Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Standards Track August 14, 2017
Expires: February 15, 2018
HTTP Authentication with SASL
draft-vanrein-httpauth-sasl-00
Abstract
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
HTTP.
Status of This Memo
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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
3. Standardising User-Aware HTTP . . . . . . . . . . . . . . . . 6
3.1. User-Aware HTTP Servers . . . . . . . . . . . . . . . . . 6
3.2. User-Aware HTTP Clients . . . . . . . . . . . . . . . . . 7
3.3. User-Aware HTTP Proxies . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. SASL Authentication Examples . . . . . . . . . . . . 11
A.1. Mechanism Inquiry . . . . . . . . . . . . . . . . . . . . 11
A.2. SASL EXTERNAL . . . . . . . . . . . . . . . . . . . . . . 11
A.3. SASL ANONYMOUS . . . . . . . . . . . . . . . . . . . . . 12
A.4. SASL SCRAM-SHA-1 . . . . . . . . . . . . . . . . . . . . 12
A.5. GS2 Kerberos5 with TLS Channel Binding . . . . . . . . . 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
and AMQP. 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 has 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.
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Sharing is beneficial in both directions. HTTP benefits by being
able to use GS2 mechanisms [RFC5801] with channel binding to TLS; for
instance Kerberos5 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, becuase it resolves the need
for configuration of initiation paths and practices. Also, it
removes authentication data from URIs, where they are not ideally
placed.
TODO: Does this do justice to current SAML over HTTP?
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 certainly not advisable to use credentials
in those places. Browsers are 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.
2. Embedding SASL in HTTP
This specification integrates the SASL framework [RFC4422] into
mainstream HTTP [RFC2616]. 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
requests.
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. According to SASL, at least tls-unique
[Section 3 of [RFC5929]] must be supported.
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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 those HTTP responses that set a status code 401
or 407, and that are sent when the HTTP server decides to initiate an
authentication exchange.
Initial Requests are those HTTP requests that a client sends to
initiate a fresh SASL authentication. User-Aware Requests are a
variation defined further below, intended for attempts to address
public resources under a given user name.
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.
Final Responses either set a 200 or 403 status code, the first
depicting success and the second depicting failure. Information in a
Final 200 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]. Note that
proper interpretation of the Final 200 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. The Final 403 Response never
contains authentication-related headers.
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 | mech,s2s,realm | text
Initial Request | mech,s2s,c2c,realm | c2s
User-Aware Request | mech,c2s |
Intermediate Response | mech,c2c,c2s,s2c,s2s | text
Intermediate Request | mech,c2c,c2s,s2c,s2s |
Final 200 Response | mech,c2c,name,realm | s2c,c2s,s2s
Final 403 Response | |
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2.2. Authentication Field Definitions
Data for SASL is transported in the following fields:
c2s SASL mechanism data from client to server, that the server MUST
reflect in an following Intermediate Response, or set to an
empty string when absent from an Initial Request.
s2c SASL mechanism data from server to client, that the client MUST
reflect in following Initial Requests or Intermediate Requests.
s2s holds opaque server data which the client MUST reflect in
Intermediate Requests, to implement stateless SASL handling in
the server. This is a requirement for the HTTP Authentication
framework [Section 5.1.2 of [RFC7235]].
c2c holds opaque client data which the server MUST reflect in
Intermediate Responses and Final 200 Responses. This helps to
also make the client stateless.
As in other protocols, it is not safe for all SASL mechanisms to
exchange c2s and s2c messages over unprotected transports. The c2c
and s2s fields 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, c2c
and s2s fields may also need to be concealed from peers and
intermediates.
Whether s2c is supplied in a Final 200 Response depends on the SASL
mechanism, which may or may not have additional data to provide in
this phase. Note that SASL requires empty s2c messages to be
distinguished from absence thereof. When the server provides c2s
and/or s2s data in a Final 200 Response, then it indicates that the
supplied fields MAY provide one-step re-authentication with an empty
s2c string, but the server MAY revoke this privilege at any time and
for any reason; it would respond with an Initial Response in case of
such revocation, but with a quick Final 200 Response if the one-step
re-authentication is still acceptable.
