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NFSv4                                                         W. Adamson
Internet-Draft                                                    NetApp
Intended status: Standards Track                             N. Williams
Expires: July 31, 2016                                      Cryptonector
                                                        January 28, 2016

             Remote Procedure Call (RPC) Security Version 3


   This document specifies version 3 of the Remote Procedure Call (RPC)
   security protocol (RPCSEC_GSS).  This protocol provides support for
   multi-principal authentication of client hosts and user principals to
   a server (constructed by generic composition), security label
   assertions for multi-level and type enforcement, structured privilege
   assertions, and channel bindings.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 31, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
     1.1.  Added Functionality . . . . . . . . . . . . . . . . . . .   4
     1.2.  XDR Code Extraction . . . . . . . . . . . . . . . . . . .   5
   2.  The RPCSEC_GSSv3 Protocol . . . . . . . . . . . . . . . . . .   5
     2.1.  Compatibility with RPCSEC_GSSv2 . . . . . . . . . . . . .   6
     2.2.  Version Negotiation . . . . . . . . . . . . . . . . . . .   6
     2.3.  New REPLY Verifier  . . . . . . . . . . . . . . . . . . .   6
     2.4.  XDR Code Preliminaries  . . . . . . . . . . . . . . . . .   7
     2.5.  RPCSEC_GSS_BIND_CHANNEL Operation . . . . . . . . . . . .   9
     2.6.  New auth_stat Values  . . . . . . . . . . . . . . . . . .   9
     2.7.  New Control Procedures  . . . . . . . . . . . . . . . . .  10
       2.7.1.  New Control Procedure - RPCSEC_GSS_CREATE . . . . . .  10
       2.7.2.  New Control Procedure - RPCSEC_GSS_LIST . . . . . . .  18
     2.8.  Extensibility . . . . . . . . . . . . . . . . . . . . . .  19
   3.  Operational Recommendation for Deployment . . . . . . . . . .  20
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  22
   Appendix B.  RFC Editor Notes . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction and Motivation

   The original RPCSEC_GSS protocol [RFC2203] provided for
   authentication of RPC clients and servers to each other using the
   Generic Security Services Application Programming Interface (GSS-API)
   [RFC2743].  The second version of RPCSEC_GSS [RFC5403]  added support
   for channel bindings [RFC5056].

   Existing GSS-API mechanisms are insufficient for communicating
   certain authorization and authentication information to a server.
   The GSS-API and its mechanisms certainly could be extended to address

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   this shortcoming.  However, here it is addressed at the application
   layer, i.e., in RPCSEC_GSS.

   A major motivation for version 3 RPCSEC_GSS (RPCSEC_GSSv3) is to add
   support for multi-level (labeled) security and server-side copy for

   Multi-Level Security (MLS) is a traditional model where subjects
   (processes) are given a security level (Unclassified, Secret, Top
   Secret, etc.) and objects (files) are given security labels that
   mandate the access of the subject to the object (see [NFSv4.2]
   Section 9.2).

   Labeled NFS (see Section 9 of [NFSv4.2]) uses an MLS policy with
   Mandatory Access Control (MAC) systems as defined in [RFC4949].
   Labeled NFS stores MAC file object labels on the NFS server and
   enables client Guest Mode MAC as described in Section 9.6.2 of
   [NFSv4.2].  RPCSEC_GSSv3 label assertions assert client MAC process
   subject labels to enable Full Mode MAC when combined with Labeled NFS
   as described in Section 9.6.1 of [NFSv4.2].

   A traditional inter-server file copy entails the user gaining access
   to a file on the source, reading it, and writing it to a file on the
   destination.  In secure NFSv4 inter-server server-side copy (see
   Section 4 of [NFSv4.2]), the user first secures access to both source
   and destination files, and then uses NFSv4.2 defined RPCSEC_GSSv3
   structured privileges to authorize the destination to copy the file
   from the source on behalf of the user.

   Multi-principal assertions can be used to address shared cache
   poisoning attacks (see Section 9 of [AFS-RXGK]) on the client cache
   by a user.  As described in Section 7 of [AFS-RXGK], multi-user
   machines with a single cache manager can fetch and cache data on a
   users' behalf, and re-display it for another user from the cache
   without re-fetching the data from the server.  The initial data
   acquisition is authenticated by the first user's credentials, and if
   only that user's credentials are used, it may be possible for a
   malicious user or users to "poison" the cache for other users by
   introducing bogus data into the cache.

   Another use of the multi-principal assertion is the secure conveyance
   of privilege information for processes running with more (or even
   with less) privilege than the user normally would be accorded.

