Network Working Group S. Kwan

Network Working Group S. Kwan
Request for Comments: 3645 P. Garg
Updates: 2845 J. Gilroy
Category: Standards Track L. Esibov
J. Westhead
Microsoft Corp.
R. Hall
Lucent Technologies
October 2003

Generic Security Service Algorithm for
Secret Key Transaction Authentication for DNS (GSS-TSIG)

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 Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

The Secret Key Transaction Authentication for DNS (TSIG) protocol
provides transaction level authentication for DNS. TSIG is
extensible through the definition of new algorithms. This document
specifies an algorithm based on the Generic Security Service
Application Program Interface (GSS-API) (RFC2743). This document
updates RFC 2845.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Algorithm Overview . . . . . . . . . . . . . . . . . . . . . . 3
2.1. GSS Details. . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Modifications to the TSIG protocol (RFC 2845). . . . . . 4
3. Client Protocol Details. . . . . . . . . . . . . . . . . . . . 5
3.1. Negotiating Context. . . . . . . . . . . . . . . . . . . 5
3.1.1. Call GSS_Init_sec_context. . . . . . . . . . . . . 6
3.1.2. Send TKEY Query to Server. . . . . . . . . . . . . 8
3.1.3. Receive TKEY Query-Response from Server. . . . . . 8
3.2. Context Established. . . . . . . . . . . . . . . . . . . 11
3.2.1. Terminating a Context. . . . . . . . . . . . . . . 11
4. Server Protocol Details. . . . . . . . . . . . . . . . . . . . 12
4.1. Negotiating Context. . . . . . . . . . . . . . . . . . . 12
4.1.1. Receive TKEY Query from Client . . . . . . . . . . 12
4.1.2. Call GSS_Accept_sec_context. . . . . . . . . . . . 12
4.1.3. Send TKEY Query-Response to Client . . . . . . . . 13
4.2. Context Established. . . . . . . . . . . . . . . . . . . 15
4.2.1. Terminating a Context. . . . . . . . . . . . . . . 15
5. Sending and Verifying Signed Messages. . . . . . . . . . . . . 15
5.1. Sending a Signed Message - Call GSS_GetMIC . . . . . . . 15
5.2. Verifying a Signed Message - Call GSS_VerifyMIC. . . . . 16
6. Example usage of GSS-TSIG algorithm. . . . . . . . . . . . . . 18
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 22
9. Conformance. . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Intellectual Property Statement. . . . . . . . . . . . . . . . 23
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References. . . . . . . . . . . . . . . . . . 24
12.2. Informative References. . . . . . . . . . . . . . . . . 24
13. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
14. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26

1. Introduction

The Secret Key Transaction Authentication for DNS (TSIG) [RFC2845]
protocol was developed to provide a lightweight authentication and
integrity of messages between two DNS entities, such as client and
server or server and server. TSIG can be used to protect dynamic
update messages, authenticate regular message or to off-load
complicated DNSSEC [RFC2535] processing from a client to a server and
still allow the client to be assured of the integrity of the answers.

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The TSIG protocol [RFC2845] is extensible through the definition of
new algorithms. This document specifies an algorithm based on the
Generic Security Service Application Program Interface (GSS-API)
[RFC2743]. GSS-API is a framework that provides an abstraction of
security to the application protocol developer. The security
services offered can include authentication, integrity, and
confidentiality.

The GSS-API framework has several benefits:

* Mechanism and protocol independence. The underlying mechanisms
that realize the security services can be negotiated on the fly
and varied over time. For example, a client and server MAY use
Kerberos [RFC1964] for one transaction, whereas that same server
MAY use SPKM [RFC2025] with a different client.

* The protocol developer is removed from the responsibility of
creating and managing a security infrastructure. For example, the
developer does not need to create new key distribution or key
management systems. Instead the developer relies on the security
service mechanism to manage this on its behalf.

The scope of this document is limited to the description of an
authentication mechanism only. It does not discuss and/or propose an
authorization mechanism. Readers that are unfamiliar with GSS-API
concepts are encouraged to read the characteristics and concepts
section of [RFC2743] before examining this protocol in detail. It is
also assumed that the reader is familiar with [RFC2845], [RFC2930],
[RFC1034] and [RFC1035].

The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", and "MAY" in this document are to be interpreted as
described in BCP 14, RFC 2119 [RFC2119].

2. Algorithm Overview

In GSS, client and server interact to create a "security context".
The security context can be used to create and verify transaction
signatures on messages between the two parties. A unique security
context is required for each unique connection between client and
server.

Creating a security context involves a negotiation between client and
server. Once a context has been established, it has a finite
lifetime for which it can be used to secure messages. Thus there are
three states of a context associated with a connection:

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+----------+
| |
V |
+---------------+ |
| Uninitialized | |
| | |
+---------------+ |
| |
V |
+---------------+ |
| Negotiating | |
| Context | |
+---------------+ |
| |
V |
+---------------+ |
| Context | |
| Established | |
+---------------+ |
| |
+----------+

Every connection begins in the uninitialized state.

