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Authentication in TurboGears 2 applications

This document describes how repoze.who is integrated into TurboGears and how you make get started with it. For more information, you may want to check repoze.who‘s website.

repoze.who is a powerful and extensible authentication package for arbitrary WSGI applications. By default TurboGears2 configures it to log using a form and retrieving the user informations through the user_name field of the User class. This is made possible by the authenticator plugin that TurboGears2 uses by default which is repoze.who.plugins.sa.SQLAlchemyAuthenticatorPlugin.

How it works in TurboGears

The authentication layer it’s a WSGI middleware which is able to authenticate the user through the method you want (e.g., LDAP or HTTP authentication), “remember” the user in future requests and log the user out.

You can customize the interaction with the user through four kinds of plugins, sorted by the order in which they are run on each request:

  • An identifier plugin, with no action required on the user’s side, is able to tell whether it’s possible to authenticate the user (e.g., if it finds HTTP Authentication headers in the HTTP request). If so, it will extract the data required for the authentication (e.g., username and password, or a session cookie). There may be many identifiers and repoze.who will run each of them until one finds the required data to authenticate the user.
  • If at least one of the identifiers could find data necessary to authenticate the current user, then an authenticator plugin will try to use the extracted data to authenticate the user. There may be many authenticators and repoze.who will run each of them until one authenticates the user.
  • When the user tries to access a protected area or the login page, a challenger plugin will come up to request an action from the user (e.g., enter a user name and password and then submit the form). The user’s response will start another request on the application, which should be caught by an identifier to extract the login data and then such data will be used by the authenticator.
  • For authenticated users, repoze.who provides the ability to load related data (e.g., real name, email) in the WSGI environment so that it can be easily used in the application. Such a functionality is provided by so-called metadata provider plugins. There may be many metadata providers and repoze.who will run them all.

When repoze.who needs to store data about the authenticated user in the WSGI environment, it uses its repoze.who.identity key, which can be accessed using the code below:

from tg import request

# The authenticated user's data kept by repoze.who:
identity = request.environ.get('repoze.who.identity')

Such a value is a dictionary and is often called “the identity dict”. It will only be defined if the current user has been authenticated.

There is a short-cut to the code above in the WSGI request, which will be defined in {yourproject}.lib.base.BaseController if you enabled authentication and authorization when you created the project.

For example, to check whether the user has been authenticated you may use:

   # ...
   from tg import request
   # ...
   if request.identity:
       flash('You are authenticated!')

``request.identity`` will equal to ``None`` if the user has not been
authenticated.

Likewise, this short-cut is also set in the template context as
``tg.identity``.

The username will be available in identity['repoze.who.userid'] (or request.identity['repoze.who.userid'], depending on the method you select).

The FastFormPlugin

By default, TurboGears 2.3.4 configures repoze.who to use tg.configuration.auth.fastform.FastFormPlugin as the first identifier and challenger – using /login as the relative URL that will display the login form, /login_handler as the relative URL where the form will be sent and /logout_handler as the relative URL where the user will be logged out. The so-called rememberer of such identifier will be an instance of repoze.who.plugins.cookie.AuthTktCookiePlugin.

All these settings can be customized through the config.app_cfg.base_config.sa_auth options in your project. Identifiers, Authenticators and Challengers can be overridden providing a different list for each of them as:

base_config.sa_auth['identifiers'] = [('myidentifier', myidentifier)]

You don’t have to use repoze.who directly either, unless you decide not to use it the way TurboGears configures it.

Customizing authentication and authorization

It’s very easy for you to customize authentication and identification settings in repoze.who from {yourproject}.config.app_cfg.base_config.sa_auth.

Customizing how user informations, groups and permissions are retrieved

TurboGears provides an easy shortcut to customize how your authorization data is retrieved without having to face the complexity of the underlying authentication layer. This is performed by the TGAuthMetadata object which is configured in your project config.app_cfg.base_config.

This object provides three methods which have to return respectively the user, its groups and its permissions. You can freely change them as you wish as they are part of your own application behavior.

Advanced Customizations

For more advanced customizations or to use repoze plugins to implement different forms of authentication you can freely customize the whole authentication layer using through the {yourproject}.config.app_cfg.base_config.sa_auth options.

The available directives are all optional:

  • form_plugin: This is a replacement for the FriendlyForm plugin and will be

    always used as a challenger. If form_identifies option is True it will also be appended to the list of identifiers.

  • ìdentifiers: A custom list of repoze.who identifiers.

    By default it contains the form_plugin and the AuthTktCookiePlugin.

  • challengers: A custom list of repoze.who challengers.

    The form_plugin is always appended to this list, so if you have only one challenger you will want to change the form_plugin instead of overridding this list.

  • authmetadata: This is the object that TG will use to fetch authorization metadata.

    Changing the authmetadata object you will be able to change how TurboGears fetches your user data, groups and permissions. Using authmetada a new repoze.who metadata provider is created.

  • mdproviders: This is a list of repoze.who metadata providers.

    If authmetadata is not None a metadata provider based on it will always be appended to the mdproviders.

Customizing the model structure assumed by the quickstart

Your auth-related model doesn’t have to be like the default one, where the class for your users, groups and permissions are, respectively, User, Group and Permission, and your users’ user name is available in User.user_name. What if you prefer Member and Team instead of User and Group, respectively?