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1.1.  Added Functionality

   RPCSEC_GSS version 3 (RPCSEC_GSSv3) is therefore described.
   RPCSEC_GSSv3 is the same as RPCSEC_GSSv2 [RFC5403], except that the
   following assertions of authority have been added.

   o  Security labels for Full Mode security type enforcement, and other
      labeled security models (See Section 9.6.2 in [NFSv4.2]).

   o  Application-specific structured privileges.  These allow an RPC
      application client to pass structured information to the
      corresponding application code in a server to control the
      applicability of the privilege and/or the conditions in which the
      privilege may be exercised.  For an example see server-side copy

   o  Multi-principal authentication of the client host and user to the
      server done by binding two RPCSEC_GSS handles.

   o  Simplified channel binding.

   Assertions of labels and privileges are evaluated by the server,
   which may then map the asserted values to other values, all according
   to server-side policy.  See [NFSv4.2].

   An option for enumerating server supported label format specifiers
   (LFS) is provided.  See Section 9.2 in [NFSv4.2].

   Note that there is no RPCSEC_GSS_CREATE payload that is REQUIRED to
   implement.  RPCSEC_GSSv3 implementations are feature driven.  Besides
   implementing the RPCSEC_GSS_CREATE operation and payloads for the
   desired features, all RPCSEC_GSSv3 implementation MUST implement:

   o  The new GSS version number (Section 2.2).

   o  The new reply verifier (Section 2.3).

   o  The new auth stat values (Section 2.6).

   RPCSEC_GSSv3 targets implementing a desired feature must also
   implement the RPCSEC_GSS_LIST operation, and the RPCSEC_GSS_CREATE
   operation replies for unsupported features.

   o  For label assertions the target indicates no support by returning
      the new RPCSEC_GSS_LABEL_PROBLEM auth stat (See Section

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   o  For structured privilege assertions the target indicates no
      support by returning the new RPCSEC_GSS_UNKNOWN_MESSAGE auth stat
      (See Section

   o  For multi-principal authentication (Section, the target
      indicates no support by not including a rgss3_gss_mp_auth value in
      the rgss3_create_res.

   o  For channel bindings (Section the target indicates no
      support by not including a rgss3_chan_binding value in the

1.2.  XDR Code Extraction

   This document contains the External Data Representation (XDR)
   ([RFC4506]) definitions for the RPCSEC_GSSv3 protocol.  The XDR
   description is provided in this document in a way that makes it
   simple for the reader to extract into ready to compile form.  The
   reader can feed this document in the following shell script to
   produce the machine readable XDR description of RPCSEC_GSSv3:


   grep "^  *///" | sed 's?^  */// ??' | sed 's?^  *///$??'


   I.e. if the above script is stored in a file called "extract.sh", and
   this document is in a file called "spec.txt", then the reader can do:


    sh extract.sh < spec.txt > rpcsec_gss_v3.x


   The effect of the script is to remove leading white space from each
   line, plus a sentinel sequence of "///".

2.  The RPCSEC_GSSv3 Protocol

   RPCSEC_GSS version 3 (RPCSEC_GSSv3) is very similar to RPCSEC_GSS
   version 2 (RPCSEC_GSSv2) [RFC5403].  The differences are the addition
   of support for assertions and channel bindings are supported via a
   different mechanism.

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   The entire RPCSEC_GSSv3 protocol is not presented here.  Only the
   differences between it and RPCSEC_GSSv2 are shown.

   The use of RPCSEC_GSSv3 is structured as follows:

   o  A client uses an existing RPCSEC_GSSv3 context handle established
      in the usual manner (See Section 5.2 [RFC2203]) to protect
      RPCSEC_GSSv3 exchanges, this will be termed the "parent" handle.

   o  The server issues a "child" RPCSEC_GSSv3 handle in the
      RPCSEC_GSS_CREATE response which uses the underlying GSS-API
      security context of the parent handle in all subsequent exchanges
      that uses the child handle.

   o  An RPCSEC_GSSv3 child handle MUST NOT be used as the parent handle
      in an RPCSEC_GSS3_CREATE control message.

2.1.  Compatibility with RPCSEC_GSSv2

   The functionality of RPCSEC_GSSv2 [RFC5403] is fully supported by
   RPCSEC_GSSv3 with the exception of the RPCSEC_GSS_BIND_CHANNEL
   operation which is not supported when RPCSEC_GSSv3 is in use (see
   Section 2.5).

2.2.  Version Negotiation

   An initiator that supports version 3 of RPCSEC_GSS simply issues an
   RPCSEC_GSS request with the rgc_version field set to
   RPCSEC_GSS_VERS_3.  If the target does not recognize
   RPCSEC_GSS_VERS_3, the target will return an RPC error per
   Section 5.1 of [RFC2203].

   The initiator MUST NOT attempt to use an RPCSEC_GSS handle returned
   by version 3 of a target with version 1 or version 2 of the same
   target.  The initiator MUST NOT attempt to use an RPCSEC_GSS handle
   returned by version 1 or version 2 of a target with version 3 of the
   same target.

