Re: p1: handling obs-fold

Mark Nottingham <mnot@mnot.net> Sat, 20 April 2013 07:44 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Sat, 20 Apr 2013 17:43:18 +1000
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To: Willy Tarreau <w@1wt.eu>
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Subject: Re: p1: handling obs-fold
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On 20/04/2013, at 5:00 PM, Willy Tarreau <w@1wt.eu> wrote:

> On Sat, Apr 20, 2013 at 02:07:39PM +1000, Mark Nottingham wrote:
>> p1 3.2.4 defines requirements for handling obs-fold:
>> 
>>> When an obs-fold is received in a message, recipients MUST do one of:
>>> 
>>> 	? accept the message and replace any embedded obs-fold whitespace with either a single SP or a matching number of SP octets (to avoid buffer copying) prior to interpreting the field value or forwarding the message downstream;
>>> 	? if it is a request, reject the message by sending a 400 (Bad Request) response with a representation explaining that obsolete line folding is unacceptable; or,
>>> 	? if it is a response, discard the message and generate a 502 (Bad Gateway) response with a representation explaining that unacceptable line folding was received.
>>> 
>>> Recipients that choose not to implement obs-fold processing (as described above) MUST NOT accept messages containing header fields with leading whitespace, as this can expose them to attacks that exploit this difference in processing.
>> 
>> This seems to repeat itself; what is the difference between choosing to reject the request in the manner described in the last two bullet points, and not accepting the message?
>> 
>> I think that the last sentence can be removed.
> 
> I think it was here before the addition above. In fact it targets a different
> audience which is not aware of OBS at all. The simple fact that we talk about
> prepending spaces before a header field means that the reader doesn't
> understand that this field is not one but the continuation of previous one.
> 
> Maybe this confusing sentence should be removed and replaced with something
> like this before the block you quoted :
> 
>  Presence of a space or tab character at the beginning of a line must not
>  be taken as a new header field but as the continuation of previous header
>  field (obs-fold). As such it cannot happen on the first header field.
> 
> That way readers looking for what to do with these spaces will find their
> response here and will be able to decide what to do with the options that
> are offered to them.


Seems reasonable; I think this one is largely editorial; the only way I'd be really concerned would be if nothing changed.

Recorded as:
  http://trac.tools.ietf.org/wg/httpbis/trac/ticket/444
with suggestions.

--
Mark Nottingham   http://www.mnot.net/