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Applications and Real-Time HTTP HTTP authentication This specification defines the "Authentication-Info" and "Proxy-Authentication-Info" response header fields for use in Hypertext Transfer Protocol (HTTP) authentication schemes that need to return information once the client's authentication credentials have been accepted. Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . Working Group information can be found at and ; source code and issues list for this draft can be found at .

This specification defines the "Authentication-Info" and "Proxy-Authentication-Info" response header fields for use in HTTP authentication schemes () that need to return information once the client's authentication credentials have been accepted. Both were previously defined in , defining the HTTP "Digest" authentication scheme. This document generalizes the description for use not only in "Digest" (), but also in other future schemes that might have the same requirements for carrying additional information during authentication.

This specification uses the Augmented Backus-Naur Form (ABNF) notation of with a list extension, defined in , that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates repetition). The ABNF production for "auth-param" is defined in .

HTTP authentication schemes can use the Authentication-Info response header field to communicate information after the client's authentication credentials have been accepted. This information can include a finalization message from the server (e.g., it can contain the server authentication). The field value is a list of parameters (name/value pairs), using the "auth-param" syntax defined in . This specification only describes the generic format; authentication schemes using Authentication-Info will define the individual parameters. The "Digest" Authentication Scheme, for instance, defines multiple parameters in .

Authentication-Info = #auth-param

The Authentication-Info header field can be used in any HTTP response, independently of request method and status code. Its semantics are defined by the authentication scheme indicated by the Authorization header field () of the corresponding request. A proxy forwarding a response is not allowed to modify the field value in any way. Authentication-Info can be used inside trailers () when the authentication scheme explicitly allows this.

Parameter values can be expressed either as "token" or as "quoted-string" (). Authentication scheme definitions need to allow both notations, both for senders and recipients. This allows recipients to use generic parsing components, independent of the authentication scheme in use. For backwards compatibility, authentication scheme definitions can restrict the format for senders to one of the two variants. This can be important when it is known that deployed implementations will fail when encountering one of the two formats.

The Proxy-Authentication-Info response header field is equivalent to Authentication-Info, except that it applies to proxy authentication () and its semantics are defined by the authentication scheme indicated by the Proxy-Authorization header field () of the corresponding request:

Proxy-Authentication-Info = #auth-param

However, unlike Authentication-Info, the Proxy-Authentication-Info header field applies only to the next outbound client on the response chain. This is because only the client that chose a given proxy is likely to have the credentials necessary for authentication. However, when multiple proxies are used within the same administrative domain, such as office and regional caching proxies within a large corporate network, it is common for credentials to be generated by the user agent and passed through the hierarchy until consumed. Hence, in such a configuration, it will appear as if Proxy-Authentication-Info is being forwarded because each proxy will send the same field value.

Adding information to HTTP responses that are sent over an unencrypted channel can affect security and privacy. The presence of the header fields alone indicates that HTTP authentication is in use. Additional information could be exposed by the contents of the authentication-scheme specific parameters; this will have to be considered in the definitions of these schemes.

HTTP header fields are registered within the "Message Headers" registry located at , as defined by . This document updates the definitions of the "Authentication-Info" and "Proxy-Authentication-Info" header fields, so the "Permanent Message Header Field Names" registry has been updated accordingly: Header Field Name Protocol Status Reference Authentication-Info http standard of this document Proxy-Authentication-Info http standard of this document

This document is based on the header field definitions in RFCs 2069 and 2617, whose authors are: John Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott D. Lawrence, Paul J. Leach, Ari Luotonen, Eric W. Sink, and Lawrence C. Stewart. Additional thanks go to the members of the HTTPAUTH and HTTPBIS Working Groups, namely, Amos Jeffries, Benjamin Kaduk, Alexey Melnikov, Mark Nottingham, Yutaka Oiwa, Rifaat Shekh-Yusef, and Martin Thomson.