Using external authentication with Keystone

updated: 2023-04-14 08:07

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. The REMOTE_USER variable is the username. This is the default if no plugin is given.

  • Domain: This plugin expects that the REMOTE_DOMAIN variable contains the domain for the user. If this variable is not present, the configured default domain will be used. The REMOTE_USER variable 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>
updated: 2023-04-14 08:07
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