Network Working Group A. Melnikov, Ed.
Request for Comments: 4752 Isode
Obsoletes: 2222 November 2006
Category: Standards Track
The Kerberos V5 ("GSSAPI")
Simple Authentication and Security Layer (SASL) Mechanism
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The IETF Trust (2006).
The Simple Authentication and Security Layer (SASL) is a framework
for adding authentication support to connection-based protocols.
This document describes the method for using the Generic Security
Service Application Program Interface (GSS-API) Kerberos V5 in the
This document replaces Section 7.2 of RFC 2222, the definition of the
"GSSAPI" SASL mechanism. This document, together with RFC 4422,
obsoletes RFC 2222.
Table of Contents
1. Introduction ....................................................21.1. Relationship to Other Documents ............................22. Conventions Used in This Document ...............................23. Kerberos V5 GSS-API Mechanism ...................................23.1. Client Side of Authentication Protocol Exchange ............33.2. Server Side of Authentication Protocol Exchange ............43.3. Security Layer .............................................64. IANA Considerations .............................................75. Security Considerations .........................................76. Acknowledgements ................................................87. Changes since RFC 2222 ..........................................88. References ......................................................88.1. Normative References .......................................88.2. Informative References .....................................91. Introduction
This specification documents currently deployed Simple Authentication
and Security Layer (SASL [SASL]) mechanism supporting the Kerberos V5
[KERBEROS] Generic Security Service Application Program Interface
([GSS-API]) mechanism [RFC4121]. The authentication sequence is
described in Section 3. Note that the described authentication
sequence has known limitations, in particular, it lacks channel
bindings and the number of round-trips required to complete
authentication exchange is not minimal. SASL WG is working on a
separate document that should address these limitations.
1.1. Relationship to Other Documents
This document, together with RFC 4422, obsoletes RFC 2222 in its
entirety. This document replaces Section 7.2 of RFC 2222. The
remainder is obsoleted as detailed in Section 1.2 of RFC 4422.
2. Conventions Used in This Document
The key words "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" [KEYWORDS].
3. Kerberos V5 GSS-API Mechanism
The SASL mechanism name for the Kerberos V5 GSS-API mechanism
[RFC4121] is "GSSAPI". Though known as the SASL GSSAPI mechanism,
the mechanism is specifically tied to Kerberos V5 and GSS-API's
Kerberos V5 mechanism.
The GSSAPI SASL mechanism is a "client goes first" SASL mechanism;
i.e., it starts with the client sending a "response" created as
described in the following section.
The implementation MAY set any GSS-API flags or arguments not
mentioned in this specification as is necessary for the
implementation to enforce its security policy.
Note that major status codes returned by GSS_Init_sec_context() or
GSS_Accept_sec_context() other than GSS_S_COMPLETE or
GSS_S_CONTINUE_NEEDED cause authentication failure. Major status
codes returned by GSS_Unwrap() other than GSS_S_COMPLETE (without any
additional supplementary status codes) cause authentication and/or
security layer failure.
3.1. Client Side of Authentication Protocol Exchange
The client calls GSS_Init_sec_context, passing in
input_context_handle of 0 (initially), mech_type of the Kerberos V5
GSS-API mechanism [KRB5GSS], chan_binding of NULL, and targ_name
equal to output_name from GSS_Import_Name called with input_name_type
of GSS_C_NT_HOSTBASED_SERVICE (*) and input_name_string of
"service@hostname" where "service" is the service name specified in
the protocol's profile, and "hostname" is the fully qualified host
name of the server. When calling the GSS_Init_sec_context, the
client MUST pass the integ_req_flag of TRUE (**). If the client will
be requesting a security layer, it MUST also supply to the
GSS_Init_sec_context a mutual_req_flag of TRUE, and a
sequence_req_flag of TRUE. If the client will be requesting a
security layer providing confidentiality protection, it MUST also
supply to the GSS_Init_sec_context a conf_req_flag of TRUE. The
client then responds with the resulting output_token. If
GSS_Init_sec_context returns GSS_S_CONTINUE_NEEDED, then the client
should expect the server to issue a token in a subsequent challenge.
The client must pass the token to another call to
GSS_Init_sec_context, repeating the actions in this paragraph.