The following fields support SASL within the HTTP Authentication
Framework:
mech selects the SASL mechanism to use. It MUST be reflected from
the preceding message, except: In an Initial Response, the
field is filled with a space-separated list of SASL mechanism
names; In an Initial Request, the client chooses one SASL
mechanism name; In a User-Aware Request, the field is fixated
to ANONYMOUS.
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name is the authorised user@domain.name or domain.name identity.
text is a user-oriented text explaining what information is needed.
realm indicates the scope of authorisation.
3. Standardising User-Aware HTTP
This section specifies an optional extension to HTTP SASL. HTTP
clients, servers and proxies that adopt it will be called User-Aware.
The purpose of User-Aware resources is to have standardised locations
for user pages, including personal well-known URIs [RFC5785].
Most protocols attach a service to a domain name or host name, and
have a form like john@example.com to zoom in on a user of the
service. But although HTTP URIs can express user names, it lacks a
standard interpretation of their meaning. Ideally, access to user
resources should support a mixture of public and protected resources.
A possible approach is hereby proposed, using a User-Aware Request as
introduced in Section 2.
Whether or not content found with a user name is dynamic or static is
orthogonal to the additional resource location information. For
caches, it is therefore possible to cache such content as soon as
they recognise the lack of impact of SASL ANONYMOUS authentication on
the content and its secrecy.
3.1. User-Aware HTTP Servers
User-Aware HTTP servers MUST NOT assume successful authentication
when presented with a User-Aware Request. It MAY however treat the
c2s field as a modifier while locating public resources. What the
impact of this modifier is, if any, is an implementation matter of
the server. Intuitively, the c2s field can be made to feel like a
user name whose data is requested, or a view on data, or just a
sequence of requests that can easily be traced. HTTP servers MAY log
a trace message containing the c2s string.
When a User-Aware Request addresses a resource that is not public,
the server MAY send a Final 403 Response, or otherwise it MUST send
an Initial Response. The HTTP authentication exchange following the
latter should then complete before deciding on resource access.
An HTTP server may redirect a User-Aware Request with the HTTP
Location header. While constructing the new HTTP URI, the server MAY
include the user field from the User-Aware Request, but only when
redirecting to the same protocol, host name and port; configuration
directives often have a relative URI notation for this purpose. A
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newly constructed HTTP URI with a different protocol, host name or
port MUST NOT copy the user name from the User-Aware Request.
3.2. User-Aware HTTP Clients
When an HTTP URI contains a user name, then User-Aware HTTP clients
MUST produce the User-Aware authentication header. The c2s string,
called a trace string in the SASL ANONYMOUS mechanism, is set to the
user name portion of the HTTP URI.
TODO: This assumes that all non-User-Aware HTTP Servers ignore the
User-Aware Request's extras. We tried Nginx, Apache and IIS, and
they agree. RFC 2616 also suggests that the client may choose to
authenticate.
At any time when provided with a User-Aware Request, the User-Aware
HTTP server may decide to request authentication through an Initial
Response. It now matters whether the HTTP URI also contains a
password. With a password available, the User-Aware client SHOULD
select a password-based SASL mechanism that it considers sufficiently
secure. Without a password available, the client is free to select
any of the mechanisms supplied. Even SASL ANONYMOUS may be chosen,
though it must then be sent as an Initial Request, rather than as a
User-Aware Request, so it can be distinguished as an explicit choice
to skip authentication.
HTTP clients may be referenced or redirected from one HTTP URI to
another. The client MUST NOT consider a reference with a username,
and possibly even a password, to be an attack; it may hold a tracing
identity, but so can paths and cookies; and the password can be
useful, especially when it is short-lived and personalised. When
processing a step to another HTTP URI through a notation for an
absolute URI, then a comparison is made of the protocol, user name,
host name and port. Only when all fields match and when no password
is specified SHOULD the User-Aware HTTP client insert a password from
the previous URI to the newly specified URI. In notations that may
hold a relative URI, such as in HTML references, the comparison is
more strict; for those, there MUST NOT be any mentioning of the
protocol, user name, password or host name if the user name and
password are to be taken along to the new URI.
