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Remote Procedure Call Encryption By Default

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9289.
Authors Trond Myklebust , Chuck Lever
Last updated 2019-09-22 (Latest revision 2019-04-25)
Replaces draft-cel-nfsv4-rpc-tls
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state Became RFC 9289 (Proposed Standard)
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Network File System Version 4                               T. Myklebust
Internet-Draft                                               Hammerspace
Updates: 5531 (if approved)                                C. Lever, Ed.
Intended status: Standards Track                                  Oracle
Expires: March 24, 2020                               September 21, 2019

              Remote Procedure Call Encryption By Default


   This document describes a mechanism that, through the use of
   opportunistic Transport Layer Security (TLS), enables encryption of
   in-transit Remote Procedure Call (RPC) transactions while
   interoperating with ONC 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.

   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

   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 March 24, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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

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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
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  RPC-Over-TLS in Operation . . . . . . . . . . . . . . . . . .   5
     4.1.  Discovering Server-side TLS Support . . . . . . . . . . .   5
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Using TLS with RPCSEC GSS . . . . . . . . . . . . . .   8
   5.  TLS Requirements  . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Base Transport Considerations . . . . . . . . . . . . . .   8
       5.1.1.  Operation on TCP  . . . . . . . . . . . . . . . . . .   8
       5.1.2.  Operation on UDP  . . . . . . . . . . . . . . . . . .   9
       5.1.3.  Operation on Other Transports . . . . . . . . . . . .   9
     5.2.  TLS Peer Authentication . . . . . . . . . . . . . . . . .   9
       5.2.1.  X.509 Certificates Using PKIX trust . . . . . . . . .   9
       5.2.2.  X.509 Certificates Using Fingerprints . . . . . . . .  11
       5.2.3.  Pre-Shared Keys . . . . . . . . . . . . . . . . . . .  11
       5.2.4.  Token Binding . . . . . . . . . . . . . . . . . . . .  11
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  11
     6.1.  DESY NFS server . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  Hammerspace NFS server  . . . . . . . . . . . . . . . . .  12
     6.3.  Linux NFS server and client . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  Limitations of an Opportunistic Approach  . . . . . . . .  13
       7.1.1.  STRIPTLS Attacks  . . . . . . . . . . . . . . . . . .  13
     7.2.  Multiple User Identity Realms . . . . . . . . . . . . . .  14
     7.3.  Security Considerations for AUTH_SYS on TLS . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Known Weaknesses of the AUTH_SYS Authentication

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                Flavor . . . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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).
   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 can be 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 each RPC header remain in clear-text, and can constitute
      a significant security exposure.

   o  Host identity management and user identity management must be
      carried out in the same security realm.  In certain environments,
      different authorities might be responsible for provisioning client
      systems versus provisioning new users.

   o  On-host cryptographic manipulation of data payloads can exact a
      significant CPU and memory bandwidth cost on RPC peers.  Offloadng
      does not appear to be practical using GSS privacy since each
      message is encrypted using its own key based on the issuing RPC

   However strong a privacy service is, it cannot provide any security
   if the challenges of 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].

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   Encrypting at the RPC transport layer enables several significant

   Encryption By Default
      In-transit encryption by itself may be enabled without additional
      administrative actions such as identifying client systems 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].

   Decoupled User and Host Identities
      TLS can be used to authenticate peer hosts while other security
      mechanisms can handle user authentictation.  Cryptographic
      authentication of hosts can be provided while still using simpler
      user authentication flavors such as AUTH_SYS.

   Encryption Offload
      Whereas hardware support for GSS privacy has not appeared in the
      marketplace, the use of a well-established transport encryption
      mechanism that is also employed by other very common network
      protocols makes it likely that a hardware encryption
      implementation will be available to offload encryption and

   Securing AUTH_SYS
      Most critically, several security issues inherent in the current
      widespread use of AUTH_SYS (i.e., acceptance of UIDs and GIDs
      generated by an unauthenticated client) can be significantly

   This document specifies the use of RPC on a TLS-protected transport
   in a fashion that is transparent to upper layer protocols based on
   RPC.  It provides policies in line with [RFC7435] that enable RPC-on-
   TLS to be deployed opportunistically in environments with RPC
   implementations that do not support TLS.  Specifications for RPC-
   based upper layer protocols are free to require stricter policies to
   guarantee that TLS with encryption or TLS with host authentication
   and encryption is used for every connection.