2.1. GSS Details

Client and server MUST be locally authenticated and have acquired
default credentials before using this protocol as specified in
Section 1.1.1 "Credentials" in RFC 2743 [RFC2743].

The GSS-TSIG algorithm consists of two stages:

I. Establish security context. The Client and Server use the
GSS_Init_sec_context and GSS_Accept_sec_context APIs to generate
the tokens that they pass to each other using [RFC2930] as a
transport mechanism.

II. Once the security context is established it is used to generate
and verify signatures using GSS_GetMIC and GSS_VerifyMIC APIs.
These signatures are exchanged by the Client and Server as a part
of the TSIG records exchanged in DNS messages sent between the
Client and Server, as described in [RFC2845].

2.2. Modifications to the TSIG protocol (RFC 2845)

Modification to RFC 2845 allows use of TSIG through signing server's
response in an explicitly specified place in multi message exchange
between two DNS entities even if client's request wasn't signed.

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Specifically, Section 4.2 of RFC 2845 MUST be modified as follows:

Replace:
"The server MUST not generate a signed response to an unsigned
request."

With:
"The server MUST not generate a signed response to an unsigned
request, except in case of response to client's unsigned TKEY
query if secret key is established on server side after server
processed client's query. Signing responses to unsigned TKEY
queries MUST be explicitly specified in the description of an
individual secret key establishment algorithm."

3. Client Protocol Details

A unique context is required for each server to which the client
sends secure messages. A context is identified by a context handle.
A client maintains a mapping of servers to handles:

(target_name, key_name, context_handle)

The value key_name also identifies a context handle. The key_name is
the owner name of the TKEY and TSIG records sent between a client and
a server to indicate to each other which context MUST be used to
process the current request.

DNS client and server MAY use various underlying security mechanisms
to establish security context as described in sections 3 and 4. At
the same time, in order to guarantee interoperability between DNS
clients and servers that support GSS-TSIG it is REQUIRED that
security mechanism used by client enables use of Kerberos v5 (see
Section 9 for more information).

3.1. Negotiating Context

In GSS, establishing a security context involves the passing of
opaque tokens between the client and the server. The client
generates the initial token and sends it to the server. The server
processes the token and if necessary, returns a subsequent token to
the client. The client processes this token, and so on, until the
negotiation is complete. The number of times the client and server
exchange tokens depends on the underlying security mechanism. A
completed negotiation results in a context handle.

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The TKEY resource record [RFC2930] is used as the vehicle to transfer
tokens between client and server. The TKEY record is a general
mechanism for establishing secret keys for use with TSIG. For more
information, see [RFC2930].

3.1.1. Call GSS_Init_sec_context

To obtain the first token to be sent to a server, a client MUST call
GSS_Init_sec_context API.

The following input parameters MUST be used. The outcome of the call
is indicated with the output values below. Consult Sections 2.2.1,
"GSS_Init_sec_context call", of [RFC2743] for syntax definitions.

INPUTS
CREDENTIAL HANDLE claimant_cred_handle = NULL (NULL specifies "use
default"). Client MAY instead specify some other valid
handle to its credentials.
CONTEXT HANDLE input_context_handle = 0
INTERNAL NAME targ_name = "DNS@<target_server_name>"
OBJECT IDENTIFIER mech_type = Underlying security
mechanism chosen by implementers. To guarantee
interoperability of the implementations of the GSS-TSIG
mechanism client MUST specify a valid underlying security
mechanism that enables use of Kerberos v5 (see Section 9 for
more information).
OCTET STRING input_token = NULL
BOOLEAN replay_det_req_flag = TRUE
BOOLEAN mutual_req_flag = TRUE
BOOLEAN deleg_req_flag = TRUE
BOOLEAN sequence_req_flag = TRUE
BOOLEAN anon_req_flag = FALSE
BOOLEAN integ_req_flag = TRUE
INTEGER lifetime_req = 0 (0 requests a default
value). Client MAY instead specify another upper bound for the
lifetime of the context to be established in seconds.
OCTET STRING chan_bindings = Any valid channel bindings
as specified in Section 1.1.6 "Channel Bindings" in [RFC2743]

OUTPUTS
INTEGER major_status
CONTEXT HANDLE output_context_handle
OCTET STRING output_token
BOOLEAN replay_det_state
BOOLEAN mutual_state
INTEGER minor_status
OBJECT IDENTIFIER mech_type
BOOLEAN deleg_state

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BOOLEAN sequence_state
BOOLEAN anon_state
BOOLEAN trans_state
BOOLEAN prot_ready_state
BOOLEAN conf_avail
BOOLEAN integ_avail
INTEGER lifetime_rec

If returned major_status is set to one of the following errors:

GSS_S_DEFECTIVE_TOKEN
GSS_S_DEFECTIVE_CREDENTIAL
GSS_S_BAD_SIG (GSS_S_BAD_MIC)
GSS_S_NO_CRED
GSS_S_CREDENTIALS_EXPIRED
GSS_S_BAD_BINDINGS
GSS_S_OLD_TOKEN
GSS_S_DUPLICATE_TOKEN
GSS_S_NO_CONTEXT
GSS_S_BAD_NAMETYPE
GSS_S_BAD_NAME
GSS_S_BAD_MECH
GSS_S_FAILURE

then the client MUST abandon the algorithm and MUST NOT use the GSS-
TSIG algorithm to establish this security context. This document
does not prescribe which other mechanism could be used to establish a
security context. Next time when this client needs to establish
security context, the client MAY use GSS-TSIG algorithm.

