Talons == Falcon Hooks

Talons is a library of WSGI middleware that is designed to work with the Falcon lightweight Python framework for building RESTful APIs. Like Falcon, Talons aims to be fast, light, and flexible.

The first middleware in Talons is authentication middleware, enabling one or more backend identity plugins to handle authentication.

What is talons.auth?

talons.auth is a namespace package that contains utilities for constructing identifying and authenticating middleware and plugins designed for applications running the Falcon WSGI micro-framework for building REST APIs.

A simple usage example

A simple Falcon API application is constructed like so:

import falcon

# falcon.API instances are callable WSGI apps
app = falcon.API()

To add middleware to a Falcon API application, we simply instantiate the desired talons.auth middleware and supply it to the falcon.API() call:

import falcon
from talons.auth import middleware
from talons.auth import basicauth, httpheader, htpasswd

# Assume getappconfig() returns a dictionary of application configuration
# options that may have been read from some INI file...
config = getappconfig()

auth_middleware = middleware.create_middleware(identify_with=[
                                                 basicauth.Identifier,
                                                 httpheader.Identifier],
                                               authenticate_with=htpasswd.Authenticator,
                                               **config)

app = falcon.API(before=[auth_middleware])

Details

There are a variety of basic plugins that handle identification of the user making an API request and authenticating credentials with a number of common backends, including LDAP and SQL data stores.

Authentication involves two main tasks:

• Identifying the user who wishes to be authenticated
• Validating credentials for the identified user

Classes that derive from talons.auth.interfaces.Identifies implement an identify method that takes the falcon.request.Request object from the WSGI pipeline and looks at elements of the request to determine who the requesting user is.

The class that stores credential information -- including a login, password/key, a set of roles or groups, as well as other metadata about the requesting user -- is the talons.auth.interfaces.Identity class. talons.auth.interfaces.Identifies subclasses store this Identity object in the WSGI environs' "wsgi.identity" bucket.

Classes that derive from talons.auth.interfaces.Authenticates implement an authenticate method that takes a single argument -- a talons.auth.interfaces.Identity object -- and attempts to validate that the identity is authentic.

To give your Falcon-based WSGI application authentication capabilities, you simply create middleware that has one or more talons.auth.identify modules and one or more talons.auth.authenticate modules. We even give you a helper method -- talons.auth.middleware.create_middleware -- to create such middleware in a single call.

Identifiers

Each class that derives from talons.auth.interfaces.Identifies is called an "Identifier". Each class implements a single method, identify(), that takes the incoming falcon.request.Request object as its sole parameter. If the identity of the authenticating user can be determined, then the Identifier object stores a talons.auth.interfaces.Identity object in the WSGI environ's wsgi.identity key and returns True.

Multiple Identifier classes can be supplied to the talons.auth.middleware.create_middleware method to support a variety of ways of gleaning identity information from the WSGI request. Each Identifier's identify() method checks to see if the wsgi.identity key is already set in the WSGI environs. If it is, the method simply returns True and does not attempt to process anything further.

talons.auth.basicauth.Identifier

The most basic identifier, talons.auth.basicauth.Identifier has no configuration options and simply looks in the Authenticate HTTP header for credential information. If the Authenticate HTTP header is found and contains valid credential information, then that identity information is stored in the wsgi.identity WSGI environs key.

talons.auth.httpheader.Identifier

Another simple identifier, talons.auth.httpheader.Identifier looks for configurable HTTP headers in the incoming WSGI request, and uses the values of the HTTP headers to construct a talons.auth.Identity object.

A set of configuration options control how this Identifier class behaves:

• httpheader_user: HTTP header to look for user/login name (required)
• httpheader_key: HTTP header to look for password/key (required)
• httpheader_$ATTRIBUTE: HTTP header that, if found, will be used to add $ATTRIBUTE to the Identity object stored in the WSGI pipeline. (optional)

The above configuration options are supplied to the constructor as keyword arguments.

Example

Suppose we wanted to extract identity information from the following HTTP Headers:

• X-Auth-User -- The value of this header will be the authenticating user's user name
• X-Auth-Password -- The value of this header will be the authenticating user's password
• X-Auth-Domain -- The value of this header should be considered the authentication domain that will be considered when authenticating the identity. We want to store this value on the talons.auth.Identity object's domain attribute.

Our configuration options would look like this:

httpheader_user=x-auth-user
httpheader_key=x-auth-password
httpheader_domain=x-auth-domain

Authenticators

Each class that derives from talons.auth.interfaces.Authenticates is called an "Authenticator". Each Authenticator implements a single method, authenticate(), that takes a talons.auth.interfaces.Identity object as its sole parameter.

The authenticate method verifies that the supplied identity can be verified (authenticated). Different implementations will rely on various backend storage systems to validate the incoming identity/credentials. If authentication was successful, the method returns True, False otherwise.

Talons comes with a few simple examples of Authenticator plugins.

talons.auth.external.Authenticator

A generic Authenticator plugin that has one main configuration option, external_authn_callable which should be the "module.function" or "module.class.method" dotted-import notation for a function or class method that accepts a single parameter. This function will be called by the instance of talons.auth.authenticate.external.Authenticator to validate the credentials of a request.