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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 use "confidentiality".  In this document the two
   terms are synonymous.

   We adhere to the convention that a "client" is a network host that
   actively initiates an association, and a "server" is a network host
   that passively accepts an association request.

   RPC documentation historically refers to the authentication of a
   connecting host as "machine authentication" or "host authentication".
   TLS documentation refers to the same as "peer authentication".  In
   this document there is little distinction between these terms.

   The term "user authentication" in this document refers specifically
   to the RPC caller's credential, provided in the "cred" and "verf"
   fields in each RPC Call.

4.  RPC-Over-TLS in Operation

4.1.  Discovering Server-side TLS Support

   The mechanism described in this document interoperates fully with RPC
   implementations that do not support TLS.  The use of TLS is
   automatically disabled in these cases.

   To achieve this, we introduce a new RPC 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.  Except for the
   modifications described in this section, the RPC protocol is largely
   unaware of security encapsulation.

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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 begin sending traffic to a server, it
   starts with 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 SHOULD send a STARTTLS.

   Once the TLS handshake is complete, the RPC client and server will
   have established a secure channel for communicating.  The client MUST
   switch to a security flavor other than AUTH_TLS within that channel,
   presumably after negotiating down redundant RPCSEC_GSS privacy and
   integrity services and applying channel binding [RFC7861].

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   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.

   If an RPC client attempts to use AUTH_TLS for anything other than the
   NULL RPC procedure, the RPC server MUST respond with a reject_stat of
   AUTH_ERROR.  If the client sends a STARTTLS after it has sent other
   non-encrypted RPC traffic or after a TLS session has already been
   negotiated, the server MUST silently discard it.

4.2.  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.

   Each RPC server that supports RPC-over-TLS MUST possess a unique
   global identity (e.g., a certificate that is signed by a well-known
   trust anchor).  Such an RPC server MUST request a TLS peer identity
   from each client upon first contact.  There are two different modes
   of client deployment:

   Server-only Host Authentication
      In this type of deployment, RPC-over-TLS clients are essentially
      anonymous; i.e., they present no globally unique identifier to the
      server peer.  In this situation, the client can authenticate the
      server host using the presented server peer TLS identity, but the
      server cannot authenticate the client.

   Mutual Host Authentication
      In this type of deployment, the client possesses a unique global
      identity (e.g., a certificate).  As part of the TLS handshake,
      both peers authenticate using the presented TLS identities.  If
      authentication of either peer fails, or if authorization based on
      those identities blocks access to the server, the client
      association SHOULD be rejected.

   In either of these modes, RPC user authentication is not affected by
   the use of transport layer security.  Once a TLS session is
   established, the server MUST NOT utilize the client peer's TLS
   identity for the purpose of authorizing individual RPC requests.

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4.2.1.  Using TLS with RPCSEC GSS

   RPCSEC GSS can provide per-request integrity or privacy (also known
   as confidentiality) services.  When operating over a TLS session,
   these services become redundant.  A TLS-capable RPC implementation
   uses GSS channel binding for detecting when GSS integrity or privacy
   is unnecessary and can therefore be avoided.  See Section 2.5 of
   [RFC7861] for details.

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

5.  TLS Requirements

   When a TLS session is negotiated for the purpose of transporting RPC,
   the following restrictions apply:

   o  Implementations MUST NOT negotiate TLS versions prior to v1.3
      [RFC8446].  Support for mandatory-to-implement ciphersuites for
      the negotiated TLS version is REQUIRED.

   o  Implementations MUST support certificate-based mutual
      authentication.  Support for TLS-PSK mutual authentication
      [RFC4279] is OPTIONAL.  See Section 4.2 for further details.

   o  Negotiation of a ciphersuite providing for confidentiality as well
      as integrity protection is REQUIRED.  Support for and negotiation
      of compression is OPTIONAL.

