WebStack

Annotated docs/login-redirect.html

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2005-11-15 paulb file changeset files shortlog [project @ 2005-11-15 15:46:01 by paulb] Added has_key method.
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  <title>LoginRedirect and Login Modules</title>
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<h1>LoginRedirect and Login Modules</h1>
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<p>The <code>LoginRedirect</code> and <code>Login</code> modules
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provide a
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"single sign-on" environment for WebStack applications. Unlike the
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authenticator-only approach, each application or part of an application
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utilising this mechanism must be wrapped inside a
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<code>LoginRedirectResource</code> object which determines whether a
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given
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transaction contains information identifying the application's user.</p>
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<h2>How the Modules Work</h2>
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<p>When a request arrives in the application, the following things
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happen:</p>
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<ol>
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  <li>The <code>LoginRedirectResource</code> examines the transaction
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and attempts to find out whether it identifies a user.</li>
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  <li>Should sufficient information be present in the transaction, the
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user is allowed to access the application and is identified in the
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normal way (ie. the <code>get_user</code> method on the transaction
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object).</li>
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  <li>Otherwise, a redirect occurs to a login screen provided by a <code>LoginResource</code>
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object which then presents a login form to be completed by the user.</li>
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  <li>The <code>LoginResource</code> object then receives the
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completed form information and verifies the identity of the user,
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testing the supplied credentials against the credentials database
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specified in the deployment of the resource.</li>
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  <li>Upon successful authentication, the user is redirected back to
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the application, guarded by <code>LoginRedirectResource</code> which
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should let the user gain access.</li>
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</ol>
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<h2>Introducing LoginRedirectResource</h2>
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<p>The easiest way of introducing <code>LoginRedirectResource</code>
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objects
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is to do so in the adapter code, as described in <a
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 href="writing-adapters.html">"Writing Adapters"</a>. The most
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significant
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difference between deploying normal resources and
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<code>LoginRedirectResource</code> objects is the special way in which
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such
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objects are initialised and that they themselves contain the actual
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resources
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of the application which provide the real functionality.</p>
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<p>Here is what the deployment of <code>LoginRedirectResource</code>
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objects
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often looks like:</p>
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<pre>from WebStack.Resources.LoginRedirect import LoginRedirectResource, LoginRedirectAuthenticator<br /><br />deploy(<br />    LoginRedirectResource(<br />        login_url="http://localhost:8081",<br />        app_url="http://localhost:8080",<br />        resource=[some resource object which provides the real application behaviour],<br />        authenticator=LoginRedirectAuthenticator(secret_key="horses"),<br />        anonymous_parameter_name="anonymous",<br />        logout_parameter_name="logout"<br />    )<br />)</pre>
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<p>Certain parts of the resource are configurable, according to which
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other
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services may exist in or alongside the deployed application.</p>
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<div class="WebStack">
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<h3>WebStack API - LoginRedirectResource Initialisation</h3>
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<p>The following parameters must be provided when initialising a
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<code>LoginRedirectResource</code> object:</p>
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<dl>
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  <dt><code>login_url</code></dt>
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  <dd>This specifies the location of the separate login application or
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resource which presents a login screen to unidentified users and logs
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them in.</dd>
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  <dt><code>app_url</code></dt>
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  <dd>This specifies the location of the application itself - it must
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therefore be updated according to where the application is eventually
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deployed.</dd>
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  <dt><code>resource</code></dt>
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  <dd>This provides the resource object which contains the application
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code, or at least the entry point into various parts of the application
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code.</dd>
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  <dt><code>authenticator</code></dt>
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  <dd>This provides the authenticator object which decides whether a
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user is recognised or not. The special <code>LoginRedirectAuthenticator</code>
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is recommended and must itself be configured using a <code>secret_key</code>
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parameter which is used to protect user-related information exchanged
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over the network - the value provided for <code>secret_key</code> must
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be unguessable and kept secret from unauthorised individuals.</dd>
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  <dt><code>anonymous_parameter_name</code></dt>
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  <dd>An optional parameter providing the name of a request parameter
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(see <a href="parameters.html">"Request Parameters and Uploads"</a>)
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which, if specified, permits a user to access an application without
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being formally identified. If omitted, all users will be required to
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identify themselves explicitly.</dd>
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  <dt><code>anonymous_username</code></dt>
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  <dd>An optional parameter providing the name given to anonymous users
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which is returned when a transaction's <code>get_user</code> method is
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called. By default, <code>anonymous</code> is used for such users.</dd>
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  <dt><code>logout_parameter_name</code></dt>
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  <dd>An optional parameter providing the name of a request parameter
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which, if specified, permits a user to log out of an application. If
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omitted, no means of logging out will be available, although deleting
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browser cookies will probably have the desired effect.</dd>
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  <dt><code>logout_url</code></dt>
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  <dd>An optional parameter which indicates the location of the
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resource visited when a user logs out. By default, the location is a