First of all we need to inform the authentication layer that our user is stored in a different class. This makes repoze.who know where to look for the user to check its password:

# what is the class you want to use to search for users in the database
base_config.sa_auth.user_class = model.Member

Then we have to tell out authmetadata how to retrieve the user, its groups and permissions:

from tg.configuration.auth import TGAuthMetadata

#This tells to TurboGears how to retrieve the data for your user
class ApplicationAuthMetadata(TGAuthMetadata):
    def __init__(self, sa_auth):
        self.sa_auth = sa_auth

    def authenticate(self, environ, identity):
        user = self.sa_auth.dbsession.query(self.sa_auth.user_class).filter_by(user_name=identity['login']).first()
        if user and user.validate_password(identity['password']):
            return identity['login']

    def get_user(self, identity, userid):
        return self.sa_auth.user_class.query.get(user_name=userid)

    def get_groups(self, identity, userid):
        return [team.team_name for team in identity['user'].teams]

    def get_permissions(self, identity, userid):
        return [p.permission_name for p in identity['user'].permissions]

base_config.sa_auth.authmetadata = ApplicationAuthMetadata(base_config.sa_auth)

Now our application is able to fetch the user from the Member table and its groups from the Team table. Using TGAuthMetadata makes also possible to introduce a caching layer to avoid performing too many queries to fetch the authentication data for each request.

BasicAuth Example

The following is an example of an advanced authentication stack customization to use browser basic authentication instead of form based authentication.

Declaring a Custom Authentication Backend

First required step is to declare that we are going to use a custom authentication backend:

base_config.auth_backend = 'htpasswd'

When this is valued to ming or sqlalchemy TurboGears will configure a default authentication stack based on users stored on the according database, if auth_backend is None the whole stack will be disabled.

Then we must remove all the simple authentication options, deleting all the basic_config.sa_auth from app_cfg.py is usually enough. Leaving unexpected options behind (options our authentication stack doesn’t use) might lead to a crash on application startup.

Using HTPasswd file for users

Next step is storing our users inside an htpasswd file, this can be achieved by using the HTPasswdPlugin authenticator:

from repoze.who.plugins.htpasswd import HTPasswdPlugin, plain_check
base_config.sa_auth.authenticators = [('htpasswd', HTPasswdPlugin('./htpasswd', plain_check))]

This will make TurboGears load users from an htpasswd file inside the directory we are starting the application from. The plain_check function is the one used to decode password stored inside the htpasswd file. In this case passwords are expected to be in plain text in the form:

manager:managepass

Challenging and Identifying users with BasicAuth

Now that we are correctly able to authenticate users from an htpasswd file, we need to use BasicAuth for identifying returning users:

from repoze.who.plugins.basicauth import BasicAuthPlugin

base_auth = BasicAuthPlugin('MyTGApp')
base_config.sa_auth.identifiers = [('basicauth', base_auth)]

This will correctly identify users that are already logged using BasicAuth, but we are still sending users to login form to perform the actual login.

As BasicAuth requires the login to be performed through the browser we must disable the login form and set the basic auth plugin as a challenger:

# Disable the login form, it won't work anyway as the credentials
# for basic auth must be provided through the browser itself
base_config.sa_auth.form_identifies = False

# Use BasicAuth plugin to ask user for credentials, this will replace
# the whole login form.
base_config.sa_auth.challengers = [('basicauth', base_auth)]

Providing User Data

The previous steps are focused on providing a working authentication layer, but we will need to also identify the authenticated user so that also request.identity and the authorization layer can work as expected.

This is achieved through the authmetadata option, which tells TurboGears how to retrieve the user and it’s informations. In this case as we don’t have a database of users we will just provide a simple user with only display_name and user_name so that most things can work. For manager user we will also provide the managers group so that user can access the TurboGears admin:

from tg.configuration.auth import TGAuthMetadata

class ApplicationAuthMetadata(TGAuthMetadata):
    def __init__(self, sa_auth):
        self.sa_auth = sa_auth

    def get_user(self, identity, userid):
        # As we use htpasswd for authentication
        # we cannot lookup the user in a database,
        # so just return a fake user object
        from tg.util import Bunch
        return Bunch(display_name=userid, user_name=userid)

    def get_groups(self, identity, userid):
        # If the user is manager we give him the
        # managers group, otherwise no groups
        if userid == 'manager':
            return ['managers']
        else:
            return []

    def get_permissions(self, identity, userid):
        return []

base_config.sa_auth.authmetadata = ApplicationAuthMetadata(base_config.sa_auth)

Removing Login Form

As the whole authentication is now performed through BasicAuth the login form is now unused, so probably want to remove the login form related urls which are now unused:

  • /login
  • /post_login
  • /post_logout

Disabling authentication and authorization

If you need more flexibility than that provided by the quickstart, or you are not going to use repoze.who, you should prevent TurboGears from dealing with authentication/authorization by removing (or commenting) the following line from {yourproject}.config.app_cfg:

base_config.auth_backend = '{whatever you find here}'

Then you may also want to delete those settings like base_config.sa_auth.* – they’ll be ignored.