Success values of major_status are GSS_S_CONTINUE_NEEDED and
GSS_S_COMPLETE. The exact success code is important during later
processing.

The values of replay_det_state and mutual_state indicate if the
security package provides replay detection and mutual authentication,
respectively. If returned major_status is GSS_S_COMPLETE AND one or
both of these values are FALSE, the client MUST abandon this
algorithm.

Client's behavior MAY depend on other OUTPUT parameters according to
the policy local to the client.

The handle output_context_handle is unique to this negotiation and is
stored in the client's mapping table as the context_handle that maps
to target_name.

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3.1.2. Send TKEY Query to Server

An opaque output_token returned by GSS_Init_sec_context is
transmitted to the server in a query request with QTYPE=TKEY. The
token itself will be placed in a Key Data field of the RDATA field in
the TKEY resource record in the additional records section of the
query. The owner name of the TKEY resource record set queried for
and the owner name of the supplied TKEY resource record in the
additional records section MUST be the same. This name uniquely
identifies the security context to both the client and server, and
thus the client SHOULD use a value which is globally unique as
described in [RFC2930]. To achieve global uniqueness, the name MAY
contain a UUID/GUID [ISO11578].

TKEY Record
NAME = client-generated globally unique domain name string
(as described in [RFC2930])
RDATA
Algorithm Name = gss-tsig
Mode = 3 (GSS-API negotiation - per [RFC2930])
Key Size = size of output_token in octets
Key Data = output_token

The remaining fields in the TKEY RDATA, i.e., Inception, Expiration,
Error, Other Size and Data Fields, MUST be set according to
[RFC2930].

The query is transmitted to the server.

Note: if the original client call to GSS_Init_sec_context returned
any major_status other than GSS_S_CONTINUE_NEEDED or GSS_S_COMPLETE,
then the client MUST NOT send TKEY query. Client's behavior in this
case is described above in Section 3.1.1.

3.1.3. Receive TKEY Query-Response from Server

Upon the reception of the TKEY query the DNS server MUST respond
according to the description in Section 4. This section specifies
the behavior of the client after it receives the matching response to
its query.

The next processing step depends on the value of major_status from
the most recent call that client performed to GSS_Init_sec_context:
either GSS_S_COMPLETE or GSS_S_CONTINUE.

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3.1.3.1. Value of major_status == GSS_S_COMPLETE

If the last call to GSS_Init_sec_context yielded a major_status value
of GSS_S_COMPLETE and a non-NULL output_token was sent to the server,
then the client side component of the negotiation is complete and the
client is awaiting confirmation from the server.

Confirmation is in the form of a query response with RCODE=NOERROR
and with the last client supplied TKEY record in the answer section
of the query. The response MUST be signed with a TSIG record. Note
that the server is allowed to sign a response to unsigned client's
query due to modification to the RFC 2845 specified in Section 2.2
above. The signature in the TSIG record MUST be verified using the
procedure detailed in section 5, Sending and Verifying Signed
Messages. If the response is not signed, OR if the response is
signed but the signature is invalid, then an attacker has tampered
with the message in transit or has attempted to send the client a
false response. In this case, the client MAY continue waiting for a
response to its last TKEY query until the time period since the
client sent last TKEY query expires. Such a time period is specified
by the policy local to the client. This is a new option that allows
the DNS client to accept multiple answers for one query ID and select
one (not necessarily the first one) based on some criteria.

If the signature is verified, the context state is advanced to
Context Established. Proceed to section 3.2 for usage of the
security context.

3.1.3.2. Value of major_status == GSS_S_CONTINUE_NEEDED

If the last call to GSS_Init_sec_context yielded a major_status value
of GSS_S_CONTINUE_NEEDED, then the negotiation is not yet complete.
The server will return to the client a query response with a TKEY
record in the Answer section. If the DNS message error is not
NO_ERROR or error field in the TKEY record is not 0 (i.e., no error),
then the client MUST abandon this negotiation sequence. The client
MUST delete an active context by calling GSS_Delete_sec_context
providing the associated context_handle. The client MAY repeat the
negotiation sequence starting with the uninitialized state as
described in section 3.1. To prevent infinite looping the number of
attempts to establish a security context MUST be limited to ten or
less.

