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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 Wed, 01 Jun 2011 23:46:47 GMT


Alex Chaffee commented on COUCHDB-431:

Sorry for using unclear language -- you're absolutely right, I'm talking about adding headers
per *service* (i.e. host-and-port), not per *database* in Couchese.

You're also quite right about the danger of '*' -- I used it as shorthand but it would be
more sensible to use the real domain name of your host -- which would actually take care of
the attack vector you mentioned.

I'm confused about your "dynamically generated header" paragraph. The CORS header must match
the domain name(s) of the valid web site(s), which seems pretty static to me. Or are you just
saying again that you'd like the header to vary per database? Note that a single Access-Control-Allow-Origin
header can contain multiple hostnames (if I read the spec at correctly)
so you could serve two sites from the same service with "Access-Control-Allow-Origin:". (Assuming the browsers implement the spec correctly, of course! :-))

Here's my use case: I want to write an app that goes directly from JavaScript in a web browser
to a CouchDB service running on localhost (and eventually, on an optional public server).
It seems like a perfect fit and it's frustrating that it doesn't work, especially since it's
just for lack of a single HTTP header. I'm currently building a simple Ruby HTTP server to
sit between the browser and Couch, but there's really no reason for that third tier.

Got any ideas on how to smuggle in a response header without your ACL patch?

> 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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