3.3. User-Aware HTTP Proxies
User-Aware HTTP proxies MAY store responses to User-Aware Requests
when no other reasons disallow it, but they MUST use the c2s string
as an additional part of the resource identity. In other words, the
stored resource MUST NOT be returned for requests that are not User-
Aware Requests, or that carry a different c2s string. This only
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concerns Authorization header fields, not those in a Proxy-
Authorization header.
TODO: Do we ever use Proxy-Authorization in User-Aware Requests?
4. Security Considerations
The SASL exchange may be at risk of tampering when the sequence of
HTTP messages is not secured to form one stream. The termination of
such a secure layer MUST also terminate an ongoing SASL handshake.
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.
Channel binding is available in some SASL mechanisms. When used with
HTTP SASL, it binds to the TLS channel, by default using the type
tls-unique [Section 3 of [RFC5929]]. 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.
5. 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
6. References
6.1. Normative References
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[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<http://www.rfc-editor.org/info/rfc2616>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<http://www.rfc-editor.org/info/rfc4422>.
[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,
<http://www.rfc-editor.org/info/rfc4559>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<http://www.rfc-editor.org/info/rfc5056>.
[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,
<http://www.rfc-editor.org/info/rfc5554>.
[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,
<http://www.rfc-editor.org/info/rfc5746>.
[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, <http://www.rfc-editor.org/info/rfc5801>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<http://www.rfc-editor.org/info/rfc5929>.
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[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,
<http://www.rfc-editor.org/info/rfc6595>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<http://www.rfc-editor.org/info/rfc7235>.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015,
<http://www.rfc-editor.org/info/rfc7615>.
[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,
<http://www.rfc-editor.org/info/rfc7627>.
6.2. Informative References
[RFC4505] Zeilenga, K., "Anonymous Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4505,
DOI 10.17487/RFC4505, June 2006,
<http://www.rfc-editor.org/info/rfc4505>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[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,
<http://www.rfc-editor.org/info/rfc5802>.
[RFC7804] Melnikov, A., "Salted Challenge Response HTTP
Authentication Mechanism", RFC 7804, DOI 10.17487/RFC7804,
March 2016, <http://www.rfc-editor.org/info/rfc7804>.
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Appendix A. SASL Authentication Examples
This section is non-normative. It shows a number of examples of SASL
exchanges over HTTP.
A.1. Mechanism Inquiry
Normally, a client is not aware of the need to authenticate, and in
these cases it is sent an Initial Response. For example, the Initial
401 Response could be
WWW-Authenticate: SASL mech="EXTERNAL ANONYMOUS GS2-KRB5"
which indicates that this resource offers a choice, but nonetheless
offers the option of ANONYMOUS authentication (which implies guest
access only) and more uplifting options through EXTERNAL and GS2-KRB5
authentication. The client now chooses a SASL mechanism name for its
Initial Request and both client and server continue to reflect it
back and forth for the remainder of the authentication.
There is no explicit mechanism for asking the server about the
available SASL mechanisms, because this may depend on request aspects
like the location and method. To find out what the options are, it
is best to simply attempt the intended access.
A.2. SASL EXTERNAL
The SASL EXTERNAL mechanism refers the context in which HTTP is run,
such as its transport protocol, and attempts to infer client
authenticity from it. The normal context that can answer to this
desire is the TLS transport; the client identity can be provided in a
TLS handshake using an X.509 certificate, an OpenPGP key, a Kerberos
ticket or an SRP handshake. Each of these allow the derivation of a
user name and its realm.
The EXTERNAL mechanism [Appendix A of [RFC4422]] can simply be stated
by the client, either when the server requests authentication or when
the client wants to provide it, and should pass through without
further interaction. To authenticate as john@example.com, the
request should contain
Authorization: SASL mech="EXTERNAL",realm="example.com",
c2s="john",c2c=data0
which would immediately lead to a Final Response. Note that a Final
200 Response mentions the realm and name that were actually
authorized, and these may differ from the requested names. SASL
EXTERNAL is a very easy-to-use HTTP authentication method, and it is
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arguably the most secure; given the dynamic nature and mixed origin
of HTTP resources, it can be helpful to rely on lower layers such as
the transport layer that is usually less involved in the dynamic
resource mix.