If the DNS message error is NO_ERROR and the error field in the TKEY
record is 0 (i.e., no error), then the client MUST pass a token
specified in the Key Data field in the TKEY resource record to

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GSS_Init_sec_context using the same parameters values as in previous
call except values for CONTEXT HANDLE input_context_handle and OCTET
STRING input_token as described below:

INPUTS
CONTEXT HANDLE input_context_handle = context_handle (this is the
context_handle corresponding to the key_name which is the
owner name of the TKEY record in the answer section in the
TKEY query response)

OCTET STRING input_token = token from Key field of
TKEY record

Depending on the following OUTPUT values of GSS_Init_sec_context

INTEGER major_status
OCTET STRING output_token

the client MUST take one of the following actions:

If OUTPUT major_status is set to one of the following values:

GSS_S_DEFECTIVE_TOKEN
GSS_S_DEFECTIVE_CREDENTIAL
GSS_S_BAD_SIG (GSS_S_BAD_MIC)
GSS_S_NO_CRED
GSS_S_CREDENTIALS_EXPIRED
GSS_S_BAD_BINDINGS
GSS_S_OLD_TOKEN
GSS_S_DUPLICATE_TOKEN
GSS_S_NO_CONTEXT
GSS_S_BAD_NAMETYPE
GSS_S_BAD_NAME
GSS_S_BAD_MECH
GSS_S_FAILURE

the client MUST abandon this negotiation sequence. This means that
the client MUST delete an active context by calling
GSS_Delete_sec_context providing the associated context_handle. The
client MAY repeat the negotiation sequence starting with the
uninitialized state as described in section 3.1. To prevent infinite
looping the number of attempts to establish a security context MUST
be limited to ten or less.

If OUTPUT major_status is GSS_S_CONTINUE_NEEDED OR GSS_S_COMPLETE
then client MUST act as described below.

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If the response from the server was signed, and the OUTPUT
major_status is GSS_S_COMPLETE,then the signature in the TSIG record
MUST be verified using the procedure detailed in section 5, Sending
and Verifying Signed Messages. If the signature is invalid, then the
client MUST abandon this negotiation sequence. This means that the
client MUST delete an active context by calling
GSS_Delete_sec_context providing the associated context_handle. The
client MAY repeat the negotiation sequence starting with the
uninitialized state as described in section 3.1. To prevent infinite
looping the number of attempts to establish a security context MUST
be limited to ten or less.

If major_status is GSS_S_CONTINUE_NEEDED the negotiation is not yet
finished. The token output_token MUST be passed to the server in a
TKEY record by repeating the negotiation sequence beginning with
section 3.1.2. The client MUST place a limit on the number of
continuations in a context negotiation to prevent endless looping.
Such limit SHOULD NOT exceed value of 10.

If major_status is GSS_S_COMPLETE and output_token is non-NULL, the
client-side component of the negotiation is complete but the token
output_token MUST be passed to the server by repeating the
negotiation sequence beginning with section 3.1.2.

If major_status is GSS_S_COMPLETE and output_token is NULL, context
negotiation is complete. The context state is advanced to Context
Established. Proceed to section 3.2 for usage of the security
context.

3.2. Context Established

When context negotiation is complete, the handle context_handle MUST
be used for the generation and verification of transaction
signatures.

The procedures for sending and receiving signed messages are
described in section 5, Sending and Verifying Signed Messages.

3.2.1. Terminating a Context

When the client is not intended to continue using the established
security context, the client SHOULD delete an active context by
calling GSS_Delete_sec_context providing the associated
context_handle, AND client SHOULD delete the established context on
the DNS server by using TKEY RR with the Mode field set to 5, i.e.,
"key deletion" [RFC2930].

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4. Server Protocol Details

As on the client-side, the result of a successful context negotiation
is a context handle used in future generation and verification of the
transaction signatures.

A server MAY be managing several contexts with several clients.
Clients identify their contexts by providing a key name in their
request. The server maintains a mapping of key names to handles:

(key_name, context_handle)

4.1. Negotiating Context

A server MUST recognize TKEY queries as security context negotiation
messages.

4.1.1. Receive TKEY Query from Client

Upon receiving a query with QTYPE = TKEY, the server MUST examine
whether the Mode and Algorithm Name fields of the TKEY record in the
additional records section of the message contain values of 3 and
gss-tsig, respectively. If they do, then the (key_name,
context_handle) mapping table is searched for the key_name matching
the owner name of the TKEY record in the additional records section
of the query. If the name is found in the table and the security
context for this name is established and not expired, then the server
MUST respond to the query with BADNAME error in the TKEY error field.
If the name is found in the table and the security context is not
established, the corresponding context_handle is used in subsequent
GSS operations. If the name is found but the security context is
expired, then the server deletes this security context, as described
in Section 4.2.1, and interprets this query as a start of new
security context negotiation and performs operations described in
Section 4.1.2 and 4.1.3. If the name is not found, then the server
interprets this query as a start of new security context negotiation
and performs operations described in Section 4.1.2 and 4.1.3.

