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From André Warnier ...@ice-sa.com>
Subject Re: request parameters mishandle utf-8 encoding
Date Fri, 01 Aug 2008 21:52:39 GMT
Christopher Schultz wrote:
[...]

Here is the definitive reference :
http://www.faqs.org/rfcs/rfc2396.html
and see 1.5. URI Transcribability and following if you are courageous.

And the HTTP 1.1 RFC 2616 makes reference to the above RFC in what 
regards URL encoding.

The point is that the URL contained in the HTTP request line (the first 
line) cannot be considered to be in any particular encoding, unless the 
client and server somehow agree on a convention in advance.
All it says in the specs, is that only certain ranges of bytes are 
allowed "as is" in URL's, and the rest should be escaped, and it says 
how they should be escaped.

To say this in lay language : you can decide to write a URL in pretty 
much any encoding of any character set you want, but then, once you have 
your encoded URL, you should scan it byte by byte, and any byte that is 
not in the accepted "as is" range should be encoded as per the spec.
The accepted range is, generally speaking, the byte values that 
correspond to the printable characters in the latin-1 alphabet, minus 
some "excluded" characters like #,<,>,/ etc...

For example, if your choice of encoding was so that, after encoding, at 
position 30 of your URL string was a byte with a hex value 0x20 (which 
in iso-8859-1 is a space), then it should be replaced by a "+".
Similarly, if after the original encoding there happened to be a byte at 
position 40 with a hex value of 0x0D (CR, a control character), it 
should be replaced by the sequence %0D.  And so on.

Now, whether the server will "understand" your URL is another matter.

The receiving HTTP server should first of all decode the received URL in 
the same way, before any further decoding is done.  Thus, from left to 
right, any "+" byte should be replaced by a byte 0x20, any sequence 
"%0D" should be replaced by the single byte with hex value 0x0D, etc..

Then, by default, it is the convention that in the absence of any other 
information or convention, the resulting string should be considered as 
being in the iso-8859-1 (latin-1) alphabet.

However, if the client and server have somehow made a convention that 
they would exchange URLs containing Unicode characters, encoded as 
UTF-8, that's fine.

After the HTTP Request line, are any number of HTTP headers.  As far as 
I remember, these should conform to the rules for MIME headers, which 
may well specify that they should be limited to ASCII, I am too lazy to 
check.

Then there may be a blank line, followed by a request content.
For that one, the situation is totally different, because a preceding 
HTTP header should specify the content-type, and if it is text, the 
character-set and encoding used.

By using the option in Tomcat that specifies "consider the request URL 
as being in the same encoding as the request body", you are making the 
big assumption that you know the client, and that you know that it will 
send requests that way.
Between a client and a server that "don't know eachother", it is very 
unsafe to make that assumption.  Specifying this parameter in Tomcat is 
not going to magically make your client respect that convention.

It's a pity, but that's the way it is with HTTP 1.1.
The people who designed the protocol and wrote the specs did a great 
job, but did not include any unambiguous way to specify, in the URL 
itself, in which character set or encoding of ditto it was written, if 
it is not the default latin-1.

In the SMTP protocol, by contrast, there exists a way to specify the 
encoding of a header value (e.g. the "Subject" header), at the beginning 
of the header value itself.

André

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