Skip to main content

HTTP Authentication with SASL

Document Type Active Internet-Draft (individual)
Author Rick van Rein
Last updated 2022-11-07
RFC stream (None)
Intended RFC status (None)
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
Network Working Group                                        R. Van Rein
Intended status: Standards Track                         8 November 2022
Expires: 12 May 2023

                     HTTP Authentication with SASL


   Most application-level protocols standardise their authentication
   exchanges under the SASL framework.  HTTP has taken another course,
   and often ends up replicating the work to allow individual
   mechanisms.  This specification adopts full SASL authentication into

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 12 May 2023.

Copyright Notice

   Copyright (c) 2022 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Van Rein                   Expires 12 May 2023                  [Page 1]
Internet-Draft                  HTTP SASL                  November 2022

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Embedding SASL in HTTP  . . . . . . . . . . . . . . . . . . .   3
     2.1.  HTTP Request and Response Messages  . . . . . . . . . . .   4
     2.2.  Authentication Field Definitions  . . . . . . . . . . . .   5
     2.3.  Caching Authentication Results  . . . . . . . . . . . . .   6
   3.  Server-Side User Name . . . . . . . . . . . . . . . . . . . .   6
   4.  Authentication Session Example  . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  HTTP Server Environment Variables  . . . . . . . . .  13
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   HTTP has historically followed its own path for client
   authentication, while many other end-user protocols standardised on
   SASL; examples of SASL protocols include SMTP, IMAP, POP, XMPP, LDAP,
   AMQP and MQTT.  This specification introduces SASL to HTTP, so it may
   share in past and future work done for SASL in general.

   Among the work that could be shared is backend authentication
   integration, which is possible due to protocol-independent SASL
   exchanges for any given method, making it easy to take them out of
   one protocol and inserting them into another.  Although HTTP has
   adopted several SASL-compatible authentication methods, it uses
   various notations and so it still needs method-specific support at
   the HTTP level to translate them to a SASL backend.

   In front-ends, a similar situation has arisen.  The varying syntaxes
   for authentication methods have made it difficult to rely on support
   in most or all HTTP clients.  When such clients could externalise
   their SASL handling to generic software such as a SASL library, then
   any extension to a library automatically spills over into the HTTP
   sphere.  It is common for developers of web clients to also produce
   email clients, so a shared code base (and credential store) is not
   difficult to imagine.

   Sharing of authentication mechanisms is beneficial in both
   directions.  HTTP benefits by being able to use anything from strong
   password mechanisms [RFC5802] without explicit support [RFC7804] in
   applications, up to GS2 mechanisms [RFC5801] with channel binding
   [RFC5056] [RFC5554] to TLS [RFC5929] based on pinning either the

Van Rein                   Expires 12 May 2023                  [Page 2]
Internet-Draft                  HTTP SASL                  November 2022

   certificate for the TLS server or even a unique part of the
   individual TLS connection; for instance Kerberos5 [RFC4120] currently
   uses Negotiate authentication [RFC4559] which is not as secure as
   GS2-KRB5-PLUS over SASL.

   SASL also benefits; had it been the norm for HTTP, then the work to
   pass SAML over it [RFC6595] would probably have been done
   immediately.  In fact, HTTP can still benefit from receiving
   standardised SAML20 inquiries over SASL, because it resolves the need
   for configuration of initiation paths and practices.  Also, it
   removes authentication data from URIs, where they are not ideally

   In terms of security for HTTP applications, it appears beneficial to
   have very good authentication capabilities in the layers below the
   application; this is specifically true for applications developed in
   HTML and JavaScript, which tend to load code from various places,
   including code that is not always in the end user's interest; since
   it already is a concern what identity information passes through
   these applications, it is not advisable to use credentials in those
   places.  The HTTP layer is in a better position to take control over
   these assets, at the protocol levels of HTTP and TLS, and conceal
   credentials and possibly also identity from applications running on
   top.  Inasfar as tokens are needed, they can be derived from session
   keys using generally accepted key derivation schemes, but the session
   keys can be isolated from dynamic layers above HTTP.

2.  Embedding SASL in HTTP

   This specification integrates the SASL framework [RFC4422] into
   mainstream HTTP [RFC7231], [RFC7232].  The SASL Authentication scheme
   follows the general structure for HTTP Authentication [RFC7235].  It
   uses the WWW-Authenticate and Proxy-Authenticate headers in responses
   from web servers and web proxies, respectively, and correspondingly
   the Authorization and Proxy-Authorization request header to answer to

   The SASL service name for the following embedding of SASL is HTTP;
   contrary to most other service names, it is spelled in uppercase, in
   line with what has become general practice in Kerberos and GSSAPI.