4.1.2. Call GSS_Accept_sec_context

The server performs its side of a context negotiation by calling
GSS_Accept_sec_context. The following input parameters MUST be used.
The outcome of the call is indicated with the output values below.
Consult Sections 2.2.2 "GSS_Accept_sec_context call" of the RFC 2743
[RFC2743] for syntax definitions.

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INPUTS
CONTEXT HANDLE input_context_handle = 0 if new negotiation,
context_handle matching
key_name if ongoing negotiation
OCTET STRING input_token = token specified in the Key
field from TKEY RR (from Additional records Section of
the client's query)

CREDENTIAL HANDLE acceptor_cred_handle = NULL (NULL specifies "use
default"). Server MAY instead specify some other valid
handle to its credentials.
OCTET STRING chan_bindings = Any valid channel bindings
as specified in Section 1.1.6 "Channel Bindings" in [RFC2743]

OUTPUTS
INTEGER major_status
CONTEXT_HANDLE output_context_handle
OCTET STRING output_token
INTEGER minor_status
INTERNAL NAME src_name
OBJECT IDENTIFIER mech_type
BOOLEAN deleg_state
BOOLEAN mutual_state
BOOLEAN replay_det_state
BOOLEAN sequence_state
BOOLEAN anon_state
BOOLEAN trans_state
BOOLEAN prot_ready_state
BOOLEAN conf_avail
BOOLEAN integ_avail
INTEGER lifetime_rec
CONTEXT_HANDLE delegated_cred_handle

If this is the first call to GSS_Accept_sec_context in a new
negotiation, then output_context_handle is stored in the server's
key-mapping table as the context_handle that maps to the name of the
TKEY record.

4.1.3. Send TKEY Query-Response to Client

The server MUST respond to the client with a TKEY query response with
RCODE = NOERROR, that contains a TKEY record in the answer section.

If OUTPUT major_status is one of the following errors the error field
in the TKEY record set to BADKEY.

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GSS_S_DEFECTIVE_TOKEN
GSS_S_DEFECTIVE_CREDENTIAL
GSS_S_BAD_SIG (GSS_S_BAD_MIC)
GSS_S_DUPLICATE_TOKEN
GSS_S_OLD_TOKEN
GSS_S_NO_CRED
GSS_S_CREDENTIALS_EXPIRED
GSS_S_BAD_BINDINGS
GSS_S_NO_CONTEXT
GSS_S_BAD_MECH
GSS_S_FAILURE

If OUTPUT major_status is set to GSS_S_COMPLETE or
GSS_S_CONTINUE_NEEDED then server MUST act as described below.

If major_status is GSS_S_COMPLETE the server component of the
negotiation is finished. If output_token is non-NULL, then it MUST
be returned to the client in a Key Data field of the RDATA in TKEY.
The error field in the TKEY record is set to NOERROR. The message
MUST be signed with a TSIG record as described in section 5, Sending
and Verifying Signed Messages. Note that server is allowed to sign a
response to unsigned client's query due to modification to the RFC
2845 specified in Section 2.2 above. The context state is advanced
to Context Established. Section 4.2 discusses the usage of the
security context.

If major_status is GSS_S_COMPLETE and output_token is NULL, then the
TKEY record received from the client MUST be returned in the Answer
section of the response. The message MUST be signed with a TSIG
record as described in section 5, Sending and Verifying Signed
Messages. Note that server is allowed to sign a response to unsigned
client's query due to modification to the RFC 2845 specified in
section 2.2 above. The context state is advanced to Context
Established. Section 4.2 discusses the usage of the security
context.

If major_status is GSS_S_CONTINUE_NEEDED, the server component of the
negotiation is not yet finished. The server responds to the TKEY
query with a standard query response, placing in the answer section a
TKEY record containing output_token in the Key Data RDATA field. The
error field in the TKEY record is set to NOERROR. The server MUST
limit the number of times that a given context is allowed to repeat,
to prevent endless looping. Such limit SHOULD NOT exceed value of
10.

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In all cases, except if major_status is GSS_S_COMPLETE and
output_token is NULL, other TKEY record fields MUST contain the
following values:

NAME = key_name
RDATA
Algorithm Name = gss-tsig
Mode = 3 (GSS-API negotiation - per [RFC2930])
Key Size = size of output_token in octets

The remaining fields in the TKEY RDATA, i.e., Inception, Expiration,
Error, Other Size and Data Fields, MUST be set according to
[RFC2930].

4.2. Context Established

When context negotiation is complete, the handle context_handle is
used for the generation and verification of transaction signatures.
The handle is valid for a finite amount of time determined by the
underlying security mechanism. A server MAY unilaterally terminate a
context at any time (see section 4.2.1).

Server SHOULD limit the amount of memory used to cache established
contexts.

The procedures for sending and receiving signed messages are given in
section 5, Sending and Verifying Signed Messages.