5.1.  Base Transport Considerations

5.1.1.  Operation on TCP

   RPC over TCP is protected by using TLS [RFC8446].  As soon as a
   client completes the TCP handshake, it uses the mechanism described
   in Section 4.1 to discover TLS support and then negotiate a TLS

   After the TLS session is established, all traffic on the connection
   is encapsulated and protected until the TLS session is terminated.
   This includes reverse-direction operations (i.e., RPC requests
   initiated on the server-end of the connection).  An RPC client
   receiving a reverse-direction operation on a connection outside of an
   existing TLS session MUST reject the request with a reject_stat of

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   An RPC peer terminates a TLS session by sending a TLS closure alert,
   or by closing the underlying TCP socket.  After TLS session
   termination, a recipient MUST reject any subsequent RPC requests over
   the same connection with a reject_stat of AUTH_ERROR.

5.1.2.  Operation on UDP

   RPC over UDP is protected using DTLS [RFC6347].  As soon as a client
   initializes a socket for use with an unfamiliar server, it uses the
   mechanism described in Section 4.1 to discover DTLS support and then
   negotiate a DTLS session.  Connected operation is RECOMMENDED.

   Using a DTLS transport does not introduce reliable or in-order
   semantics to RPC on UDP.  Also, DTLS does not support fragmentation
   of RPC messages.  One RPC message fits in a single DTLS datagram.
   DTLS encapsulation has overhead which reduces the effective Path MTU
   (PMTU) and thus the maximum RPC payload size.

   DTLS does not detect STARTTLS replay.  A DTLS session can be
   terminated by sending a TLS closure alert.  Subsequent RPC messages
   passing between the client and server will no longer be protected
   until a new TLS session is established.

5.1.3.  Operation on Other Transports

   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.  Because there might not be provisions to
   exchange client and server certificates, authentication material
   could be provided by facilites within a future RPC-over-RDMA

   Transports that provide intrinsic TLS-level security (e.g., QUIC)
   would need to be accommodated separately from the current document.
   In such cases, use of TLS might not be opportunitic as it is for TCP
   or UDP.

5.2.  TLS Peer Authentication

   Peer authentication can be performed by TLS using any of the
   following mechanisms:

5.2.1.  X.509 Certificates Using PKIX trust

   Implementations are REQUIRED to support this mechanism.  In this
   mode, an RPC peer is uniquely identified by the tuple (serial number
   of presented certificate;Issuer).

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   o  Implementations MUST allow the configuration of a list of trusted
      Certification Authorities for incoming connections.

   o  Certificate validation MUST include the verification rules as per

   o  Implementations SHOULD indicate their trusted Certification
      Authorities (CAs).

   o  Peer validation always includes a check on whether the locally
      configured expected DNS name or IP address of the server that is
      contacted matches its presented certificate.  DNS names and IP
      addresses can be contained in the Common Name (CN) or
      subjectAltName entries.  For verification, only one of these
      entries is to be considered.  The following precedence applies:
      for DNS name validation, subjectAltName:DNS has precedence over
      CN; for IP address validation, subjectAltName:iPAddr has
      precedence over CN.  Implementors of this specification are
      advised to read Section 6 of [RFC6125] for more details on DNS
      name validation.

   o  Implementations MAY allow the configuration of a set of additional
      properties of the certificate to check for a peer's authorization
      to communicate (e.g., a set of allowed values in
      subjectAltName:URI or a set of allowed X509v3 Certificate

   o  When the configured trust base changes (e.g., removal of a CA from
      the list of trusted CAs; issuance of a new CRL for a given CA),
      implementations MAY renegotiate the TLS session to reassess the
      connecting peer's continued authorization.

   Authenticating a connecting entity does not mean the RPC server
   necessarily wants to communicate with that client.  For example, if
   the Issuer is not in a trusted set of Issuers, the RPC server may
   decline to perform RPC transactions with this client.
   Implementations that want to support a wide variety of trust models
   should expose as many details of the presented certificate to the
   administrator as possible so that the trust model can be implemented
   by the administrator.  As a suggestion, at least the following
   parameters of the X.509 client certificate should be exposed:

   o  Originating IP address

   o  Certificate Fingerprint

   o  Issuer

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   o  Subject

   o  all X509v3 Extended Key Usage

   o  all X509v3 Subject Alternative Name

   o  all X509v3 Certificate Policies

5.2.2.  X.509 Certificates Using Fingerprints

   This mechanism is OPTIONAL to implement.  In this mode, an RPC peer
   is uniquely identified by the fingerprint of the presented

