[Cookbook][Security] Added doc for x509 pre authenticated listener

cookbook/security/index.rst

@@ -16,5 +16,6 @@ Security
     securing_services
     custom_provider
     custom_authentication_provider
+    pre_authenticated
     target_path
     csrf_in_login_form

cookbook/security/pre_authenticated.rst

@@ -0,0 +1,73 @@
+.. index::
+   single: Security; Pre authenticated providers
+
+Using pre authenticated security firewalls
+==========================================
+
+A lot of authentication modules are already provided by some webservers,
+including Apache. These modules generally set some environment variables
+that can be used to know which user is accessing your application. Out of the 
+box, Symfony supports most authentication mecanisms.
+These are called *pre authenticated* requests because the user is already
+authenticated when reaching your application.
+
+.. note::
+
+    An authentication provider will only inform the user provider of the username
+    that made the request. You will need to either use an available
+    :class:`Symfony\\Component\\Security\\Core\\User\\UserProviderInterface`
+    or implement your own:
+
+    * :doc:`/cookbook/security/entity_provider`
+    * :doc:`/cookbook/security/custom_provider`
+
+X.509 Client certificate authentication
+---------------------------------------
+
+When using client certificate, your webserver is doing all the authentication
+process itself. For Apache, on your VirtualHost, you may use the 
+``SSLVerifyClient Require`` directive.
+
+On your Symfony2 application security configuration, you can enable the x509
+authentication firewall:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/security.yml
+        security:
+            firewalls:
+                secured_area:
+                    pattern: ^/
+                    x509:
+                        provider: your_user_provider
+
+    .. code-block:: xml
+
+        <!-- app/config/security.xml -->
+        <config>
+            <firewall name="secured_area" pattern="^/">
+                <x509 provider="your_user_provider"/>
+            </firewall>
+        </config>
+
+    .. code-block:: php
+
+        // app/config/security.php
+        $container->loadFromExtension('security', array(
+            'firewalls' => array(
+                'secured_area' => array(
+                    'pattern' => '^/'
+                    'x509'    => array(
+                        'provider' => 'your_user_provider',
+                    ),
+                ),
+            ),
+        ));
+
+By default, the firewall will provide the ``SSL_CLIENT_S_DN_Email`` variable to
+your user provider, and set the ``SSL_CLIENT_S_DN`` as credentials in the 
+:class:`Symfony\\Component\\Security\\Core\\Authentication\\Token\\PreAuthenticatedToken`.
+You can override these by setting respectively the ``user`` and the ``credentials`` keys
+in the x509 firewall configuration.