(*) Clients MAY use name types other than GSS_C_NT_HOSTBASED_SERVICE
to import servers' acceptor names, but only when they have a priori
knowledge that the servers support alternate name types. Otherwise
clients MUST use GSS_C_NT_HOSTBASED_SERVICE for importing acceptor
(**) Note that RFC 2222 [RFC2222] implementations will not work with
GSS-API implementations that require integ_req_flag to be true. No
implementations of RFC 1964 [KRB5GSS] or RFC 4121 [RFC4121] that
require integ_req_flag to be true are believed to exist and it is
expected that any future update to [RFC4121] will require that
integrity be available even in not explicitly requested by the
When GSS_Init_sec_context returns GSS_S_COMPLETE, the client examines
the context to ensure that it provides a level of protection
permitted by the client's security policy. In particular, if the
integ_avail flag is not set in the context, then no security layer
can be offered or accepted.
If the conf_avail flag is not set in the context, then no security
layer with confidentiality can be offered or accepted. If the
context is acceptable, the client takes the following actions: If the
last call to GSS_Init_sec_context returned an output_token, then the
client responds with the output_token, otherwise the client responds
with no data. The client should then expect the server to issue a
token in a subsequent challenge. The client passes this token to
GSS_Unwrap and interprets the first octet of resulting cleartext as a
bit-mask specifying the security layers supported by the server and
the second through fourth octets as the maximum size output_message
the server is able to receive (in network byte order). If the
resulting cleartext is not 4 octets long, the client fails the
negotiation. The client verifies that the server maximum buffer is 0
if the server does not advertise support for any security layer.
The client then constructs data, with the first octet containing the
bit-mask specifying the selected security layer, the second through
fourth octets containing in network byte order the maximum size
output_message the client is able to receive (which MUST be 0 if the
client does not support any security layer), and the remaining octets
containing the UTF-8 [UTF8] encoded authorization identity.
(Implementation note: The authorization identity is not terminated
with the zero-valued (%x00) octet (e.g., the UTF-8 encoding of the
NUL (U+0000) character)). The client passes the data to GSS_Wrap
with conf_flag set to FALSE and responds with the generated
output_message. The client can then consider the server
3.2. Server Side of Authentication Protocol Exchange
A server MUST NOT advertise support for the "GSSAPI" SASL mechanism
described in this document unless it has acceptor credential for the
Kerberos V GSS-API mechanism [KRB5GSS].
The server passes the initial client response to
GSS_Accept_sec_context as input_token, setting input_context_handle
to 0 (initially), chan_binding of NULL, and a suitable
acceptor_cred_handle (see below). If GSS_Accept_sec_context returns
GSS_S_CONTINUE_NEEDED, the server returns the generated output_token
to the client in challenge and passes the resulting response to
another call to GSS_Accept_sec_context, repeating the actions in this
Servers SHOULD use a credential obtained by calling GSS_Acquire_cred
or GSS_Add_cred for the GSS_C_NO_NAME desired_name and the Object
Identifier (OID) of the Kerberos V5 GSS-API mechanism [KRB5GSS](*).
Servers MAY use GSS_C_NO_CREDENTIAL as an acceptor credential handle.
Servers MAY use a credential obtained by calling GSS_Acquire_cred or
GSS_Add_cred for the server's principal name(s) (**) and the Kerberos
V5 GSS-API mechanism [KRB5GSS].
(*) Unlike GSS_Add_cred the GSS_Acquire_cred uses an OID set of GSS-
API mechanism as an input parameter. The OID set can be created by
using GSS_Create_empty_OID_set and GSS_Add_OID_set_member. It can be
freed by calling the GSS_Release_oid_set.
(**) Use of server's principal names having
GSS_C_NT_HOSTBASED_SERVICE name type and "service@hostname" format,
where "service" is the service name specified in the protocol's
profile, and "hostname" is the fully qualified host name of the
server, is RECOMMENDED. The server name is generated by calling
GSS_Import_name with input_name_type of GSS_C_NT_HOSTBASED_SERVICE
and input_name_string of "service@hostname".
Upon successful establishment of the security context (i.e.,
GSS_Accept_sec_context returns GSS_S_COMPLETE), the server SHOULD
verify that the negotiated GSS-API mechanism is indeed Kerberos V5
[KRB5GSS]. This is done by examining the value of the mech_type
parameter returned from the GSS_Accept_sec_context call. If the
value differs, SASL authentication MUST be aborted.
Upon successful establishment of the security context and if the
server used GSS_C_NO_NAME/GSS_C_NO_CREDENTIAL to create acceptor
credential handle, the server SHOULD also check using the
GSS_Inquire_context that the target_name used by the client matches
- the GSS_C_NT_HOSTBASED_SERVICE "service@hostname" name syntax,
where "service" is the service name specified in the application
- the GSS_KRB5_NT_PRINCIPAL_NAME [KRB5GSS] name syntax for a two-
component principal where the first component matches the service
name specified in the application protocol's profile.
