Django CAS

django_cas is a CAS 1.0 and CAS 2.0 authentication backend for Django. It allows you to use Django's built-in authentication mechanisms and User model while adding support for CAS.

It also includes a middleware that intercepts calls to the original login and logout pages and forwards them to the CASified versions, and adds CAS support to the admin interface.

Installation

Run python setup.py install, or place the django_cas directory in your PYTHONPATH directly. (Note: If you're using Python 2.4 or older, you'll need to install ElementTree to use CAS 2.0 functionality.)

Now add it to the middleware and authentication backends in your settings. Make sure you also have the authentication middleware installed. Here's what mine looks like:

MIDDLEWARE_CLASSES = (
    'django.middleware.common.CommonMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django_cas.middleware.CASMiddleware',
    'django.middleware.doc.XViewMiddleware',
)

AUTHENTICATION_BACKENDS = (
    'django.contrib.auth.backends.ModelBackend',
    'django_cas.backends.CASBackend',
)

Set the following required setting in settings.py:

• CAS_SERVER_URL: This is the only setting you must explicitly define. Set it to the base URL of your CAS source (e.g. http://sso.some.edu/cas/).

Optional settings include:

• CAS_ADMIN_PREFIX: The URL prefix of the Django administration site. If undefined, the CAS middleware will check the view being rendered to see if it lives in django.contrib.admin.views.
• CAS_IGNORE_REFERER: If True, logging out of the application will always send the user to the URL specified by CAS_REDIRECT_URL.
• CAS_LOGOUT_COMPLETELY: If False, logging out of the application won't log the user out of CAS as well.
• CAS_REDIRECT_URL: Where to send a user after logging in or out if there is no referrer and no next page set. Default is /.
• CAS_VERSION: The CAS protocol version to use. '1' and '2' are supported, with '2' being the default.

Make sure your project knows how to log users in and out by adding these to your URL mappings:

(r'^accounts/login/$', 'django_cas.views.login'),
(r'^accounts/logout/$', 'django_cas.views.logout'),

Users should now be able to log into your site (and staff into the administration interface) using CAS.

Populating User Data

To add user data, subclass CASBackend and specify that as your application's backend.

For example:

from django_cas.backend import CASBackend

class PopulatedCASBackend(CASBackend):
    """CAS authentication backend with user data populated from AD"""

    def authenticate(self, ticket, service):
        """Authenticates CAS ticket and retrieves user data"""

        user = super(PopulatedCASBackend, self).authenticate(
            ticket, service)

        # Connect to AD, modify user object, etc.

        return user

Preventing Infinite Redirects

Django's current implementation of its permission_required and user_passes_test decorators (in django.contrib.auth.decorators) has a known issue that can cause users to experience infinite redirects. The decorators return the user to the login page, even if they're already logged in, which causes a loop with SSO services like CAS.

django_cas provides fixed versions of these decorators in django_cas.decorators. Usage is unchanged, and in the event that this issue is fixed, the decorators should still work without issue.

For more information see http://code.djangoproject.com/ticket/4617.

Customizing the 403 Error Page

Django doesn't provide a simple way to customize 403 error pages, so you'll have to make a response middleware that handles HttpResponseForbidden.

For example, in views.py:

from django.http import HttpResponseForbidden
from django.template import RequestContext, loader

def forbidden(request, template_name='403.html'):
    """Default 403 handler"""

    t = loader.get_template(template_name)
    return HttpResponseForbidden(t.render(RequestContext(request)))

And in middleware.py:

from django.http import HttpResponseForbidden
from yourapp.views import forbidden

class Custom403Middleware(object):
      """Catches 403 responses and renders 403.html"""

      def process_response(self, request, response):

          if isinstance(response, HttpResponseForbidden):
             return forbidden(request)
          else:
             return response

Now add yourapp.middleware.Custom403Middleware to your MIDDLEWARE_CLASSES setting and create a template named 403.html.

CAS 2.0 support

The CAS 2.0 protocol is supported in the same way that 1.0 is; no extensions or new features from the CAS 2.0 specification are implemented. elementtree is required to use this functionality. (elementtree is also included in Python 2.5's standard library.)

Note: The CAS 3.x server uses the CAS 2.0 protocol. There is no CAS 3.0 protocol, though the CAS 3.x server does allow extensions to the protocol.

Differences Between Django CAS 1.0 and 2.0

Version 2.0 of django_cas breaks compatibility in some small ways, in order simplify the library. The following settings have been removed:

• CAS_LOGIN_URL and CAS_LOGOUT_URL: Version 2.0 is capable of determining these automatically.
• CAS_POPULATE_USER: Subclass CASBackend instead (see above).
• CAS_REDIRECT_FIELD_NAME: Django's own REDIRECT_FIELD_NAME is now used unconditionally.