updating the cors article with some feedback

content/python/cornice-cors.rst

@@ -0,0 +1,254 @@
+Implementing CORS in Cornice
+############################
+
+:date: 22-01-2013
+:slug: cross-origin-in-cornice
+:status: draft
+
+For security reasons, it's not possible to do cross-domain requests. In other
+words, if you have a page served from the domain `lolnet.org`, it will not be
+possible for it to get data from `notmyidea.org`.
+
+Well, it's possible, using tricks and techniques like `JSONP
+<http://en.wikipedia.org/wiki/JSONP>`_, but that doesn't work all the time (see
+`the section below <#how-this-is-different-from-jsonp>`_). I remember myself
+doing some simple proxies on my domain server to be able to query other's API.
+
+Hopefuly, there is a nicer way to do this, namely, "Cross Origin
+Resource-Sharing", or `CORS <http://www.w3.org/TR/cors/>`_.
+
+You want an icecream? Go ask your dad first.
+============================================
+
+If you want to use CORS, you need the API you're querying to support it; on the
+server side.
+
+The HTTP server need to answer to the `OPTIONS` verb, and with the appropriate
+response headers.
+
+`OPTIONS` is sent as what the authors of the spec call a "preflight request";
+just before doing a request to the API, the *User-Agent* (the browser most of
+the time) asks the permission to the resource, with an `OPTIONS` call.
+
+The server answers, and tell what is available and what isn't:
+
+.. image:: |filename|/images/cors_flow.png
+    :alt: The CORS flow (from the HTML5 CORS tutorial)
+
+- 1a. The User-Agent, rather than doing the call directly, asks the server, the
+  API, the permission to do the request. It does so with the following headers:
+
+  - **Access-Control-Request-Headers**, contains the headers the User-Agent
+    want to access.
+  - **Access-Control-Request-Method** contains the method the User-Agent want
+    to access.
+
+- 1b. The API answers what is authorized:
+
+  - **Access-Control-Allow-Origin** the origin that's accepted. Can be `*` or
+    the domain name.
+  - **Access-Control-Allow-Methods** a *list* of allowed methods. This can be
+    cached. Note than the request asks permission for one method and the
+    server should return a list of accepted methods.
+  - **Access-Allow-Headers** a list of allowed headers, for all of the
+    methods, since this can be cached as well.
+
+- 2. The User-Agent can do the "normal" request.
+
+
+So, if you want to access the `/icecream` resource, and do a PUT there, you'll
+have the following flow::
+
+    > OPTIONS /icecream
+    > Access-Control-Request-Methods = PUT
+    > Origin: notmyidea.org 
+    < Access-Control-Allow-Origin = notmyidea.org 
+    < Access-Control-Allow-Methods = PUT,GET,DELETE
+    200 OK
+
+You can see that we have an `Origin` Header in the request, as well as
+a `Access-Control-Request-Methods`. We're here asking if we have the right, as
+`notmyidea.org`, to do a `PUT` request on `/icecream`.
+
+And the server tells us that we can do that, as well as `GET` and `DELETE`.
+
+I'll not cover all the details of the CORS specification here, but bear in mind
+than with CORS, you can control what are the authorized methods, headers,
+origins, and if the client is allowed to send authentication information or
+not.
+
+
+A word about security
+=====================
+
+CORS is not an answer for every cross-domain call you want to do, because you
+need to control the service you want to call. For instance, if you want to
+build a feed reader and access the feeds on different domains, you can be
+pretty much sure that the servers will not implement CORS, so you'll need to
+write a proxy yourself, to provide this.
+
+Secondly, CORS, if misunderstood, can be unsecure, and cause some security
+problems. Because the rules apply when a client want to do a request to
+a server, you need to be extra careful about who you're authorizing.
+
+A CORS uncorrectly-secured server can be accessed by a client very easily,
+bypassing the network security. For instance, if a service runs on an intranet,
+only available from behind a VPN, and accepts every cross-origin call, then
+anyone service javascript files to the browser of an user with access to this
+service could make calls there, which is probably not what you want.
+
+
+How this is different from JSONP?
+=================================
+
+You may know the `JSONP <http://en.wikipedia.org/wiki/JSONP>`_ protocol. JSONP
+Allows to do cross origin, but for a particular use case, and do have some
+drawbacks (for instance, it's not possible to do DELETEs or PUTs with JSONP.
+
+JSONP exploits the fact that's possible to get information from another domain
+when you are asking for javascript code, using the `<script>` element.
+
+    Exploiting the open policy for <script> elements, some pages use them to
+    retrieve JavaScript code that operates on dynamically generated
+    JSON-formatted data from other origins. This usage pattern is known as
+    JSONP. Requests for JSONP retrieve not JSON, but arbitrary JavaScript code.
+    They are evaluated by the JavaScript interpreter, not parsed by a JSON
+    parser.
+
+Using CORS in Cornice
+=====================
+
+Okay, things are hopefully clearer by now about CORS, so let's see how we
+implemented it on the server-side.
+
+Cornice is a toolkit that let you define resources in python, and takes care of
