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

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.

Copyright Notice

Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

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/