Proxy Web Servers and Authentication > How Proxies Work > Cache headers

Response

Headers used only in HTTP Responses.

WWW-Authenticate

Returned by the server when credentials are required to retrieve an resource, but the credentials have not been supplied or are invalid. The header contains parameters instructing the client how to respond with a correctly formatted "Authorization" header.

Proxy-authenticate

Returned by a proxy when credentials are required to use the services of the proxy.

Warning

Used by an origin server or a proxy to send more detailed warning messages back to the client.