2.3.  New REPLY Verifier

   A new reply verifier is needed for RPCSEC_GSSv3 because of a
   situation that arises from the use of the same GSS context by child
   and parent handles.  Because the RPCSEC_GSSv3 child handle uses the
   same GSS context as the parent handle, a child and parent
   RPCSEC_GSSv3 handle could have the same RPCSEC_GSS sequence numbers.
   Since the reply verifier of previous versions of RPCSEC_GSS computes
   a Message Integrity Code (MIC) on just the sequence number, this
   provides opportunities for man in the middle attacks.

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   This issue is addressed in RPCSEC_GSS version 3 by computing the
   verifier using the exact same input as is used to compute the request
   verifier, except that the mtype is changed from CALL to REPLY.  The
   new reply verifier computes a MIC over the following RPC reply header

     unsigned int xid;
     msg_type mtype;    /* set to REPLY */
     unsigned int rpcvers;
     unsigned int prog;
     unsigned int vers;
     unsigned int proc;
     opaque_auth  cred; /* captures the RPCSEC_GSS handle */

2.4.  XDR Code Preliminaries

   The following code fragment replaces the corresponding preliminary
   code shown in Figure 1 of [RFC5403].  The values in the code fragment
   in Section 2.6 are additions to the auth_stat enumeration.
   Subsequent code fragments are additions to the code for version 2
   that support the new procedures defined in version 3.


      ///  /*
      ///   * Copyright (c) 2013 IETF Trust and the persons
      ///   * identified as the document authors. All rights
      ///   * reserved.
      ///   *
      ///   * The document authors are identified in [RFC2203],
      ///   * [RFC5403], and [RFCTBD].
      ///   *
      ///   * Redistribution and use in source and binary forms,
      ///   * with or without modification, are permitted
      ///   * provided that the following conditions are met:
      ///   *
      ///   * o Redistributions of source code must retain the above
      ///   *   copyright notice, this list of conditions and the
      ///   *   following disclaimer.
      ///   *
      ///   * o Redistributions in binary form must reproduce the
      ///   *   above copyright notice, this list of
      ///   *   conditions and the following disclaimer in
      ///   *   the documentation and/or other materials
      ///   *   provided with the distribution.
      ///   *
      ///   * o Neither the name of Internet Society, IETF or IETF
      ///   *   Trust, nor the names of specific contributors, may be

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      ///   *   used to endorse or promote products derived from this
      ///   *   software without specific prior written permission.
      ///   *
      ///   */
      ///  /*
      ///   * This code was derived from RFC2203, RFC5403, and RFCTBD.
      ///   * Please reproduce this note if possible.
      ///   */
      ///  enum rpc_gss_service_t {
      ///          /* Note: the enumerated value for 0 is reserved. */
      ///          rpc_gss_svc_none         = 1,
      ///          rpc_gss_svc_integrity    = 2,
      ///          rpc_gss_svc_privacy      = 3,
      ///          rpc_gss_svc_channel_prot = 4
      ///  };
      ///  enum rpc_gss_proc_t {
      ///           RPCSEC_GSS_DATA          = 0,
      ///           RPCSEC_GSS_INIT          = 1,
      ///           RPCSEC_GSS_CONTINUE_INIT = 2,
      ///           RPCSEC_GSS_DESTROY       = 3,
      ///           RPCSEC_GSS_BIND_CHANNEL  = 4, /* not used */
      ///           RPCSEC_GSS_CREATE        = 5, /* new */
      ///           RPCSEC_GSS_LIST          = 6  /* new */
      ///  };
      ///  struct rpc_gss_cred_vers_1_t {
      ///          rpc_gss_proc_t    gss_proc; /* control procedure */
      ///          unsigned int      seq_num;  /* sequence number */
      ///          rpc_gss_service_t service;  /* service used */
      ///          opaque            handle<>; /* context handle */

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      ///  };
      ///  const RPCSEC_GSS_VERS_1 = 1;
      ///  const RPCSEC_GSS_VERS_2 = 2;
      ///  const RPCSEC_GSS_VERS_3 = 3; /* new */
      ///  union rpc_gss_cred_t switch (unsigned int rgc_version) {
      ///  case RPCSEC_GSS_VERS_1:
      ///  case RPCSEC_GSS_VERS_2:
      ///  case RPCSEC_GSS_VERS_3: /* new */
      ///          rpc_gss_cred_vers_1_t rgc_cred_v1;
      ///  };


   As seen above, the RPCSEC_GSSv3 credential has the same format as the
   RPCSEC_GSSv1 [RFC2203] and RPCSEC_GSSv2 [RFC5403] credential.
   Setting the rgc_version field to 3 indicates that the initiator and
   target support the new RPCSEC_GSSv3 control procedures.


