HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields

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.¶

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This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.¶

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This specification defines the "Authentication-Info" and "Proxy-Authentication-Info" response header fields for use in HTTP authentication schemes ([RFC7235]) that need to return information once the client's authentication credentials have been accepted.¶

Both were previously defined in Section 3 of [RFC2617], defining the HTTP "Digest" authentication scheme. This document generalizes the description for use not only in "Digest" ([RFC7616]), 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 [RFC5234] with a list extension, defined in Section 7 of [RFC7230], 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 Section 2.1 of [RFC7235].¶

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 Section 2.1 of [RFC7235]. 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 Section 3.5 of [RFC7616].¶

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 ([RFC7235], Section 4.2) 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 ([RFC7230], Section 4.1.2) when the authentication scheme explicitly allows this.¶

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 <http://www.iana.org/assignments/message-headers>, as defined by [BCP90].¶

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.¶