INTERNET-DRAFT Magnus Nystrom
July, 2004 RSA Security
Expires: January, 2005 Alexey Melnikov
Intended category: Standards track Isode Ltd.
SASL in HTTP/1.1
draft-nystrom-http-sasl-12.txt
Status of this Memo
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This memo suggest the use of SASL [RFC2222] as a framework to enable
the use of strong authentication mechanisms in HTTP/1.1 [RFC2616],
and describes one approach to accomplish this.
Please send comments on this document directly to authors or to the
relevant mailing lists, e.g. ietf-sasl@imc.org.
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Table of contents
1 Conventions used in this memo ... ......................... 3
2 Introduction .............................................. 4
2.1 The HTTP/1.1 challenge-response framework ................ X
3 Relationship with the HTTP/1.1 specification .............. X
4 SASL framework ............................................ 4
4.1 Introduction and examples ................................ X
4.1.1 Introduction ........................................... X
4.1.2 Example sequence diagrams .............................. X
4.2 SASL authentication scheme ............................... X
4.2.1 Recognition of the scheme .............................. X
4.2.2 SASL authentication response header sent by server ..... X
4.2.3 SASL authorization request header sent by client ....... X
4.3 Usage model .............................................. X
4.3.1 SASL handshake initiation .............................. X
4.3.2 Client response ........................................ X
4.3.3 Server behavior upon receiving a "SASL" <auth-scheme>
token ................................................. XX
4.3.4 Client behavior upon receiving a "SASL" <auth-scheme>
token ................................................. XX
4.3.5 Subsequent requests ................................... XX
4.3.6 Client aborting a handshake ........................... XX
4.3.7 Pipelining considerations ............................. XX
4.3.8 Caching considerations................................. XX
4.3.9 "Web farm" considerations ............................. XX
4.3.10 HTTP header and state management ..................... XX
4.4 Request/response encoding ............................... XX
4.4.1 SASL challenge/response encoding ...................... XX
4.4.2 Security layer......................................... XX
4.4.3 Interaction with TLS................................... XX
4.4.4 Mandatory to implement SASL mechanism.................. XX
4.5 Status codes and error handling ......................... XX
4.5.1 HTTP/1.1 status codes.................................. XX
4.5.2 Client error handling.................................. XX
4.6 Authorization identity .................................. XX
4.7 Examples ................................................ XX
4.7.1 Example 1 - Server requires authentication ............ XX
4.7.2 Example 2 - Initial response .......................... XX
4.7.3 Example 3 - One mechanism only ........................ XX
4.7.4 Example 4 - Server sends additional data .............. XX
4.7.5 Example 5 - Abort ..................................... XX
4.7.6 Example 6 - Client requires authentication ............ XX
4.7.7 Example 7 - Client requires authentication, server
supports multiple realm ............................... XX
4.7.8 Example 8 - Client uses POST request .................. XX
4.7.9 Example 9 - Client authentication fails ............... XX
4.8 Interoperability with existing HTTP/1.1 clients and
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servers ................................................. XX
4.9 Preferences ............................................. XX
4.10 SASL mechanism recommendations ......................... XX
5 IANA considerations ...................................... XX
5.1 GSSAPI/SASL service name ................................ XX
5.2 HTTP/1.1 Status codes ................................... XX
6 Security considerations .................................. XX
6.1 Introduction ............................................ XX
6.2 Active attacks .......................................... XX
6.2.1 Man-in-the-middle ..................................... XX
6.2.2 Denial of service ..................................... XX
6.2.3 Replay ................................................ XX
6.3 Passive attacks ......................................... XX
6.4 Protecting the body of POST/PUT requests ................ XX
6.5 Other considerations .................................... XX
7 Implementation considerations (informative)............... XX
7.1 The SASL authentication exchange context ................ XX
7.2 SASL security layer handling ............................ XX
7.3 SASL Profile Checklist .................................. XX
8 Acknowledgements ......................................... XX
9 References ............................................... XX
9.1 Normative references .................................... XX
9.2 Informative references .................................. XX
10 Authors' addresses ...................................... XX
11 IPR Disclosure Acknowledgement .......................... XX
12 Intellectual Property Statement ......................... XX
13 Full Copyright Statement ................................ XX
Appendix A. Changes since previous revisions ................ XX
1 Conventions used in this memo
The keywords "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in
this document are to be interpreted as defined in "Key words for use
in RFCs to Indicate Requirement Levels" [RFC2119].
General understanding of SASL [RFC2222] is required before reading of
this document. [RFC2222] defines several terms used through out this
document, in particular "authorization identity" and "security
layer".
In the examples, "C:" and "S:" indicate lines sent by a client and a
server respectively; "CP:" and "SP:" indicate lines sent by a client
and a server respectively with a SASL security layer active.
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2 Introduction
The Hypertext Transfer Protocol, HTTP/1.1 [RFC2616], supports only
two authentication schemes, namely the "Basic Access Authentication
Scheme" and the "Digest Access Authentication Scheme" [RFC2617].
Neither of these can be considered to be strong authentication
schemes. The former is extremely insecure unless used in conjunction
with a lower-level protocol offering security services, since it
sends cleartext passwords. The latter is an improvement, but is still
vulnerable to man-in-the-middle attacks.
The Simple Authentication and Security Layer (SASL [RFC2222])
provides a method for adding authentication and security services to
connection-oriented protocols in a flexible manner, enabling a
variety of authentication and security mechanisms (e.g. mechanisms
based on one-time-passwords, public key technology or password-based
public-key cryptography), and also a flexible means to negotiate
these mechanisms subject to local policies and security requirements.
This memo therefore suggests a method to use SASL in HTTP/1.1 and
solicit comments on the suggested approach.
This document is using the HTTP/1.1 challenge-response framework to
implement SASL in HTTP/1.1. The challenge-response framework is
outlines in Section 2.1.
2.1 The HTTP/1.1 challenge-response framework
HTTP/1.1 provides a simple challenge-response mechanism that can be
used by a server or proxy to challenge a client request and by a
client to provide authentication information. The reader is referred
to [RFC2616] and [RFC2617] for a more detailed description of this
mechanism. The relevant ABNF productions are:
challenge = auth-scheme 1*SP 1#auth-param
auth-scheme = token
auth-param = token "=" (token | quoted-string)
The challenge will be found in a WWW-Authenticate or a Proxy-
Authenticate header field.
The client response, containing the client's credentials is defined
as follows:
credentials = auth-scheme 1*SP 1#auth-param
The response will be found in an Authorization or a Proxy-
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Authorization header field.
3 Relationship with the HTTP/1.1 specification
This memo relies on the HTTP/1.1 [RFC2616] specification. As with RFC
2616, it uses the ABNF [RFC2234] grammar of that document and relies
on both non-terminals and other aspects of it.
Further, this memo REQUIRES persistent connections whenever a SASL
security layer (see Section 4.4.2) is negotiated. Note, that a SASL
security layer is an optional (to negotiate) feature of SASL,
however, once negotiated it can't be turned off (or not used), until
a subsequent reauthentication completes successfully on the same TCP
connection. It is also RECOMMENDED to use a persistent connection
while performing a SASL authentication exchange. See also Section
4.3.10 for additional discussions of this issue.
4 SASL framework
4.1 Introduction and examples
4.1.1 Introduction
The SASL protocol itself is relatively straightforward. It consists
of a number of exchanges between the client and the server.
Typically, the initial exchange negotiates the authentication
mechanism and then remaining exchanges actually authenticate the
client to the server.
The following figure shows, in schematic fashion a typical SASL
authentication handshake which authenticates the client using the
CRAM-MD5 mechanism. See Section 4.7.1 for the detailed example on how
this will look in HTTP.