In addition, there are two other configuration options that indicate whether the external_authfn function may set the roles or groups attributes on the supplied identity:

• external_sets_roles: Boolean (defaults to False). A True value indicates the plugin may set the roles attribute on the identity object.

• external_sets_groups: Boolean (defaults to False). A True value indicates the plugin may set the groups attribute on the identity object.

Example

Suppose we have some application code that looks up a stored password for a user in a Redis Key-Value Store. Salted, encrypted passwords for each user are stored in the Redis KVS, along with a comma-separated list of roles the user belongs to.

Our application has a Python file called /application/auth.py that looks like this:

import hashlib

import redis

_AUTH_DB = redis.StrictRedis(host='localhost', port=6379, db=0)


def _pass_matches_stored_pass(pass, stored_pass):
    # Assume that passwords are stored in Redis in the following format:
    # salt:hashedpass
    # and that the passwords have been hashed with SHA-256
    salt, stored_hashed_pass = stored_pass.split(':')
    hashed_pass = hashlib.sha256(salt.encode() + pass.encode()).hexdigest()
    return hashed_pass == stored_hashed_pass


def authenticate(identity):
    user = identity.login
    pass = identity.key

    # Assume that user "records" are stored in Redid in the following format:
    # salt:hashedpass#roles
    # Where roles is a comma-separated list of roles
    auth_success = False
    user_record = _AUTH_DB.get(user)
    if user_record:
        stored_pass, role_list = user_record.split('#')
        auth_success = _pass_matches_stored_pass(pass, stored_pass)
        if auth_success:
            identity.roles = role_list.split(',')
    return auth_success

To use the above application.auth.authenticate method for authenticating identities, we'd supply the following configuration options to the talons.auth.external.Authenticator constructor:

• external_authn_callable=application.auth.authenticate
• external_sets_roles=True

talons.auth.htpasswd.Authenticator

An Authenticator plugin that queries an Apache htpasswd file to check the credentials of a request. The plugin has a single configuration option:

• htpasswd_path: The filepath to the Apache htpasswd file to use for authentication checks.

Authorizers

Each class that derives from talons.auth.interfaces.Authorizes is called an "Authorizer". Each Authorizer implements a single method, authorize(), that takes a talons.auth.interfaces.Identity object, and a talons.auth.interfaces.ResourceAction object.

The ResourceAction object currently has a single method, to_string, that returns a "dotted-notation" string describing the requested HTTP resource.

For instance, let's say the identity made an HTTP request to:

POST /users/12345/groups

The ResourceAction.to_string method that is supplied to the authorize function would yield the string "users.12345.groups.post". This string is useful to plugins that compare the string with the supplied identity object. See below for an example that makes this more clear.

At present, there is only a single Authorizer built in to Talons: the talons.auth.external.Authorizer class. Like its sister, the talons.auth.external.Authenticator, it accepts an external callable that accepts the identity and resource action parameters and returns whether the identity is allowed to perform the action on the resource. The single configuration parameter is called external_authz_callable.

Let's continue the example from above and add an external callable that will be used as an authorizer. This callable will compare the result of the ResourceAction's to_string method against the supplied identity object and a hashmap of regular expressions in order to determine if the user is permitted to perform an action.

Assuming our application has a Python file called /application/auth.py that contains the above authenticate code, as well as this: like this:

import re


def self_or_admin(match, identity):
    """
    Returns True if the identity has an admin role or the identity
    matches the requesting user.
    """
    if "admin" in identity.roles:
        return True
    return match.groups(1) == identity.login


def anyone(*args):
    return True


_POLICY_RULES = [
    (r'^users\.(^\.)+\.get$', self_or_admin),
    (r'^users\.post$', anyone),
]
POLICIES = []
for regex, fn in _POLICY_RULES:
    POLICIES.append((re.compile(regex), fn))


def authorize(identity, resource_action):
    user = identity.login
    res_string = resource_action.to_string()
    for p, fn in _POLICIES:
        m = p.match(res_string)
        if m:
            return fn(m, identity)

To use the above application.auth.authorize method for authorizing the identity that was authenticated, we'd supply the following configuration options to the talons.auth.external.Authorizer constructor:

• external_authz_callable=application.auth.authorize

Why talons.auth?

Why not just use middleware like repose.who for authentication plugins? Why re-invent the wheel here?

A few reasons, in no particular order:

• Use of the Webob library. I'm not a fan of it, as I've run into numerous issues with this library over the years.
• Use of zope.interfaces. Also not a fan of it. It's a library that seems to be designed for traditional C++ programmers instead of feeling like it's designed for Python developers. Just use the abc module if you absolutely must have strict interface enforcement.
• Trying to override things like logging setup in constructors of middleware.
• No Paste.
• Wanted something that fit Falcon's app construction paradigm.

But hey, there's nothing inherently wrong with repoze.who. If you like it, and it works for you, use it.

Contributing

Jay Pipes maintains the Talons library. You can usually find him on the Freenode IRC #openstack-dev channel. Interested in improving and enhancing Talons? Pull requests are always welcome.

License and Copyright

Copyright 2013-2014, Jay Pipes

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.