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Boot Stages

In order to be able to provide the functionality that it does, cloud-init must be integrated into the boot in fairly controlled way. There are five stages to boot:

  1. Generator
  2. Local
  3. Network
  4. Config
  5. Final

Generator

When booting under systemd, a generator will run that determines if cloud-init.target should be included in the boot goals. By default, this generator will enable cloud-init. It will not enable cloud-init if either:

  • The file /etc/cloud/cloud-init.disabled exists
  • The kernel command line as found in /proc/cmdline contains cloud-init=disabled. When running in a container, the kernel command line is not honored, but cloud-init will read an environment variable named KERNEL_CMDLINE in its place.

Again, these mechanisms for disabling cloud-init at runtime currently only exist in systemd.

Local

systemd service cloud-init-local.service
runs as soon as possible with / mounted read-write
blocks as much of boot as possible, must block network
modules none

The purpose of the local stage is to:

  • locate "local" data sources.
  • apply networking configuration to the system (including "Fallback")

In most cases, this stage does not do much more than that. It finds the datasource and determines the network configuration to be used. That network configuration can come from:

  • datasource: cloud provided network configuration via metadata
  • fallback: cloud-init's fallback networking consists of rendering the equivalent to "dhcp on eth0", which was historically the most popular mechanism for network configuration of a guest
  • none: network configuration can be disabled by writing the file /etc/cloud/cloud.cfg with the content: network: {config: disabled}

If this is an instance's first boot, then the selected network configuration is rendered. This includes clearing of all previous (stale) configuration including persistent device naming with old mac addresses.

This stage must block network bring-up or any stale configuration might already have been applied. That could have negative effects such as DHCP hooks or broadcast of an old hostname. It would also put the system in an odd state to recover from as it may then have to restart network devices.

Cloud-init then exits and expects for the continued boot of the operating system to bring network configuration up as configured.

Note: In the past, local data sources have been only those that were available without network (such as 'ConfigDrive'). However, as seen in the recent additions to the DigitalOcean datasource, even data sources that require a network can operate at this stage.

Network

systemd service cloud-init.service
runs after local stage and configured networking is up
blocks as much of remaining boot as possible
modules cloud_init_modules in /etc/cloud/cloud.cfg

This stage requires all configured networking to be online, as it will fully process any user-data that is found. Here, processing means:

  • retrieve any #include or #include-once (recursively) including http
  • decompress any compressed content
  • run any part-handler found.

This stage runs the disk_setup and mounts modules which may partition and format disks and configure mount points (such as in /etc/fstab). Those modules cannot run earlier as they may receive configuration input from sources only available via network. For example, a user may have provided user-data in a network resource that describes how local mounts should be done.

On some clouds such as Azure, this stage will create filesystems to be mounted, including ones that have stale (previous instance) references in /etc/fstab. As such, entries /etc/fstab other than those necessary for cloud-init to run should not be done until after this stage.

A part-handler will run at this stage, as will boot-hooks including cloud-config bootcmd. The user of this functionality has to be aware that the system is in the process of booting when their code runs.

Config

systemd service cloud-config.service
runs after network
blocks nothing
modules cloud_config_modules in /etc/cloud/cloud.cfg

This stage runs config modules only. Modules that do not really have an effect on other stages of boot are run here.

Final

systemd service cloud-final.service
runs as final part of boot (traditional "rc.local")
blocks nothing
modules cloud_final_modules in /etc/cloud/cloud.cfg

This stage runs as late in boot as possible. Any scripts that a user is accustomed to running after logging into a system should run correctly here. Things that run here include

  • package installations
  • configuration management plugins (puppet, chef, salt-minion)
  • user-scripts (including runcmd).

For scripts external to cloud-init looking to wait until cloud-init is finished, the cloud-init status subcommand can help block external scripts until cloud-init is done without having to write your own systemd units dependency chains. See :ref:`cli_status` for more info.

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