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Re: [Recipes] Puzzle with HTTP Accept, Firefox header plug-in and RDF

From: Antoine Isaac <aisaac@few.vu.nl>
Date: Fri, 13 Feb 2009 10:31:59 +0100
Message-ID: <49953E0F.20202@few.vu.nl>
To: Diego Berrueta <diego.berrueta@fundacionctic.org>
CC: SWD WG <public-swd-wg@w3.org>, sergio.fernandez@fundacionctic.org, Lourens van der Meij <lourens@cs.vu.nl>

Hi,

Just a pointer to a blog post related to my issue, spotted by Lourens (CCed)
http://fgiasson.com/blog/index.php/2007/07/06/content-negotiation-bad-use-cases-i-recently-observed/

Antoine


> 
> Hi Diego,
> 
>> Antoine,
>>
>> El jue, 12-02-2009 a las 16:39 +0100, Antoine Isaac escribió:
>>> But when it queries for the SKOS 2008 file [3], Firefox gets 
>>> redirected to [4] and not to [5].
>>>
>>> How come? What is puzzling is that Vapour [6] reports that everything 
>>> is ok with both URIs [1] and [3], even though the behaviours differ 
>>> when there is no content negociation. Interestingly, Vapour itself 
>>> recognizes my Accept headers: when I activate the application/rdf+xml 
>>> only, it sends its reports in RDF, while it sends the HTML if I do 
>>> nothing.
>>>
>>> Is there something more than Accept header involved? Has my plug-in 
>>> bugs, or am I just too dumb to think of myself playing with that kind 
>>> of stuff?
>>
>> This problem is a nice example of the risks of using the User-Agent
>> header for content negotiation. The server is issuing a redirect to the
>> HTML document when you use Firefox/IE, regardless of the content of the
>> Accept header. Compare these two requests:
>>
>> $ curl -I -H "Accept: application/rdf+xml"
>> http://www.w3.org/2008/05/skos
>>
>> HTTP/1.1 303 See Other
>> ...
>> Location: http://www.w3.org/TR/2008/WD-skos-reference-20080829/skos.rdf
>> ...
>>
>>
>> $ curl -I -H "Accept: application/rdf+xml" -H "User-agent: Mozilla/5.0
>> (X11; U; Linux i686; en-US; rv:1.9.0.6) Gecko/2009020409 Iceweasel/3.0.6
>> (Debian-3.0.6-1)" http://www.w3.org/2008/05/skos
>>
>> HTTP/1.1 303 See Other
>> ...
>> Location: http://www.w3.org/TR/2008/WD-skos-reference-20080829/skos.html
>> ...
>>
>>
>> Best,
>>
>> (Thanks to Sergio, in CC, for the HTTP traces)
> 
> Thanks! That indeed explains everything.
> 
> I'd be curious, though, why such a rule has been set up in the Recipes. 
> Once this document acknowledges that:
>> It is accepted as a principle of good practice that HTTP clients 
>> SHOULD include an 'Accept:' field in a request header,
> then the rewrite rules could have been specified based on the Accept 
> header only, couldn't they?
> Of course you can say rightfully that this assumes that some tricky 
> minds will use Mozilla to get RDF. But I would then reply that having 
> such a flaw in the approach is likely to encourage people to adapt 
> variants of the recipes.
> As an example I guess neither Vapour nor dbPedia follow strictly one of 
> the Recipes, as the ones that do involve serving RDF or HMTL seem to 
> include the User-Agent header in the rewriting.
> 
> Cheers,
> 
> Antoine
> 
> 
Received on Friday, 13 February 2009 09:32:32 GMT

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