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path to the root resource in the server environment of the application.</dd>
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  <dt><code>use_logout_redirect</code></dt>
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  <dd>An optional parameter which determines whether users logging out
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of an application will be redirected to the <code>logout_url</code> or
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not. By default, users are redirected, but if a false value is given
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for this parameter, a simple page is presented to the user informing
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them of the situation - it is recommended that a subclass of <code>LoginRedirectResource</code>
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be employed should more informative pages be required.</dd>
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</dl>
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</div>
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<h3>Redirection from/to the Application</h3>
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<p>Some server/framework environments do not permit automatic
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redirection
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back to the application, notably Apache/mod_python. In such cases, a
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success
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screen is presented to the user with a link to the application they
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were
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attempting to access.</p>
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<h3>The Role of Authenticators</h3>
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<p>In this mechanism, authenticators are employed, but only to verify
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the
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credentials of users when <code>LoginResource</code> or
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<code>LoginRedirectResource</code> objects are accessed. Although it
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should
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be possible to reuse <a href="authenticators.html">application-wide
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authenticator</a> classes in conjunction with <code>LoginResource</code>,
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such classes will not provide the additional functionality required to
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support the "single sign-on" aspects of this mechanism - mixing in such
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classes with <code>LoginAuthenticator</code> may provide a solution to
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this
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issue, however.</p>
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<h2>Deploying a Login Application</h2>
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<p>In order for this authentication mechanism to function in its
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entirety, a
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login application (or resource) must be available to authenticate
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unidentified users. It may already be the case that such an application
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has
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been deployed and is available at a certain URL - if so, it should be
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sufficient for a <code>LoginRedirectResource</code> object to be
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configured
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as described above, making sure that the <code>login_url</code>
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actually
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refers to the location of the existing login application, and that the
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<code>authenticator</code> object's <code>secret_key</code> is
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consistent
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with the way the existing login application has been set up.</p>
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<p>However, if no existing login application (or resource) exists, one
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may be
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deployed using adapter code similar to the following:</p>
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<pre>from WebStack.Adapters.BaseHTTPRequestHandler import deploy<br />from WebStack.Resources.Login import LoginResource, LoginAuthenticator<br /><br />deploy(<br />    LoginResource(                   # This is the login application's main resource.<br />        LoginAuthenticator(          # This provides authentication support.<br />            secret_key="horses",<br />            credentials=(<br />                ("badger", "abc"),<br />                ("vole", "xyz"),<br />            )<br />        )<br />    ),<br />    address=("", 8081)<br />)</pre>
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<p>The above code merely starts a login application in the
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BaseHTTPRequestHandler environment at a specific address (which
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corresponds
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to that specified in the <code>login_url</code> of the
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<code>LoginRedirectResource</code> used above) and provides a
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<code>LoginAuthenticator</code> object configured to use a
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<code>secret_key</code> (which corresponds to the <code>secret_key</code>
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used in the <code>authenticator</code> of the
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<code>LoginRedirectResource</code> above) and some user credentials.
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The user
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credentials define which users are to be recognised for applications
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which
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employ this login application, along with the password details of each
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user.</p>
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<div class="WebStack">
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<h3>WebStack API - LoginAuthenticator Credentials</h3>
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<p>When initialising a <code>LoginAuthenticator</code> object with
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credentials, the supplied credentials object must support tests on its
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contents of the following form:</p>
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<pre>(username, password) in credentials</pre>
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<p>In other words, the credentials object must either be a sequence of
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username and password tuples, or it must implement the
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<code>__contains__</code> method and accept such tuples as arguments to
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that
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method.</p>
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</div>
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<h2>Anonymous Access</h2>
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<p>With the <code>LoginRedirect</code> and <code>Login</code>
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modules, it is
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possible to declare a particular request parameter (see
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<code>anonymous_parameter_name</code> above) which must be present in
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the URL
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used to access a particular application for the client to be given
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anonymous
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access. Consequently, anonymous users are then identified specially
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with a
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special username that can also be configured (see
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<code>anonymous_username</code> above).</p>
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<h2>Logout Functions</h2>
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<p>With the <code>LoginRedirect</code> and <code>Login</code>
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modules, it is
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possible to declare a particular request parameter (see
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<code>logout_parameter_name</code> above) which must be present in the
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URL
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used to access a particular application for the client to be logged
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out. A
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special logout confirmation URL may also be configured (see
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<code>logout_url</code> and <code>use_logout_redirect</code> above).</p>
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</body>
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</html>