   RPCSEC_GSSv3 provides a channel binding assertion that replaces the

   The RPCSEC_GSS_BIND_CHANNEL operation is not supported on RPCSEC_GSS
   version 3 handles.  If a server receives an RPCSEC_GSS_BIND_CHANNEL
   operation on an RPCSEC_GSSv3 handle, it MUST return a reply status of
   MSG_ACCEPTED with an accept stat of PROC_UNAVAIL.

2.6.  New auth_stat Values

   RPCSEC_GSSv3 requires the addition of several values to the auth_stat
   enumerated type definition.  The use of these new auth_stat values is
   explained throughout this document.

              enum auth_stat {
                       * RPCSEC_GSSv3 errors
                      RPCSEC_GSS_INNER_CREDPROBLEM = 15,
                      RPCSEC_GSS_LABEL_PROBLEM = 16,
                      RPCSEC_GSS_PRIVILEGE_PROBLEM = 17,
                      RPCSEC_GSS_UNKNOWN_MESSAGE = 18

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2.7.  New Control Procedures

   There are two new RPCSEC_GSSv3 control procedures: RPCSEC_GSS_CREATE,

   The RPCSEC_GSS_CREATE procedure binds any combination of assertions:
   multi-principal authentication, labels, structured privileges, or
   channel bindings to a new RPCSEC_GSSv3 context returned in the
   rgss3_create_res rcr_handle field.

   The RPCSEC_GSS_LIST procedure queries the target for supported

   RPCSEC_GSS version 3 control messages are similar to the RPCSEC_GSS
   version 1 and version 2 RPCSEC_GSS_DESTROY control message (see
   section 5.4 [RFC2203]) in that the sequence number in the request
   must be valid, and the header checksum in the verifier must be valid.
   As in RPCSEC_GSS version 1 and version 2, the RPCSEC_GSS version 3
   control messages may contain call data following the verifier in the
   body of the NULLPROC procedure.  In other words, they look a lot like
   an RPCSEC_GSS data message with the header procedure set to NULLPROC.

   The client MUST use one of the following security services to protect
   the RPCSEC_GSS_CREATE or RPCSEC_GSS_LIST control message:

   o  rpc_gss_svc_integrity

   o  rpc_gss_svc_privacy

   Specifically the client MUST NOT use rpc_gss_svc_none.

   RPCSEC_GSS_LIST can also use rpc_gss_svc_channel_prot (see
   RPCSEC_GSSv2 [RFC5403]) if the request is sent using an RPCSEC_GSSv3
   child handle with channel bindings enabled as described in

2.7.1.  New Control Procedure - RPCSEC_GSS_CREATE


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      ///  struct rgss3_create_args {
      ///          rgss3_gss_mp_auth    *rca_mp_auth;
      ///          rgss3_chan_binding   *rca_chan_bind_mic;
      ///          rgss3_assertion_u     rca_assertions<>;
      ///  };
      ///  struct rgss3_create_res {
      ///          opaque               rcr_handle<>;
      ///          rgss3_gss_mp_auth    *rcr_mp_auth;
      ///          rgss3_chan_binding   *rcr_chan_bind_mic;
      ///          rgss3_assertion_u     rcr_assertions<>;
      ///  };
      ///  enum rgss3_assertion_type {
      ///          LABEL = 0,
      ///          PRIVS = 1
      ///  };
      ///  union rgss3_assertion_u
      ///        switch (rgss3_assertion_type atype) {
      ///  case LABEL:
      ///          rgss3_label  rau_label;
      ///  case PRIVS:
      ///          rgss3_privs  rau_privs;
      ///  default:
      ///          opaque       rau_ext<>;
      ///  };


   The call data for an RPCSEC_GSS_CREATE request consists of an
   rgss3_create_args which binds one or more items of several kinds to
   the returned rcr_handle RPCSEC_GSSv3 context handle called the
   "child" handle:

   o  Multi-principal authentication: another RPCSEC_GSS context handle

   o  A channel binding

   o  Authorization assertions: labels and or privileges

   The reply to this message consists of either an error or an
   rgss3_create_res structure.  As noted in Section and
   Section successful rgss3_assertions are enumerated in
   rcr_assertions, and are REQUIRED to be enumerated in the same order
   as they appeared in the rca_assertions argument.

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   Upon successful RPCSEC_GSS_CREATE, both the client and the server
   need to associate the resultant child rcr_handle context handle with
   the parent context handle in their GSS context caches so as to be
   able to reference the parent context given the child context handle.

   RPCSEC_GSSv3 child handles MUST be destroyed upon the destruction of
   the associated parent handle.

