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:
- Generator
- Local
- Network
- Config
- 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
containscloud-init=disabled
. When running in a container, the kernel command line is not honored, but cloud-init will read an environment variable namedKERNEL_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-defined 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 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. [1] 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. [2] 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 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.
[1] | 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. |
[2] | A couple of ways in which this strict reliance on the presence of a datasource has been observed to cause problems:
|