Internet Engineering Task Force (IETF) J. Reschke
Request for Comments: 7615 greenbytes
Obsoletes: 2617 September 2015
Category: Standards Track
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
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 32. Notational Conventions . . . . . . . . . . . . . . . . . . . 33. The Authentication-Info Response Header Field . . . . . . . . 33.1. Parameter Value Format . . . . . . . . . . . . . . . . . 44. The Proxy-Authentication-Info Response Header Field . . . . . 45. Security Considerations . . . . . . . . . . . . . . . . . . . 46. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 57. References . . . . . . . . . . . . . . . . . . . . . . . . . 57.1. Normative References . . . . . . . . . . . . . . . . . . 57.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
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
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.
[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,
[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
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
Julian F. Reschke
Muenster, NW 48155