   Since SASL prescribes channel binding to occur relative to TLS
   instead of to the application protocol, we can add that when the
   HTTPS transport is used.  Whether channel binding is used SHOULD
   remain a configuration choice in HTTP software, as it might interfere
   with intentional HTTPS proxying.  Unintended proxying on the other
   hand, might lead to tapping of credentials under certain SASL
   mechanisms, and it may be considered helpful to prevent such

Van Rein                   Expires 12 May 2023                  [Page 3]
Internet-Draft                  HTTP SASL                  November 2022

   situations by requiring channel binding for those situations.  HTTP
   in general allows a user session to hop between connections, and
   browsers are likely to do this; to support this, the support of tls-
   server-end-point channel binding [Section 4 of [RFC5929]] is
   RECOMMENDED.  Specific HTTP clients may exercise more control over
   connections to achieve stronger security; for those use cases, tls-
   unique channel binding [Section 3 of [RFC5929]] is RECOMMENDED.
   Generic web servers SHOULD support both forms of channel binding.

2.1.  HTTP Request and Response Messages

   This section defines a few names for HTTP request and response
   messages, to be used in the remainder of this specification.

   Initial Responses are HTTP responses that normally set a status code
   401 or 407, and that are sent when the HTTP server decides to
   initiate an authentication exchange.  In addition, the server MAY
   send Initial Responses in other responses, to indicate to the client
   that it MAY try again to achieve better results [Section 4.1 of

   Initial Requests are those HTTP requests that a client sends to
   initiate a fresh SASL authentication.  The identity SHOULD be
   selected by the user independently from the URI; prior settings MAY
   however be remembered by a client for the combination of resource
   authority (scheme, host and possibly a separately communicated
   resource user name) with the server-sent realm string.  The server
   can support a mixture of client identities for various roles or
   access levels through variation of realm strings.  There is no
   current practice of server-side resource names in HTTP, but the
   generic URI schema presents this logic and it is easy to imagine an
   HTTP User header that a client could support.

   Intermediate Responses are HTTP responses to SASL authentication,
   with a status code set to 401 or 407.  Intermediate Requests are
   those HTTP requests that a client sends to continue a SASL
   authentication after an Intermediate Response.

   Positive Responses set a 200 status code to depict success.
   Information in this response is provided in an Authentication-Info or
   Proxy-Authentication-Info header [RFC7615] instead of the headers
   used in Initial Responses and Intermediate Responses [RFC7235].
   Proper interpretation of a Positive Response requires client state
   indicating that SASL authentication was used, or else the optional
   fields are not completely reliable information sources; cryptographic
   markers in the c2c field MAY be used to overcome this in a manner
   that defies abuse by rogue servers.

Van Rein                   Expires 12 May 2023                  [Page 4]
Internet-Draft                  HTTP SASL                  November 2022

   Negative Responses also set a 401 or 407 status code and will often
   return the client to an earlier state that it recognises as one it
   has tried before.  These responses should therefore offer
   authentication to start again.  In contrast to the Initial Response,
   there is now a c2c field that helps the client evaluate the request.

   The following fields, defined in upcoming sections, MUST and MAY be
   present in HTTP authentication exchanges for SASL:

   Request or Response   | MUST have fields | MAY have fields
   Initial Response      | s2s,mech         |     realm
   Initial Request       |     mech         | c2s,realm,s2s
   Intermediate Response | s2s              | s2c
   Intermediate Request  | s2s              | c2s
   Positive Response     |                  | s2s
   Negative Response     | s2s,mech         |     realm

2.2.  Authentication Field Definitions

   Data for SASL is transported in the following fields:

   c2s   holds SASL token data from client to server.  This field is
         transmitted with base64 encoding.  The field is absent when the
         SASL client sends no token.

   s2c   holds SASL token data from server to client.  This field is
         transmitted with base64 encoding.  The field is absent when the
         SASL server sends no token.

   s2s   holds opaque server data which the client MUST reflect in
         Intermediate Requests and, when responding to an Initial
         Response, in the Initial Request.  This is a necessity for a
         stateless HTTP Authentication framework [Section 5.1.2 of
         [RFC7235]].  It MAY be used in a Positive Response to pass a
         cacheable Section 2.3 authentication token in a future Initial

   The following fields support SASL within the HTTP Authentication

   realm  optionally names a scope of authorisation under the
         combination of scheme, server host name and possibly a HTTP
         user to implement the semantics of the generic URI username for
         resource selection.  The realm does not necessarily match a
         domain name, which is used elsewhere as a realm notation.

   mech  In an Initial Response, the field is filled with a space-

Van Rein                   Expires 12 May 2023                  [Page 5]
Internet-Draft                  HTTP SASL                  November 2022

         separated list of SASL mechanism names; In an Initial Request,
         the client chooses one SASL mechanism name.

