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what does "unrecognized header fields" mean?

From: Dave Kristol <dmk@research.bell-labs.com>
Date: Tue, 26 May 1998 11:48:49 -0400 (EDT)
Message-Id: <199805261548.LAA22824@aleatory.research.bell-labs.com>
To: http-wg@cuckoo.hpl.hp.com
The LPWA proxy truncates the Referer header, in most cases, to read
        Referer:
with no value, and therefore violates the HTTP/1.1 specification.

I received a report about a site that didn't work with the LPWA HTTP
proxy.  After much investigation, I discovered that the site's server
software was barfing because Referer had no value.  Add a value, or
remove the header altogether, and their server worked okay.

I was all set to go off in high dudgeon about how the specification
*says* that headers that aren't understood should be ignored.  What it
says (7.1 Entity Header Fields) is that "unrecognized header fields
SHOULD be ignored...."

The question I have is, what does "unrecognized" mean?  Does it just
mean a header whose name is unfamiliar, or does it also mean a
recognized header for which the value is in some way invalid (such as
my example above)?  I realize that "be liberal in what you accept" is
on my side, here, but it's not clear that the *letter* of the
specification is also on my side.

Dave Kristol
Received on Tuesday, 30 June 1998 11:42:16 EDT

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