Internet Engineering Task Force (IETF)                        J. Reschke
Request for Comments: 7615                                    greenbytes
Obsoletes: 2617                                           September 2015
Category: Standards Track
ISSN: 2070-1721


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

Abstract

   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.

Status of This Memo

   This is an Internet Standards Track document.

   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.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7615.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  The Authentication-Info Response Header Field . . . . . . . .   3
     3.1.  Parameter Value Format  . . . . . . . . . . . . . . . . .   4
   4.  The Proxy-Authentication-Info Response Header Field . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   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.

2.  Notational Conventions

   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].

3.  The Authentication-Info Response Header Field

   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].

     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 ([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.

3.1.  Parameter Value Format

   Parameter values can be expressed either as "token" or as "quoted-
   string" (Section 3.2.6 of [RFC7230]).

   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.

4.  The Proxy-Authentication-Info Response Header Field

   The Proxy-Authentication-Info response header field is equivalent to
   Authentication-Info, except that it applies to proxy authentication
   ([RFC7235], Section 2) and its semantics are defined by the
   authentication scheme indicated by the Proxy-Authorization header
   field ([RFC7235], Section 4.4) 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.

5.  Security Considerations

   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.

6.  IANA Considerations

   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 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 | Section 3 of    |
   |                           |          |          | this document   |
   | Proxy-Authentication-Info | http     | standard | Section 4 of    |
   |                           |          |          | this document   |
   +---------------------------+----------+----------+-----------------+

7.  References

7.1.  Normative References

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

7.2.  Informative References

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004, <http://www.rfc-editor.org/info/bcp90>.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <http://www.rfc-editor.org/info/rfc2617>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <http://www.rfc-editor.org/info/rfc7616>.

Acknowledgements

   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.

Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Email: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/