Nova presents configuration information to instances it starts via a mechanism called metadata. This metadata is made available via either a configdrive, or the metadata service. These mechanisms are widely used via helpers such as cloud-init to specify things like the root password the instance should use. There are three separate groups of people who need to be able to specify metadata for an instance.

User provided data

The user who booted the instance can pass metadata to the instance in several ways. For authentication keypairs, the keypairs functionality of the Nova APIs can be used to upload a key and then specify that key during the Nova boot API request. For less structured data, a small opaque blob of data may be passed via the user-data feature of the Nova API. Examples of such unstructured data would be the puppet role that the instance should use, or the HTTP address of a server to fetch post-boot configuration information from.

Nova provided data

Nova itself needs to pass information to the instance via its internal implementation of the metadata system. Such information includes the network configuration for the instance, as well as the requested hostname for the instance. This happens by default and requires no configuration by the user or deployer.

Deployer provided data

There is however a third type of data. It is possible that the deployer of OpenStack needs to pass data to an instance. It is also possible that this data is not known to the user starting the instance. An example might be a cryptographic token to be used to register the instance with Active Directory post boot – the user starting the instance should not have access to Active Directory to create this token, but the Nova deployment might have permissions to generate the token on the user’s behalf.

Nova supports a mechanism to add “vendordata” to the metadata handed to instances. This is done by loading named modules, which must appear in the nova source code. We provide two such modules:

  • StaticJSON: a module which can include the contents of a static JSON file loaded from disk. This can be used for things which don’t change between instances, such as the location of the corporate puppet server.
  • DynamicJSON: a module which will make a request to an external REST service to determine what metadata to add to an instance. This is how we recommend you generate things like Active Directory tokens which change per instance.

Tell me more about DynamicJSON

To use DynamicJSON, you configure it like this:

  • Add “DynamicJSON” to the vendordata_providers configuration option. This can also include “StaticJSON” if you’d like.
  • Specify the REST services to be contacted to generate metadata in the vendordata_dynamic_targets configuration option. There can be more than one of these, but note that they will be queried once per metadata request from the instance, which can mean a fair bit of traffic depending on your configuration and the configuration of the instance.

The format for an entry in vendordata_dynamic_targets is like this:


Where name is a short string not including the ‘@’ character, and where the URL can include a port number if so required. An example would be:


Metadata fetched from this target will appear in the metadata service at a new file called vendordata2.json, with a path (either in the metadata service URL or in the configdrive) like this:


For each dynamic target, there will be an entry in the JSON file named after that target. For example:

    "testing": {
        "value1": 1,
        "value2": 2,
        "value3": "three"

Do not specify the same name more than once. If you do, we will ignore subsequent uses of a previously used name.

The following data is passed to your REST service as a JSON encoded POST:

Key Description
project-id The ID of the project that owns this instance.
instance-id The UUID of this instance.
image-id The ID of the image used to boot this instance.
user-data As specified by the user at boot time.
hostname The hostname of the instance.
metadata As specified by the user at boot time.

Deployment considerations

Nova provides authentication to external metadata services in order to provide some level of certainty that the request came from nova. This is done by providing a service token with the request – you can then just deploy your metadata service with the keystone authentication WSGI middleware. This is configured using the keystone authentication parameters in the vendordata_dynamic_auth configuration group.