   Server implementation and policy MAY result in labels, privileges,
   and identities being mapped to concepts and values that are local to
   the server.  Server policies should take into account the identity of
   the client and/or user as authenticated via the GSS-API.  Multi-principal Authentication


      ///  struct rgss3_gss_mp_auth {
      ///          opaque          rgmp_handle<>; /* inner handle */
      ///          opaque          rgmp_rpcheader_mic<>;
      ///  };


   RPCSEC_GSSv3 clients MAY assert a multi-principal authentication of
   the RPC client host principal and a user principal.  This feature is
   needed, for example, when an RPC client host wishes to use authority
   assertions that the server may only grant if a user and an RPC client
   host are authenticated together to the server.  Thus a server may
   refuse to grant requested authority to a user acting alone (e.g., via
   an unprivileged user-space program), or to an RPC client host acting
   alone (e.g., when an RPC client host is acting on behalf of a user)
   but may grant requested authority to an RPC client host acting on
   behalf of a user if the server identifies the user and trusts the RPC
   client host.

   It is assumed that an unprivileged user-space program would not have
   access to RPC client host credentials needed to establish a GSS-API
   security context authenticating the RPC client host to the server,
   therefore an unprivileged user-space program could not create an
   RPCSEC_GSSv3 RPCSEC_GSS_CREATE message that successfully binds an RPC
   client host and a user security context.

   In addition to the parent handle (Section 2), the multi-principal
   authentication call data has an RPCSEC_GSS version 3 handle
   referenced via the rgmp_handle field termed the "inner" handle.

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   Clients using RPCSEC_GSSv3 multi-principal authentication MUST use an
   RPCSEC_GSSv3 context handle that corresponds to a GSS-API security
   context that authenticates the RPC client host for the parent handle.
   For the inner context handle with RPCSEC_GSSv3 it MUST use a context
   handle to authenticate a user.  The reverse (parent handle
   authenticates user, inner authenticates an RPC client host) MUST NOT
   be used.  Other multi-principal parent and inner context handle uses
   might eventually make sense, but would need to be introduced in a new
   revision of the RPCSEC_GSS protocol.

   The child context handle returned by a successful multi-principal
   assertion binds the inner RPCSEC_GSSv3 context handle to the parent
   RPCSEC_GSS context and MUST be treated by servers as authenticating
   the GSS-API initiator principal authenticated by the inner context
   handle's GSS-API security context.  This principal may be mapped to a
   server-side notion of user or principal.

   Multi-principal binding is done by including an assertion of type
   rgss3_gss_mp_auth in the RPCSEC_GSS_CREATE rgss3_create_args call
   data.  The inner context handle is placed in the rgmp_handle field.
   A MIC of the RPC call header up to and including the credential is
   computed using the GSS-API security context associated with the inner
   context handle and is placed in rgmp_rpcheader_mic field.  Note that
   the rgmp_rpcheader_mic only identifies the client host GSS context by
   it's context handle (the parent context handle) in the rpc header.

   An RPCSEC_GSS_CREATE control procedure with a multi-principal
   authentication payload MUST use the rpc_gss_svc_privacy security
   service for protection.  This is to prevent an attacker from
   intercepting the RPCSEC_GSS_CREATE control procedure, re-assigning
   the (parent) context handle, and stealing the user's identity.

   The target verifies the multi-principal authentication by first
   confirming that the parent context used is an RPC client host
   context, and then verifies the rgmp_rpcheader_mic using the GSS-API
   security context associated with the rgmp_handle field.

   On a successful verification, the rgss3_gss_mp_auth field in the
   rgss3_create_res reply MUST be filled in with the inner RPCSEC_GSSv3
   context handle as the rgmp_handle, and a MIC computed over the RPC
   reply header (see section Section 2.3) using the GSS-API security
   context associated with the inner handle.

   On failure, the rgss3_gss_mp_auth field is not sent
   (rgss3_gss_mp_auth is an optional field).  A MSG_DENIED reply to the
   RPCSEC_GSS_CREATE call is formulated as usual.

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   As described in Section of [RFC2203] the server maintains a
   list of contexts for the clients that are currently in session with
   it.  When a client request comes in, there may not be a context
   corresponding to its handle.  When this occurs on an
   RPCSEC_GSS3_CREATE request processing of the parent handle, the
   server rejects the request with a reply status of MSG_DENIED with the
   reject_stat of AUTH_ERROR and with an auth_stat value of

   A new value, RPCSEC_GSS_INNER_CREDPROBLEM, has been added to the
   auth_stat type.  With a multi-pricipal authorization request, the
   server must also have a context corresponding to the inner context
   handle.  When the server does not have a context handle corresponding
   to the inner context handle of a multi-pricipal authorization
   request, the server sends a reply status of MSG_DENIED with the
   reject_stat of AUTH_ERROR and with an auth_stat value of

   When processing the multi-principal authentication request, if the
   GSS_VerifyMIC() call on the rgmp_rpcheader_mic fails to return
   GSS_S_COMPLETE, the server sends a reply status of MSG_DENIED with
   the reject_stat of AUTH_ERROR and with an auth_stat value of


      ///  typedef opaque rgss3_chan_binding<>;


   RPCSEC_GSSv3 provides a different way to do channel binding than
   RPCSEC_GSSv2 [RFC5403].  Specifically:

   a.  RPCSEC_GSSv3 builds on RPCSEC_GSSv1 by reusing existing,
       established context handles rather than providing a different RPC
       security flavor for establishing context handles,

   b.  channel bindings data are not hashed because there is now general
       agreement that it is the secure channel's responsibility to
       produce channel bindings data of manageable size.

