Add docs for 401 vs 403 responses

docs/api-guide/authentication.md

@@ -8,7 +8,7 @@
 
 Authentication is the mechanism of associating an incoming request with a set of identifying credentials, such as the user the request came from, or the token that it was signed with.  The [permission] and [throttling] policies can then use those credentials to determine if the request should be permitted.
 
-REST framework provides a number of authentication policies out of the box, and also allows you to implement custom policies.
+REST framework provides a number of authentication schemes out of the box, and also allows you to implement custom schemes.
 
 Authentication will run the first time either the `request.user` or `request.auth` properties are accessed, and determines how those properties are initialized.
 
@@ -18,21 +18,21 @@ The `request.auth` property is used for any additional authentication informatio
 
 ---
 
-**Note:** Don't forget that authentication by itself wont allow or disallow an incoming request, it simply identifies the credentials that the request was made with.  For information on how to setup the permission polices for your API please see the [permissions documentation][permission].
+**Note:** Don't forget that **authentication by itself won't allow or disallow an incoming request**, it simply identifies the credentials that the request was made with.  For information on how to setup the permission polices for your API please see the [permissions documentation][permission].
 
 ---
 
 ## How authentication is determined
 
-The authentication policy is always defined as a list of classes.  REST framework will attempt to authenticate with each class in the list, and will set `request.user` and `request.auth` using the return value of the first class that successfully authenticates.
+The authentication schemes are always defined as a list of classes.  REST framework will attempt to authenticate with each class in the list, and will set `request.user` and `request.auth` using the return value of the first class that successfully authenticates.
 
 If no class authenticates, `request.user` will be set to an instance of `django.contrib.auth.models.AnonymousUser`, and `request.auth` will be set to `None`.
 
 The value of `request.user` and `request.auth` for unauthenticated requests can be modified using the `UNAUTHENTICATED_USER` and `UNAUTHENTICATED_TOKEN` settings.
 
-## Setting the authentication policy
+## Setting the authentication scheme
 
-The default authentication policy may be set globally, using the `DEFAULT_AUTHENTICATION` setting.  For example.
+The default authentication schemes may be set globally, using the `DEFAULT_AUTHENTICATION` setting.  For example.
 
     REST_FRAMEWORK = {
         'DEFAULT_AUTHENTICATION': (
@@ -41,7 +41,7 @@ The default authentication policy may be set globally, using the `DEFAULT_AUTHEN
         )
     }
 
-You can also set the authentication policy on a per-view basis, using the `APIView` class based views.
+You can also set the authentication scheme on a per-view basis, using the `APIView` class based views.
 
     class ExampleView(APIView):
         authentication_classes = (SessionAuthentication, UserBasicAuthentication)
@@ -66,24 +66,43 @@ Or, if you're using the `@api_view` decorator with function based views.
         }
         return Response(content)
 
+## Unauthorized and Forbidden responses
+
+When an unauthenticated request is denied permission there are two different error codes that may be appropriate.
+
+* [HTTP 401 Unauthorized][http401]
+* [HTTP 403 Permission Denied][http403]
+
+The kind of response that will be used depends on the type of authentication scheme in use, and the ordering of the authentication classes.
+
+Although multiple authentication schemes may be in use, only one scheme may be used to determine the type of response.  **The first authentication class set on the view is given priority when determining the type of response**.
+
+Note that when a *successfully authenticated* request is denied permission, a `403 Permission Denied` response will always be used, regardless of the authentication scheme.
+
+---
+
 # API Reference
 
 ## BasicAuthentication
 
-This policy uses [HTTP Basic Authentication][basicauth], signed against a user's username and password.  Basic authentication is generally only appropriate for testing.
+This authentication scheme uses [HTTP Basic Authentication][basicauth], signed against a user's username and password.  Basic authentication is generally only appropriate for testing.
 
 If successfully authenticated, `BasicAuthentication` provides the following credentials.
 
 * `request.user` will be a `django.contrib.auth.models.User` instance.
 * `request.auth` will be `None`.
 
+Unauthenticated responses that are denied permission will result in an `HTTP 401 Unauthenticated` response with an appropriate WWW-Authenticate header.  For example:
+
+    WWW-Authenticate: Basic realm="api"
+
 **Note:** If you use `BasicAuthentication` in production you must ensure that your API is only available over `https` only.  You should also ensure that your API clients will always re-request the username and password at login, and will never store those details to persistent storage.
 