4.2.1. Terminating a Context

A server can terminate any established context at any time. The
server MAY hint to the client that the context is being deleted by
including a TKEY RR in a response with the Mode field set to 5, i.e.,
"key deletion" [RFC2930]. An active context is deleted by calling
GSS_Delete_sec_context providing the associated context_handle.

5. Sending and Verifying Signed Messages

5.1. Sending a Signed Message - Call GSS_GetMIC

The procedure for sending a signature-protected message is specified
in [RFC2845]. The data to be passed to the signature routine
includes the whole DNS message with specific TSIG variables appended.
For the exact format, see [RFC2845]. For this protocol, use the
following TSIG variable values:

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TSIG Record
NAME = key_name that identifies this context
RDATA
Algorithm Name = gss-tsig

Assign the remaining fields in the TSIG RDATA appropriate values as
described in [RFC2845].

The signature is generated by calling GSS_GetMIC. The following
input parameters MUST be used. The outcome of the call is indicated
with the output values specified below. Consult Sections 2.3.1
"GSS_GetMIC call" of the RFC 2743[RFC2743] for syntax definitions.

INPUTS
CONTEXT HANDLE context_handle = context_handle for key_name
OCTET STRING message = outgoing message plus TSIG
variables (per [RFC2845])
INTEGER qop_req = 0 (0 requests a default
value). Caller MAY instead specify other valid value (for
details see Section 1.2.4 in [RFC2743])

OUTPUTS
INTEGER major_status
INTEGER minor_status
OCTET STRING per_msg_token

If major_status is GSS_S_COMPLETE, then signature generation
succeeded. The signature in per_msg_token is inserted into the
Signature field of the TSIG RR and the message is transmitted.

If major_status is GSS_S_CONTEXT_EXPIRED, GSS_S_CREDENTIALS_EXPIRED
or GSS_S_FAILURE the caller MUST delete the security context, return
to the uninitialized state and SHOULD negotiate a new security
context, as described above in Section 3.1

If major_status is GSS_S_NO_CONTEXT, the caller MUST remove the entry
for key_name from the (target_ name, key_name, context_handle)
mapping table, return to the uninitialized state and SHOULD negotiate
a new security context, as described above in Section 3.1

If major_status is GSS_S_BAD_QOP, the caller SHOULD repeat the
GSS_GetMIC call with allowed QOP value. The number of such
repetitions MUST be limited to prevent infinite loops.

5.2. Verifying a Signed Message - Call GSS_VerifyMIC

The procedure for verifying a signature-protected message is
specified in [RFC2845].

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The NAME of the TSIG record determines which context_handle maps to
the context that MUST be used to verify the signature. If the NAME
does not map to an established context, the server MUST send a
standard TSIG error response to the client indicating BADKEY in the
TSIG error field (as described in [RFC2845]).

For the GSS algorithm, a signature is verified by using
GSS_VerifyMIC:

INPUTS
CONTEXT HANDLE context_handle = context_handle for key_name
OCTET STRING message = incoming message plus TSIG
variables (per [RFC2845])
OCTET STRING per_msg_token = Signature field from TSIG RR

OUTPUTS
INTEGER major_status
INTEGER minor_status
INTEGER qop_state

If major_status is GSS_S_COMPLETE, the signature is authentic and the
message was delivered intact. Per [RFC2845], the timer values of the
TSIG record MUST also be valid before considering the message to be
authentic. The caller MUST not act on the request or response in the
message until these checks are verified.

When a server is processing a client request, the server MUST send a
standard TSIG error response to the client indicating BADKEY in the
TSIG error field as described in [RFC2845], if major_status is set to
one of the following values

GSS_S_DEFECTIVE_TOKEN
GSS_S_BAD_SIG (GSS_S_BAD_MIC)
GSS_S_DUPLICATE_TOKEN
GSS_S_OLD_TOKEN
GSS_S_UNSEQ_TOKEN
GSS_S_GAP_TOKEN
GSS_S_CONTEXT_EXPIRED
GSS_S_NO_CONTEXT
GSS_S_FAILURE

If the timer values of the TSIG record are invalid, the message MUST
NOT be considered authentic. If this error checking fails when a
server is processing a client request, the appropriate error response
MUST be sent to the client according to [RFC2845].

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6. Example usage of GSS-TSIG algorithm

This Section describes an example where a Client, client.example.com,
and a Server, server.example.com, establish a security context
according to the algorithm described above.

I. Client initializes security context negotiation

To establish a security context with a server, server.example.com, the
Client calls GSS_Init_sec_context with the following parameters.
(Note that some INPUT and OUTPUT parameters not critical for this
algorithm are not described in this example.)

CONTEXT HANDLE input_context_handle = 0
INTERNAL NAME targ_name = "[email protected]"
OCTET STRING input_token = NULL
BOOLEAN replay_det_req_flag = TRUE
BOOLEAN mutual_req_flag = TRUE

The OUTPUTS parameters returned by GSS_Init_sec_context include
INTEGER major_status = GSS_S_CONTINUE_NEEDED
CONTEXT HANDLE output_context_handle context_handle
OCTET STRING output_token output_token
BOOLEAN replay_det_state = TRUE
BOOLEAN mutual_state = TRUE

Client verifies that replay_det_state and mutual_state values are
TRUE. Since the major_status is GSS_S_CONTINUE_NEEDED, which is a
success OUTPUT major_status value, client stores context_handle that
maps to "[email protected]" and proceeds to the next step.

