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From "Roy T. Fielding" <field...@liege.ICS.UCI.EDU>
Subject Re: Host header
Date Thu, 23 Jan 1997 15:47:50 GMT
> 2) As Roy pointed out a month or so ago, we really should be sending
> "Vary: Host" when we have Host-based virtual servers. Although the
> HTTP spec is pretty clear in implying this is neccessary, my problem
> with it is mainly that a minimally HTTP/1.1-compliant proxy will
> probably simply not cache anything with a Vary: header (the spec
> actually reccomends this, I believe), whereas in this case, there's no
> reason really not to.

I disagree -- a minimally compliant cache would have the sense not
to do something that stupid, particularly if Apache sent Vary: Host
on a regular basis.

> And, to be HTTP/1.1-compliant, we'd have to send Vary: Host for every
> response, even those not based on the Host: header (because they're
> contingent on the lack of it, which is the same as it being
> there). Even if the server isn't configured to use any Host-based
> servers explictly (since Apache will allow *any* of its virtual
> servers to be called by a request to the main server with a Host:
> header).

I'm not sure I followed that, but the basic discrimination is whether the
same request with a different Host value would result in a different
resource being selected by the server.  If a particular server does not
look at the Host value for requests on a particular IP address, then it
does not need to send Vary: Host.

BTW, the only reason that this is needed is to support server-side caches.
It would be reasonable to include a directive that disabled sending
Vary: Host, since that is an optimization which should be under the control
of the administrator.

I haven't done this myself because I don't use virtual hosts and 
don't understand the code that supports them.

.....Roy

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