Network Working Group                                          M. Eisler
Internet Draft                                              January 1999
Document: draft-ietf-nfsv4-nfssec-00.txt

   NFS Version 2 and Version 3 Security Issues and the NFS Protocol's
                   Use of RPCSEC_GSS and Kerberos V5

Status of this Memo

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   Comments on this document should be sent to
   "", the NFS Version 4 Working Group
   discussion list.


   This memorandum clarifies various security issues involving the NFS
   protocol (Version 2 and Version 3 only) and then describes how the
   Version 2 and Version 3 of the NFS protocol use the RPCSEC_GSS
   security flavor protocol and Kerberos V5.  This memorandum is
   provided so that people can write compatible implementations.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
   1.1.  Overview of RPC Security Architecture  . . . . . . . . . . . 3
   2.  Overview of NFS Security . . . . . . . . . . . . . . . . . . . 4
   2.1.  Port Monitoring  . . . . . . . . . . . . . . . . . . . . . . 4
   2.1.1.  MOUNT Protocol . . . . . . . . . . . . . . . . . . . . . . 5
   2.2.  RPC Security Flavors . . . . . . . . . . . . . . . . . . . . 5
   2.2.1.  AUTH_SYS . . . . . . . . . . . . . . . . . . . . . . . . . 5
   2.2.2.  AUTH_DH and AUTH_KERB4 . . . . . . . . . . . . . . . . . . 5
   2.2.3.  RPCSEC_GSS . . . . . . . . . . . . . . . . . . . . . . . . 6
   2.3.  Authentication for NFS Procedures  . . . . . . . . . . . . . 6
   2.3.1.  NULL Procedure . . . . . . . . . . . . . . . . . . . . . . 6
   2.3.2.  NFS Procedures Used at Mount Time  . . . . . . . . . . . . 7
   2.4.  Binding Security Flavors to Exports  . . . . . . . . . . . . 7
   2.5.  Anonymous Mapping  . . . . . . . . . . . . . . . . . . . . . 8
   2.6.  Host-based Access Control  . . . . . . . . . . . . . . . . . 8
   2.7.  Security Flavor Negotiation  . . . . . . . . . . . . . . . . 9
   2.8.  Registering Flavors  . . . . . . . . . . . . . . . . . . . . 9
   3.  The NFS Protocol's Use of RPCSEC_GSS . . . . . . . . . . . .  10
   3.1.  Server Principal . . . . . . . . . . . . . . . . . . . . .  10
   3.2.  Negotiation  . . . . . . . . . . . . . . . . . . . . . . .  10
   3.3.  Changing RPCSEC_GSS Parameters . . . . . . . . . . . . . .  11
   3.4.  Registering Pseudo Flavors and Mappings  . . . . . . . . .  11
   4.  The NFS Protocol over Kerberos V5  . . . . . . . . . . . . .  12
   4.1.  Issues with Kerberos V5 QOPs . . . . . . . . . . . . . . .  12
   4.2.  The NFS Protocol over Kerberos V5 Pseudo Flavor
         Registration Entry . . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . .  14
   References . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   The NFS protocol provides transparent remote access to shared file
   systems across networks. The NFS protocol is designed to be machine,
   operating system, network architecture, and security mechanism, and
   transport protocol independent. This independence is achieved through
   the use of Remote Procedure Call (RPC) primitives built on top of an
   eXternal Data Representation (XDR).  NFS protocol Version 2 is
   specified in the Network File System Protocol Specification
   [RFC1094]. A description of the initial implementation can be found
   in [Sandberg]. NFS protocol Version 3 is specified in the NFS Version
   3 Protocol Specification [RFC1813]. A description of some initial
   implementations can be found in [Pawlowski].

   For the remainder of this document, whenever it refers to the NFS
   protocol, it means NFS Version 2 and Version 3, unless otherwise

   The RPC protocol is specified in the Remote Procedure Call Protocol
   Specification Version 2 [RFC1831]. The XDR protocol is specified in
   External Data Representation Standard [RFC1832].

