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Re: Host header

From: Adam M. Costello BOGUS address, see signature <BOGUS@BOGUS.nicemice.net>
Date: Mon, 16 Feb 2004 21:22:54 -0500
Message-Id: <>
To: uri@w3.org

Stefan Eissing <stefan.eissing@greenbytes.de> wrote:

 > how does the escaping/punying of hostnames affect the HTTP host
 > header?  As an implementor of HTTP client code, I wonder in which
 > format I have to send the header value.
 > At the moment I would just use the hostname as it appears in the URI.

I think the HTTP spec (RFC-2616) is clear on this point:

   3.2.1 General Syntax

     For definitive information on URL syntax and semantics, see "Uniform
     Resource Identifiers (URI):  Generic Syntax and Semantics,"
     RFC 2396...  This specification adopts the definitions of ...
     "host", ... from that specification.

   3.2.2 http URL

     http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]

   14.23 Host

     Host = "Host" ":" host [ ":" port ] ; Section 3.2.2

The host in the Host: field is the very same token as the host in the
URL.  Therefore it is correct to simply literally copy the host from
the URL into the host field.  RFC-2396 does not permit percent-escapes
in host.  If the new URI spec defines a new syntax that allows
percent-escapes in host, will that retroactively alter the HTTP spec?  I
don't know, but either way, it's still the same token in the Host: field
and the URL.  If percent-escapes become valid in the URL host, they also
become valid in the Host: host.  In any case, a simple copy from the URL
to the Host: field conforms to the HTTP spec.

If your application wants to actively support IDNs (as opposed to merely
letting them pass through in blissful ignorance), then it needs to
follow the additional rules of IDNA (RFC-3490), one of which is that
domain names being put into IDN-unaware slots need to be in ASCII form.
Since the HTTP Host: field is IDN-unaware, you'd need to apply ToASCII
to the host name before constructing the Host: field.

This brings up in interesting failure mode that could arise if
percent-escapes are allowed in host.  Suppose an IDN-unaware browser
tries to follow an href to http://jos%C3%A9.net/.  Suppose it gets past
the first failure opportunity by decoding the percent-escapes before
calling the host resolver.  Let's even suppose that the host resolver is
IDN-aware and somehow knows that the query is UTF-8, and can therefore
perform ToASCII on the application's behalf without being asked (I'm
not sure that's wise, but let's just suppose that's what happens).  The
lookup will succeed, and the application will connect to the server, and
issue the request:

GET / HTTP/1.1
Host: jos%C3%A9.net

At this point, the server, which is also IDN-unaware, fails to recognize
the virtual server.

It's not unreasonable to suppose that the server is IDN-unaware.  IDNA
was designed so that I could register xn--jos-dma.net, and use any
existing web hosting service to host xn--jos-dma.net, and the hosting
service need not know or care that xn--jos-dma.net is an IDN.

If the original href had used the ACE form rather than a percent-encoded
UTF-8 form, all three opportunities for failure would have been

Received on Monday, 16 February 2004 21:23:22 UTC

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