   (a) is useful in keeping RPCSEC_GSSv3 simple in general, not just for
   channel binding.  (b) is useful in keeping RPCSEC_GSSv3 simple
   specifically for channel binding.

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   Channel binding is accomplished as follows.  The client prefixes the
   channel bindings data octet string with the channel type as described
   in [RFC5056], then the client calls GSS_GetMIC() to get a MIC of
   resulting octet string, using the parent RPCSEC_GSSv3 context
   handle's GSS-API security context.  The MIC is then placed in the
   rca_chan_bind_mic field of RPCSEC_GSS_CREATE arguments

   If the rca_chan_bind_mic field of the arguments of a
   RPCSEC_GSS_CREATE control message is set, then the server MUST verify
   the client's channel binding MIC if the server supports this feature.
   If channel binding verification succeeds then the server MUST
   generate a new MIC of the same channel bindings and place it in the
   rcr_chan_bind_mic field of the RPCSEC_GSS_CREATE rgss3_create_res
   results.  If channel binding verification fails or the server doesn't
   support channel binding then the server MUST indicate this in its
   reply by not including a rgss3_chan_binding value in rgss3_create_res
   (rgss3_chan_binding is an optional field).

   The client MUST verify the result's rcr_chan_bind_mic value by
   calling GSS_VerifyMIC() with the given MIC and the channel bindings
   data (including the channel type prefix).  If client-side channel
   binding verification fails then the client MUST call
   RPCSEC_GSS_DESTROY.  If the client requested channel binding but the
   server did not include an rcr_chan_binding_mic field in the results,
   then the client MAY continue to use the resulting context handle as
   though channel binding had never been requested.  If the client
   considers channel binding critical, it MUST call RPCSEC_GSS_DESTROY.

   As per-RPCSEC_GSSv2 [RFC5403]:

      "Once a successful [channel binding] procedure has been performed
      on an [RPCSEC_GSSv3] context handle, the initiator's
      implementation may map application requests for rpc_gss_svc_none
      and rpc_gss_svc_integrity to rpc_gss_svc_channel_prot credentials.
      And if the secure channel has privacy enabled, requests for
      rpc_gss_svc_privacy can also be mapped to

   Any RPCSEC_GSSv3 child context handle that has been bound to a secure
   channel in this way SHOULD be used only with the
   rpc_gss_svc_channel_prot, and SHOULD NOT be used with
   rpc_gss_svc_none nor rpc_gss_svc_integrity -- if the secure channel
   does not provide privacy protection then the client MAY use
   rpc_gss_svc_privacy where privacy protection is needed or desired.

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      ///  struct rgss3_label {
      ///          rgss3_lfs       rl_lfs;
      ///          opaque          rl_label<>;
      ///  };
      ///  struct rgss3_lfs {
      ///          unsigned int rlf_lfs_id;
      ///          unsigned int rlf_pi_id;
      ///  };


   The client discovers which label format specifiers (LFS) the server
   supports via the RPCSEC_GSS_LIST control message.  Full mode MAC is
   enabled when an RPCSEC_GSS version 3 process subject label assertion
   is combined with a file object label provided by the NFSv4.2
   sec_label attribute.

   Label encoding is specified to mirror the NFSv4.2 sec_label attribute
   described in Section 12.2.4 of [NFSv4.2].  The label format specifier
   (LFS) is an identifier used by the client to establish the syntactic
   format of the security label and the semantic meaning of its
   components.  The policy identifier (PI) is an optional part of the
   definition of an LFS which allows for clients and server to identify
   specific security policies.  The opaque label field of rgss3_label is
   dependent on the MAC model to interpret and enforce.

   If a label itself requires privacy protection (i.e., that the user
   can assert that label is a secret) then the client MUST use the
   rpc_gss_svc_privacy protection service for the RPCSEC_GSS_CREATE

   RPCSEC_GSSv3 clients MAY assert a set of subject security labels in
   some LSF by binding a label assertion to the RPCSEC_GSSv3 child
   context handle.  This is done by including an assertion of type
   rgss3_label in the RPCSEC_GSS_CREATE rgss3_create_args rca_assertions
   call data.  The label assertion payload is the set of subject labels
   asserted by the calling NFS client process.  The resultant child
   context is used for NFS requests asserting the client process subject
   labels.  The NFS server process that handles such requests then
   asserts the (client) process subject label(s) as it attempts to
   access a file that has associated LNFS object labels.