   A new RPC security flavor, RPCSEC_GSS, has been specified [RFC2203].
   This new flavor allows application protocols built on top of RPC to
   access security mechanisms that adhere to the GSS-API specification

   The purpose of this document is to clarify NFS security issues and to
   specify how the NFS protocol uses RPCSEC_GSS. This document will also
   describe how NFS works over Kerberos V5, via RPCSEC_GSS.

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

1.1.  Overview of RPC Security Architecture

   The RPC protocol includes a slot for security parameters (referred to
   as an authentication flavor in the RPC specification [RFC1831]) on
   every call.  The contents of the security parameters are determined
   by the type of authentication used by the server and client. A server
   may support several different flavors of authentication at once.
   Some of the better known flavors are summarized as follows:

   *    The AUTH_NONE flavor provides null authentication, that is, no
        authentication information is passed.

   *    The AUTH_SYS flavor provides a UNIX-style user identifier, group

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        identifier, and an array of supplemental group identifiers with
        each call.

   *    The AUTH_DH (sometimes referred to as AUTH_DES [RFC1057]) flavor
        provides DES-encrypted authentication parameters based on a
        network-wide string name, with session keys exchanged via the
        Diffie-Hellman public key scheme.

   *    The AUTH_KERB4 flavor provides DES encrypted authentication
        parameters based on a network-wide string name (the name is a
        Kerberos Version 4 principal identifier) with session keys
        exchanged via Kerberos Version 4 secret keys.

   The NFS protocol is not limited to the above list of security

2.  Overview of NFS Security

2.1.  Port Monitoring

   Many NFS servers will require that the client send its NFS requests
   from UDP or TCP source ports with values < 1024. The theory is that
   binding to ports < 1024 is a privileged operation on the client, and
   so the client is enforcing file access permissions on its end. The
   theory breaks down because:

   *    On many operating systems, there are no constraints on what port
        what user can bind to.

   *    Just because the client host enforces the privilege on binding
        to ports < 1024 does not necessarily mean that a non-privileged
        user cannot gain access to the port binding privilege. For
        example with a single-user desk-top host running a UNIX
        operating system, the user may have knowledge of the root user
        password. And even if he does not have that knowledge, with
        physical access to the desk-top machine, root privileges are
        trivially acquired.

   In some rare cases, when the system administrator can be certain that
   the clients are trusted and under control (in particular, protected
   from physical attack), relying of trusted ports MAY be a reliable
   form of security.

   In most cases, the use of privileged ports and port monitoring for
   security is at best an inconvenience to the attacker and SHOULD NOT
   be depended on.

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   To maximize interoperability:

   *    NFS clients SHOULD attempt to bind to ports < 1024. In some
        cases, if they fail to bind (because either the user does not
        have the privilege to do so, or there is no free port < 1024),
        the NFS client MAY wish to attempt the NFS operation over a port
        >= 1024.

   *    NFS servers that implement port monitoring SHOULD provide a
        method to turn it off.

   *    Whether port monitoring is enabled or not, NFS servers SHOULD
        NOT reject NFS requests to the NULL procedure (procedure number
        0). See subsection 2.3.1, "NULL procedure" for a complete

2.1.1.  MOUNT Protocol

   The port monitoring issues and recommendations apply to the MOUNT
   protocol as well.

2.2.  RPC Security Flavors

   The NFS server checks permissions by taking the credentials from the
   RPC security information in each remote request. Each flavor packages
   credentials differently.

2.2.1.  AUTH_SYS

   Using the AUTH_SYS flavor of authentication, the server gets the
   client's effective user identifier, effective group identifier and
   supplemental group identifiers on each call, and uses them to check
   access. Using user identifiers and group identifiers implies that the
   client and server either share the same identifier name space or do
   local user and group identifier mapping.