II. Client sends a query with QTYPE = TKEY to server

Client sends a query with QTYPE = TKEY for a client-generated globally
unique domain name string, 789.client.example.com.server.example.com.
Query contains a TKEY record in its Additional records section with
the following fields. (Note that some fields not specific to this
algorithm are not specified.)

NAME = 789.client.example.com.server.example.com.
RDATA
Algorithm Name = gss-tsig
Mode = 3 (GSS-API negotiation - per [RFC2930])
Key Size = size of output_token in octets
Key Data = output_token

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After the key_name 789.client.example.com.server.example.com.
is generated it is stored in the client's (target_name, key_name,
context_handle) mapping table.

III. Server receives a query with QTYPE = TKEY

When server receives a query with QTYPE = TKEY, the server verifies
that Mode and Algorithm fields in the TKEY record in the Additional
records section of the query are set to 3 and "gss-tsig" respectively.
It finds that the key_name 789.client.example.com.server.example.com.
is not listed in its (key_name, context_handle) mapping table.

IV. Server calls GSS_Accept_sec_context

To continue security context negotiation server calls
GSS_Accept_sec_context with the following parameters. (Note that
some INPUT and OUTPUT parameters not critical for this algorithm
are not described in this example.)

INPUTS
CONTEXT HANDLE input_context_handle = 0
OCTET STRING input_token = token specified in the Key
field from TKEY RR (from Additional
records section of the client's query)

The OUTPUTS parameters returned by GSS_Accept_sec_context include
INTEGER major_status = GSS_S_CONTINUE_NEEDED
CONTEXT_HANDLE output_context_handle context_handle
OCTET STRING output_token output_token

Server stores the mapping of the
789.client.example.com.server.example.com. to OUTPUT context_handle
in its (key_name, context_handle) mapping table.

V. Server responds to the TKEY query

Since the major_status = GSS_S_CONTINUE_NEEDED in the last server's
call to GSS_Accept_sec_context, the server responds to the TKEY query
placing in the answer section a TKEY record containing output_token in
the Key Data RDATA field. The error field in the TKEY record is set
to 0. The RCODE in the query response is set to NOERROR.

VI. Client processes token returned by server

When the client receives the TKEY query response from the server, the
client calls GSS_Init_sec_context with the following parameters.
(Note that some INPUT and OUTPUT parameters not critical for this
algorithm are not described in this example.)

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CONTEXT HANDLE input_context_handle = the context_handle stored
in the client's mapping table entry ([email protected].,
789.client.example.com.server.example.com., context_handle)
INTERNAL NAME targ_name = "[email protected]"
OCTET STRING input_token = token from Key field of TKEY
record from the Answer section of the server's response
BOOLEAN replay_det_req_flag = TRUE
BOOLEAN mutual_req_flag = TRUE

The OUTPUTS parameters returned by GSS_Init_sec_context include
INTEGER major_status = GSS_S_COMPLETE
CONTEXT HANDLE output_context_handle = context_handle
OCTET STRING output_token = output_token
BOOLEAN replay_det_state = TRUE
BOOLEAN mutual_state = TRUE

Since the major_status is set to GSS_S_COMPLETE the client side
security context is established, but since the output_token is not
NULL client MUST send a TKEY query to the server as described below.

VII. Client sends a query with QTYPE = TKEY to server

Client sends to the server a TKEY query for the
789.client.example.com.server.example.com. name. Query contains a
TKEY record in its Additional records section with the following
fields. (Note that some INPUT and OUTPUT parameters not critical to
this algorithm are not described in this example.)

NAME = 789.client.example.com.server.example.com.
RDATA
Algorithm Name = gss-tsig
Mode = 3 (GSS-API negotiation - per [RFC2930])
Key Size = size of output_token in octets
Key Data = output_token

VIII. Server receives a TKEY query

When the server receives a TKEY query, the server verifies that Mode
and Algorithm fields in the TKEY record in the Additional records
section of the query are set to 3 and gss-tsig, respectively. It
finds that the key_name 789.client.example.com.server.example.com. is
listed in its (key_name, context_handle) mapping table.

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IX. Server calls GSS_Accept_sec_context

To continue security context negotiation server calls
GSS_Accept_sec_context with the following parameters (Note that some
INPUT and OUTPUT parameters not critical for this algorithm are not
described in this example)

INPUTS
CONTEXT HANDLE input_context_handle = context_handle from the
(789.client.example.com.server.example.com., context_handle)
entry in the server's mapping table
OCTET STRING input_token = token specified in the Key
field of TKEY RR (from Additional records Section of
the client's query)

The OUTPUTS parameters returned by GSS_Accept_sec_context include
INTEGER major_status = GSS_S_COMPLETE
CONTEXT_HANDLE output_context_handle = context_handle
OCTET STRING output_token = NULL

Since major_status = GSS_S_COMPLETE, the security context on the
server side is established, but the server still needs to respond to
the client's TKEY query, as described below. The security context
state is advanced to Context Established.