   Implementations SHOULD allow the configuration of a list of trusted
   certificates, identified via fingerprint of the DER encoded
   certificate octets.  Implementations MUST support SHA-1 as the hash
   algorithm for the fingerprint.  To prevent attacks based on hash
   collisions, support for a more contemporary hash function, such as

5.2.3.  Pre-Shared Keys

   This mechanism is OPTIONAL to implement.  In this mode, an RPC peer
   is uniquely identified by key material that has been shared out-of-
   band or by a previous TLS-protected connection (see [RFC8446]
   Section 2.2).  At least the following parameters of the TLS
   connection should be exposed:

   o  Originating IP address

   o  TLS Identifier

5.2.4.  Token Binding

   This mechanism is OPTIONAL to implement.  In this mode, an RPC peer
   is uniquely identified by a token.

   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].

6.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to

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   assist the IETF in its decision processes in progressing drafts to

   Please note that the listing of any individual implementation here
   does not imply endorsement by the IETF.  Furthermore, no effort has
   been spent to verify the information presented here that was supplied
   by IETF contributors.  This is not intended as, and must not be
   construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

6.1.  DESY NFS server

   Organization:  DESY


   Maturity:  Prototype software based on early versions of this

   Coverage:  The bulk of this specification is implemented.  The use of
              DTLS functionality is not implemented.

   Licensing: LGPL

   Implementation experience:  No comments from implementors.

6.2.  Hammerspace NFS server

   Organization:  Hammerspace


   Maturity:  Prototype software based on early versions of this

   Coverage:  The bulk of this specification is implemented.  The use of
              DTLS functionality is not implemented.

   Licensing: Proprietary

   Implementation experience:  No comments from implementors.

6.3.  Linux NFS server and client

   Organization:  The Linux Foundation


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   Maturity:  Prototype software based on early versions of this

   Coverage:  The bulk of this specification is implemented.  The use of
              DTLS functionality is not implemented.

   Licensing: GPLv2

   Implementation experience:  No comments from implementors.

7.  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.

7.1.  Limitations of an Opportunistic Approach

   A range of options is allowed by the opportunistic approach described
   in this document, from "no peer authentication or encryption" to
   "server-only authentication with encryption" to "mutual
   authentication with encryption".  The security level may indeed be
   selected without user intervention based on a policy.
   Implementations must take care to accurately represent to all RPC
   consumers the level of security that is actually in effect.

7.1.1.  STRIPTLS Attacks

   A classic form of attack on network protocols that initiate an
   association in plain-text to discover support for TLS is a man-in-
   the-middle that alters the plain-text handshake to make it appear as
   though TLS support is not available on one or both peers.  Clients
   implementers can choose from the following to mitigate STRIPTLS

   o  A TLSA record [RFC6698] can alert clients that TLS is expected to
      work, and provides a binding of hostname to x.509 identity.  If
      TLS cannot be negotiated or authentication fails, the client
      disconnects and reports the problem.

   o  Client security policy can be configured to require that a TLS
      session is established on every connection.  If an attacker spoofs
      the handshake, the client disconnects and reports the problem.  If

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      TLSA records are not available, this approach is strongly

7.2.  Multiple User Identity Realms

   To maintain the privacy of RPC users on a single client belonging to
   multiple distinct security realms, the client MUST establish an
   independent TLS session for each user identity domain, each using a
   distinct globally unique identity.  The purpose of this separation is
   to prevent even privileged users in each security realm from
   monitoring RPC traffic emitted on behalf of users in other security
   realms on the same peer.

7.3.  Security Considerations for AUTH_SYS on TLS

   The use of a TLS-protected transport when the AUTH_SYS authentication
   flavor is in use addresses a number of longstanding weaknesses (as
   detailed in Appendix A).  TLS augments AUTH_SYS by providing both
   integrity protection and a privacy service that AUTH_SYS lacks.  This
   protects data payloads, RPC headers, and user identities against
   monitoring or alteration while in transit.  TLS guards against the
   insertion or deletion of messages, thus also ensuring the integrity
   of the message stream between RPC client and server.

