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.. _kernel_cmdline:
Kernel command line
*******************
Providing configuration data via the kernel command line is somewhat of a last
resort, since this method only supports
:ref:`cloud config<user_data_formats-cloud_config>` starting with
`#cloud-config`, and many datasources do not support injecting kernel
command line arguments without modifying the bootloader.
Despite the limitations of using the kernel command line, cloud-init supports
some use-cases.
Note that this page describes kernel command line behavior that applies
to all clouds. To provide a local configuration with an image using kernel
command line, see :ref:`datasource NoCloud<datasource_nocloud>` which provides
more configuration options.
.. _kernel_datasource_override:
Datasource discovery override
=============================
During boot, cloud-init must identify which datasource it is running on
(OpenStack, AWS, Azure, GCP, etc). This discovery step can be optionally
overriden by specifying the datasource name, such as:
.. code-block:: text
root=/dev/sda ro ds=openstack
Kernel cloud-config-url configuration
=====================================
In order to allow an ephemeral, or otherwise pristine image to receive some
configuration, ``cloud-init`` can read a URL directed by the kernel command
line and proceed as if its data had previously existed.
This allows for configuring a metadata service, or some other data.
When :ref:`the local stage<boot-Local>` runs, it will check to see if
``cloud-config-url`` appears in key/value fashion in the kernel command line,
such as:
.. code-block:: text
root=/dev/sda ro cloud-config-url=http://foo.bar.zee/abcde
``Cloud-init`` will then read the contents of the given URL. If the content
starts with ``#cloud-config``, it will store that data to the local filesystem
in a static filename :file:`/etc/cloud/cloud.cfg.d/91_kernel_cmdline_url.cfg`,
and consider it as part of the config from that point forward.
.. note::
If :file:`/etc/cloud/cloud.cfg.d/91_kernel_cmdline_url.cfg` already exists,
cloud-init will not overwrite the file, and the ``cloud-config-url``
parameter is completely ignored.
This is useful, for example, to be able to configure the MAAS datasource by
controlling the kernel command line from outside the image, you can append:
.. code-block:: text
cloud-config-url=http://your.url.here/abcdefg
Then, have the following content at that url:
.. code-block:: yaml
#cloud-config
datasource:
MAAS:
metadata_url: http://mass-host.localdomain/source
consumer_key: Xh234sdkljf
token_key: kjfhgb3n
token_secret: 24uysdfx1w4
.. warning::
``url`` kernel command line key is deprecated.
Please use ``cloud-config-url`` parameter instead.
.. note::
Since ``cloud-config-url=`` is so generic, in order to avoid false
positives, only :ref:`cloud config<user_data_formats-cloud_config>` user
data starting with ``#cloud-config`` is supported.
.. note::
The ``cloud-config-url=`` is unencrypted http GET, and may contain
credentials. Care must be taken to ensure this data is only
transferred via trusted channels (i.e., within a closed system).
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