X. Server responds to the TKEY query

Since the major_status = GSS_S_COMPLETE in the last server's call to
GSS_Accept_sec_context and the output_token is NULL, the server
responds to the TKEY query placing in the answer section a TKEY record
that was sent by the client in the Additional records section of the
client's latest TKEY query. In addition, this server places a
TSIG record in additional records section of its response. Server
calls GSS_GetMIC to generate a signature to include it in the TSIG
record. The server specifies the following GSS_GetMIC INPUT
parameters:

CONTEXT HANDLE context_handle = context_handle from the
(789.client.example.com.server.example.com., context_handle)
entry in the server's mapping table
OCTET STRING message = outgoing message plus TSIG
variables (as described in [RFC2845])

The OUTPUTS parameters returned by GSS_GetMIC include
INTEGER major_status = GSS_S_COMPLETE
OCTET STRING per_msg_token

Signature field in the TSIG record is set to per_msg_token.

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XI. Client processes token returned by server

Client receives the TKEY query response from the server. Since the
major_status was GSS_S_COMPLETE in the last client's call to
GSS_Init_sec_context, the client verifies that the server's response
is signed. To validate the signature, the client calls
GSS_VerifyMIC with the following parameters:

INPUTS
CONTEXT HANDLE context_handle = context_handle for
789.client.example.com.server.example.com. key_name
OCTET STRING message = incoming message plus TSIG
variables (as described in [RFC2845])
OCTET STRING per_msg_token = Signature field from TSIG RR
included in the server's query response

Since the OUTPUTS parameter major_status = GSS_S_COMPLETE, the
signature is validated, security negotiation is complete and the
security context state is advanced to Context Established. These
client and server will use the established security context to sign
and validate the signatures when they exchange packets with each
other until the context expires.

7. Security Considerations

This document describes a protocol for DNS security using GSS-API.
The security provided by this protocol is only as effective as the
security provided by the underlying GSS mechanisms.

All the security considerations from RFC 2845, RFC 2930 and RFC 2743
apply to the protocol described in this document.

8. IANA Considerations

The IANA has reserved the TSIG Algorithm name gss-tsig for the use in
the Algorithm fields of TKEY and TSIG resource records. This
Algorithm name refers to the algorithm described in this document.
The requirement to have this name registered with IANA is specified
in RFC 2845.

9. Conformance

The GSS API using SPNEGO [RFC2478] provides maximum flexibility to
choose the underlying security mechanisms that enables security
context negotiation. GSS API using SPNEGO [RFC2478] enables client
and server to negotiate and choose such underlying security
mechanisms on the fly. To support such flexibility, DNS clients and
servers SHOULD specify SPNEGO mech_type in their GSS API calls. At

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the same time, in order to guarantee interoperability between DNS
clients and servers that support GSS-TSIG it is required that

- DNS servers specify SPNEGO mech_type
- GSS APIs called by DNS client support Kerberos v5
- GSS APIs called by DNS server support SPNEGO [RFC2478] and
Kerberos v5.

In addition to these, GSS APIs used by DNS client and server MAY also
support other underlying security mechanisms.

10. 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.

11. Acknowledgements

The authors of this document would like to thank the following people
for their contribution to this specification: Chuck Chan, Mike
Swift, Ram Viswanathan, Olafur Gudmundsson, Donald E. Eastlake, 3rd
and Erik Nordmark.

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RFC 3645 GSS-TSIG October 2003

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2478] Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
Negotiation Mechanism", RFC 2478, December 1998.

[RFC2743] Linn, J., "Generic Security Service Application Program
Interface, Version 2 , Update 1", RFC 2743, January 2000.

[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D. and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.

[RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.

12.2. Informative References

[ISO11578] "Information technology", "Open Systems Interconnection",
"Remote Procedure Call", ISO/IEC 11578:1996,
http://www.iso.ch/cate/d2229.html.

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1034, November 1987.

[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
1964, June 1996.

[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism
(SPKM)", RFC 2025, October 1996.

[RFC2137] Eastlake 3rd, D., "Secure Domain Name System Dynamic
Update", RFC 2137, April 1997.

[RFC2535] Eastlake 3rd, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.

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13. Authors' Addresses

Stuart Kwan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: [email protected]

Praerit Garg
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: [email protected]

James Gilroy
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: [email protected]

Levon Esibov
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: [email protected]

Randy Hall
Lucent Technologies
400 Lapp Road
Malvern PA 19355
USA
EMail: [email protected]

Jeff Westhead
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: [email protected]

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14. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.

Kwan, et al. Standards Track [Page 26]