine with a custom script you’ve written:

maas $PROFILE machine deploy $SYSTEM_ID user_data=<base-64-encoded-script>
  • $PROFILE: Your MAAS login. E.g. admin
  • $SYSTEM_ID: The machine’s system ID.
  • <base-64-encoded-script>: A base-64 encoded copy of your customisation script. See below for an example.

E.g.:

Suppose you would like to import an SSH key immediately after your machine deployment. You might use this script, called import_key.sh:

#!/bin/bash
(
echo === $date ===
ssh-import-id foobar_user
) | tee /ssh-key-import.log

This script echos the date in addition to the output of the ssh-import-key command. It also adds that output to a file, /ssh-key-import.log.

Base-64 encoding is required because the MAAS command-line interacts with the MAAS API, and base-64 encoding allows MAAS to send the script inside a POST HTTP command.

Use the base64 command to output a base-64 encoded version of your script:

base64 ./import_key.sh

Putting it together:

maas $PROFILE machine deploy $SYSTEM_ID user_data=$(base64 ./import_key.sh)

After MAAS deploys the machine, you’ll find /ssh-key-import.log on the node you deployed.