   For those sites that do not implement a consistent user identifier
   and group identifier space, NFS implementations must agree on the
   mapping of user and group identifiers between NFS clients and

2.2.2.  AUTH_DH and AUTH_KERB4

   The AUTH_DH and AUTH_KERB4 styles of security are based on a
   network-wide name. They provide greater security through the use of
   DES encryption and public keys in the case of AUTH_DH, and DES
   encryption and Kerberos secret keys (and tickets) in the AUTH_KERB4
   case. Again, the server and client must agree on the identity of a

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   particular name on the network, but the name to identity mapping is
   more operating system independent than the user identifier and group
   identifier mapping in AUTH_SYS. Also, because the authentication
   parameters are encrypted, a malicious user must know another user's
   network password or private key to masquerade as that user.
   Similarly, the server returns a verifier that is also encrypted so
   that masquerading as a server requires knowing a network password.

2.2.3.  RPCSEC_GSS

   The RPCSEC_GSS style of security is based on a security-mechanism-
   specific principal name. GSS-API mechanisms provide security through
   the use of cryptography. The cryptographic protections are used in
   the construction of the credential on calls, and in the verifiers on
   replies. Optionally, cryptographic protections will be in the body of
   the calls and replies.

   Note that the discussion of AUTH_NONE, AUTH_SYS, AUTH_DH, AUTH_KERB4,
   and RPCSEC_GSS does not imply that the NFS protocol is limited to
   using those five flavors.

2.3.  Authentication for NFS Procedures

2.3.1.  NULL Procedure

   The NULL procedure is typically used by NFS clients to determine if
   an NFS server is operating and responding to requests (in other
   words, to "ping" the NFS server). Some NFS servers require that a
   client using the NULL procedure:

   *    send the request from TCP or UDP port < 1024.  There does not
        seem to be any value in this because the NULL procedure is of
        very low overhead and certainly no more overhead than the cost
        of processing a NULL procedure and returning an authentication
        error. Moreover, by sending back an authentication error, the
        server has confirmed the information that the client was
        interested in: is the server operating?

   *    be authenticated with a flavor stronger than AUTH_SYS. This is a
        problem because the RPCSEC_GSS protocol uses NULL for control

   NFS servers SHOULD:

   *    accept the NULL procedure ping over AUTH_NONE and AUTH_SYS, in
        addition to other RPC security flavors, and

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   *    NOT require that the source port be < 1024 on a NULL procedure

2.3.2.  NFS Procedures Used at Mount Time

   Certain NFS procedures are used at the time the NFS client mounts a
   file system from the server.  Some NFS server implementations will
   not require authentication for these NFS procedures.  For NFS
   protocol Version 2, these procedures are GETATTR and STATFS. For
   Version 3, the procedure is FSINFO.

   The reason for not requiring authentication is described as follows.
   When the NFS client mounts a NFS server's file system, the identity
   of the caller on the client is typically an administrative entity (in
   UNIX operating systems, this is usually the "root" user).  It is
   often the case that, for unattended operation in concert with an
   automounter [Callaghan], the AUTH_DH, AUTH_KERB4, or RPCSEC_GSS
   credentials for the administrative entity associated with an
   automounter are not available. If so, the NFS client will use
   AUTH_NONE or AUTH_SYS for the initial NFS operations used to mount a
   file system.  While an attacker could exploit this implementation
   artifact, the exposure is limited to gaining the attributes of a file
   or a file system's characteristics. This OPTIONAL trade off favors
   the opportunity for improved ease of use.

2.4.  Binding Security Flavors to Exports

   NFS servers MAY export file systems with specific security flavors
   bound to the export.  In the event a client uses a security flavor
   that is not the one of the flavors the file system was exported with,
   NFS server implementations MAY:

   *    reject the request with an error (either an NFS error or an RPC
        level authentication error), or

   *    allow the request, but map the user's credentials to a user
        other than the one the client intended. Typically the user that
        is the result of this mapping is a user with limited access on
        the system, such as user "nobody" on UNIX systems.

   If a client uses AUTH_NONE, the server's options are the same as the
   above, except that AUTH_NONE carries with it no user identity. In
   order to allow the request, on many operating systems the server will
   assign a user identity. Typically this assignment will be a user with
   limited access on the system, such as user "nobody" on UNIX systems.