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   Servers that support labeling in the requested LFS MAY map the
   requested subject label to a different subject label as a result of
   server-side policy evaluation.

   The labels that are accepted by the target and bound to the
   RPCSEC_GSSv3 context MUST be enumerated in the rcr_assertions field
   of the rgss3_create_res RPCSEC_GSS_CREATE reply.

   Servers that do not support labeling or that do not support the
   requested LFS reject the label assertion with a reply status of
   MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of
   RPCSEC_GSS_LABEL_PROBLEM.  Structured Privilege Assertions


      ///  typedef opaque utf8string<>;   /* UTF-8 encoding */
      ///  typedef utf8string utf8str_cs; /* Case-sensitive UTF-8 */
      ///  struct rgss3_privs {
      ///          utf8str_cs      rp_name<>;
      ///          opaque          rp_privilege<>;
      ///  };


   A structured privilege is a capability defined by a specific RPC
   application.  To support the assertion of this privilege, by a client
   using the application, in a server that also supports the
   application, the application may define a private data structure that
   is understood by clients and servers implementing the RPC

   RPCSEC_GSSv3 clients MAY assert a structured privilege by binding the
   privilege to the RPCSEC_GSSv3 context handle.  This is done by
   including an assertion of type rgss3_privs in the RPCSEC_GSS_CREATE
   rgss3_create_args rca_assertions call data.

   The privilege is identified by the description string that is used by
   RPCSEC_GSSv3 to identify the privilege and communicate the private
   data between the relevant RPC application-specific code without
   needing to be aware of the details of the structure used.  Thus, as
   far as RPCSEC_GSSv3 is concerned, the defined structure is passed
   between client and server as opaque data encoded in the
   rpc_gss3_privs rp_privilege field.

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   Encoding, server verification and any server policies for structured
   privileges are described by the RPC application definition.  The
   rp_name field of rpc_gss3_privs carries the description string used
   to identify and list the privilege.  The utf8str_cs definition is
   from [RFC7530].

   A successful structured privilege assertion MUST be enumerated in the
   rcr_assertions field of the rgss3_create_res RPCSEC_GSS_CREATE reply.

   If a server receives a structured privilege assertion that it does
   not recognize, the assertion is rejected with a reply status of
   MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of

   It is assumed that a client asserting more than one structured
   privilege to be bound to a context handle would require all the
   privilege assertions to succeed.

   If a server receives an RPCSEC_GSS_CREATE request containing one or
   more structured privilege assertions, any of which it fails to verify
   according to the requirements of the RPC application defined
   behavior, the request is rejected with a reply status of MSG_DENIED,
   a reject_status of AUTH_ERROR, and an auth_stat of

   Section  "Inter-Server Copy via ONC RPC with RPCSEC_GSSv3"
   of [NFSv4.2] shows an example of structured privilege definition and

2.7.2.  New Control Procedure - RPCSEC_GSS_LIST


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      ///  enum rgss3_list_item {
      ///          LABEL = 0,
      ///          PRIVS = 1
      ///  };
      ///  struct rgss3_list_args {
      ///          rgss3_list_item      rla_list_what<>;
      ///  };
      ///  union rgss3_list_item_u
      ///        switch (rgss3_list_item itype) {
      ///  case LABEL:
      ///          rgss3_label          rli_labels<>;
      ///  case PRIVS:
      ///          rgss3_privs          rli_privs<>;
      ///  };
      ///  typedef rgss3_list_item_u rgss3_list_res<>;


   The call data for an RPCSEC_GSS_LIST request consists of a list of
   integers (rla_list_what) indicating what assertions are to be listed,
   and the reply consists of an error or the requested list.

   The result of requesting a list of rgss3_list_item LABEL is a list of
   LFSs supported by the server.  The client can then use the LFS list
   to assert labels via the RPCSEC_GSS_CREATE label assertions.  See

2.8.  Extensibility

   Assertion types may be added in the future by adding arms to the
   'rgss3_assertion_u' union.  Examples of other potential assertion
   types include:

   o  Client-side assertions of identity:

      *  Primary client/user identity

      *  Supplementary group memberships of the client/user, including
         support for specifying deltas to the membership list as seen on
         the server.

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3.  Operational Recommendation for Deployment

   RPCSEC_GSSv3 is a superset of RPCSEC_GSSv2 [RFC5403] which in turn is
   a superset of RPCSEC_GSSv1 [RFC2203], and so can be used in all
   situations where RPCSEC_GSSv2 is used, or where RPCSEC_GSSv1 is used
   and channel bindings functionality is not needed.  RPCSEC_GSSv3
   should be used when the new functionality is needed.

