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From "Alex Chaffee (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-431) Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
Date Mon, 30 May 2011 16:15:47 GMT


Alex Chaffee commented on COUCHDB-431:

I just encountered this issue myself, and I haven't thought through all the implications,
but isn't there an easier way to crack this nut? If we add a "response_headers" config setting
to the normal database httpd config, and the server adds those headers to every HTTP response,
then that's it. Admins can then implement a coarse form of CORS on a per-database level with
a simple "{response_headers: {'Access-Control-Allow-Origin': '*'}}". 

If more fine-grained control is needed (e.g. allow CORS for some client IP#s but not others)
then a patch like Jason's would be needed. But ACL systems are notoriously difficult to design
and implement. My solution leaves access control up to the local admin, and also makes it
clear just how simple CORS actually is -- it's not hard security, just a message from the
server that tells the client "here's the data, and here's a hint about how I think you should
use it" (which hint is ignored by everybody except web browsers). 

(cf. Mozilla: "The Cross-Origin Resource Sharing standard works by adding new HTTP headers
that allow servers to describe the set of origins that are permitted to read that information
using a web browser.  Firefox supports these headers and enforces the restrictions they establish." )


> Support cross domain XMLHttpRequest (XHR) calls by implementing Access Control spec
> -----------------------------------------------------------------------------------
>                 Key: COUCHDB-431
>                 URL:
>             Project: CouchDB
>          Issue Type: New Feature
>          Components: HTTP Interface
>    Affects Versions: 0.9
>            Reporter: James Burke
>            Assignee: Randall Leeds
>            Priority: Minor
>         Attachments: A_0001-Generalize-computing-the-appropriate-headers-for-any.patch,
A_0002-Send-server-headers-for-externals-responses.patch, A_0003-Usably-correct-w3c-CORS-headers-for-valid-requests.patch,
A_0004-Respond-to-CORS-preflight-checks-HTTP-OPTIONS.patch, cors.html
> Historically, browsers have been restricted to making XMLHttpRequests (XHRs) to the same
origin (domain) as the web page making the request. However, the latest browsers now support
cross-domain requests by implementing the Access Control spec from the W3C:
> In order to keep older servers safe that assume browsers only do same-domain requests,
the Access Control spec requires the server to opt-in to allow cross domain requests by the
use of special HTTP headers and supporting some "pre-flight" HTTP calls.
> Why should CouchDB support this: in larger, high traffic site, it is common to serve
the static UI files from a separate, differently scaled server complex than the data access/API
server layer. Also, there are some API services that are meant to be centrally hosted, but
allow API consumers to use the API from different domains. In these cases, the UI in the browser
would need to do cross domain requests to access CouchDB servers that act as the API/data
access server layer.
> JSONP is not enough in these cases since it is limited to GET requests, so no POSTing
or PUTing of documents.
> Some information from Firefox's perspective (functionality available as of Firefox 3.5):
> And information on Safari/Webkit (functionality in latest WebKit and Safari 4):
> IE 8 also uses the Access Control spec, but the requests have to go through their XDomainRequest
object (XDR):
> and I thought IE8 only allowed GET or POST requests through their XDR.
> But as far as CouchDB is concerned, implementing the Access Control headers should be
enough, and hopefully IE 9 will allow normal xdomain requests via XHR.

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