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Subject [Bug 61706] New: ProxyBadHeader Ignore does not ignore headers that fail check_header()
Date Tue, 31 Oct 2017 19:59:46 GMT

            Bug ID: 61706
           Summary: ProxyBadHeader Ignore does not ignore headers that
                    fail check_header()
           Product: Apache httpd-2
           Version: 2.4.28
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: mod_proxy_http
  Target Milestone: ---

I have a reverse proxy to my cable modem.  The modem returns an invalid header
field "Cache-Control : no-cache".  So, to work around the problem, I enabled
ProxyBadHeader Ignore in my configuration.

However, apache returns the same 500 Internal Server Error I receive when
ProxyBadHeader is the default IsError (and not even the 502 Bad Gateway
documented for that setting)

Tracking the issue through the source, the problem appears that
ap_proxy_read_headers() only applies ProxyBadHeader if the header is missing a
":".  When the proxied header is passed as a response to the request,
http_filters.c:check_header() fails for the header, and the connection is
aborted with 500.

The solution is that the same key/value checks that check_header() performs
should be done in ap_proxy_read_headers() before the header is considered valid
and added to r->headers_out (and could provide better trace logging when a bad
header is encountered there).

NOTE: Ideally, there should be a new value for ProxyBadHeader which would allow
a distinction between "ignore and discard" and "ignore and pass through" (so
bad headers could be filtered)

To summarize:
* IsError should return 502 on any header that fails check_header()
* Ignore should probably discard bad headers as documented (better name would
be "Discard" :).
* StartBody should start body based on the same check_header() check..

...and I would suggest a new value:
* Pass which should actually allow the header to be filtered (so the original
values might be fixed and retained, but only if configured to do so!).

I could provide a patch if needed for the above, but perhaps there's another
preferred alternative?

For references, here's my proxy configuration:

ProxyBadHeader Ignore                                                           
<Location "/cable/">                                                            

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