Network File System Version 4                               T. Myklebust
Internet-Draft                                               Hammerspace
Updates: 5531 (if approved)                                C. Lever, Ed.
Intended status: Standards Track                                  Oracle
Expires: August 15, 2019                               February 11, 2019

              Remote Procedure Call Encryption By Default


   This document describes a mechanism that enables encryption of in-
   transit Remote Procedure Call (RPC) transactions with minimal
   administrative overhead and full interoperation with RPC
   implementations that do not support this mechanism.  This document
   updates RFC 5531.

Status of This Memo

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

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   This Internet-Draft will expire on August 15, 2019.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  RPC-Over-TLS in Operation . . . . . . . . . . . . . . . . . .   4
     4.1.  Discovering Server-side TLS Support . . . . . . . . . . .   4
     4.2.  Streams and Datagrams . . . . . . . . . . . . . . . . . .   6
     4.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .   6
       4.3.1.  No Client Authentication  . . . . . . . . . . . . . .   6
       4.3.2.  Client Authentication . . . . . . . . . . . . . . . .   7
       4.3.3.  Advanced Forms of RPC Authentication  . . . . . . . .   7
       4.3.4.  Other Forms of TLS Authentication . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.1.  Implications for AUTH_SYS . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   In 2014 the IETF published [RFC7258] which recognized that
   unauthorized observation of network traffic had become widespread and
   was a subversive threat to all who make use of the Internet at large.
   It strongly recommended that newly defined Internet protocols make a
   real effort to mitigate monitoring attacks.  Typically this
   mitigation is done by encrypting data in transit.

   The Remote Procedure Call version 2 protocol has been a Proposed
   Standard for three decades (see [RFC5531] and its antecedants).

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   Eisler et al. first introduced an in-transit encryption mechanism for
   RPC with RPCSEC GSS over twenty years ago [RFC2203].  However,
   experience has shown that RPCSEC GSS is difficult to deploy:

   o  Per-client deployment and administrative costs are not scalable.
      Keying material must be provided for each RPC client, including
      transient clients.

   o  Parts of the RPC header remain in clear-text, and can constitute a
      significant security exposure.

   o  On-host cryptographic manipulation of data payloads can exact a
      significant CPU cost on both clients and the server.

   o  Host identity management must be carried out in a security realm
      that is separate from user identity management.  In certain
      environments, for example, different authorities might be
      responsible for provisioning client systems versus provisioning
      new users.

   However strong a privacy service is, it can not provide any security
   if the difficulties of deploying and using it result in it not being
   used at all.

   An alternative approach is to employ a transport layer security
   mechanism that can protect the privacy of each RPC connection
   transparently to RPC and Upper Layer protocols.  The Transport Layer
   Security protocol [RFC8446] (TLS) is a well-established Internet
   building block that protects many common Internet protocols such as
   the Hypertext Transport Protocol (http) [RFC2818].

   Encrypting at the RPC transport layer enables several significant

   Encryption By Default
      In-transit encryption can be enabled immediately after
      installation without additional administrative actions such as
      identifying the host system to a trust authority, generating
      additional key material, or provisioning a secure network tunnel.

   Protection of Existing Protocols
      The imposition of encryption at the transport layer protects any
      Upper Layer protocol that employs RPC without alteration of that
      protocol.  RPC transport layer encryption can protect recent
      versions of NFS such as NFS version 4.2 [RFC7862] and indeed
      legacy NFS versions such as NFS version 3 [RFC1813] and NFS side-
      band protocols such as the MNT protocol [RFC1813].

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   Decoupled User and Host Identities
      RPCSEC GSS provides a framework for cryptographically protecting
      user and host identities but assumes that both are managed by the
      same security authority.

   Encryption Offload
      The use of a well-established transport encryption mechanism that
      is also employed by other very common network protocols makes it
      possible to use hardware encryption implementations so that the
      host CPU is not burdened with the work of encrypting and
      decrypting large RPC arguments and results.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   This document adopts the terminology introduced in Section 3 of
   [RFC6973] and assumes a working knowledge of the Remote Procedure
   Call (RPC) version 2 protocol [RFC5531] and the Transport Layer
   Security (TLS) version 1.3 protocol [RFC8446].

   Note also that the NFS community uses the term "privacy" where other
   Internet communities might use "confidentiality".  In this document
   the two terms are synonymous.

4.  RPC-Over-TLS in Operation

   In this section we cleave to the convention that a "client" is the
   peer host that actively initiates a connection, and a "server" is the
   peer host that passively accepts a connection request.

