.. _kernel_cmdline:

Kernel command line
*******************

In order to allow an ephemeral, or otherwise pristine image to receive some
configuration, ``cloud-init`` will 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.

.. note::
   Usage of the kernel command line is somewhat of a last resort,
   as it requires knowing in advance the correct command line or modifying
   the boot loader to append data.

For example, when :command:`cloud-init init --local` runs, it will check to
see if ``cloud-config-url`` appears in key/value fashion in the kernel command
line, as in:

.. 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.

If that file exists already, it will not be overwritten, and the
``cloud-config-url`` parameter is completely ignored.

Then, when the datasource runs, it will find that config already available.

So, 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, ``cloud-init`` requires the content to start with
   ``#cloud-config`` for it to be considered.

.. note::

   The ``cloud-config-url=`` is un-authed http GET, and contains credentials.
   It could be set up to be randomly generated and also to check the source
   address in order to be more secure.