+the heavy lifting for you, so I wanted it to take care of the CORS support as
+well.
+
+In Cornice, you define a service like this:
+
+.. code-block:: python
+
+   from cornice import Service
+
+   foobar = Service(name="foobar", path="/foobar")
+
+   # and then you do something with it
+   @foobar.get()
+   def get_foobar(request):
+       # do something with the request.
+
+To add CORS support to this resource, you can go this way, with the
+`cors_origins` parameter:
+
+.. code-block:: python
+    
+    foobar = Service(name='foobar', path='/foobar', cors_origins=('*',))
+
+Tadam, you have enabled CORS for your service. **Be aware that here, you're
+authorizing anyone to query your server, that may not be what you want.**
+
+Of course, you can specify a list of origins you trust, and you don't need
+to stick with `*`, which means "authorize everyone".
+
+Headers
+-------
+
+You can define the headers you want to expose for the service:
+
+.. code-block:: python
+
+    foobar = Service(name='foobar', path='/foobar', cors_origins=('*',))
+
+    @foobar.get(cors_headers=('X-My-Header', 'Content-Type'))
+    def get_foobars_please(request):
+        return "some foobar for you"
+
+I've done some testing and it wasn't working on Chrome because I wasn't
+handling the headers the right way (The missing one was `Content-Type`, that
+chrome was asking for). With my first version of the implementation, I needed
+the service implementers to explicitely list all the headers that should be
+exposed. And of course this is not what we want.
+
+So I introduced an `expose_all_headers` flag, which is set to `True` by
+default, if the service supports CORS.
+
+Cookies / Credentials
+---------------------
+
+By default, the requests you do to your API endpoint don't include the
+credential information, ,for security reasons. If you really want to do that,
+you need to enable it using the `cors_credentials` parameter. You can activate
+this one on a per-service basis, or on a per-method basis.
+
+Caching
+-------
+
+When you do a preflight request, the information returned by the server can be
+cached by the User-Agent so that it's not redone before each actual call.
+
+The caching period is defined by the server, using the `Access-Control-Max-Age`
+header. You can configure this timing using the `cors_max_age` parameter.
+
+Simplifying the API
+-------------------
+
+We have cors_headers, cors_enabled, cors_origins, cors_credentials,
+cors_max_age, cors_expose_all_headers … It start to be a fair number of
+parameters. If you want to have a specific CORS-policy for your services, that
+can be a bit tedious to pass these to your services all the time.
+
+I introduced another way to pass the CORS policy, so you can do something like
+that:
+
+.. code-block:: python
+
+    policy = dict(enabled=False,
+                  headers=('X-My-Header', 'Content-Type'),
+                  origins=('*.notmyidea.org'),
+                  credentials=True,
+                  max_age=42)
+
+    foobar = Service(name='foobar', path='/foobar', cors_policy=policy)
+
+Comparison with other implementations
+=====================================
+
+I was curious enough to have a look at the other implementations of CORS for
+django, for instance, and I found `a gist about it
+<https://gist.github.com/426829.js>`_.
+
+Basically, this adds a middleware that adds the "rights" headers to the answer,
+depending on the request.
+
+While this approach works, It's not implementing the specification completely:
+you need to add support for all the resources at once.
+
+We can think about a nice way to implement this, with definition of what's
+supposed to be exposed via CORS and what shouldn't, directly in your settings,
+but CORS support should in my opinion be handled at the service definition level,
+and not anywhere else (for instance in the settings), appart for the list of
+authorized hosts, because you don't know exactly what's going on when you look
+at the definition of the service.
+
+Resources
+=========
+
+There is a number of good resources that can be useful to you if you want to
+either understand how CORS work, or if you want to implement it yourself.
+
+- http://enable-cors.org/ is useful to get started when you don't know anything
+  about CORS.
+- There is a W3C wiki page containing information that may be useful about
+  clients, common pitfalls etc: http://www.w3.org/wiki/CORS_Enabled
+- *HTML5 rocks* has a tutorial explaining how to implement CORS, with `a nice
+  section about the server-side
+  <http://www.html5rocks.com/en/tutorials/cors/#toc-adding-cors-support-to-the-server>`_.
+- Be sure to have a look at the `clients support-matrix for this feature
+  <http://caniuse.com/#search=cors>`_.
+- About security, `check out this page
+  <https://code.google.com/p/html5security/wiki/CrossOriginRequestSecurity>`_
+
+Of course, the W3C specification is the best resource to rely on. This
+specification isn't hard to read, so you may want to go through it. Especially
+the `"resource processing model" section <http://www.w3.org/TR/cors/#resource-processing-model>`_

pelican.conf.py

@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
+
 PATH = "content"
-AUTHOR = u'Alexis Métaireau'
+AUTHOR = 'Alexis Métaireau'
-SITENAME = u"Alexis' log"
+SITENAME = "Alexis' log"
 THEME = "theme"
 CSS_FILE = "wide.css"