A.3. SASL ANONYMOUS
The ANONYMOUS mechanism is another simple method. It indicates a
lacking desire to authenticate as a particular user. This SASL
method defines the c2s data as trace information [RFC4505] so the
Initial Request
Authorization: SASL mech="ANONYMOUS",realm="example.com",
c2c=data0,c2s="knock, knock"
would probably log the "knock, knock" string but lead to guest-level
access rights. An important use of this SASL method is that it
reflects an explicit client choice to access a resource as a guest,
which brings it across the Initial Response that may be returned from
the HTTP server when a resource requires authentication.
Note the difference with a User-Aware SASL ANONYMOUS header
Authorization: SASL mech="ANONYMOUS",c2s="knocker"
which would not be considered an authentication attempt because the
s2s and c2c fields are missing. Instead, an attempt is made to serve
public data for user knocker for the requested resource location.
The server may decide to initiate an authentication exchange for the
resource location if it deems the contents to not be public.
A.4. SASL SCRAM-SHA-1
SCRAM-SHA-1 is available as a SASL mechanism [RFC5802] as well as an
HTTP mechanism [RFC7804] with identical data. This means that a
translation of the dedicated authentication mechanism onto a SASL
backend of the HTTP server would be possible. This helps to support
the original HTTP mechanism even when only a SASL authentication
backend is available.
SCRAM mechanisms start off with data from the client, so c2s is
already set in the Initial Request. To demonstrate an existing
example [Section 5 of [RFC5802]], we can set user name user with
password pencil,
Authorize: SASL mech="SCRAM-SHA-1",realm="example.com",c2c=data1,
c2s="n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL"
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or alternatively, using base-64 encoding for the c2s field,
Authorize: SASL mech="SCRAM-SHA-1",realm="example.com",c2c=data1,
c2s=biwsbj11c2VyLHI9ZnlrbytkMmxiYkZnT05Sdjlxa3hkYXdM
to which the server would send Intermediate Response
WWW-Authenticate: SASL mech="SCRAM-SHA-1",c2c=data1,s2s=data2,
c2s="n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL",
s2c="r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096",
text="Password for user"
which causes the client to use the password for the Intermediate
Request
Authorize: SASL mech="SCRAM-SHA-1",c2c=data3,s2s=data2,
s2c="r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
s=QSXCR+Q6sek8bf92,i=4096"
c2s="c=biws,r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,
p=v0X8v3Bz2T0CJGbJQyF0X+HI4Ts="
to which the server responds with a Final 200 Response and an
informative header
Authentication-Info: mech="SCRAM-SHA-1",c2c=data3,
s2c="v=rmF9pqV8S7suAoZWja4dJRkFsKQ=",
name="user@example.com",realm="example.com"
and in addition, the server will have performed the requested work.
As with the SASL EXTERNAL mechanism, it is arguably more secure when
the password is entered into a browser popup then in a dynamically
composed application that may not be under full control of the end
user.
A.5. GS2 Kerberos5 with TLS Channel Binding
When a client decides to perform Kerberos5 [RFC4120] authentication,
it should procure a service ticket for HTTP/
www.example.com@EXAMPLE.COM when www.example.com is the web server
host name and EXAMPLE.COM is its realm. Clients usually derive the
host name from a URI, but then need to find the realm; Kerberos5
implementations and standards offer a few methods for that; clients
MUST NOT use insecure mechanisms to derive the realm from the host
name; clients MUST NOT accept realms from HTTP or even TLS for use
with Kerberos5.
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The client now provides an Initial Request to the www.example.com
with the GS2 token in the c2s field, possibly with base-64 encoding
Authorize: SASL mech="GS2-KRB5-PLUS",realm="example.com",
c2c=data0,c2s=base64blob
where the SASL mechanism name indicates Kerberos5 over the GS2 bridge
[RFC5801], with channel binding [RFC5056] [RFC5554] [RFC5929],
presumably to a TLS wrapper. This usually suffices for the server to
produce a Final Response.
Appendix B. Acknowledgements
Author's Address
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
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