2.3.  Caching Authentication Results

   When an HTTP server sends a Positive Response, it MAY include an
   "s2s" field.  If it does this, then it should be prepared to accept
   the field value for authentication in an Initial Request.  However,
   credentials can expire or fall in disgrace for other reasons, so the
   server MAY still choose to reject the provided field.

   When an HTTP client receives a Positive Response with an "s2s" field,
   it MAY memorise the fields for future reuse in an Initial Request,
   either with or without preceding Initial Response from the server.
   The HTTP client MUST use the realm as part of the decision which
   cached result to use, but it MAY extrapolate the results from one
   resource retrieval in an attempt to authenticate another.

   When cached fields result in a Negative Response then the HTTP client
   SHOULD remove the failing cache entry, and it SHOULD try again by
   going through a full SASL authentication cycle.  The stateless nature
   of HTTP authentication is helpful in the sense that a new Initial
   Request can be sent to an older Initial Response.

3.  Server-Side User Name

   HTTP does not define a mechanism to specifically select the user as
   an authoritative resource name space on the server.  Local syntax
   conventions exist, but lack universally reliable semantics.  Basic
   authentication has been used to this effect, but this conflates the
   client identity with the server-side name space, which is not
   necessarily the same.

   To allow HTTP servers to zoom in on user-specific information, the
   User header is hereby introduced.  Its syntax matches the userinfo
   part of a URI, up to but excluding any colons in it:

   User = *( unreserved / pct-encoded / sub-delims )

   The value of the header MUST be percent-decoded before the server can
   use it to identify a local user.

   The User header MAY be sent by clients, and HTTP servers MAY ignore
   it for any reason, including local user identities that do not comply
   to a more restrictive local user name syntax.

Van Rein                   Expires 12 May 2023                  [Page 6]
Internet-Draft                  HTTP SASL                  November 2022

   When an HTTP server makes use of the User header, it MUST include a
   Vary header in its response, with either a single "*" in it or the
   name "User".  This informs caches that the response must be
   considered specific to the User header value in the matching request.

   The User header may be used with or without any form of
   authentication.  When used with authentication, the value of the
   percent-decoded header is considered part of the authority component
   of the resource, and therefore of the naming scope for the realm.
   Clients can use this refined notion of realm to select an
   authentication identity; when the value is known early enough, this
   may even help to select an X.509 client certificate.  Note that the
   User header might be used together with the aforementioned practice
   of Basic authentication, but it can also replace it with an even
   simpler mechanism to free up the authentication exchange for HTTP

   The distinction of a client-side user from a server-side user can
   benefit the use of credential schemes that are not tied to the HTTP
   server.  A specific example of this is the current work on realm
   crossover with GS2-SXOVER-PLUS.  The use of such a mechanism may
   offload security concerns from the application layer.

4.  Authentication Session Example

   This section is non-normative.

   When an HTTP server receives a request for a protected page, it will
   send an Initial Response to ask for authentication with a special
   status code 401; for proxy access that would be 407, and header names
   change accordingly.  Stripped down to the bare essentials, the server
   sends (this section adds whitespace for clarity)

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: SASL
       realm="members only",
       mech="SCRAM-SHA-256 SCRAM-SHA-256-PLUS
             SCRAM-SHA-1 SCRAM-SHA-1-PLUS
             GS2-KRB5-PLUS GS2-KRB5",

   The server offers SCRAM-* and GS2-KRB5 mechanisms.  The variants with
   -PLUS provide additional channel binding, to ensure that
   authentication is specific to the current HTTPS connection, thus
   avoiding replay of the session across connections.  Clients aware of
   HTTP connections may use connection-specific channel binding (tls-
   unique) while those that abstract from the connections must resort to
   weaker name-based channel binding (tls-server-end-point).