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2.5.  Anonymous Mapping

   The following passage is excerpted verbatim from RFC 1813, section 
   4.4 "Permission Issues" (except that "may" has been changed to

      In most operating systems, a particular user (on UNIX, the uid 0)
      has access to all files, no matter what permission and ownership
      they have. This superuser permission MAY not be allowed on the
      server, since anyone who can become superuser on their client
      could gain access to all remote files. A UNIX server by default
      maps uid 0 to a distinguished value (UID_NOBODY), as well as
      mapping the groups list, before doing its access checking. A
      server implementation MAY provide a mechanism to change this
      mapping. This works except for NFS version 3 protocol root file
      systems (required for diskless NFS version 3 protocol client
      support), where superuser access cannot be avoided.  Export
      options are used, on the server, to restrict the set of clients
      allowed superuser access.

   The issues identified as applying to NFS protocol Version 3 in the
   above passage also apply to Version 2.

2.6.  Host-based Access Control

   In some NFS server implementations, a host-based access control
   method is used whereby file systems can be exported to lists of
   clients.  File systems may also be exported for read-only or read-
   write access.  Several of these implementations will check access
   only at mount time, during the request for the file handle via the
   MOUNT protocol handshake.  The lack of authorization checking during
   subsequent NFS requests has the following consequences:

   *    NFS servers are not able to repudiate access to the file system
        by an NFS client after the client has mounted the file system.

   *    An attacker can circumvent the MOUNT server's access control to
        gain access to a file system that the attacker is not authorized
        for. The circumvention is accomplished by either stealing a file
        handle (usually by snooping the network traffic between an
        legitimate client and server) or guessing a file handle.  For
        this attack to succeed, the attacker must still be able
        impersonate a user's credentials, which is simple for AUTH_SYS,
        but harder for AUTH_DH, AUTH_KERB4, and RPCSEC_GSS.

   *    WebNFS clients that use the public file handle lookup [RFC2054]
        will not go through the MOUNT protocol to acquire initial file
        handle of the NFS file system. Enforcing access control via the

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        MOUNT protocol is going to be a little use. Granted, some WebNFS
        server implementations cope with this by limiting the use of the
        public file handle to file systems exported to every client on
        the Internet.

   Thus, NFS server implementations SHOULD check the client's
   authorization on each NFS request.

2.7.  Security Flavor Negotiation

   Any application protocol that supports multiple styles of security
   will have the issue of negotiating the security method to be used.
   NFS Version 2 had no support for security flavor negotiation.  It was
   up to the client to guess, or depend on prior knowledge.  Often the
   prior knowledge would be available in the form of security options
   specified in a directory service used for the purpose of

   The MOUNT Version 3 protocol, associated with NFS Version 3, solves
   the problem by having the response to the MNT procedure include a
   list of flavors in the MNT procedure. Note that because some NFS
   servers will export file systems to specific lists of clients, with
   different access (read-only versus read-write), and with different
   security flavors, it is possible a client might get back multiple
   security flavors in the list returned in the MNT response. The use of
   one flavor instead of another might imply read-only instead of read-
   write access, or perhaps some other degradation of access. For this
   reason, a NFS client SHOULD use the first flavor in the list that it
   supports, on the assumption that the best access is provided by the
   first flavor. NFS servers that support the ability to export file
   systems with multiple security flavors SHOULD either present the best
   accessing flavor first to the client, or leave the order under the
   control of the system administrator.

2.8.  Registering Flavors

   When one develops a new RPC security flavor, MUST be
   contacted to get a unique flavor assignment. To simplify NFS client
   and server administration, having a simple ASCII string name for the
   flavor is useful. Currently, the following assignments exist:

      flavor       string name

      AUTH_NONE    none
      AUTH_SYS     sys
      AUTH_DH      dh
      AUTH_KERB4   krb4

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   A string name for a new flavor SHOULD be assigned.  String name
   assignments can be registered by contacting In the
   future, this function may be transferred to

3.  The NFS Protocol's Use of RPCSEC_GSS

3.1.  Server Principal

   When using RPCSEC_GSS, the NFS server MUST identify itself in GSS-API
   via a GSS_C_NT_HOSTBASED_SERVICE name type.
   GSS_C_NT_HOSTBASED_SERVICE names are of the form:


   For NFS, the "service" element is


3.2.  Negotiation

   RPCSEC_GSS is a single security flavor over which different security
   mechanisms can be multiplexed. Within a mechanism, GSS-API provides
   for the support of multiple quality of protections (QOPs), which are
   pairs of cryptographic algorithms. Each algorithm in the QOP consists
   of an encryption algorithm for privacy and a checksum algorithm for
   integrity.  RPCSEC_GSS lets one protect the RPC request/response pair
   with plain header authentication, message integrity, and message
   privacy.  Thus RPCSEC_GSS effectively supports M * Q * 3 different
   styles of security, where M is the number of mechanisms supported, Q
   is the average number of QOPs supported for each mechanism, and 3
   enumerates authentication, integrity, and privacy.

   Because RPCSEC_GSS encodes many styles of security, just adding
   RPCSEC_GSS to the list of flavors returned in MOUNT Version 3's MNT
   response is not going to be of much use to the NFS client.

   The solution is the creation of a concept called "pseudo flavors."
   Pseudo flavors are 32 bit integers that are allocated out of the same
   number space as regular RPC security flavors like AUTH_NONE,
   AUTH_SYS, AUTH_DH, AUTH_KERB4, and RPCSEC_GSS. The idea is that each
   pseudo flavor will map to a specific triple of security mechanism,
   quality of protection, and service. The service will be one of
   authentication, integrity, and privacy. Note that integrity includes
   authentication, and privacy includes integrity. RPCSEC_GSS uses
   constants named rpc_gss_svc_none, rpc_gss_svc_integrity, and
   rpc_gss_svc_privacy, for authentication, integrity, and privacy

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   Thus, instead of returning RPCSEC_GSS, a MOUNT Version 3 server will
   instead return one or more pseudo flavors if the NFS server supports
   RPCSEC_GSS and if the file system has been exported with one or more
   <mechanism, QOP, service> triples.  See section 4, "The NFS Protocol
   over Kerberos V5" for an example of pseudo flavor to triple mapping.

3.3.  Changing RPCSEC_GSS Parameters

   Once an RPCSEC_GSS session or context has been set up (via the
   RPCSEC_GSS_INIT and RPCSEC_GSS_CONTINUE_INIT control procedures of
   RPCSEC_GSS), the NFS server MAY lock the <mechanism, QOP, service>
   triple for the duration of the session.  While RPCSEC_GSS allows for
   the use of different QOPs and services on each message, it would be
   expensive for the NFS server to re-consult its table of exported file
   systems to see if the triple was allowed. Moreover, by the time the
   NFS server's dispatch routine was reached, the typical RPC subsystem
   would already have performed the appropriate GSS-API operation,
   GSS_VerifyMIC() or GSS_Unwrap(), if the respective integrity or
   privacy services were selected. If the file system being accessed
   were not exported with integrity or privacy, or with the particular
   QOP used to perform the integrity or privacy service, then it would
   be possible to execute a denial of service attack, whereby the
   objective of the caller is to deny CPU service to legitimate users of
   the NFS server's machine processors.

   Thus, in general, clients SHOULD NOT assume that they will be
   permitted to alter the <mechanism, QOP, service> triple once the data
   exchange phase of RPCSEC_GSS has started.

3.4.  Registering Pseudo Flavors and Mappings

   Pseudo flavor numbers MUST be registered via same method as regular
   RPC security flavor numbers via

   Once the pseudo flavor number has been assigned, registrants SHOULD
   register the mapping with The mapping registration
   MUST contain:

   *    the pseudo flavor number, an ASCII string name for the flavor
        (for example "none" has been assigned for AUTH_NONE), and

   *    the <mechanism, algorithm(s), service> triple.  As per the GSS-
        API specification, the mechanism MUST be identified with a
        unique ISO object identifier (OID). The reason why the second
        component of the triple is not necessarily a QOP value is that
        GSS-API allows mechanisms much latitude in the mapping of the
        algorithm used in the default quality of protection (See
        subsection 4.1, "Issues with Kerberos V5 QOPs," for a detailed

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        discussion). With some mechanisms, the second component of the
        triple will be a QOP. Internally, on the NFS implementation, it
        is expected that the triple would use a QOP for the second

   The mapping registration SHOULD also contain:

   *    A reference to an RFC (typically an Informational RFC)
        describing how the NFS protocol MUST work over the pseudo
        flavor(s), including the pseudo flavor number(s), string name(s)
        for the flavor(s), and any other issues, including how the
        registrant is interpreting the GSS-API mechanism.