When GSS_Accept_sec_context returns GSS_S_COMPLETE, the server
examines the context to ensure that it provides a level of protection
permitted by the server's security policy. In particular, if the
integ_avail flag is not set in the context, then no security layer
can be offered or accepted. If the conf_avail flag is not set in the
context, then no security layer with confidentiality can be offered
If the context is acceptable, the server takes the following actions:
If the last call to GSS_Accept_sec_context returned an output_token,
the server returns it to the client in a challenge and expects a
reply from the client with no data. Whether or not an output_token
was returned (and after receipt of any response from the client to
such an output_token), the server then constructs 4 octets of data,
with the first octet containing a bit-mask specifying the security
layers supported by the server and the second through fourth octets
containing in network byte order the maximum size output_token the
server is able to receive (which MUST be 0 if the server does not
support any security layer). The server must then pass the plaintext
to GSS_Wrap with conf_flag set to FALSE and issue the generated
output_message to the client in a challenge.
The server must then pass the resulting response to GSS_Unwrap and
interpret the first octet of resulting cleartext as the bit-mask for
the selected security layer, the second through fourth octets as the
maximum size output_message the client is able to receive (in network
byte order), and the remaining octets as the authorization identity.
The server verifies that the client has selected a security layer
that was offered and that the client maximum buffer is 0 if no
security layer was chosen. The server must verify that the src_name
is authorized to act as the authorization identity. After these
verifications, the authentication process is complete. The server is
not expected to return any additional data with the success
3.3. Security Layer
The security layers and their corresponding bit-masks are as follows:
1 No security layer
2 Integrity protection.
Sender calls GSS_Wrap with conf_flag set to FALSE
4 Confidentiality protection.
Sender calls GSS_Wrap with conf_flag set to TRUE
Other bit-masks may be defined in the future; bits that are not
understood must be negotiated off.
When decoding any received data with GSS_Unwrap, the major_status
other than the GSS_S_COMPLETE MUST be treated as a fatal error.
Note that SASL negotiates the maximum size of the output_message to
send. Implementations can use the GSS_Wrap_size_limit call to
determine the corresponding maximum size input_message.
4. IANA Considerations
IANA modified the existing registration for "GSSAPI" as follows:
Family of SASL mechanisms: NO
SASL mechanism name: GSSAPI
Security considerations: See Section 5 of RFC 4752
Published specification: RFC 4752
Person & email address to contact for further information:
Alexey Melnikov <Alexey.Melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: email@example.com
Additional information: This mechanism is for the Kerberos V5
mechanism of GSS-API.
5. Security Considerations
Security issues are discussed throughout this memo.
When constructing the input_name_string, the client SHOULD NOT
canonicalize the server's fully qualified domain name using an
insecure or untrusted directory service.
For compatibility with deployed software, this document requires that
the chan_binding (channel bindings) parameter to GSS_Init_sec_context
and GSS_Accept_sec_context be NULL, hence disallowing use of GSS-API
support for channel bindings. GSS-API channel bindings in SASL is
expected to be supported via a new GSS-API family of SASL mechanisms
(to be introduced in a future document).
Additional security considerations are in the [SASL] and [GSS-API]
specifications. Additional security considerations for the GSS-API
mechanism can be found in [KRB5GSS] and [KERBEROS].
This document replaces Section 7.2 of RFC 2222 [RFC2222] by John G.
Myers. He also contributed significantly to this revision.
Lawrence Greenfield converted text of this document to the XML
Contributions of many members of the SASL mailing list are gratefully
acknowledged, in particular comments from Chris Newman, Nicolas
Williams, Jeffrey Hutzelman, Sam Hartman, Mark Crispin, and Martin
7. Changes since RFC 2222
RFC 2078 [RFC2078] specifies the version of GSS-API used by RFC 2222
[RFC2222], which provided the original version of this specification.
That version of GSS-API did not provide the integ_integ_avail flag as
an input to GSS_Init_sec_context. Instead, integrity was always
requested. RFC 4422 [SASL] requires that when possible, the security
layer negotiation be integrity protected. To meet this requirement
and as part of moving from RFC 2078 [RFC2078] to RFC 2743 [GSS-API],
this specification requires that clients request integrity from
GSS_Init_sec_context so they can use GSS_Wrap to protect the security
layer negotiation. This specification does not require that the
mechanism offer the integrity security layer, simply that the
security layer negotiation be wrapped.
8.1. Normative References
[GSS-API] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[KERBEROS] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[KRB5GSS] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
1964, June 1996.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121, July
[SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
8.2. Informative References
[RFC2078] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
[RFC2222] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC 2222, October 1997.
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