Client Server
------ ------
<- Please authenticate, I speak
CRAM-MD5, GSSAPI, and
DIGEST-MD5
I choose CRAM-MD5 ->
<- Go ahead, your challenge
is "abcdef"
I am user "user8", and ->
my response is
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"0123456789ABCDEF"
<- Ok user "user8", your are
authenticated.
Note that other mechanisms may require a larger number of round
trips.
This document describes how to use SASL as an authentication
mechanism for HTTP. Standard HTTP authentication headers are used,
but they contain SASL data. SASL messages sent by the client are
carried in the Authorization header. SASL messages sent by the server
are carried in the WWW-Authenticate header.
4.1.2 Example sequence diagrams
Server initiated authentication:
Client Server
----------------- Initial Request ----------------------->
<------ 401 WWW-Authenticate SASL (mechanisms,realm,id) --
--- Authorization (mechanism,id[,realm]) ---------------->
<------ 401 WWW-Authenticate SASL (id,challenge) ---------
--- Authorization (id,credential)------------------------>
[
<------ 401 WWW-Authenticate SASL (id,challenge) ---------
--- Authorization (id,credential)------------------------>
](0 or more times depending on the SASL mechanism)
<------ 235 WWW-Authenticate SASL (id) -------------------
----------------- Initial Request (retry) --------------->
<------ 200 Server performs the requested operation ------
Client initiated authentication:
Client Server
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--- OPTIONS request with Authorization ([realm]) -------->
<------ 401 WWW-Authenticate SASL (mechanisms,realm,id) --
--- Authorization (mechanism,id) ------------------------>
<------ 401 WWW-Authenticate SASL (id,challenge) ---------
--- Authorization (id,credential)------------------------>
[
<------ 401 WWW-Authenticate SASL (id,challenge) ---------
--- Authorization (id,credential)------------------------>
](0 or more times depending on the SASL mechanism)
<------ 235 WWW-Authenticate SASL (id) -------------------
----------------- Initial Request ----------------------->
<------ 200 Server performs the requested operation ------
<<All subsequent requests are carried out as usual.>>
4.2 SASL authentication scheme
4.2.1 Recognition of the scheme
A server MUST use the auth-scheme token "SASL" if it supports SASL
and is willing to perform authentication using a SASL-based
mechanism.
4.2.2 SASL authentication response header sent by server
For the "SASL" <auth-scheme>, the authentication response header is
as follows:
challenge = SASL 1*SP sasl-response-parameters
sasl-response-parameters
= [sasl-mechanisms WSAC] [realm WSAC] sasl-sid
[WSAC sasl-challenge] [WSAC sasl-status]
[WSAC http-authzid]
sasl-mechanisms = "mechanisms" "=" <"> 1#sasl-mech-name <">
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realm = "realm" "=" quoted-string
; See RFC 2617
sasl-sid = "id" "=" quoted-string
sasl-challenge = "challenge" "=" <"> base64-string <">
sasl-status = "status" "=" quoted-string
http-authzid = "http-authzid" "=" sasl-authzid
sasl-authzid = <"> URI <">
; Usually a URI using the "http" scheme.
; URI is defined in [RFC2396]
sasl-mech-name = 1*20 SASLCHAR
; Name must be from IANA set of registered SASL mechanisms,
; e.g. "SECURID"
base64-string = *base64-group [base64-fingroup]
; Encoding must be in accordance with Section 3 of [RFC3548],
; except not limited to 76 chars/line.
; Spaces are not allowed.
base64-group = 4*BASE64
base64-fingroup = 4*BASE64 | (3*BASE64 "=") | (2*BASE64 "==")
SASLCHAR = UPALPHA | DIGIT | "-" | "_"
; Characters allowed in SASL mechanism name
BASE64 = DIGIT | ALPHA | "+" | "/"
WSAC = *LWS "," *LWS
Note: All directives ("mechanisms", "id", "realm", "challenge", etc.)
are case-insensitive. All directive values are case-sensitive.
The meanings of the values of the directives used above are as
follows:
sasl-mechanisms
A list of registered SASL mechanisms acceptable to the
server. MUST be sent by the server unless a mechanism already has
been agreed upon (see example 2 in Section 4.7.2). A server should
list supported SASL mechanisms in its preferred order - from the
most preferred to the least preferred. However a client MUST NOT
blindly trust the order of the mechanisms in the received
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sasl-mechanisms directive. The client must enforce own mechanism
selection policy first, e.g. "only use mechanisms that provide
mutual authentication", and only use the order specified by the
server if everything else is equal.
realm
As defined in [RFC2617]. The directive MUST be present in initial
challenges and when the realm otherwise would not be known by the
client.
sasl-sid
A session identifier identifying a particular SASL authentication
exchange (handshake) context (see also Section 7.1). MUST always be
present. Sasl-sids are chosen by the server and at any given point
in time MUST be unique for each established connection.
sasl-challenge
A Base64-encoded challenge (or server credentials, at the end
of an authentication exchange) in accordance with a selected SASL
mechanism. MUST NOT be sent unless there is exactly one SASL
mechanism in the <sasl-mechanisms> directive.
sasl-status
A string indicating the resulting status of a SASL authentication
exchange. For this version of this profile, this parameter is only
used when client authentication has failed, in which case the
parameter's value shall be "failed" (see further Section 4.3.3).
http-authzid
Upon successful authentication the server MAY (and if the client
specified options="http-authzid" the server MUST) return the
resulting protocol-specific authorization identifier for the
authenticated client. The returned identifier informs the client of
the established HTTP/1.1 authorization identity.
4.2.3 SASL authorization request header sent by client
For the SASL scheme, the authorization request header is as follows:
credentials = SASL [1*SP sasl-request-parameters]
sasl-request-parameters
= [sasl-mechanism WSAC] [sasl-sid WSAC]
[realm WSAC] [sasl-options WSAC] [sasl-
credentials]
sasl-mechanism = "mechanism" "=" <"> sasl-mech-name <">
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sasl-credentials = "credentials" "="
<"> (base64-string <"> | cancel-token) <">
sasl-options = "options" "=" <"> 1#token <">
cancel-token = "*"
The meanings of the values of the directives used above are as
follows:
sasl-mechanism
A SASL mechanism acceptable to the client, chosen from the list
provided by the server or set by some configuration. MUST be sent
by the client unless a mechanism already has been agreed upon.
sasl-sid
A session identifier identifying a particular SASL authentication
exchange context, previously set by a server. MUST always be sent
by
the client except for the case of "initial responses," see Section
4.3.1 below.
realm
As defined in [RFC2617]. MUST always be sent by the client unless
the realm is possible to determine by other means.
sasl-credentials
Base64-encoded credentials in accordance with a selected SASL
mechanism, or a <cancel-token> ("*"). MUST be sent if a
<sasl-challenge> directive has been received by the client.
sasl-options
Allows the client to request SASL specific options. Currently
only a single option "http-authzid" is defined. Sending of the
"http-authzid" option instructs the server to return a
<http-authzid> directive upon successful authentication (see also
Section 4.2.2). The "http-authzid" option MUST only be sent in the
request in which the client selects the authentication mechanism to
be used. Other options may have other restrictions.
4.3 Usage model
4.3.1 SASL handshake initiation
4.3.1.1 Server initiated authentication
When a client makes a request for a resource on a server that
requires SASL-based authentication, the server MUST respond with a
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401 - Unauthorized (407 - Proxy Authentication Required) response
including a WWW-Authenticate (or Proxy-Authenticate) header field
that contains a "SASL" <auth-scheme>.
The server MUST list all supported and acceptable SASL mechanisms in
the <sasl-mechanisms> directive. If the server only supports one SASL
mechanism, it MAY include a <sasl-challenge> directive in order to
reduce the number of roundtrips (see the example in Section 4.7.3).