 ## TokenAuthentication
 
-This policy uses a simple token-based HTTP Authentication scheme.  Token authentication is appropriate for client-server setups, such as native desktop and mobile clients.
+This authentication scheme uses a simple token-based HTTP Authentication scheme.  Token authentication is appropriate for client-server setups, such as native desktop and mobile clients.
 
-To use the `TokenAuthentication` policy, include `rest_framework.authtoken` in your `INSTALLED_APPS` setting.
+To use the `TokenAuthentication` scheme, include `rest_framework.authtoken` in your `INSTALLED_APPS` setting.
 
 You'll also need to create tokens for your users.
 
@@ -101,31 +120,56 @@ If successfully authenticated, `TokenAuthentication` provides the following cred
 * `request.user` will be a `django.contrib.auth.models.User` instance.
 * `request.auth` will be a `rest_framework.tokenauth.models.BasicToken` instance.
 
+Unauthenticated responses that are denied permission will result in an `HTTP 401 Unauthenticated` response with an appropriate WWW-Authenticate header.  For example:
+
+    WWW-Authenticate: Token
+
 **Note:** If you use `TokenAuthentication` in production you must ensure that your API is only available over `https` only.
 
-## OAuthAuthentication
+## OAuth2Authentication
 
-This policy uses the [OAuth 2.0][oauth] protocol to authenticate requests.  OAuth is appropriate for server-server setups, such as when you want to allow a third-party service to access your API on a user's behalf.
+This authentication scheme uses the [OAuth 2.0][oauth] protocol to authenticate requests.  OAuth is appropriate for server-server setups, such as when you want to allow a third-party service to access your API on a user's behalf.
 
-If successfully authenticated, `OAuthAuthentication` provides the following credentials.
+If successfully authenticated, `OAuth2Authentication` provides the following credentials.
 
 * `request.user` will be a `django.contrib.auth.models.User` instance.
 * `request.auth` will be a `rest_framework.models.OAuthToken` instance.
 
+**TODO**: Note type of response (401 vs 403)
+
+**TODO**: Implement OAuth2Authentication, using django-oauth2-provider.
+
 ## SessionAuthentication
 
-This policy uses Django's default session backend for authentication.  Session authentication is appropriate for AJAX clients that are running in the same session context as your website.
+This authentication scheme uses Django's default session backend for authentication.  Session authentication is appropriate for AJAX clients that are running in the same session context as your website.
 
 If successfully authenticated, `SessionAuthentication` provides the following credentials.
 
 * `request.user` will be a `django.contrib.auth.models.User` instance.
 * `request.auth` will be `None`.
 
+Unauthenticated responses that are denied permission will result in an `HTTP 403 Forbidden` response.
+
+---
+
 # Custom authentication
 
-To implement a custom authentication policy, subclass `BaseAuthentication` and override the `.authenticate(self, request)` method.  The method should return a two-tuple of `(user, auth)` if authentication succeeds, or `None` otherwise.
+To implement a custom authentication scheme, subclass `BaseAuthentication` and override the `.authenticate(self, request)` method.  The method should return a two-tuple of `(user, auth)` if authentication succeeds, or `None` otherwise.
+
+In some circumstances instead of returning `None`, you may want to raise an `Unauthenticated` exception from the `.authenticate()` method.
+
+Typically the approach you should take is:
+
+* If authentication is not attempted, return `None`.  Any other authentication schemes also in use will still be checked.
+* If authentication is attempted but fails, raise an `Unauthenticated` exception.  An error response will be returned immediately, without checking any other authentication schemes.
+
+You *may* also override the `.authentication_header(self, request)` method.  If implemented, it should return a string that will be used as the value of the `WWW-Authenticate` header in a `HTTP 401 Unauthenticated` response.
+
+If the `.authentication_header()` method is not overridden, the authentication scheme will return `HTTP 403 Forbidden` responses when an unauthenticated request is denied access.
 
 [cite]: http://jacobian.org/writing/rest-worst-practices/
+[http401]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.2
+[http403]: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.4
 [basicauth]: http://tools.ietf.org/html/rfc2617
 [oauth]: http://oauth.net/2/
 [permission]: permissions.md

rest_framework/authentication.py

@@ -21,6 +21,14 @@ class BaseAuthentication(object):
         """
         raise NotImplementedError(".authenticate() must be overridden.")
 
+    def authenticate_header(self, request):
+        """
+        Return a string to be used as the value of the `WWW-Authenticate`
+        header in a `401 Unauthenticated` response, or `None` if the
+        authentication scheme should return `403 Permission Denied` responses.
+        """
+        pass
+
 
 class BasicAuthentication(BaseAuthentication):
     """