Van Rein                   Expires 12 May 2023                  [Page 7]
Internet-Draft                  HTTP SASL                  November 2022

   The server might have additionally offered the ANONYMOUS mechanism to
   allow the client to select "guest mode" access; the interaction would
   continue as authenticated, but presumably with limited access to HTTP
   resources and continued WWW-Authenticate headers to continue to offer
   authentication to improve resource information content.  The server
   might have offered EXTERNAL to allow the client to incorporate a TLS
   credential for authentication and possibly change to an authorization
   identity.  The server might have offered GS2-SXOVER-PLUS if it is
   willing to connect to the client's home realm over Diameter, and
   thereby support realm crossover of SASL credentials.

   The client initiates the SCRAM-SHA-256-PLUS mechanism, and to that
   end sends an Initial Request (this section shows square brackets
   around text that is transmitted with base64-encoding)

   Authorization: SASL
       realm="members only",

   This mechanism is initiated by the client, hence the inclusion of the
   c2s token in the Initial Request.  The contents of this field are
   specific to the selected mechanism, so SCRAM-SHA-256-PLUS in this

   The SCRAM mechanism implementation is now initiated with the c2s
   token, and the server produces a followup challenge in a s2c token.
   To be able to validate future client messages against server-side
   state, it includes such state in an s2s token.  This token is
   presumably protected from abuse with a signature and/or encryption,
   and it would likely identify the selected mechanism to validate
   during later rounds.  The server packs all this in an Intermediate

   HTTP/1.1 401 Unauthorized
   WWW-Authentication: SASL

   Given that all server state is contained in this message, the client
   is free at any time to give up authentication and perhaps try another
   method.  Normally however, it would proceed with the ongoing

Van Rein                   Expires 12 May 2023                  [Page 8]
Internet-Draft                  HTTP SASL                  November 2022

   The SCRAM mechanism continues with another round.  The client engages
   in the prescribed cryptographic computations and packs an
   Intermediate Request along with updated state in the new c2s token

   Authorization: SASL

   When the client has performed authentication properly, as determined
   by a server-side check of the c2s response token with the prior state
   in the s2s token, it can send a Positive Response along with the
   requested resource

   HTTP/1.1 200 OK
   WWW-Authentication: SASL

   The s2s token in a Positive Response is an optional extension.  It is
   presented by the server to allow the client to speed up
   authentication in future requests.  The client may send it whenever
   the server asks for the same realm string under the same scheme and
   authority; the client may make proactive assumpions about the realm
   string for new requests.  Authentication must never be reused in
   another context than bound by channel binding.  When used, the client
   immediate sends an Intermediate Response holding

   Authorization: SASL
       realm="members only",

   The server always has an option to refuse repeated authentication and
   forcing the client into a new authentication round.  One reason for
   this could be that a session timed out.  Another might be that the
   client is trying to use a credential outside a scope set by channel

5.  Security Considerations

   It is not generally safe for SASL mechanisms to exchange c2s and s2c
   messages over unprotected transports.  Furthermore, the SASL exchange
   may be at risk of tampering when the sequence of HTTP messages is not
   secured to form one stream.  This means that a secure transport layer
   must be used, like TLS.  The termination of such a secure layer MUST
   also terminate any ongoing SASL handshakes.

Van Rein                   Expires 12 May 2023                  [Page 9]
Internet-Draft                  HTTP SASL                  November 2022

   The s2s field MUST be protected against tampering by rogue peers, and
   such protection also protects against tampering by rogue
   intermediates when using an unprotected transport.  In addition, but
   dependent on the mechanism used, the s2s field may also need
   encryption to conceal their data from peers and intermediates.

   SASL EXTERNAL can be a very efficient mechanism to combine with a
   secure transport layer if that includes authentication.  This may be
   the case for TLS, especially when client-side authentication is
   deployed.  Mechanisms other than EXTERNAL should take into account
   that a relation may exist between identities negotiated in the
   protective layer and the SASL exchange over HTTP.  For example, a
   login account may be exchanged for an alias or group identity.

   Channel binding is available in some SASL mechanisms.  When used with
   HTTP SASL over TLS, it binds to the TLS channel.  When doing so, it
   is vital that either there be no renegotiation of the TLS handshake,
   or both secure renegotiation [RFC5746] and the extended master secret
   [RFC7627] are used.

   The User header field as defined herein is orthogonal to issues of
   authentication and authorisation, and adds no security concerns.

