The X-Accept-Proxy-Authentication header specifies the HTTP authentication schemes that the client supports when challenged by a proxy server. The authentication schemes that are supported by proxy servers, if any, are implementation-specific and MAY be enabled arbitrarily by a server administrator.<48>
An example of how HTTP authentication works is specified in [RFC1945] section 11.
This header is defined for use only in requests sent to a server.
The syntax of the X-Accept-Proxy-Authentication header is defined as follows.
X-Accept-Proxy-Authentication = "X-Accept-Proxy-Authentication: "
auth-scheme *("," auth-scheme) CRLF
auth-scheme ; as defined in section 11 of [RFC1945]
The following is an example.
X-Accept-Proxy-Authentication: Negotiate, NTLM, Digest, Basic