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RFC 3645

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

Pages: 26
Proposed Standard
Updates:  2845

Top   ToC   RFC3645 - Page 1
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.
Top   ToC   RFC3645 - Page 2

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.
Top   ToC   RFC3645 - Page 3
   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:
Top   ToC   RFC3645 - Page 4
                              +----------+
                              |          |
                              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.
Top   ToC   RFC3645 - Page 5
   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.
Top   ToC   RFC3645 - Page 6
   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
Top   ToC   RFC3645 - Page 7
     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.
Top   ToC   RFC3645 - Page 8

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
Top   ToC   RFC3645 - Page 10
   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.
Top   ToC   RFC3645 - Page 11
   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].
Top   ToC   RFC3645 - Page 12

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.
Top   ToC   RFC3645 - Page 13
   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.
Top   ToC   RFC3645 - Page 14
        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.
Top   ToC   RFC3645 - Page 15
   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 = "DNS@server.example.com" 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 "DNS@server.example.com" 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 (DNS@server.example.com.,
          789.client.example.com.server.example.com., context_handle)
     INTERNAL NAME  targ_name             = "DNS@server.example.com"
     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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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: skwan@microsoft.com Praerit Garg Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail: praeritg@microsoft.com James Gilroy Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail: jamesg@microsoft.com Levon Esibov Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail: levone@microsoft.com Randy Hall Lucent Technologies 400 Lapp Road Malvern PA 19355 USA EMail: randyhall@lucent.com Jeff Westhead Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail: jwesth@microsoft.com
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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.