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Middleware Architecture

Abstract

The Keystone middleware architecture supports a common authentication protocol in use between the OpenStack projects. By using keystone as a common authentication and authorization mechanism, the OpenStack project can plug in to existing authentication and authorization systems in use by existing environments.

In this document, we describe the architecture and responsibilities of the authentication middleware which acts as the internal API mechanism for OpenStack projects based on the WSGI standard.

This documentation describes the implementation in keystonemiddleware.auth_token

Specification Overview

'Authentication' is the process of determining that users are who they say they are. Typically, 'authentication protocols' such as HTTP Basic Auth, Digest Access, public key, token, etc, are used to verify a user's identity. In this document, we define an ''authentication component'' as a software module that implements an authentication protocol for an OpenStack service. OpenStack is using a token based mechanism to represent authentication and authorization.

At a high level, an authentication middleware component is a proxy that intercepts HTTP calls from clients and populates HTTP headers in the request context for other WSGI middleware or applications to use. The general flow of the middleware processing is:

  • clear any existing authorization headers to prevent forgery
  • collect the token from the existing HTTP request headers
  • validate the token
    • if valid, populate additional headers representing the identity that has been authenticated and authorized
    • if invalid, or no token present, reject the request (HTTPUnauthorized) or pass along a header indicating the request is unauthorized (configurable in the middleware)
    • if the keystone service is unavailable to validate the token, reject the request with HTTPServiceUnavailable.

Authentication Component

Figure 1. Authentication Component

An Authentication Component

The middleware may also be configured to operate in a 'delegated mode'. In this mode, the decision to reject an unauthenticated client is delegated to the OpenStack service, as illustrated in authComponentDelegated.

Here, requests are forwarded to the OpenStack service with an identity status message that indicates whether the client's identity has been confirmed or is indeterminate. It is the OpenStack service that decides whether or not a reject message should be sent to the client.

Authentication Component (Delegated Mode)

Figure 2. Authentication Component (Delegated Mode)

An Authentication Component (Delegated Mode)

Deployment Strategy

The middleware is intended to be used inline with OpenStack wsgi components, based on the Oslo WSGI middleware class. It is typically deployed as a configuration element in a paste configuration pipeline of other middleware components, with the pipeline terminating in the service application. The middleware conforms to the python WSGI standard [PEP-333]. In initializing the middleware, a configuration item (which acts like a python dictionary) is passed to the middleware with relevant configuration options.

Configuration

The middleware is configured within the config file of the main application as a WSGI component. Example for the auth_token middleware:

[app:myService]
paste.app_factory = myService:app_factory

[pipeline:main]
pipeline = authtoken myService

[filter:authtoken]
paste.filter_factory = keystonemiddleware.auth_token:filter_factory

_static/keystonemiddleware.conf.sample

If the auth_plugin configuration option is set, you may need to refer to the Authentication Plugins <http://docs.openstack.org/developer/ python-keystoneclient/authentication-plugins.html> document for how to configure the auth_token middleware.

For services which have a separate paste-deploy ini file, auth_token middleware can be alternatively configured in [keystone_authtoken] section in the main config file. For example in Nova, all middleware parameters can be removed from api-paste.ini:

[filter:authtoken]
paste.filter_factory = keystonemiddleware.auth_token:filter_factory

and set in nova.conf:

[DEFAULT]
auth_strategy=keystone

[keystone_authtoken]
auth_host = 127.0.0.1
auth_port = 35357
auth_protocol = http
admin_user = admin
admin_password = SuperSekretPassword
admin_tenant_name = service
# Any of the options that could be set in api-paste.ini can be set here.

Note

Middleware parameters in paste config take priority and must be removed to use options in the [keystone_authtoken] section.

The following is an example of a service's auth_token middleware configuration when auth_plugin is set to password.

[keystone_authtoken]
auth_plugin = password
project_domain_name = Default
project_name = service
user_domain_name = Default
username = nova
password = ServicePassword
auth_url = http://127.0.0.1:35357
# Any of the options that could be set in api-paste.ini can be set here.

