Kernel command line

Providing configuration data via the kernel command line is somewhat of a last resort, since this method only supports 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 datasource NoCloud which provides more configuration options.

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 overridden by specifying the datasource name, such as:

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 the local stage runs, it will check to see if cloud-config-url appears in key/value fashion in the kernel command line, such as:

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 /etc/cloud/cloud.cfg.d/91_kernel_cmdline_url.cfg, and consider it as part of the config from that point forward.

Note

If /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:

cloud-config-url=http://your.url.here/abcdefg

Then, have the following content at that url:

#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 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).