4.1.  Discovering Server-side TLS Support

   The mechanism described in this document interoperates fully with
   implementations that do not support it.  The use of TLS is
   automatically disabled in these cases.  To achieve this, we introduce
   a new authentication flavor called AUTH_TLS.  This new flavor is used
   to signal that the client wants to initiate TLS negotiation if the
   server supports it.

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   enum auth_flavor {
           AUTH_NONE       = 0,
           AUTH_SYS        = 1,
           AUTH_SHORT      = 2,
           AUTH_DH         = 3,
           AUTH_KERB       = 4,
           AUTH_RSA        = 5,
           RPCSEC_GSS      = 6,
           AUTH_TLS        = 7,

           /* and more to be defined */


   The length of the opaque data constituting the credential sent in the
   call message MUST be zero.  The verifier accompanying the credential
   MUST be an AUTH_NONE verifier of length zero.

   The flavor value of the verifier received in the reply message from
   the server MUST be AUTH_NONE.  The bytes of the verifier's string
   encode the fixed ASCII characters "STARTTLS".

   When an RPC client is ready to initiate a TLS handshake, it sends a
   NULL RPC request with an auth_flavor of AUTH_TLS.  The NULL request
   is made to the same port as if TLS were not in use.

   The RPC server can respond in one of three ways:

   o  If the RPC server does not recognise the AUTH_TLS authentication
      flavor, it responds with a reject_stat of AUTH_ERROR.  The RPC
      client then knows that this server does not support TLS.

   o  If the RPC server accepts the NULL RPC procedure, but fails to
      return an AUTH_NONE verifier containing the string "STARTTLS", the
      RPC client knows that this server does not support TLS.

   o  If the RPC server accepts the NULL RPC procedure, and returns an
      AUTH_NONE verifier containing the string "STARTTLS", the RPC
      client MAY proceed with TLS negotiation.

   If an RPC client attempts to use AUTH_TLS for anything other than the
   NULL RPC procedure, the RPC server responds with a reject_stat of

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   Once the TLS handshake is complete, the RPC client and server will
   have established a secure channel for communicating and can proceed
   to use standard security flavors within that channel, presumably
   after negotiating down the irrelevant RPCSEC_GSS privacy and
   integrity services and applying channel binding [RFC7861].

   If TLS negotiation fails for any reason -- say, the RPC server
   rejects the certificate presented by the RPC client, or the RPC
   client fails to authenticate the RPC server -- the RPC client reports
   this failure to the calling application the same way it would report
   an AUTH_ERROR rejection from the RPC server.

4.2.  Streams and Datagrams

   RPC operates on several different types of transports.  RPC on a
   stream transport is protected by using TLS [RFC8446]; on a datagram
   transport, RPC must use DTLS [RFC6347].

   RPC-over-RDMA can make use of Transport Layer Security below the RDMA
   transport layer [RFC8166].  The exact mechanism is not within the
   scope of this document.

4.3.  Authentication

   Both RPC and TLS have their own variants of authentication, and there
   is some overlap in capability.  The goal of interoperability with
   implementations that do not support TLS requires that we limit the
   combinations that are allowed and precisely specify the role that
   each layer plays.  We also want to handle TLS such that an RPC
   implementation can make the use of TLS invisible to existing RPC
   consumer applications.

   Toward these ends, there are two main deployment modes.

4.3.1.  No Client Authentication

   In a basic deployment, a server possesses a certificate that is self-
   signed or signed by a well-known trust anchor, while its clients
   might not possess a certificate.  In this situation, the client MAY
   authenticate the server host, but the server cannot authenticate
   connecting clients.  Here, encryption of the transport connection is
   established and the RPC requests in transit carry user and group
   identities according to the conventions of the ONC RPC protocol.

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4.3.2.  Client Authentication

   In this type of deployment, both the server and its clients possess
   valid certificates.  As part of the TLS handshake, both peers MAY
   authenticate.  Should authentication of either peer fail, or should
   authorization based on those identities block access to the server,
   the connection can be rejected.  However, once encryption of the
   transport connection is established, the server MUST NOT utilize TLS
   identity for the purpose of authorizing RPC requests.

   In some cases, a client might choose to present a certificate that
   represents a user rather than one that is bound to the client host.
   As above, the server MUST NOT utilize this identity for the purpose
   of authorizing RPC requests.

