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Re: HTTPbis -10 drafts published : Connection header

From: Willy Tarreau <w@1wt.eu>
Date: Wed, 14 Jul 2010 10:55:14 +0200
To: Julian Reschke <julian.reschke@gmx.de>
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <20100714085514.GE2717@1wt.eu>

On Tue, Jul 13, 2010 at 08:18:10AM +0200, Julian Reschke wrote:
> <http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-10#appendix-D.11>

Here I have found something that ought to be clarified concerning
the Connection header :

9.1. Connection
   HTTP/1.1 proxies MUST parse the Connection header field before a
   message is forwarded and, for each connection-token in this field,
   remove any header field(s) from the message with the same name as the
   connection-token.  Connection options are signaled by the presence of
   a connection-token in the Connection header field, not by any
   corresponding additional header field(s), since the additional header
   field may not be sent if there are no parameters associated with that
   connection option.

   Message headers listed in the Connection header MUST NOT include end-
   to-end headers, such as Cache-Control.

The last sentence is already very important, but some side effects
remain on some implementations, because it is not stated that only
headers that were given by the client must be removed. If you take
Apache 2.2 as a proxy for instance, by default it will add an
"X-Forwarded-For" header when forwarding the connection to the
server, to indicate the client's address. If the client says
"Connection: X-Forwarded-For", then this header is removed from the
output and the server does not get the client's address. I've not
checked the code, but I think this is because the header cleaning
happen just before forwarding the connection, and after header
addition. This can permit a client to alter the semantics of the
communication between a proxy and a server, possibly bypassing some
filtering or hiding its activities. I think that adding a sentence
such as the following would be fine :

   If an HTTP/1.1 proxy intends to modify or add headers to the
   message being forwarded, it may only do so after the headers
   above have been removed.

I've also tried to remove "Connection" (which is a hop-by-hop header)
with Apache and fortunately it did not work. Out of curiosity I tried
with "Content-Length" and "Host" and they did not work either.
However, when I try to remove "Transfer-Encoding" (hop-by-hop) on a
POST with empty body, I observe that "Content-Length: 0" is not added
to the request, which is harmless (the request will be rejected due
to missing content length).

I've just found that other end-to-end headers such as "cache-control",
"content-encoding" etc... can still be removed via Apache, possibly
leading to differences in contents interpretation between the proxy
and the server (eg: when content filtering is performed), though this
is purely implementation-specific. However, implementation-specific
differences or limitations sometimes indicate a difficulty in fully
understanding or respecting a standard.

Received on Wednesday, 14 July 2010 08:55:51 UTC

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