If the service doesn't use the global oslo.config object (CONF), then the olso config project name can be set it in paste config and keystonemiddleware will load the project configuration itself. Optionally the location of the configuration file can be set if oslo.config is not able to discover it.

[filter:authtoken]
paste.filter_factory = keystonemiddleware.auth_token:filter_factory
oslo_config_project = nova
# oslo_config_file = /not_discoverable_location/nova.conf

Configuration Options

  • auth_plugin: This is the plugin used for authentication, such as password and token. For example, if the auth_plugin configuration option is set to password then set username, password, project_name, project_domain_name, user_domain_name and auth_url accordingly.
  • auth_admin_prefix: Prefix to prepend at the beginning of the path
  • auth_host: (required) the host providing the keystone service API endpoint for validating and requesting tokens
  • auth_port: (optional, default 35357) the port used to validate tokens
  • auth_protocol: (optional, default https)
  • auth_uri: (optional, defaults to `auth_protocol`://`auth_host`:auth_port)
  • auth_version: API version of the admin Identity API endpoint
  • delay_auth_decision: (optional, default 0) (off). If on, the middleware will not reject invalid auth requests, but will delegate that decision to downstream WSGI components.
  • http_connect_timeout: (optional) Request timeout value for communicating with Identity API server.
  • http_request_max_retries: (default 3) How many times are we trying to reconnect when communicating with Identity API Server.
  • http_handler: (optional) Allows to pass in the name of a fake http_handler callback function used instead of httplib.HTTPConnection or httplib.HTTPSConnection. Useful for unit testing where network is not available.
  • admin_token: either this or the following three options are required. If set, this is a single shared secret with the keystone configuration used to validate tokens.
  • admin_user, admin_password, admin_tenant_name: if admin_token is not set, or invalid, then admin_user, admin_password, and admin_tenant_name are defined as a service account which is expected to have been previously configured in Keystone to validate user tokens.
  • cache: (optional) Env key for the swift cache
  • certfile: (required, if Keystone server requires client cert)
  • keyfile: (required, if Keystone server requires client cert) This can be the same as the certfile if the certfile includes the private key.
  • cafile: (optional, defaults to use system CA bundle) the path to a PEM encoded CA file/bundle that will be used to verify HTTPS connections.
  • insecure: (optional, default False) Don't verify HTTPS connections (overrides cafile).
  • signing_dir: (optional) Directory used to cache files related to PKI tokens
  • memcached_servers: (optional) If defined, the memcached server(s) to use for caching
  • token_cache_time: (default 300) In order to prevent excessive requests and validations, the middleware uses an in-memory cache for the tokens the Keystone API returns. This is only valid if memcache_servers s defined. Set to -1 to disable caching completely.
  • memcache_security_strategy: (optional) if defined, indicate whether token data should be authenticated or authenticated and encrypted. Acceptable values are MAC or ENCRYPT. If MAC, token data is authenticated (with HMAC) in the cache. If ENCRYPT, token data is encrypted and authenticated in the cache. If the value is not one of these options or empty, auth_token will raise an exception on initialization.
  • memcache_secret_key: (mandatory if memcache_security_strategy is defined)

    this string is used for key derivation.

  • include_service_catalog: (optional, default True) Indicate whether to set the X-Service-Catalog header. If False, middleware will not ask for service catalog on token validation and will not set the X-Service-Catalog header.
  • enforce_token_bind: (default permissive) Used to control the use and type of token binding. Can be set to: "disabled" to not check token binding. "permissive" (default) to validate binding information if the bind type is of a form known to the server and ignore it if not. "strict" like "permissive" but if the bind type is unknown the token will be rejected. "required" any form of token binding is needed to be allowed. Finally the name of a binding method that must be present in tokens.

