407 Proxy Authentication Required

This code is similar to 401 (Unauthorized), but indicates that the
   client must first authenticate itself with the proxy. The proxy MUST
   return a Proxy-Authenticate header field containing a
   challenge applicable to the proxy for the requested resource. The
   client MAY repeat the request with a suitable Proxy-Authorization
   header field. HTTP access authentication is explained
   in "HTTP Authentication: Basic and Digest Access Authentication".