   *    A reference to the GSS-API mechanism used.

   An example of a complete registration is provided in subsection 4.2,
   "The NFS Protocol over Kerberos V5 Pseudo Flavor Registration Entry."

4.  The NFS Protocol over Kerberos V5

   The NFS protocol uses Kerberos V5 security using the RPCSEC_GSS
   security flavor.  The GSS-API security mechanism for Kerberos V5 that
   the NFS/RPCSEC_GSS protocol stack uses is described in the Kerberos
   V5 GSS-API description [RFC 1964].

4.1.  Issues with Kerberos V5 QOPs

   The Kerberos V5 GSS-API description defines three algorithms for

   *    DES MAC MD5

   *    MD2.5

   *    DES-MAC

   RFC 1964 states that MD2.5 "may be significantly weaker than DES MAC
   MD5." RFC 1964 also states that DES-MAC "may not be present in all

   Thus the description of operation of NFS clients and servers over
   Kerberos V5 is limited to the DES MAC MD5 integrity algorithm.

   NFS clients and servers operating over Kerberos V5 MUST support the
   DES MAC MD5 integrity algorithm. RFC 1964 lists a single algorithm
   for privacy: 56 bit DES.  NFS clients and servers SHOULD support the
   56 bit DES privacy algorithm.

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   GSS-API has the concept of a default QOP of zero which means
   different integrity and privacy algorithms to different GSS-API
   mechanisms. In Kerberos V5, the default QOP of zero means to use the
   56 bit DES algorithm (when doing a GSS_Wrap() operation with the
   conf_req_flag set to 1).

   For Kerberos V5, the default QOP of zero means different integrity
   algorithms to different implementations of Kerberos V5.  Furthermore,
   during the processing of a token in GSS_Unwrap(), and
   GSS_VerifyMIC(), at least one reference implementation of the
   Kerberos V5 GSS-API mechanism [MIT], always returns a QOP of zero,
   regardless of integrity algorithm encoded in the token.  For such
   implementations, it means that the caller of GSS_Unwrap() and
   GSS_VerifyMIC() cannot know the actual integrity algorithm used.
   Given that each integrity algorithm has a different degree of
   security, this situation may not be acceptable to the user of GSS-
   API. An implementation of Kerberos V5 under GSS-API for use under NFS
   MUST NOT do this.

   For the purposes of NFS, as a simplification, some Kerberos V5 GSS-
   API mechanisms MAY map QOP 0 to always mean DES MAC MD5 integrity,
   and when using GSS_VerifyMIC() and GSS_Unwrap(), always map the DES
   MAC MD5 integrity that is specified to QOP 0.

4.2.  The NFS Protocol over Kerberos V5 Pseudo Flavor Registration Entry

   Here are the pseudo flavor mappings for the NFS protocol using
   Kerberos V5 security:


 1 == number of pseudo flavor
 2 == name of pseudo flavor
 3 == mechanism's OID
 4 == mechanism's algorithm(s)
 5 == RPCSEC_GSS service

 1      2     3                    4              5
 390003 krb5  1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_none
 390004 krb5i 1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_integrity
 390005 krb5p 1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_privacy
                                   for integrity,
                                   and 56 bit DES
                                   for privacy.