Caching for improved response

In order to prevent excessive requests and validations, the middleware uses an in-memory cache for the tokens the keystone API returns. Keep in mind that invalidated tokens may continue to work if they are still in the token cache, so token_cache_time is configurable. For larger deployments, the middleware also supports memcache based caching.

  • memcached_servers: (optional) if defined, the memcached server(s) to use for caching. It will be ignored if Swift MemcacheRing is used instead.
  • token_cache_time: (optional, default 300 seconds) Set to -1 to disable caching completely.

When deploying auth_token middleware with Swift, user may elect to use Swift MemcacheRing instead of the local Keystone memcache. The Swift MemcacheRing object is passed in from the request environment and it defaults to 'swift.cache'. However it could be different, depending on deployment. To use Swift MemcacheRing, you must provide the cache option.

  • cache: (optional) if defined, the environment key where the Swift MemcacheRing object is stored.

Memcached dependencies

In order to use memcached it is necessary to install the python-memcached library. If data stored in memcached will need to be encrypted it is also necessary to install the pycrypto library. These libs are not listed in the requirements.txt file.

Memcached and System Time

When using memcached with auth_token middleware, ensure that the system time of memcached hosts is set to UTC. Memcached uses the host's system time in determining whether a key has expired, whereas Keystone sets key expiry in UTC. The timezone used by Keystone and memcached must match if key expiry is to behave as expected.

Memcache Protection

When using memcached, we are storing user tokens and token validation information into the cache as raw data. Which means that anyone who has access to the memcached servers can read and modify data stored there. To mitigate this risk, auth_token middleware provides an option to authenticate and optionally encrypt the token data stored in the cache.

  • memcache_security_strategy: (optional) if defined, indicate whether token data should be authenticated or authenticated and encrypted. Acceptable values are MAC or ENCRYPT. If MAC, token data is authenticated (with HMAC) in the cache. If ENCRYPT, token data is encrypted and authenticated in the cache. If the value is not one of these options or empty, auth_token will raise an exception on initialization.
  • memcache_secret_key: (optional, mandatory if memcache_security_strategy is defined) this string is used for key derivation. If memcache_security_strategy is defined and memcache_secret_key is absent, auth_token will raise an exception on initialization.

Exchanging User Information

The middleware expects to find a token representing the user with the header X-Auth-Token or X-Storage-Token. X-Storage-Token is supported for swift/cloud files and for legacy Rackspace use. If the token isn't present and the middleware is configured to not delegate auth responsibility, it will respond to the HTTP request with HTTPUnauthorized, returning the header WWW-Authenticate with the value Keystone uri='...' to indicate where to request a token. The auth_uri returned is configured with the middleware.

The authentication middleware extends the HTTP request with the header X-Identity-Status. If a request is successfully authenticated, the value is set to Confirmed. If the middleware is delegating the auth decision to the service, then the status is set to Invalid if the auth request was unsuccessful.

An X-Service-Token header may also be included with a request. If present, and the value of X-Auth-Token or X-Storage-Token has not caused the request to be denied, then the middleware will attempt to validate the value of X-Service-Token. If valid, the authentication middleware extends the HTTP request with the header X-Service-Identity-Status having value Confirmed and also extends the request with additional headers representing the identity authenticated and authorised by the token.

If X-Service-Token is present and its value is invalid and the delay_auth_decision option is True then the value of X-Service-Identity-Status is set to Invalid and no further headers are added. Otherwise if X-Service-Token is present and its value is invalid then the middleware will respond to the HTTP request with HTTPUnauthorized, regardless of the validity of the X-Auth-Token or X-Storage-Token values.

Extended the request with additional User Information

:pykeystonemiddleware.auth_token.AuthProtocol extends the request with additional information if the user has been authenticated. See the "What we add to the request for use by the OpenStack service" section in :pykeystonemiddleware.auth_token for the list of fields set by the auth_token middleware.

References

PEP-333

pep0333 Phillip J Eby. 'Python Web Server Gateway Interface v1.0.'' http://www.python.org/dev/peps/pep-0333/.