The server MUST include a <sasl-sid> directive to identify the
particular authenticaton exchange context. This value MUST be the
same for all messages associated with that authentication exchange.
Further, the server MUST include a <realm> directive in accordance
with [RFC2617], however if a particular SASL mechanism defines its
own "realm" as a part of its authentication exchange, the mechanism
specific version of "realm" MUST be used by the mechanism.
If the server supports multiple realms for the requested resource, it
MUST return multiple SASL challenges formatted as described above,
each including different <realm>s (and potentially different <sasl-
sid>s for different realms).
The server MAY also return additional challenges if Basic and/or
Digest [RFC2617] access authentication is supported for the requested
resource.
4.3.1.2 Client initiated authentication
A client, which is about to issue a request to a server, and knows
that the server requires a certain SASL mechanism, MAY include a a
"SASL" <auth-scheme> token in an Authorization (or Proxy-
Authorization) header field in its request. If the client chooses to
do so, it MUST include a <sasl-mechanism> directive identifying the
used SASL mechanism, but MUST NOT include a <sasl-sid> directive, as
session identifiers are chosen by the server. The client MAY also
specify a <realm> directive (if it is known) and a <sasl-options>
directive in the request. If the chosen SASL mechanism requires that
the client sends data first, the client MUST also include a <sasl-
credentials> directive, c.f. the "initial response" in [RFC2222] (see
also the example in Section 4.7.2). This minimizes the number of
roundtrips, since otherwise the server would be required to send an
empty challenge.
If the client requires authentication, but doesn't know which
mechanisms are supported by the server, the client SHOULD issue an
OPTIONS request that includes a Request-URI header for the desired
resource and an Authorization (or Proxy-Authorization) header field
containing a "SASL" <auth-scheme> token that MAY contain <realm>, but
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MUST NOT contain any of the <sasl-mechanism>, <sasl-sid> or <sasl-
credentials> directives. This provides a way for the client to query
the server about supported SASL mechanisms for the requested
resource.
This document REQUIRES that a compliant SASL-aware server handles an
OPTIONS request with the "SASL" <auth-scheme> token described in the
previous paragraph by listing all supported and acceptable SASL
mechanisms in the <sasl-mechanisms> directive in the WWW-Authenticate
(or Proxy-Authenticate) header field as described in Section 4.3.1.1.
When replying to OPTIONS request the server SHALL use the 401 -
Unauthorized (407 - Proxy Authentication Required) response, if the
requested resource requires client authentication. <<Note that
Section 10.4.2 of HTTP/1.1 requires that a 401 response includes a
WWW-Authenticate header>> The server SHALL use the 200 - OK response,
if unauthenticated users are allowed to see the resource. In both
cases, the presense of the WWW-Authenticate (or Proxy-Authenticate)
header field containing "SASL" <auth-scheme> signifies that SASL
authentication is supported for the requested resource; the absence
of any WWW-Authenticate (or Proxy-Authenticate) header field with
"SASL" <auth-scheme> signifies that SASL authentication is not
supported for the requested resource. For example, the server SHALL
use the 200 - OK response including a WWW-Authenticate (or Proxy-
Authenticate) header field with "SASL" <auth-scheme>, if
unauthenticated users are allowed to see the resource and SASL
authentication is supported for the resource. See also an example in
Section 4.7.6.
<<Do we need a summary table here?>>
If the client has specified a wrong <realm> value (i.e. a <realm>
value that is not recognized by the server or a <realm> value that
doesn't control access to the requested resource) or has not provided
any <realm> value and the server supports multiple realms for the
requested resource, than the server MUST ignore data sent in the
client's request and respond with a 401 - Unauthorized (407 - Proxy
Authentication Required) response containing multiple SASL challenges
formatted as described in section 4.3.1.1, each SASL challenge
including different <realm>s. The client can than select a proper
<realm> value and retry the authentication request. See also example
in Section 4.7.7.
4.3.2 Client response
A client, which receives a "SASL" <auth-scheme> authentication
response token containing the <sasl-mechanisms> directive in a WWW-
Authenticate (Proxy-Authenticate) header in a 401 - Unauthorized (407
- Proxy Authentication Required) response, examines the list of the
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available SASL mechanism found in the <sasl-mechanisms> directive. If
the client can't find a supported and otherwise appropriate (for
accessing the resource) SASL mechanism (see also note below), it MUST
NOT continue the authentication exchange using a SASL mechanism not
on the provided list. If no acceptable SASL mechanism is found, the
client MAY try Digest and/or Basic authentication [RFC2617]. <<Should
we mention other obvious possibilities like dropping connection?>> If
the client has found an acceptable SASL mechanism, it constructs a
new request as described below. This request MAY contain the headers
from the original request, MUST contain an Authorization (Proxy-
Authorization) header containing a "SASL" <auth-scheme> token, but
SHOULD NOT contain the body of the original request (if any). We will
reference any such request as a "SASL request". The purpose of SASL
requests is to avoid sending the body of a request with each
authentication step.
Note: In cases where the 401 - Unauthorized (407 - Proxy
Authentication Required) response also contains a WWW-Authenticate
(Proxy-Authenticate) header with a "Basic" and/or a "Digest" <auth-
scheme> token, the selected authentication scheme will be subject to
local client policy. Clients are RECOMMENDED never to select Basic
authentication over any other server-suggested method.
The "SASL" <auth-scheme> token in the SASL request MUST include the
<sasl-sid> value provided by the server and a <sasl-mechanism>
directive with the chosen SASL mechanism name. If the chosen
mechanism allows for "initial response" type messages, the client
MUST also include the initial response in a <sasl-credentials>
directive. If the client is transmitting an initial response of zero
length, it MUST transmit the response as the empty token (i.e.
credentials=""). This indicates that the response is present, but
contains no data. The client MAY also include a <sasl-options>
directive.
If the client is able and willing to negotiate a SASL security layer,
it MUST establish an end-to-end tunnel using the CONNECT method as
described in Section 5.3 of [RFC2817] before starting an
authentication exchange. The Authorization header MUST NOT be used in
a CONNECT request. However, in order to save round trips, a Proxy-
Authorization header MAY be used in a CONNECT request.
Note: A direct connection (any intermediate proxies operating in
tunnel mode) is required whenever a security layer is in effect,
since at that point complete HTTP/1.1 messages may be encrypted.
When two or more authentication exchanges are performed in parallel
on the same connection ("mixed"), the client MUST NOT negotiate a
security layer on more than one of them. Multiple <sasl-sid>
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directives SHOULD NOT be "mixed" on the same connection, except for
the case when a client starts an authentication exchange with the
target server and an intervening proxy server asks the client to
authenticate to it first. In this case, the client must perform an
authentication exchange to the proxy first and then resume
authentication to the end server.
The following diagram demonstrates a "mixed" authentication exchange:
Client Server
----------- Start Authentication Exchange 1 ------------->
<--------- Reply to Authentication Exchange 1 ------------
----------- Start Authentication Exchange 2 ------------->
<--------- Reply to Authentication Exchange 2 ------------
----------- Continue Authentication Exchange 1 ---------->
If the client receives a "SASL" <auth-scheme> authentication response
token containing a <sasl-challenge> directive in a WWW-Authenticate
(Proxy-Authenticate) header for a 401 - Unauthorized (407 - Proxy
Authentication Required) response, the client should behave as
described in Section 4.3.4.
4.3.3 Server behavior upon receiving a "SASL" <auth-scheme> token
If the <auth-scheme> token contains a <sasl-sid> directive, then the
server MUST check if the SASL authentication exchange context
identified by <sasl-sid> is valid. If it is not, the server SHALL
reply with a 401 - Unauthorized (407 - Proxy Authentication Required)
response, that contains a new <sasl-sid> value and the session
continues as described in Section 4.3.1.1, i.e. the server MUST list
all supported and acceptable SASL mechanisms in the <sasl-mechanisms>
directive.
