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.. _boot_stages:
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 <https://www.freedesktop.org/software/systemd/man/systemd.generator.html>`_
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, including ``runcmd``.
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 (i.e. shell scripts passed as user-data)
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.
First Boot Determination
************************
cloud-init has to determine whether or not the current boot is the first boot
of a new instance or not, so that it applies the appropriate configuration. On
an instance's first boot, it should run all "per-instance" configuration,
whereas on a subsequent boot it should run only "per-boot" configuration. This
section describes how cloud-init performs this determination, as well as why it
is necessary.
When it runs, cloud-init stores a cache of its internal state for use across
stages and boots.
If this cache is present, then cloud-init has run on this system before.
[#not-present]_ There are two cases where this could occur. Most commonly,
the instance has been rebooted, and this is a second/subsequent boot.
Alternatively, the filesystem has been attached to a *new* instance, and this
is an instance's first boot. The most obvious case where this happens is when
an instance is launched from an image captured from a launched instance.
By default, cloud-init attempts to determine which case it is running in by
checking the instance ID in the cache against the instance ID it determines at
runtime. If they do not match, then this is an instance's first boot;
otherwise, it's a subsequent boot. Internally, cloud-init refers to this
behavior as ``check``.
This behavior is required for images captured from launched instances to
behave correctly, and so is the default which generic cloud images ship with.
However, there are cases where it can cause problems. [#problems]_ For these
cases, cloud-init has support for modifying its behavior to trust the instance
ID that is present in the system unconditionally. This means that cloud-init
will never detect a new instance when the cache is present, and it follows that
the only way to cause cloud-init to detect a new instance (and therefore its
first boot) is to manually remove cloud-init's cache. Internally, this
behavior is referred to as ``trust``.
To configure which of these behaviors to use, cloud-init exposes the
``manual_cache_clean`` configuration option. When ``false`` (the default),
cloud-init will ``check`` and clean the cache if the instance IDs do not match
(this is the default, as discussed above). When ``true``, cloud-init will
``trust`` the existing cache (and therefore not clean it).
Manual Cache Cleaning
=====================
cloud-init ships a command for manually cleaning the cache: ``cloud-init
clean``. See :ref:`cli_clean`'s documentation for further details.
Reverting ``manual_cache_clean`` Setting
========================================
Currently there is no support for switching an instance that is launched with
``manual_cache_clean: true`` from ``trust`` behavior to ``check`` behavior,
other than manually cleaning the cache.
.. warning:: If you want to capture an instance that is currently in ``trust``
mode as an image for launching other instances, you **must** manually clean
the cache. If you do not do so, then instances launched from the captured
image will all detect their first boot as a subsequent boot of the captured
instance, and will not apply any per-instance configuration.
This is a functional issue, but also a potential security one: cloud-init is
responsible for rotating SSH host keys on first boot, and this will not
happen on these instances.
.. [#not-present] It follows that if this cache is not present, cloud-init has
not run on this system before, so this is unambiguously this instance's
first boot.
.. [#problems] A couple of ways in which this strict reliance on the presence
of a datasource has been observed to cause problems:
* If a cloud's metadata service is flaky and cloud-init cannot obtain the
instance ID locally on that platform, cloud-init's instance ID
determination will sometimes fail to determine the current instance ID,
which makes it impossible to determine if this is an instance's first or
subsequent boot (`#1885527`_).
* If cloud-init is used to provision a physical appliance or device and an
attacker can present a datasource to the device with a different instance
ID, then cloud-init's default behavior will detect this as an instance's
first boot and reset the device using the attacker's configuration
(this has been observed with the NoCloud datasource in `#1879530`_).
.. _#1885527: https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1885527
.. _#1879530: https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1879530
.. vi: textwidth=79
|
