HTTP proxy - expect 100 and authentication

This question was asked already on stackoverflow.com ( https://stackoverflow.com/questions/3889574/apache-and-mod-proxy-not-handling-http-100-continue-from-client-http-417 ):

You could configure it this way:

<IfModule mod_headers.c>
  RequestHeader unset Expect early
</IfModule>

This will remove the Expect header, and the communication should work.

EDIT Please make sure, that the remote side is HTTP/1.1. In the same RFC you linked there is just one page after your cite this:

  - If the proxy knows that the version of the next-hop server is
    HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
    respond with a 417 (Expectation Failed) status.