4.3.3.  Advanced Forms of RPC Authentication

   RPCSEC GSS can provide integrity or privacy (also known as
   confidentiality) services.  When operating over an encrypted TLS
   session, these services become redundant.  Each RPC implementation is
   responsible for using channel binding for detecting when GSS
   integrity or privacy is unnecessary and can therefore be disabled See
   Section 2.5 of [RFC7861] for details.

   Note that a GSS service principal is still required on the server,
   and mutual authentication of server and client still occurs after the
   TLS session is established.

4.3.4.  Other Forms of TLS Authentication

   Versions of TLS subsequent to TLS 1.2 feature a token binding
   mechanism which is nominally more secure than using certificates.
   This is discussed in further detail in [RFC8471].  When such versions
   of TLS are used to encrypted RPC traffic, token binding may replace
   the use of certificates, but the restrictions specified earlier in
   this section still apply.

5.  Security Considerations

   One purpose of the mechanism described in this document is to protect
   RPC-based applications against threats to the privacy of RPC
   transactions and RPC user identities.  A taxonomy of these threats
   appears in Section 5 of [RFC6973].  In addition, Section 6 of
   [RFC7525] contains a detailed discussion of technologies used in
   conjunction with TLS.  Implementers should familiarize themselves
   with these materials.

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   The NFS version 4 protocol permits more than one user to use an NFS
   client at the same time [RFC7862].  Typically that NFS client will
   conserve connection resources by routing RPC transactions from all of
   its users over a few or a single connection.  In circumstances where
   the users on that NFS client belong to multiple distinct security
   domains, it may be necessary to establish separate TLS-protected
   connections that do not share the same encryption parameters.

5.1.  Implications for AUTH_SYS

   Ever since the IETF NFSV4 Working Group took over the maintenance of
   the NFSv4 family of protocols (currently specified in [RFC7530],
   [RFC5661], and [RFC7863], among others), it has encouraged the use of
   RPCSEC GSS over AUTH_SYS.  For various reasons, unfortunately
   AUTH_SYS continues to be the primary authentication mechanism
   deployed by NFS administrators.  As a result, NFS security remains in
   an unsatisfactory state.

   A deeper purpose of this document is to attempt to address some of
   the shortcomings of AUTH_SYS so that, where it has been impractical
   to deploy RPCSEC GSS, better NFSv4 security can nevertheless be

   When AUTH_SYS is used with TLS and no client certificate is
   available, the RPC server is still acting on RPC requests for which
   there is no trustworthy authentication.  In-transit traffic is
   protected, but the client itself can still misrepresent user identity
   without detection.  This is an improvement from AUTH_SYS without
   encryption, but it leaves a critical security exposure.

   Therefore, the RECOMMENDED deployment mode is that both servers and
   clients have certificate material available so that servers can have
   a degree of trust that clients are acting responsibly.

6.  IANA Considerations

   In accordance with Section 6 of [RFC7301], the authors request that
   IANA allocate the following value in the "Application-Layer Protocol
   Negotiation (ALPN) Protocol IDs" registry.  The "sunrpc" string
   identifies SunRPC when used over TLS.


   Identification Sequence:
      0x73 0x75 0x6e 0x72 0x70 0x63 ("sunrpc")


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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
              May 2009, <>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

7.2.  Informative References

   [LJNL]     Fisher, C., "Encrypting NFSv4 with Stunnel TLS", August
              2018, <

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   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813,
              DOI 10.17487/RFC1813, June 1995,

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, DOI 10.17487/RFC2203, September
              1997, <>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <>.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <>.

   [RFC7863]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 External Data Representation Standard (XDR)
              Description", RFC 7863, DOI 10.17487/RFC7863, November
              2016, <>.

   [RFC8166]  Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call Version
              1", RFC 8166, DOI 10.17487/RFC8166, June 2017,

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   [RFC8471]  Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
              "The Token Binding Protocol Version 1.0", RFC 8471,
              DOI 10.17487/RFC8471, October 2018,


   Special mention goes to Charles Fisher, author of "Encrypting NFSv4
   with Stunnel TLS" [LJNL].  His article inspired the mechanism
   described in this document.

   The authors are grateful to Bill Baker, David Black, Lars Eggert,
   Benjamin Kaduk Greg Marsden, Alex McDonald, David Noveck, Justin
   Mazzola Paluska, and Tom Talpey for their input and support of this

   Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
   Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
   Working Group Secretary Thomas Haynes for their guidance and

Authors' Addresses

   Trond Myklebust
   Hammerspace Inc
   4300 El Camino Real Ste 105
   Los Altos, CA  94022
   United States of America


   Charles Lever (editor)
   Oracle Corporation
   United States of America


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