6.  IANA Considerations

   This specification extends the "Hypertext Transfer Protocol (HTTP)
   Authentication Scheme Registry" with an "Authentication Scheme Name"
   SASL, referencing this specification.

   This specification defines an additional entry in the registry
   "Generic Security Service Application Program Interface
   (GSSAPI)/Kerberos/Simple Authentication and Security Layer (SASL)
   Service Names" namely:

   Service Name: HTTP
   Usage:        Web authentication using the SASL framework
   Reference:    TBD:this specification

   The capitalisation of the service name has historic origins and is
   now the preferred spelling for reasons of compatibility.

   Please add the following entry to the Message Headers registry:

   Header Field Name   Template   Protocol   Status    Reference
   ------------------  ---------  ---------  -------   ----------
   User                           http       TBD       TBD:THIS_SPEC

7.  References

Van Rein                   Expires 12 May 2023                 [Page 10]
Internet-Draft                  HTTP SASL                  November 2022

7.1.  Normative References

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

   [RFC5554]  Williams, N., "Clarifications and Extensions to the
              Generic Security Service Application Program Interface
              (GSS-API) for the Use of Channel Bindings", RFC 5554,
              DOI 10.17487/RFC5554, May 2009,

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
              July 2010, <>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,

   [RFC6595]  Wierenga, K., Lear, E., and S. Josefsson, "A Simple
              Authentication and Security Layer (SASL) and GSS-API
              Mechanism for the Security Assertion Markup Language
              (SAML)", RFC 6595, DOI 10.17487/RFC6595, April 2012,

Van Rein                   Expires 12 May 2023                 [Page 11]
Internet-Draft                  HTTP SASL                  November 2022

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,

7.2.  Informative References

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,

   [RFC7804]  Melnikov, A., "Salted Challenge Response HTTP
              Authentication Mechanism", RFC 7804, DOI 10.17487/RFC7804,
              March 2016, <>.

              van Rein, R., "Declaring Kerberos Realm Names in DNS
              (_kerberos TXT)", Work in Progress, Internet-Draft, draft-
              vanrein-dnstxt-krb1-10, 21 October 2022,

Van Rein                   Expires 12 May 2023                 [Page 12]
Internet-Draft                  HTTP SASL                  November 2022

Appendix A.  HTTP Server Environment Variables

   We define a number of variables that SHOULD be passed from an HTTP
   SASL stack (and from User header processing) to applications run on
   top of it.  The intention of defining these is to obtain maximum
   interoperability between these layers of software.

   The following variables MUST NOT be available until SASL
   authentication is successful; it would be available when the server
   could send a 200 OK response:

   SASL_SECURE  is only "yes" (without the quotes) when a client is
         authenticated to the current resource.  It never has another
         value; it is simply undefined when not secured by SASL.

   SASL_REALM  is the realm for which the secure exchange succeeded.  A
         realm is not always used, because sites only need it when there
         are more than one in the same name space.  When undefined in
         the SASL flow, this variable will not be set.

   REMOTE_USER  is the client identity as confirmed through SASL
         authentication.  Its content is formatted like an email
         address, and includes a domain name.  That domain need not be
         related to the web server; it is possible for a web server to
         welcome foreign clients.

   SASL_MECH  indicates the mechanism used, and is one of the
         standardised SASL mechanism names.  It may be used to detect
         the level of security.

   SASL_S2S  holds the accepted s2s field, and could be used as a random
         session identifier.  It would normally be encrypted

   SASL_S2S_  is a prefix for extra information that the server may
         extract from the s2s field in the HTTP SASL protocol flow.
         This depends on the authentication stack used in the web

   The following variable SHOULD be available while processing a request
   with a User header with locally acceptable syntax:

   LOCAL_USER  gives the HTTP User header value after syntax checking
         and percent-decoding.  If used at all, it MUST be treated as a
         resource name space selector.  This header does not describe
         the authenticated client identity, which is usually passed in a
         variable REMOTE_USER.

Van Rein                   Expires 12 May 2023                 [Page 13]
Internet-Draft                  HTTP SASL                  November 2022

Appendix B.  Acknowledgements

   Thanks to Henri Manson for making the first implementation of this
   specification and for feedback on the header formats.  The
   specification also benefited from input by Daniel Stenberg.

   This work was supported with an open source development fund from

Author's Address

   Rick van Rein
   Haarlebrink 5

Van Rein                   Expires 12 May 2023                 [Page 14]