If the <auth-scheme> token contains a <sasl-mechanism> directive, the
server MUST check if it mechanism is acceptable. If it is not, the
server MUST reply with a 450 - "Authentication mechanism not
accepted" response and, if the request included a <sasl-sid>
directive, delete the SASL authentication context identified by the
<sasl-sid>.
If the <auth-scheme> token contains a <sasl-credentials> directive,
the server MUST check if the supplied credentials authenticates the
client. If the <sasl-credentials> directive contains a <cancel-
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token> then the server MUST reject the exchange with a 401 -
Unauthorized reply.
Otherwise, the server uses the value of <sasl-credentials> directive
to check if the client is authenticated. If the client is not (yet)
authenticated, the server uses the supplied credential value to
calculate a new <sasl-challenge> as per the currently selected SASL
mechanism. If the new <sasl-challenge> is successfully calculated, it
is returned in the WWW-Authenticate (or Proxy-Authenticate) header of
a new 401 - Unauthorized (407 - Proxy Authentication Required)
response.
If the client authentication failed, the server SHALL reply with a
401 - Unauthorized (407 - Proxy Authentication Required) response
containing a WWW-Authenticate (or Proxy-Authenticate) header
containing a "SASL" <auth-scheme> authentication token with exactly
two <auth-params> directives: <sasl-sid> and <sasl-status>. The value
for the <sasl-status> directive shall be "failed". When receiving a
message with a WWW-Authenticate (Proxy-Authenticate) header of this
type, the client shall interpret the response in accordance with
Section 10.4.2 of [RFC2616]. For an example of this, see Section
4.7.9.
Note: This method of conveying information about a failed
authentication differs slightly from that defined in [RFC2616]. The
reason for this discrepancy is twofold: There may be SASL methods for
which two consecutive challenges are identical, and the method
defined in 10.6.2 of [RFC2616] was not designed for multiple-step
authentication exchanges.
Whenever an authentication exchange fails, both the client and the
server MUST return to their previous authentication state, i.e. as if
the authentication attempt never took place.
The server MAY also choose to reply with a 432 - Transition Needed
response, which indicates that the user name is valid, but the entry
in the authentication database needs to be updated in order to permit
authentication with the specified SASL mechanism. A client, which
receives a 432 - Transition Needed response, MAY retry authentication
using the SASL PLAIN mechanism. This SHOULD NOT be done unless an
appropriate TLS protection is in place. An interactive client MUST
NOT perform PLAIN authentication automatically and MUST warn the user
before proceeding.
If the client is authenticated, the server MUST at least include the
<sasl-sid> directive with its "SASL" <auth-scheme> authentication
response token. If the chosen SASL mechanism requires that further
challenge/response data (i.e. "server returns success with additional
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data" in [RFC2222]) be sent by the server, the server MUST respond
with a 401 - Unauthorized (407 - Proxy Authentication Required)
response containing a <sasl-challenge> directive with its "SASL"
<auth-scheme> authentication response token in a WWW-Authenticate (or
Proxy-Authenticate) header. Unless the server fails authentication,
the client MUST reply to this with a new SASL request containing an
Authorization header with a <sasl-sid> directive and an empty <sasl-
credentials> directive. The server will reply to this with a 235 -
Authentication Completed (236 - Proxy Authentication Completed)
response and at this point authentication is complete, and a SASL
security layer may take effect (see Section 4.4.2).
If the client is authenticated and the server does not need to send
any further challenge information, the server replies with a 235 -
Authentication Completed (236 - Proxy Authentication Completed)
response.
In both cases, when the server replies with a 235 - Authentication
Completed (236 - Proxy Authentication Completed) response, it MAY
include an <http-authzid> directive in the "SASL" <auth-scheme>
authentication response token. The <http-authzid> SHALL contain the
authorization identity of the authenticated client in the form of a
URI. Note that the content of a 235 - Authentication Completed (236 -
Proxy Authentication Completed) response (and thus the <http-authzid>
directive) is not protected by a SASL security layer. In some
deployments, the value of the <http-authzid> directive may also
contain confidential information which might require privacy
protection.
Upon receipt of a 235/236 response the client shall consider
authentication successful and may retry the original request (with
the body of the request, if any), possibly protected by a negotiated
security session (see Section 4.4.2).
4.3.4 Client behavior upon receiving a "SASL" <auth-scheme> token
The client, upon receipt of a "SASL" <auth-scheme> authentication
response token containing a <sasl-challenge> directive in a WWW-
Authenticate (Proxy-Authenticate) header for a 401 - Unauthorized
(407 - Proxy Authentication Required) response, calculates its
credentials and responds with a new SASL request containing a
(possibly empty, see previous section) <sasl-credentials> directive
and a "SASL" <auth-scheme> token in an Authorization (Proxy-
Authorization) header. The client repeats this until the
authentication exchange is successful or the server responds with a
401 (407) message without the SASL <auth-scheme> token (see previous
section).
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4.3.5 Subsequent requests
The same HTTP server (host identifier) may serve data governed by
multiple realms that may have separate associated authentication
databases. If the client leaves the authentication realm it is
currently authenticated in, e.g. by issuing a request for a resource
in a different realm, the server MAY force the client to re-
authenticate in the new realm. In this case a new authentication
exchange is started as described in 4.3.1. However there is a change
in how the security layer is established (see Section 4.4.2). If a
security layer is currently active and the new authentication
exchange negotiates a new security layer, it MUST replace the
existing one. This includes the case when the new security layer is
the NULL layer, i.e. the connection reverts to a state where no SASL
security layer is present). See Section 4.4.2 for a description of
when the security layer is being replaced/dropped.
4.3.6 Client aborting a handshake
A client may abort a handshake by letting the value of the <sasl-
credentials> field consist of the <cancel-token>, "*". For an example
of this, see Section 4.7.5.
4.3.7 Pipelining considerations
When pipelining multiple authentication requests (or authentication
requests together with other requests), the client MUST observe the
rules established in Section 4.4.2. This means that an authentication
request that completes a SASL authentication exchange and activates a
SASL security layer, MUST be the last request in a group of requests.
If this rule is not followed, the client will start sending cleartext
data that may be interpreted by the server as encrypted. This can
lead to a packet decode error on the server side and dropped
connections.
When a SASL security layer has been negotiated clients MAY put
multiple HTTP requests (and server may put multiple HTTP responses)
inside a single SASL buffer of protected data. See also Section
4.4.2.
4.3.8 Caching considerations
As described in [RFC2616] Section 14.8, a shared cache MUST NOT
return a response to a request containing an Authorization header to
any other requests unless special circumstances apply. To ensure that
these circumstances do not apply here, the server MUST send a "Cache-
Control: no-store" header together with the "WWW-Authenticate" header
in all handshake responses.
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4.3.9 "Web farm" considerations
Implementation and configuration of the SASL negotiation mechanism
described in this memo requires special considerations in the case of
"web farm" environments where several servers may serve client
requests since authentication state information otherwise may be
lost. In particular, means for sharing of authentication negotiation
state must be available.
4.3.10 HTTP header and state management
There MUST NOT be more than one WWW-Authenticate or Proxy-
Authenticate header field containing a SASL authentication response
in any HTTP response. The WWW-Authenticate or Proxy-Authenticate
header MUST NOT contain more than one SASL authentication response.
The only exception to these rules is when the server lists available
SASL mechanisms and the access to the requested resource is
controlled by more than one realm (see section 4.3.1).
There MUST NOT be more than one Authorization or Proxy-Authorization
header field containing a SASL authorization request in any HTTP
request.
