HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields
Response headers for authentication schemes to return server information after credentials are accepted.
Reschke, J., "HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields", RFC 7615, DOI 10.17487/RFC7615, September 2015, <https://www.rfc-editor.org/info/rfc7615>.
@misc{rfc7615,
series = {Request for Comments},
number = 7615,
howpublished = {RFC 7615},
publisher = {RFC Editor},
doi = {10.17487/RFC7615},
url = {https://www.rfc-editor.org/info/rfc7615},
author = {J. Reschke},
title = {{HTTP Authentication-Info and Proxy-Authentication-Info Response Header Fields}},
pagetotal = 6,
year = 2015,
month = sep,
} RFC text
Contents
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.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
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Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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Without obtaining an adequate license from the person(s) controlling
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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/
AI ELI5
On this page
§1The Need for Post-Authentication Communication
HTTP authentication schemes enable a client to prove its identity to a server. The basic flow is: client sends credentials, server validates them, and responds with the requested resource. But some schemes need to send information after accepting credentials—such as the server proving its identity to the client (mutual authentication), a session token, or parameters the client needs for the next request.
RFC 2617, which defined the HTTP Digest authentication scheme, included two header fields for this purpose: Authentication-Info and Proxy-Authentication-Info. This RFC generalizes those headers, extracting them from Digest-specific documentation so other authentication schemes can use them too.
When you show your password, sometimes the server needs to send you back more than just the webpage. It might prove to you that it's really the real server, or give you a ticket you'll need next time. This RFC says how the server should send that extra information.
§2The Authentication-Info Header Field
The Authentication-Info response header field is a list of parameters (name/value pairs), using the same format as authentication parameters in RFC 7235. Each parameter's name and meaning are defined by the specific authentication scheme in use. For example, the Digest scheme defines parameters such as nextnonce, qop, and rspauth in RFC 7616.
The header can be sent in any HTTP response, regardless of the request method or status code. Its semantics are entirely determined by the authentication scheme indicated in the client's original Authorization request header. A proxy forwarding the response must not modify the header field value.
Parameters can be expressed as either tokens or quoted strings, and authentication scheme definitions must allow both formats for interoperability.
The Authentication-Info header is a list of facts the server tells you after you log in. What facts it includes depends on the type of password scheme. The format is simple: pairs of name and value, either as plain words or quoted text.
§3Proxy-Authentication-Info for Proxy Authentication
The Proxy-Authentication-Info response header field has the same syntax and semantics as Authentication-Info, but applies specifically to proxy authentication (as opposed to origin-server authentication). It is sent by a proxy in response to a client authentication request and conveys information defined by the proxy's authentication scheme.
An important distinction: while Authentication-Info may be forwarded through the proxy hierarchy, Proxy-Authentication-Info applies only to the next outbound client on the response chain—only the client that directly chose that proxy is likely to have the credentials and need the information. However, in some scenarios with multiple proxies within a single administrative domain (such as office and regional caching proxies), each proxy may send the same field value, making it appear as if the header is being forwarded.
When you connect through a proxy server instead of directly to the website, the proxy might also ask for a password. If it does, it can send back Proxy-Authentication-Info to tell you extra things, just like the website does. But that information is only meant for you, not for proxies downstream.
§4Security and Privacy Implications
Adding information to HTTP responses sent over an unencrypted channel—such as HTTP without TLS—can affect security and privacy. The presence of either Authentication-Info or Proxy-Authentication-Info header fields alone indicates that HTTP authentication is in use, which may reveal information to passive observers.
The contents of authentication-scheme-specific parameters in these headers can expose sensitive information. Authentication scheme designers must carefully consider what information is safe to expose in these headers and may choose to restrict their use to encrypted channels. The RFC does not define specific parameters for these headers; each authentication scheme must document the parameters it uses and their privacy implications.
If you're not using a secure connection (HTTPS), the extra information the server sends back might be seen by someone listening to your connection. So scheme designers need to be careful about what they put in these headers. Sensitive information might need a locked connection.
§5Standardization and Current Use
RFC 7615 is a Standards Track document that updates RFC 2617. It generalizes the Authentication-Info and Proxy-Authentication-Info headers from the Digest authentication scheme to be usable by any HTTP authentication scheme. This allows future schemes to return post-authentication information in a consistent, standardized way.
The headers are registered in the IANA Message Headers registry. RFC 7615 was later updated by RFC 9110, which incorporates these definitions into the broader HTTP Semantics specification. Implementations should follow RFC 9110 for the current authoritative definition, though RFC 7615 remains a valid reference for the core concepts.
This RFC officially says that all password schemes can use these headers, not just the Digest scheme. It was later updated into a bigger RFC about HTTP, but the core idea is the same. Servers can now use this standard way to send back information after you log in.