Using external authentication with Keystone¶
When Keystone is executed in a web server like Apache HTTPD, it is possible to have the web server also handle authentication. This enables support for additional methods of authentication that are not provided by the identity store backend and the authentication plugins that Keystone supports.
Having the web server handle authentication is not exclusive, and both Keystone and the web server can provide different methods of authentication at the same time. For example, the web server can provide support for X.509 or Kerberos authentication, while Keystone provides support for password authentication (with SQL or an identity store as the backend).
When the web server authenticates a user, it sets environment variables,
usually REMOTE_USER, which can be used in the underlying application.
Keystone can be configured to use these environment variables to determine the
identity of the user.
Configuration¶
In order to activate the external authentication mechanism for Identity API v3,
the external method must be in the list of enabled authentication methods.
By default it is enabled, so if you don’t want to use external authentication,
remove it from the methods option in the auth section.
To configure the plugin that should be used set the external option again
in the auth section. There are two external authentication method plugins
provided by Keystone:
DefaultDomain: This plugin won’t take into account the domain information that the external authentication method may pass down to Keystone and will always use the configured default domain. TheREMOTE_USERvariable is the username. This is the default if no plugin is given.Domain: This plugin expects that theREMOTE_DOMAINvariable contains the domain for the user. If this variable is not present, the configured default domain will be used. TheREMOTE_USERvariable is the username.
Caution
You should disable the external auth method if you are currently using
federation. External auth and federation both use the REMOTE_USER
variable. Since both the mapped and external plugin are being invoked to
validate attributes in the request environment, it can cause conflicts.
For example, imagine there are two distinct users with the same username
foo, one in the Default domain while the other is in the BAR domain.
The external Federation modules (i.e. mod_shib) sets the REMOTE_USER
attribute to foo. The external auth module also tries to set the
REMOTE_USER attribute to foo for the Default domain. The
federated mapping engine maps the incoming identity to foo in the BAR
domain. This results in user_id conflict since both are using different
user_ids to set foo in the Default domain and the BAR domain.
To disable this, simply remove external from the methods option in keystone.conf:
methods = external,password,token,oauth1
Using HTTPD authentication¶
Web servers like Apache HTTP support many methods of authentication. Keystone
can profit from this feature and let the authentication be done in the web
server, that will pass down the authenticated user to Keystone using the
REMOTE_USER environment variable. This user must exist in advance in the
identity backend to get a token from the controller.
To use this method, Keystone should be running on HTTPD.
X.509 example¶
The following snippet for the Apache conf will authenticate the user based on a valid X.509 certificate from a known CA:
<VirtualHost _default_:5000>
SSLEngine on
SSLCertificateFile /etc/ssl/certs/ssl.cert
SSLCertificateKeyFile /etc/ssl/private/ssl.key
SSLCACertificatePath /etc/ssl/allowed_cas
SSLCARevocationPath /etc/ssl/allowed_cas
SSLUserName SSL_CLIENT_S_DN_CN
SSLVerifyClient require
SSLVerifyDepth 10
(...)
</VirtualHost>
Developing a WSGI middleware for authentication¶
In addition to the method described above, it is possible to implement other
custom authentication mechanisms using the REMOTE_USER WSGI environment
variable.
Attention
Please note that even if it is possible to develop a custom authentication module, it is preferable to use the modules in the HTTPD server. Such authentication modules in webservers like Apache have normally undergone years of development and use in production systems and are actively maintained upstream. Developing a custom authentication module that implements the same authentication as an existing Apache module likely introduces a higher security risk.
If you find you must implement a custom authentication mechanism, you will need
to develop a custom WSGI middleware pipeline component. This middleware should
set the environment variable REMOTE_USER to the authenticated username.
Keystone then will assume that the user has been already authenticated upstream
and will not try to authenticate it. However, as with HTTPD authentication, the
user must exist in advance in the identity backend so that a proper token can
be issued.
Your code should set the REMOTE_USER if the user is properly authenticated,
following the semantics below:
from keystone.common import wsgi
from keystone import exception
class MyMiddlewareAuth(wsgi.Middleware):
def __init__(self, *args, **kwargs):
super(MyMiddlewareAuth, self).__init__(*args, **kwargs)
def process_request(self, request):
if request.environ.get('REMOTE_USER', None) is not None:
# Assume that it is authenticated upstream
return self.application
if not self.is_auth_applicable(request):
# Not applicable
return self.application
username = self.do_auth(request)
if username is not None:
# User is authenticated
request.environ['REMOTE_USER'] = username
else:
# User is not authenticated, render exception
raise exception.Unauthorized("Invalid user")
Pipeline configuration¶
Once you have your WSGI middleware component developed you have to add it to
your pipeline. The first step is to add the middleware to your configuration
file. Assuming that your middleware module is
keystone.middleware.MyMiddlewareAuth, you can configure it in your
keystone-paste.ini as:
[filter:my_auth]
paste.filter_factory = keystone.middleware.MyMiddlewareAuth.factory
The second step is to add your middleware to the pipeline. The exact place
where you should place it will depend on your code (i.e. if you need for
example that the request body is converted from JSON before perform the
authentication you should place it after the json_body filter) but it
should be set before the public_service (for the public_api pipeline)
or admin_service (for the admin_api pipeline), since they consume
authentication.
For example, if the original pipeline looks like this:
[pipeline:public_api]
pipeline = url_normalize token_auth json_body debug ec2_extension user_crud_extension public_service
Your modified pipeline might then look like this:
[pipeline:public_api]
pipeline = url_normalize token_auth json_body my_auth debug ec2_extension user_crud_extension public_service
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