   An implementation of NFS over RPCSEC_GSS/GSS-API/Kerberos V5 that

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   maps the default QOP to DES MAC MD5 (and vice versa), would implement
   a mapping of:


      1 == number of pseudo flavor
      2 == name of pseudo flavor
      3 == mechanism's OID
      4 == QOP
      5 == RPCSEC_GSS service

      1      2     3                     4  5
      390003 krb5  1.2.840.113554.1.2.2  0  rpc_gss_svc_none
      390004 krb5i 1.2.840.113554.1.2.2  0  rpc_gss_svc_integrity
      390005 krb5p 1.2.840.113554.1.2.2  0  rpc_gss_svc_privacy

   The reference for the GSS-API mechanism with the above OID is RFC

   The reference for how the NFS protocol MUST work over Kerberos V5 is
   this document.

5.  Security Considerations

   Version 3 of the MOUNT protocol is used to negotiate the security
   flavor to be used by the NFS Version 3 client. If the NFS client uses
   a weak security flavor like AUTH_SYS to query a Version 3 MOUNT
   server, then the following attacks are possible by an attacker in the

   *    The attacker in the middle can coax the NFS client into using a
        weaker form of security than what the real NFS server requires.
        However, once the NFS client selects a security flavor when it
        sends a request to real NFS server, if the flavor is
        unacceptable, the NFS client's NFS request will be rejected. So
        at worst, a denial of service attack is possible. In theory, the
        NFS client could contact the MOUNT server using a stronger
        security flavor, but this would require that the client know in
        advance what security flavors the MOUNT server supports.

   *    If the client and server support a common set of security
        flavors, such that the client considers one preferable to the
        other (for example, one might have privacy and other not),
        unless the client uses a strong security flavor in the MOUNT
        protocol query, an attacker in the middle could cause the client
        to use the weaker form of security.  Again, a client could
        contact the MOUNT server using a stronger form of security.

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   [RFC1094] Sun Microsystems, Inc. (1989). "NFS: Network File System
             Protocol specification," RFC 1094.

             Sandberg, R., Goldberg, D., Kleiman, S., Walsh, D., Lyon,
             B.. (1985). "Design and Implementation of the Sun Network
             Filesystem,"  Proceedings of the 1985 Summer USENIX
             Technical Conference.

   [RFC1813] Callaghan, B., Pawlowski, B., Staubach, P. (1995). "NFS
             Version 3 Protocol Specification," RFC 1813.

   [RFC1831] Srinivasan, R. (1995). "RPC: Remote Procedure Call Protocol
             Specification Version 2," RFC 1831.

   [RFC1832] Srinivasan, R. (1995). "XDR: External Data Representation
             Standard," RFC 1832.

             Pawlowski, B., Juszczak, C., Staubach, P., Smith, C.,
             Lebel, D., Hitz, D. (1994). "NFS Version 3 Design and
             Implementation," Proceedings of the USENIX Summer 1994
             Technical Conference.

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

   [RFC2078] Linn, J. (1997). "Generic Security Service Application
             Program Interface, Version 2," RFC 2078.

   [RFC1057] Sun Microsystems, Inc. "RPC: Remote Procedure Call Protocol
             Specification Version 2," RFC 1057. This RFC is being
             referenced for its description of the AUTH_DH (AUTH_DES)
             RPC security flavor.

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

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INTERNET-DRAFT  NFS Security, RPCSEC_GSS, and Kerberos V5   January 1999

             Callaghan, B., Singh, S. (1993). "The Autofs Automounter,"
             Proceedings of the 1993 Summer USENIX Technical Conference.

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

   [RFC2054] Callaghan, B. (1996). "WebNFS Client Specification," RFC

   [MIT]     Massachusetts Institute of Technology (1998). "Kerberos:
             The Network Authentication Protocol." The Web site for
             downloading MIT's implementation of Kerberos V5, including
             implementations of RFC 1510 and RFC 1964.


   The author thanks:

   *    Brent Callaghan, John Hawkinson, Jack Kabat, Lin Ling, Steve
        Nahm, and David Robinson for their review comments.

   *    John Linn, for his explanation of QOP handling in RFC 1964.

Author's Address

   Address comments related to this memorandum to:

   Mike Eisler
   Sun Microsystems, Inc.
   5565 Wilson Road
   Colorado Springs, CO 80919

   Phone: 1-719-599-9026

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