   The use of TLS enables strong authentication of the communicating RPC
   peers, providing a degree of non-repudiation.  When AUTH_SYS is used
   with TLS but the RPC client is unauthenticated, the RPC server is
   still acting on RPC requests for which there is no trustworthy
   authentication.  In-transit traffic is protected, but the RPC client
   itself can still misrepresent user identity without server detection.
   This is an improvement from AUTH_SYS without encryption, but it
   leaves a critical security exposure.

   In light of the above, it is RECOMMENDED that when AUTH_SYS is used,
   RPC clients present authentication material necessary for RPC servers
   they contact to have a degree of trust that the clients are acting

   The use of TLS does not enable detection of compromise on RPC clients
   that leads to impersonation of RPC users.  In addition, there
   continues to be a requirement that the mapping of 32-bit user and
   group ID values to user identities is the same on both the RPC client
   and server.

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8.  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")


9.  References

9.1.  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,

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

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

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <>.

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   [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, <>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 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,

9.2.  Informative References

   [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,

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   [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,

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

   [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,

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

   [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,

   [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,

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9.3.  URIs


Appendix A.  Known Weaknesses of the AUTH_SYS Authentication Flavor

   The ONC RPC protocol as specified in [RFC5531] provides several modes
   of security, traditionally referred to as "authentication flavors",
   though some of these flavors provide much more than an authentication
   service.  We will refer to these as authentication flavors, security
   flavors, or simply, flavors.  One of the earliest and most basic
   flavor is AUTH_SYS, also known as AUTH_UNIX.  AUTH_SYS is currently
   specified in Appendix A of [RFC5531].

   AUTH_SYS assumes that both the RPC client and server use POSIX-style
   user and group identifiers (each user and group can be distinctly
   represented as a 32-bit unsigned integer), and that both client and
   server use the same mapping of user and group to integer.  One user
   ID, one main group ID, and up to 16 supplemental group IDs are
   associated with each RPC request.  The combination of these identify
   the entity on the client that is making the request.

   Peers are identified by a string in each RPC request.  RFC 5531 does
   not specify any requirements for this string other than that is no
   longer than 255 octets.  It does not have to be the same from request
   to request, nor does it have to match the name of the sending host.
   For these reasons, though most implementations do fill in their
   hostname in this field, receivers typically ignore its content.

   RFC 5531 Appendix A contains a brief explanation of security

      It should be noted that use of this flavor of authentication does
      not guarantee any security for the users or providers of a
      service, in itself.  The authentication provided by this scheme
      can be considered legitimate only when applications using this
      scheme and the network can be secured externally, and privileged
      transport addresses are used for the communicating end-points (an
      example of this is the use of privileged TCP/UDP ports in UNIX
      systems -- note that not all systems enforce privileged transport
      address mechanisms).

   It should be clear, therefore, that AUTH_SYS by itself offers little
   to no communication security:

   1.  It does not protect the privacy or integrity of RPC requests,
       users, or payloads, relying instead on "external" security.

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   2.  It also does not provide actual authentication of RPC peer
       machines, other than an unprotected domain name.

   3.  The use of 32-bit unsigned integers as user and group identifiers
       is problematic because these simple data types are not signed or
       otherwise verified by any authority.

   4.  Because the user and group ID fields are not integrity-protected,
       AUTH_SYS does not offer non-repudiation.


   Special mention goes to Charles Fisher, author of "Encrypting NFSv4
   with Stunnel TLS" [1].  His article inspired the mechanism described
   in this document.

   Many thanks to Tigran Mkrtchyan for his work on the DESY prototype
   and resulting feedback to this document.

   The authors are grateful to Bill Baker, David Black, Alan DeKok, Lars
   Eggert, Benjamin Kaduk, Olga Kornievskaia, Greg Marsden, Alex
   McDonald, David Noveck, Justin Mazzola Paluska, Tom Talpey, and
   Martin Thomson for their input and support of this work.

   Lastly, special thanks go to Transport Area Director Magnus
   Westerlund, NFSV4 Working Group Chairs Spencer Shepler and Brian
   Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes for their
   guidance and oversight.

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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