Since support for persistent connections is optional in HTTP/1.1, all
servers MUST implement some method for state management of SASL
authentication exchanges. This may include (but is not limited to)
session caching, session expiration, dealing with duplicated
authentication requests.
This document does not specify methods for servers to manage session
state once the client has been authenticated. For an example of such
methods, see [RFC2965].
4.4 Request/response encoding
4.4.1 SASL challenge/response Encoding
The <sasl-challenge> directive and the <sasl-credentials> directive
contain SASL challenges and responses respectively. The challenges
and responses MUST be base64 ([RFC3548], section 3) encoded before
being placed in these fields. The base64 string may in general be
arbitrarily long. Clients and servers MUST be able to support
challenges and responses that are as long as are generated by the
authentication mechanisms they support, independent of any line
length limitations the client or server may have in other parts of
its protocol implementation.
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Note that, as described in Section 4.3.6, instead of containing a
base64-encoded string, a <sasl-credentials> value may consist of the
single "*" character, indicating to the server that the client aborts
the handshake.
4.4.2 Security layer
If a protection mechanism is negotiated as part of the SASL security
session, then it MUST be applied to all subsequent requests and
responses sent between the server and the client for the given realm.
Any negotiated security layer takes effect immediately following the
<message-body> that concludes the authentication exchange for the
client, and the <message-body> of 235 (236) response for the server.
I.e., for later requests (and responses) all data - including the
status line and headers - will be protected by the new security
layer.
The same rules apply in a case of reauthentication. Whenever a new
security layer (including the empty one) is negotiated due to
reauthentication, the current layer gets replaced (dropped)
immediately after transmission (receipt) of the 235 (236) response.
A client that requires a security layer MUST check, after successful
authentication, that such a layer indeed was negotiated.
Note that a security layer requires HTTP/1.1 persistent connection.
4.4.3 Interaction with TLS
A client may not perform an HTTP/1.1 "Upgrade" to TLS [RFC2817] while
conducting a SASL negotiation, but is free to do so after, or before,
the SASL negotiation takes place.
This document allows for both a TLS and a SASL security layer to be
active at the same time. No matter in which order they were
negotiated, any data will be transformed by the SASL security layer
first and then by TLS, i.e. the relevant protocol stack will be as
follows:
+---------+
| HTTP |
+---------+
| SASL |
+---------+
| TLS |
+---------+
| TCP |
+---------+
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4.4.4 Mandatory to implement SASL mechanism
In order to guarantee interoperability, all client and server
implementations conformant to this document MUST support the DIGEST-
MD5 [RFC2831] SASL mechanism. Since support for persistent
connections is optional in HTTP/1.1, this implies that all clients
and servers MUST support DIGEST-MD5 in non-persistent mode.
4.5 Status codes and error handling
4.5.1 HTTP/1.1 status codes
HTTP/1.1 status codes which apply to SASL-based mechanisms are:
-235 - Authentication Completed
This status code indicates that SASL authentication with the server
is complete and the client may retry sending the original request.
-236 - Proxy Authentication Completed
This status code indicates that SASL authentication with the proxy
is complete and the client may retry sending the original request.
-401 - Unauthorized
An HTTP/1.1 server will use this status code when credentials
supplied by a client could not be validated, in addition to the use
described in Section 4.3 above.
-407 - Proxy Authentication Required
An HTTP/1.1 proxy will use this status code when credentials
supplied by a client could not be validated, in addition to the use
described in Section 4.3 above.
-432 - Transition Needed
This status codes indicates that the user name is valid, but the
entry in the authentication database needs to be updated in
order to permit authentication with the specified SASL mechanism.
This typically is done by authenticating once using the PLAIN
authentication mechanism. See Section 4.3.4.
This status code can be sent, for example, if a user has an entry in
a system authentication database such as Unix /etc/passwd, but does
not have credentials suitable for use by the specified mechanism.
-450 - Authentication mechanism not accepted
An HTTP/1.1 server will use this status code when a client suggests
an authentication mechanism which is not supported or accepted by
the server.
4.5.2 Client error handling
When a client does not support any of the security mechanisms
suggested by a server, or is otherwise unable to complete a SASL
mechanism handshake with a server, it shall close the connection.
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(instead of closing the connection the client MAY also cancel the
SASL exchange by specifying a "*" in a <sasl-credentials> directive
as described in Section 4.3.6). User-oriented clients SHOULD provide
the user with information about the failed handshake, and MUST fail
in a controlled, predictable manner.
4.6 Authorization identity
This document defines an authorization identity in the HTTP profile
of SASL to be a sequence of Unicode characters (excluding NUL),
encoded in UTF-8. This sequence is further prepared using the
"SASLPrep" profile [SASLPrep] of the "stringprep" algorithm
[RFC3454]. The latter restriction is required in order to have a
predictable result when comparing two authorization identities
entered by two different individuals, potentially using different
input mechanisms. This is also required as many SASL mechanisms use
authorization identities to produce hash values.
Clients MUST use the algorithm described above on authorization
identities entered by a user (for interactive clients) or read from a
configuration file. Servers MUST verify that a received authorization
identity is in the correct form. If the preparation of the
authorization identity fails or results in an empty string, the
server MUST fail the authentication exchange. The only exception to
this rule is when the received authorization identity is already the
empty string.
4.7 Examples
Note: In the examples, some lines are wrapped for readability
reasons.
4.7.1 Example 1 - Server requires authentication
This example illustrates a client requesting a URL and a server
responding with a list of supported SASL mechanisms. The client
selects one of these and responds with a new request containing an
initial-response type <sasl-credentials> directive. The server then
issues a <sasl-challenge> directive back to the client which once
again responds with a <sasl-credentials> directive in the
Authorization header field.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
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mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="CRAM-MD5",
id="jfkasdgru42705"
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705",
challenge="PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials="dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNG
QzODkw"
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
Client now retries the original request:
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
...Requested Document follows...
4.7.2 Example 2 - Initial response
In this example a client knows in advance that a certain SASL
mechanism is required. The mechanism allows for an initial-response
type message and the client therefore includes a <sasl-credentials>
directive in its Authorization header. The server accepts the
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credentials and responds with the requested information.
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="SECURID",
credentials="AG1hZ251cwAxMjM0NTY3OAA="
(the client doesn't know if authentication is complete at this point,
as certain SASL mechanisms have a variable number of steps.)
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
...Requested Document follows...
4.7.3 Example 3 - One mechanism only
In this example a server supports only one SASL mechanism, which
allows for sending of an initial challenge to a client.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="CRAM-MD5",
realm="testrealm@example.com",
id="jfkasdgru42705",
challenge="PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
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credentials="dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNG
QzODkw"
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
...Requested Document follows...
4.7.4 Example 4 - Server sends additional data
This example demonstrates the use of an integrity/privacy layer.
Note that the client is using the CONNECT method, as it is willing to
negotiate integrity/privacy protection provided by the DIGEST-MD5
SASL mechanism.
In its third message, the client specifies options="http-authzid",
which instructs the server to return an <http-authzid> directive upon
successful authentication.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="0001"
C: CONNECT classified.example.com:80 HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="DIGEST-MD5",
id="0001", options="http-authzid"
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S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
challenge="cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIscW9wPSJhdXRoIixhbGdv
cml0aG09bWQ1LXNlc3MsY2hhcnNldD11dGYtOA=="
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001",
credentials="Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLH
JlYWxtPSJlbHdvb2QuaW5ub3NvZnQuY29tIixub25jZT
0iT0E2TUc5dEVRR20yaGgiLG5jPTAwMDAwMDAxLGNub
25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9
ImltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9
uc2U9ZDM4OGRhZDkwZDRiYmQ3NjBhMTUyMzIxZjIxND
NhZjcscW9wPWF1dGg="
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
challenge="cnNwYXV0aD00YjJiYjM3ZjA0OTEwNTA1Nzc3YzJmNjM
4YzkyMjcyNQ=="
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001"
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
http-authzid="http://example.com/testrealm/users/lisa"
CP: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
SP: HTTP/1.1 200 OK
...Requested Document follows...