4.  Security Considerations

   This entire document deals with security issues.

   The RPCSEC_GSSv3 protocol allows for client-side assertions of data
   that is relevant to server-side authorization decisions.  These
   assertions must be evaluated by the server in the context of whether
   the client and/or user are authenticated, whether multi-principal
   authentication was used, whether the client is trusted, what ranges
   of assertions are allowed for the client and the user (separately or
   together), and any relevant server-side policy.

   The security semantics of assertions carried by RPCSEC_GSSv3 are
   application protocol-specific.

   Note that RPSEC_GSSv3 is not a complete solution for labeling: it
   conveys the labels of actors, but not the labels of objects.  RPC
   application protocols may require extending in order to carry object
   label information.

   There may be interactions with NFSv4's callback security scheme and
   NFSv4.1's [RFC5661] GSS-API "SSV" mechanisms.  Specifically, the
   NFSv4 callback scheme requires that the server initiate GSS-API
   security contexts, which does not work well in practice, and in the
   context of client-side processes running as the same user but with
   different privileges and security labels the NFSv4 callback security
   scheme seems particularly unlikely to work well.  NFSv4.1 has the
   server use an existing, client-initiated RPCSEC_GSS context handle to
   protect server-initiated callback RPCs.  The NFSv4.1 callback
   security scheme lacks all the problems of the NFSv4 scheme, however,
   it is important that the server pick an appropriate RPCSEC_GSS
   context handle to protect any callbacks.  Specifically, it is
   important that the server use RPCSEC_GSS context handles which
   authenticate the client to protect any callbacks relating to server
   state initiated by RPCs protected by RPCSEC_GSSv3 contexts.

   As described in Section 2.10.10 [RFC5661] the client is permitted to
   associate multiple RPCSEC_GSS handles with a single SSV GSS context.
   RPCSEC_GSSv3 handles will work well with SSV in that the man-in-the-
   middle attacks described in Section 2.10.10 [RFC5661] are solved by

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   the new reply verifier (Section 2.3).  Using an RPCSEC_GSSv3 handle
   backed by a GSS-SSV mechanism context as a parent handle in an
   RPCSEC_GSS_CREATE call while permitted is complicated by the lifetime
   rules of SSV contexts and their associated RPCSEC_GSS handles.

5.  IANA Considerations

   The following new IANA RPC Authentication Status Numbers have been

   o  RPCSEC_GSS_INNER_CREDPROBLEM (15) "No credentials for multi-
      principal assertion inner context user".  See Section

   o  RPCSEC_GSS_LABEL_PROBLEM (16) "Problem with label assertion".  See

   o  RPCSEC_GSS_PRIVILEGE_PROBLEM (17) "Problem with structured
      privilege assertion".  See Section

   o  RPCSEC_GSS_UNKNOWN_MESSAGE (18) "Unknown structured privilege
      assertion".  See Section

6.  References

6.1.  Normative References

   [NFSv4.2]  Haynes, T., "NFS Version 4 Minor Version 2", draft-ietf-
              nfsv4-minorversion2-29 (Work In Progress), December 2014.

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

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, September 1997.

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

   [RFC4506]  Eisler, M., "XDR: External Data Representation Standard",
              RFC 4506, May 2006.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC5403]  Eisler, M., "RPCSEC_GSS Version 2", RFC 5403, February

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   [RFC5661]  Shepler, S., Eisler, M., and D. Noveck, "Network File
              System (NFS) Version 4 Minor Version 1 Protocol", RFC
              5661, January 2010.

   [RFC7530]  Haynes, T. and D. Noveck, "Network File System (NFS)
              Version 4 Protocol", RFC 7530, March 2015.

6.2.  Informative References

              Wilkinson, S. and B. Kaduk, "Integrating rxgk with AFS",
              draft-wilkinson-afs3-rxgk-afs (work in progress), April

   [RFC4949]  Shirley, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

Appendix A.  Acknowledgments

   Andy Adamson would like to thank NetApp, Inc. for its funding of his
   time on this project.

   We thank Lars Eggert, Mike Eisler, Ben Kaduk, Bruce Fields, Tom
   Haynes, and Dave Noveck for their most helpful reviews.

Appendix B.  RFC Editor Notes

   [RFC Editor: please remove this section prior to publishing this
   document as an RFC]

   [RFC Editor: prior to publishing this document as an RFC, please
   replace all occurrences of RFCTBD with RFCxxxx where xxxx is the RFC
   number of this document]

Authors' Addresses

   William A. (Andy) Adamson
   3629 Wagner Ridge Ct
   Ann Arbor, MI  48103

   Phone: +1 734 665 1204
   Email: andros@netapp.com

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   Nico Williams
   13115 Tamayo Dr
   Austin, TX  78729

   Email: nico@cryptonector.com

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