Middleware Architecture

Abstract

The keystonemiddleware architecture supports a common authentication protocol in use between the OpenStack projects. By using keystone as a common authentication and authorization mechanism, various OpenStack projects can leverage the existing authentication and authorization systems in use.

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. Bearer tokens are currently the most common authentication protocol used within OpenStack.

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

The following shows the default behavior of an Authentication Component deployed in front of an OpenStack service.

An Authentication Component

The Authentication Component, or middleware, will reject any unauthenticated requests, only allowing authenticated requests through to the OpenStack service.

Authentication Component (Delegated Mode)

The Authentication Component may be configured to operate in a ‘delegated mode’. In this mode, the decision to reject or accept an unauthenticated client is delegated to the OpenStack service.

Here, requests are forwarded to the OpenStack service with an identity status message that indicates whether the identity of the client has been confirmed or is indeterminate. The consuming OpenStack service decides whether or not a rejection message should be sent to the client.

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

[DEFAULT]


[keystone_authtoken]

#
# From keystonemiddleware.auth_token
#

# Complete "public" Identity API endpoint. This endpoint should not be an
# "admin" endpoint, as it should be accessible by all end users.
# Unauthenticated clients are redirected to this endpoint to authenticate.
# Although this endpoint should  ideally be unversioned, client support in the
# wild varies.  If you're using a versioned v2 endpoint here, then this  should
# *not* be the same endpoint the service user utilizes  for validating tokens,
# because normal end users may not be  able to reach that endpoint. (string
# value)
#auth_uri = <None>

# API version of the admin Identity API endpoint. (string value)
#auth_version = <None>

# Do not handle authorization requests within the middleware, but delegate the
# authorization decision to downstream WSGI components. (boolean value)
#delay_auth_decision = false

# Request timeout value for communicating with Identity API server. (integer
# value)
#http_connect_timeout = <None>

# How many times are we trying to reconnect when communicating with Identity
# API Server. (integer value)
#http_request_max_retries = 3

# Request environment key where the Swift cache object is stored. When
# auth_token middleware is deployed with a Swift cache, use this option to have
# the middleware share a caching backend with swift. Otherwise, use the
# ``memcached_servers`` option instead. (string value)
#cache = <None>

# Required if identity server requires client certificate (string value)
#certfile = <None>

# Required if identity server requires client certificate (string value)
#keyfile = <None>

# A PEM encoded Certificate Authority to use when verifying HTTPs connections.
# Defaults to system CAs. (string value)
#cafile = <None>

# Verify HTTPS connections. (boolean value)
#insecure = false

# The region in which the identity server can be found. (string value)
#region_name = <None>

# Directory used to cache files related to PKI tokens. (string value)
#signing_dir = <None>

# Optionally specify a list of memcached server(s) to use for caching. If left
# undefined, tokens will instead be cached in-process. (list value)
# Deprecated group/name - [keystone_authtoken]/memcache_servers
#memcached_servers = <None>

# In order to prevent excessive effort spent validating tokens, the middleware
# caches previously-seen tokens for a configurable duration (in seconds). Set
# to -1 to disable caching completely. (integer value)
#token_cache_time = 300

# Determines the frequency at which the list of revoked tokens is retrieved
# from the Identity service (in seconds). A high number of revocation events
# combined with a low cache duration may significantly reduce performance. Only
# valid for PKI tokens. (integer value)
#revocation_cache_time = 10

# (Optional) If defined, indicate whether token data should be authenticated or
# authenticated and encrypted. 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. (string value)
# Allowed values: None, MAC, ENCRYPT
#memcache_security_strategy = None

# (Optional, mandatory if memcache_security_strategy is defined) This string is
# used for key derivation. (string value)
#memcache_secret_key = <None>

# (Optional) Number of seconds memcached server is considered dead before it is
# tried again. (integer value)
#memcache_pool_dead_retry = 300

# (Optional) Maximum total number of open connections to every memcached
# server. (integer value)
#memcache_pool_maxsize = 10

# (Optional) Socket timeout in seconds for communicating with a memcached
# server. (integer value)
#memcache_pool_socket_timeout = 3

# (Optional) Number of seconds a connection to memcached is held unused in the
# pool before it is closed. (integer value)
#memcache_pool_unused_timeout = 60

# (Optional) Number of seconds that an operation will wait to get a memcached
# client connection from the pool. (integer value)
#memcache_pool_conn_get_timeout = 10

# (Optional) Use the advanced (eventlet safe) memcached client pool. The
# advanced pool will only work under python 2.x. (boolean value)
#memcache_use_advanced_pool = false

# (Optional) 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. (boolean value)
#include_service_catalog = true

# 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.
# (string value)
#enforce_token_bind = permissive

# If true, the revocation list will be checked for cached tokens. This requires
# that PKI tokens are configured on the identity server. (boolean value)
#check_revocations_for_cached = false

# Hash algorithms to use for hashing PKI tokens. This may be a single algorithm
# or multiple. The algorithms are those supported by Python standard
# hashlib.new(). The hashes will be tried in the order given, so put the
# preferred one first for performance. The result of the first hash will be
# stored in the cache. This will typically be set to multiple values only while
# migrating from a less secure algorithm to a more secure one. Once all the old
# tokens are expired this option should be set to a single value for better
# performance. (list value)
#hash_algorithms = md5

# Authentication type to load (string value)
# Deprecated group/name - [keystone_authtoken]/auth_plugin
#auth_type = <None>

# Config Section from which to load plugin specific options (string value)
#auth_section = <None>

If the auth_plugin configuration option is set, you may need to refer to the Authentication Plugins 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 oslo 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

Improving response time

Validating the identity of every client on every request can impact performance for both the OpenStack service and the identity service. As a result, keystonemiddleware is configurable to cache authentication responses from the identity service in-memory. It is worth noting that tokens invalidated after they’ve been stored in the cache may continue to work. Deployments using memcached may use the following keystonemiddleware configuration options instead of an in-memory cache.

  • 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, tokens and authentication responses are stored in the cache as raw data. In the event the cache is compromised, all token and authentication responses will be readable. 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

keystonemiddleware.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 keystonemiddleware.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/.

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