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CP: ...Any subsequent request for a data on the same server,
unless the server requests reauthentication...
4.7.5 Example 5 - Abort
The following example shows how a client can abort an authentication
exchange.
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="0001"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="DIGEST-MD5",
id="0001"
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",
challenge="cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNl
PSJPQTZNRzl0RVFHbTJoaCIscW9wPSJhdXRoIixhbGdv
cml0aG09bWQ1LXNlc3MsY2hhcnNldD11dGYtOA=="
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001",
credentials="*"
S: HTTP/1.1 401 Authentication Canceled
...
4.7.6 Example 6 - Client requires authentication
The following example is almost identical to Example 1, but here the
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client requires authentication to the server.
C: OPTIONS http://classified.example.com/classified.html HTTP/1.1
Authorization: SASL
Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanism="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="CRAM-MD5",
id="jfkasdgru42705"
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705",
challenge="PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials="dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNG
QzODkw"
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
Upon receipt of a 235 response the client submits the request it
originally intended to submit:
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
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S: HTTP/1.1 200 OK
Cache-Control: no-store
...Requested Document follows...
4.7.7 Example 7 - Client requires authentication, server supports
multiple realm
The following example is almost identical to Example 2, but here the
server supports multiple realms.
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="SECURID",
credentials="AG1hZ251cwAxMjM0NTY3OAA="
As the server supports multiple realms for the requested resource,
it forces the client to select the proper realm
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,SECURID",
realm="testrealm@sales.example.com",
id="jfkasdgru42705"
WWW-Authenticate: SASL
mechanisms="SECURID",
realm="testrealm@example.com",
id="jfkasdgru42705"
(Note that the server may choose to return multiple SASL challenges
in a single WWW-Authenticate response, in this case the last server
response may also look like:
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,SECURID",
realm="testrealm@sales.example.com",
id="jfkasdgru42705", SASL
mechanisms="SECURID",
realm="testrealm@example.com",
id="jfkasdgru42705"
Also note, that different SASL challenges may use the same or
different "id".)
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C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="SECURID", id="jfkasdgru42705",
realm="testrealm@sales.example.com",
credentials="AG1hZ251cwAxMjM0NTY3OAA="
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Host: classified.example.com
S: HTTP/1.1 200 OK
Cache-Control: no-store
...Requested Document follows...
4.7.8 Example 8 - Client uses POST request
In this example the client is willing to perform a POST request but
the server requires authentication and the establishment of a
security layer.
Note that since the client sends its information unprotected in the
initial POST message, in effect only the server's response (and any
later messages) will benefit from this security layer.
C: POST http://classified.example.com/update_classified.php
HTTP/1.1
Host: classified.example.com
Content-Type: ...
Content-Length: ...
...request body...
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,OTP",
realm="testrealm@example.com",
id="0001"
C: CONNECT classified.example.com:80 HTTP/1.1
Host: classified.example.com
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S: HTTP/1.1 200 OK
C: POST http://classified.example.com/update_classified.php
HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="OTP",id="0001",credentials="AHRpbQ=="
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="0001",challenge="b3RwLW1kNSAxMjMga2UxMjM0IGV4dA=="
C: POST http://classified.example.com/update_classified.php
HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="0001",credentials="aGV4OjExZDRjMTQ3ZTIyN2MxZjE="
S: HTTP/1.1 235 OK
Cache-Control: no-store
WWW-Authenticate: SASL id="0001"
CP: POST http://classified.example.com/update_classified.php
HTTP/1.1
Host: classified.example.com
Content-Type: ...
Content-Length: ...
...request body...
SP: HTTP/1.1 200 OK
...Response to POST, if any...
CP: ...Any subsequent request for a data on the same server,
unless the server requests reauthentication...
4.7.9 Example 9 - Client authentication fails.
In this example the client authentication fails and the server
indicates this in its final message using the <sasl-status>
directive.
C: GET http://classified.example.com/classified.html HTTP/1.1
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Host: classified.example.com
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
mechanisms="DIGEST-MD5,GSSAPI,CRAM-MD5",
realm="testrealm@example.com",
id="jfkasdgru42705"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
mechanism="CRAM-MD5",
id="jfkasdgru42705"
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705",
challenge="PDE4OTYuNjk3MTcwOTUyQHBvc3RvZmZpY2UucmVzdG9u
Lm1jaS5uZXQ+"
C: GET http://classified.example.com/classified.html HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: classified.example.com
Authorization: SASL
id="jfkasdgru42705",
credentials="dGltIGI5MTNhNjAyYzdlZGE3YTQ5NWI0ZTZlNzMzNG
QzODkw"
S: HTTP/1.1 401 Unauthorized
Cache-Control: no-store
WWW-Authenticate: SASL
id="jfkasdgru42705"
status="failed"
At this point, both the server and the client shall return to their
initial state wrt. the SASL authentication.
4.8 Interoperability with existing HTTP/1.1 clients and servers
A client supporting a certain SASL-based authentication mechanism
allowing for initial responses MUST NOT include a <sasl-credentials>
directive in a "SASL" <auth-scheme> authorization request in an
Authorization or Proxy-Authorization header unless it knows that the
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server supports the SASL mechanism in question. The client SHOULD use
an OPTIONS request to discover the server's SASL capabilities (see
Section 4.3.1.2 for more details).
A server supporting SASL-based authentication SHOULD include a
"Basic" and a "Digest Access" <auth-scheme> token in a WWW-
Authenticate or Proxy-Authenticate header field, if these
authentication methods are acceptable to the server. This ensures
proper interworking with clients only capable of performing a "Basic"
or "Digest Access" authentication. Since these authentication
mechanisms does not offer strong security, the risk of downgrading
attacks should be carefully considered (see also the "Security
Considerations" section in this memo and Section 4.1 and 4.2 in
[RFC2617]).
4.9 Preferences
Servers MUST list authentication mechanisms in the WWW-Authenticate
(Proxy-Authenticate) header field in preferred order.
4.10 SASL mechanism recommendations
It is RECOMMENDED that an SASL mechanism that supports the
negotiation of a security layer with integrity protection be used,
and that this protection be enabled to avoid the connection being
hijacked after authentication has taken place. [RFC2222] discusses
some of the security issues related to SASL mechanisms.
5 IANA considerations
5.1 GSSAPI/SASL service name
For use with SASL [RC2222], a protocol must specify a service name to
be used with various SASL mechanisms, such as GSSAPI. For HTTP, the
service name shall be "http".
5.2 HTTP/1.1 Status codes
This memo defines the following HTTP/1.1 status codes:
-235 "Authentication Completed"
-236 "Proxy Authentication Completed"
-432 "Transition Needed"
-450 "Authentication mechanism not accepted"
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6 Security considerations
6.1 Introduction
This memo describes a method to integrate the SASL framework in
HTTP/1.1. SASL as such allows a wide variety of mechanism, each with
their own security characteristics. The following sections represent
an attempt to discuss threats that can be regarded to be generic in
the sense that they apply to the integration itself rather than
specific SASL mechanisms. Security services offered by, and security
considerations applying to, particular SASL mechanisms can be found
through the IANA SASL mechanism registry.
