Much of the design is precipitated from the expectation that the auth backends for most deployments will actually be shims in front of existing user systems.
Keystone is organized as a group of internal services exposed on one or many endpoints. Many of these services are used in a combined fashion by the frontend, for example an authenticate call will validate user/project credentials with the Identity service and, upon success, create and return a token with the Token service.
The Identity service provides auth credential validation and data about Users, Groups.
In the basic case all this data is managed by the service, allowing the service to manage all the CRUD associated with the data.
In other cases from an authoritative backend service. An example of this would be when backending on LDAP. See LDAP Backend below for more details.
The Resource service provides data about Projects and Domains.
Like the Identity service, this data may either be managed directly by the service or be pulled from another authoritative backend service, such as LDAP.
The Assignment service provides data about Roles and Role assignments to the entities managed by the Identity and Resource services. Again, like these two services, this data may either be managed directly by the Assignment service or be pulled from another authoritative backend service, such as LDAP.
The Token service validates and manages Tokens used for authenticating requests once a user’s credentials have already been verified.
The Catalog service provides an endpoint registry used for endpoint discovery.
The Policy service provides a rule-based authorization engine and the associated rule management interface.
Keystone is an HTTP front-end to several services. Like other OpenStack applications, this is done using python WSGI interfaces and applications are configured together using Paste. The application’s HTTP endpoints are made up of pipelines of WSGI middleware, such as:
[pipeline:api_v3]
pipeline = sizelimit url_normalize build_auth_context token_auth admin_token_auth
json_body ec2_extension_v3 s3_extension service_v3
These in turn use a subclass of keystone.common.wsgi.ComposingRouter to link URLs to Controllers (a subclass of keystone.common.wsgi.Application). Within each Controller, one or more Managers are loaded (for example, see keystone.catalog.core.Manager), which are thin wrapper classes which load the appropriate service driver based on the Keystone configuration.
- keystone.assignment.controllers.GrantAssignmentV3
- keystone.assignment.controllers.ProjectAssignmentV3
- keystone.assignment.controllers.TenantAssignment
- keystone.assignment.controllers.Role
- keystone.assignment.controllers.RoleAssignmentV2
- keystone.assignment.controllers.RoleAssignmentV3
- keystone.assignment.controllers.RoleV3
Each of the services can be configured to use a backend to allow Keystone to fit a variety of environments and needs. The backend for each service is defined in the keystone.conf file with the key driver under a group associated with each service.
A general class exists under each backend to provide an abstract base class for any implementations, identifying the expected service implementations. The classes are named after the keystone release in which they were introduced. For eg. DriverV8 for keystone release version 8. The corresponding drivers for the services are:
If you implement a backend driver for one of the Keystone services, you’re expected to subclass from these classes.
A SQL based backend using SQLAlchemy to store data persistently. The keystone-manage command introspects the backends to identify SQL based backends when running “db_sync” to establish or upgrade schema. If the backend driver has a method db_sync(), it will be invoked to sync and/or migrate schema.
Largely designed for a common use case around service catalogs in the Keystone project, a Catalog backend that simply expands pre-configured templates to provide catalog data.
Example paste.deploy config (uses $ instead of % to avoid ConfigParser’s interpolation):
[DEFAULT]
catalog.RegionOne.identity.publicURL = http://localhost:$(public_port)s/v2.0
catalog.RegionOne.identity.adminURL = http://localhost:$(public_port)s/v2.0
catalog.RegionOne.identity.internalURL = http://localhost:$(public_port)s/v2.0
catalog.RegionOne.identity.name = 'Identity Service'
The LDAP backend stores Users and Projects in separate Subtrees. Roles are recorded as entries under the Projects.
Keystone was designed from the ground up to be amenable to multiple styles of backends and as such many of the methods and data types will happily accept more data than they know what to do with and pass them on to a backend.
There are a few main data types:
- User: has account credentials, is associated with one or more projects or domains
- Group: a collection of users, is associated with one or more projects or domains
- Project: unit of ownership in OpenStack, contains one or more users
- Domain: unit of ownership in OpenStack, contains users, groups and projects
- Role: a first-class piece of metadata associated with many user-project pairs.
- Token: identifying credential associated with a user or user and project
- Extras: bucket of key-value metadata associated with a user-project pair.
- Rule: describes a set of requirements for performing an action.
While the general data model allows a many-to-many relationship between Users and Groups to Projects and Domains; the actual backend implementations take varying levels of advantage of that functionality.
While it is expected that any “real” deployment at a large company will manage their users, groups, projects and domains in their existing user systems, a variety of CRUD operations are provided for the sake of development and testing.
CRUD is treated as an extension or additional feature to the core feature set in that it is not required that a backend support it. It is expected that backends for services that don’t support the CRUD operations will raise a keystone.exception.NotImplemented.
Various components in the system require that different actions are allowed based on whether the user is authorized to perform that action.
For the purposes of Keystone there are only a couple levels of authorization being checked for:
- Require that the performing user is considered an admin.
- Require that the performing user matches the user being referenced.
Other systems wishing to use the policy engine will require additional styles of checks and will possibly write completely custom backends. By default, Keystone leverages Policy enforcement that is maintained in Oslo-Incubator, found in keystone/openstack/common/policy.py.
Given a list of matches to check for, simply verify that the credentials contain the matches. For example:
credentials = {'user_id': 'foo', 'is_admin': 1, 'roles': ['nova:netadmin']}
# An admin only call:
policy_api.enforce(('is_admin:1',), credentials)
# An admin or owner call:
policy_api.enforce(('is_admin:1', 'user_id:foo'), credentials)
# A netadmin call:
policy_api.enforce(('roles:nova:netadmin',), credentials)
Credentials are generally built from the user metadata in the ‘extras’ part of the Identity API. So, adding a ‘role’ to the user just means adding the role to the user metadata.
(Not yet implemented.)
Another approach to authorization can be action-based, with a mapping of roles to which capabilities are allowed for that role. For example:
credentials = {'user_id': 'foo', 'is_admin': 1, 'roles': ['nova:netadmin']}
# add a policy
policy_api.add_policy('action:nova:add_network', ('roles:nova:netadmin',))
policy_api.enforce(('action:nova:add_network',), credentials)
In the backend this would look up the policy for ‘action:nova:add_network’ and then do what is effectively a ‘Simple Match’ style match against the credentials.