6.2 Active attacks
6.2.1 Man-in-the-middle
Users of SASL in HTTP/1.1 SHOULD recognize that certain man-in-the-
middle attacks are possible since the negotiation of the particular
SASL security mechanism to be used is not necessarily protected. For
example, if the server suggests SASL mechanisms A, B and C in a
"SASL" <auth-scheme> token where A is a "strong" mechanism (for some
definition of "strong") but B and C are "weak" or provide fewer
security attributes than A, then an attacker could simply remove A
from the list. This forces the client to choose a "weaker" mechanism
and neither side will necessarily detect the changes made by the
attacker.
To mitigate these attacks, servers SHOULD only suggest SASL
mechanisms that will provide adequate security for the task at hand.
Similarly, the SASL <auth-scheme> token may be removed from the WWW-
Authenticate (Proxy-Authenticate) header, thus forcing use of either
the Basic or Digest Access method. For this reason, and unless other
precautions (such as only accepting certain SASL mechanisms) are
taken, it is RECOMMENDED that this authentication mechanism be used
only in conjunction with a transport, e.g. TLS, providing protection
against these attacks (server authentication and integrity protection
of messages). Note however that when using client authentication
mechanisms within a server-authenticated TLS tunnel, care must be
taken to avoid the attack described in [MITM].
6.2.2 Denial of service
Since HTTP/1.1 requests and responses are not protected against
modification per se, an attacker may, by removing SASL elements from
HTTP/1.1 headers hinder a client from accessing a certain service.
This is however a generic threat and not specific to the mechanism
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described herein.
6.2.3 Replay
Use of the "Cache-Control: no-store" and "Pragma: no-cache" headers
when indicated in requests and responses ensures that proxies do not
inadvertently store and/or deliver SASL handshake messages that
otherwise could be used in replay attacks.
6.3 Passive attacks
Unless a transport security providing confidentiality is employed,
the method described in this memo is susceptible to passive attacks
where an attacker wants to find out about the mechanisms that are
supported by a particular client.
6.4 Protecting the body of POST/PUT requests
When the client performs a POST/PUT request in the clear and gets
Unauthorized response back from the server it is already too late to
protect the body of the POST/PUT request, as it was already sent in
the clear. Arguably, if the client sent some data in the clear with
the user's permission, the user doesn't find the information being
sent worth protecting. However, existing web clients are able to
warn users about sending data in the clear, but don't have an option
to establish a secure connection first.
The described problem is not specific to this document. HTTP over TLS
uses a different URL schema to notify the client that it has to
establish a secure connection first with TLS.
So, one way to mitigate the problem would be to define a new URL
schema (or an extension to the existing URL schema) for SASL in HTTP.
This is however outside of the scope for this document.
A client wishing to protect body of a POST/PUT request from
modification and/or disclosure should first establish a channel
protection using TLS and/or SASL. In general, an interactive client
SHOULD ask a user (or be configurable) to establish channel
protection before performing any POST/PUT.
6.5 Other considerations
Section 8.2 of [RFC2817] contains relevant security considerations
for the CONNECT method.
Note that SASL mechanisms offering confidentiality and integrity
protection of messages are only usable in conjunction with the
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CONNECT method as described, since a proxy otherwise would be unable
to handle the messages properly.
Section 6.3 ("Multiple authentications") of [RFC2222bis] contains
security considerations regarding replacing a SASL security layer
with no layer on reauthentication.
7 Implementation considerations
This section is informative.
7.1 The SASL authentication exchange context
This memo assumes the existence of a SASL authentication exchange
context during the lifetime of a SASL handshake. The SASL
authentication exchange context is a SASL structure that represents
all SASL state associated with the authentication exchange identified
by sasl-sid. It may include (but is not limited to): the current step
in a multiple-step authentication exchange, an authentication id, any
material derived from password, private key, etc.
The context should be kept for some period of time after the
connection goes away. This period is implementation defined. The SASL
context should be deleted once the session expires, and must be
deleted once the authentication exchange completes with success or
failure, or the session otherwise becomes invalid (e.g. when a
duplicated authentication exchange was received for the same
session).
Although, a particular implementation may choose to store any SASL
security layer state (e.g. encryption/decryption keys) as a part of
the SASL context, this document considers a SASL security layer state
to be a separate entity from the corresponding SASL context. The SASL
security layer state is deleted when the connection it is protecting
is closed or the corresponding authentication exchange fails. In the
latter case we are talking about partially created SASL security
layer states. However, as opposed to the SASL context, the SASL
security layer state is not deleted when the authentication exchange
completes successfully.
7.2 SASL security layer handling
This section attempts to summarize client and server behaviour with
regards to SASL security layer negotiation.
A client willing to negotiate a SASL security layer must perform all
of the following steps:
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a) Use persistent connection to perform a SASL authentication
exchange (Section 4.4.2). A SASL security layer (if supported
by the server and negotiated) can only be used on the TCP
connection that was used for the final "round" (i.e. C->S:
client response, S->C: server confirms that authentication
was successful) of the authentication exchange. Note that some
SASL mechanisms use IP addresses in authentication exchange,
which effectively requires the use of a persistent connection
during the whole authentication exchange.
b) Use CONNECT to establish an end to end tunnel through proxies,
unless the client has a prior knowledge that it talks directly
to the target server (Section 4.3.2).
c) Notify the SASL layer/library being used that it supports
channel integrity and/or confidentiality.
As the SASL security layer is an optional feature of SASL, the rules
a)-c) do not guarantee that a security layer will be negotiated. A
client that requires a security layer must check, after successful
authentication, that such a layer indeed was negotiated.
Regarding c) above, if a client is not able and/or not willing to
negotiate a SASL security layer it must notify the SASL layer/library
being used that it doesn't support channel integrity or
confidentiality. Failure to do so may result in a situation when both
parties negotiate a SASL security layer, but the client is unable to
use it. The client doesn't have to do step b) and may not do step
a).
Similarly, a server willing to negotiate a SASL security layer must
perform all of the following steps:
a) Use a persistent connection to perform a SASL authentication
exchange (Section 4.4.2). A SASL security layer (if supported
by the client and negotiated) can only be used on the TCP
connection that was used for the final "round" of the
authentication exchahge.
b) Support the CONNECT method (Section 4.3.2).
c) Notify the SASL layer/library being used that it supports
channel integrity and/or confidentiality.
As for clients above, rules a)-c) do not guarantee that a security
layer will be negotiated. A server, which requires a security layer,
must check, after successful authentication, that such a layer indeed
was negotiated.
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If a server is not able and/or not willing to negotiate a SASL
security layer it must notify the SASL layer/library being used that
it doesn't support channel integrity or confidentiality. Failure to
do so may result in a situation when both ends negotiate a SASL
security layer, but the server is unable to use it.
7.3 SASL Profile Checklist
The profiling requirements of [SASL] require that the following
information be supplied by a protocol definition:
service name: "http" (section 5.1)
authentication protocol exchange initiation: section 4.3.1
listing supported SASL mechanisms:
a) if server requires authentication: section 4.3.1.1
b) client request the list: section 4.3.1.2
Initial client response: sections 4.3.1.2, 4.3.2
Initial server challenge: section 4.3.1.1
exchange sequence: client -> server: section 4.3.3
server -> client : section 4.3.2, 4.3.4
server sends failure: sections 4.3.3, 4.4.1
server sends success: section 4.3.3
client aborts exchange: section 4.3.6, also sections 4.4.1, 4.2.3
optional data with success: not supported, see section 4.3.3
security layer negotiation: section 4.4.2
order of SASL security layer and TLS,
if both are negotiated: section 4.4.3
use of the authorization identity: section 4.6
multiple authentications: yes, see section 4.3.5, also section 4.4.2
Interaction of SASL exchange with line lenght limits: section 4.4.1
Specific Issues:
multiple realms: sections 4.3.1.1 and 4.3.1.2
persistent connection: sections 3 and 4.3.10
mixing multiple authentications on the same connection: section 4.3.2
OPTIONS method: 4.3.1.2
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8 Acknowledgements
Text for Section 4.6 was borrowed from [RFC2829]. Thanks to Keith
Burdis, Raif S. Naffah, Mark Nottingham, Joe Orton, John P. Speno,
Lisa Dusseault and Eric Rescorla for providing useful feedback and
suggestions.
Robert Zuccherato, Entrust Inc., made significant contributions to
earlier drafts of this work.
A large part of this document was written while Alexey was working
for MessagingDirect.
9 References
9.1 Normative references
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3," IETF RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," IETF RFC 2119, March 1997.
[RFC2222] Myers, J., "Simple Authentication and Security Layer," IETF
RFC 2222, October 1997.
[RFC2234] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF," IETF RFC 2234, November 1997.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax," IETF RFC 2396, August
1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P., Berners-Lee, T., "Hypertext Transfer Protocol --
HTTP/1.1," IETF RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., Stewart, L., "HTTP Authentication: Basic and
Digest Access Authentication," IETF RFC 2617, June 1999.
[RFC2817] Khare, R., Lawrence, S., "Upgrading to TLS Within
HTTP/1.1," IETF RFC 2817, May 2000.
[RFC2831] Leach, P. C. Newman, "Using Digest Authentication as a SASL
Mechanism," IETF RFC 2831, May 2000.
[RFC3454] P. Hoffman, M. Blanchet, "Preparation of Internationalized
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Strings ("stringprep")," IETF RFC 3454, December 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings," IETF RFC 3548, July 2003
[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords," Work in progress, draft-ietf-sasl-saslprep-XX.txt.
9.2 Informative references
[MITM] Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle in
Tunnelled Authentication Protocols." Available from
http://eprint.iacr.org/2002/163/
[RFC2222bis] Melnikov, A., "Simple Authentication and Security Layer
(SASL)", Work in progress, draft-ietf-sasl-rfc2222bis-XX.txt.
[RFC2246] Dierks, T., and C. Allen, "The TLS Protocol Version 1.0,"
IETF RFC 2246, January 1999.
[RFC2829] Wahl, M., Alvestrand, H., Hodges, J., and R. Morgan,
"Authentication Methods for LDAP," IETF RFC 2829, May 2000.
[RFC2965] Kristol, D., L. Montulli, "HTTP State Management
Mechanism," IETF RFC 2965, October 2000.
10 Authors' addresses
Magnus Nystrom Email: magnus@rsasecurity.com
RSA Security
Box 10704
121 29 Stockholm
Sweden
Alexey Melnikov Email: Alexey.Melnikov@isode.com
Isode Limited
5 Castle Business Village,
36 Station Road,
Hampton, Middlesex,
United Kingdom, TW12 2BX
11 IPR Disclosure Acknowledgement
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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12 Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
13 Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Appendix A. Changes since previous revisions
Changes since -11
Editorial changes: Made "Conventioned used in this document" the
first section. Moved 4.1.1 into new section 1. Moved 4.3.7 at the
beginning of Section 4 (now section 4.1.2).
Added clarification note that a SASL security layer is an optional to
negotiate feature of SASL.
Added Introduction text as suggested by Eric Rescorla.
Clarified significance and handling of the order of SASL mechanisms
in the sasl-mechanisms directive as per comment by Eric.
Clarified server behaviour when receiving and OPTIONS request as per
comments by Lisa (Sections 4.3.1.2 and 4.8.1)
Clarified the meaning of "a client ... MUST choose one of the
available mechanisms" in Section 4.3.2. (List/Eric)
Other minor editorial changes as suggested by Lisa/Eric.
Updated Copyright/IPR as per new IETF policy.
Changes since -10
Added text on client prioritization when receiving both a "SASL"
auth-scheme and a "Basic" or "Digest" auth-scheme in a 401 or 407
response.
Replaced "SASL block" with "SASL buffer of protected data". The
latter is defined in RFC 2222.
Other editorial changes based on feedback by Lisa Dusseault.
Changed ABNF and updated examples in order to allow for an empty
challenge/response.
Added http-authzid directive as suggested by Lisa Dusseault. Added
sasl-options directive.
Changes since -09
Added empty initial credentials
New method for specifying failed authentication, including an
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example.
Rewording of 4.3.8.
Added mandatory to support SASL mechanism.
Added explanatory text for multiple SASL challenges and for client
abort of handshakes.
Added reference to [RFC2222bis] and [MITM].
Added an example when server supports multiple realms.
Added "SASL Profile Checklist" section.
Editorial clarifications and corrections.
Changes since -08
Editorial clarifications and corrections.
Changes since -07
Added "Implementation consideration" section with big discussion on
how to correctly implement a SASL security layer. (Comment by Keith
Burdis)
Moved the biggest part of "SASL Context" definition to the
"Implementation consideration".
Added text describing that SASLPrep should be used on authorization
identities.
Added section describing ways to protect/help protect body of a
POST/PUT request. (Comment by Keith Burdis)
Several minor fixes.
Changes since -06
Changed 102 status code back to 401.
"credentials" directive is no longer returned by the server, only
"challenge" is used.
Added text about SASL context.
Split "SASL handshake initiation" section into Client and Server
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initiated.
Added text about performing multiple authentications in parallel.
Clarified the use of persistent connection with SASL. Added warnings
about session caching and expiration. Updated text to tell when SASL
context is destroyed.
Added new status codes: 450 "Authentication mechanism not accepted".
Expired session is denoted by a 401 (407) response with a new <sasl-
sid> value.
Clarified when security layer is replaced/dropped on
reauthentication.
Added warning that the server is required to keep track of
authenticated clients. Removed the text that was saying that the
server must return sasl-sid in 200 responses when authentication is
complete.
Updated examples as a result of the changes mentioned above.
Other minor clarifications.
Changes since -05
Replaced "Cache-Control: no-cache" with "Cache-Control: no-store" as
per Mark Nottingham comment.
ABNF corrections from Joe Orton and John P Speno.
More corrections from Joe Orton.
Changed 401 to a new status code 102 used solely for authentication.
Added Transition Needed status code (432). Should check if this code
conflicts with anything.
Added new "Expect: 102-continue" header.
Reworked Section 4.3 to describe more error cases and more detailed
implementation instructions.
Disallow TLS Upgrade during SASL authentication (it is fine before or
after). Clarified order of security layers.
Clarified that Authorization header with SASL response MUST NOT be
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used with CONNECT.
Relaxed restriction for mixing SASL session ids on the same
connection in certain cases.
Added new 235/236 status codes for successfully completed
authentication.
Clarified that the body of the original request MUST NOT be sent
until authentication is complete. Updated examples to reflect that.
Added an example with a POST request.
Changes since -04
Reworked the Introduction section.
Updated example 4.7.4 to include Authorization header in CONNECT
request. This saves a round trip.
Added text that the client must use OPTIONS to find out which SASL
mechanisms are supported by the server. Added an example.
Added text regarding the server requiring reauthentication when the
client leaves the realm it authenticated in.
Some clarification about the CONNECT method. Added text that a
CONNECT request should start the authentication exchange.
Incorporated comments from Raif S. Naffah and Keith Burdis.
Changes since -03
Fixed several errors in examples due to change from "sasl-mechanism"
to "sasl-mechanisms".
More comments from Keith Burdis.
Changes since -02
Added discussions about CONNECT and session protection.
Added "Proxy servers considerations" Section. Updated examples to
include headers that prevent caching.
Added Web farm considerations section that talks about a next
response going to a different backend web-server.
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Incorporated many suggestions/corrections from Keith Burdis.
Editorial changes. Cleanup some SHOULDs and MUSTs.
Changes since -01
Added examples
Split ABNF into client and server side. ABNF cleanup.
Many editorial changes.
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