Network File System Version 4                               T. Myklebust
Internet-Draft                                               Hammerspace
Updates: 5531 (if approved)                                C. Lever, Ed.
Intended status: Standards Track                                  Oracle
Expires: November 1, 2020                                 April 30, 2020

          Towards 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 November 1, 2020.

Copyright Notice

   Copyright (c) 2020 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.

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 . . . . . . . . . . .   6
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Using TLS with RPCSEC GSS . . . . . . . . . . . . . .   8
   5.  TLS Requirements  . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Base Transport Considerations . . . . . . . . . . . . . .   9
       5.1.1.  Protected Operation on TCP  . . . . . . . . . . . . .   9
       5.1.2.  Protected Operation on UDP  . . . . . . . . . . . . .   9
       5.1.3.  Protected Operation on Other Transports . . . . . . .  10
     5.2.  TLS Peer Authentication . . . . . . . . . . . . . . . . .  11
       5.2.1.  X.509 Certificates Using PKIX trust . . . . . . . . .  11
       5.2.2.  X.509 Certificates Using Fingerprints . . . . . . . .  12
       5.2.3.  Pre-Shared Keys . . . . . . . . . . . . . . . . . . .  12
       5.2.4.  Token Binding . . . . . . . . . . . . . . . . . . . .  13
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  13
     6.1.  DESY NFS server . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Hammerspace NFS server  . . . . . . . . . . . . . . . . .  14
     6.3.  Linux NFS server and client . . . . . . . . . . . . . . .  14
     6.4.  FreeBSD NFS server and client . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Limitations of an Opportunistic Approach  . . . . . . . .  15
       7.1.1.  STRIPTLS Attacks  . . . . . . . . . . . . . . . . . .  15
       7.1.2.  Privacy Leakage Before Session Establishment  . . . .  16
     7.2.  TLS Identity Management on Clients  . . . . . . . . . . .  16
     7.3.  Security Considerations for AUTH_SYS on TLS . . . . . . .  17
     7.4.  Best Security Policy Practices  . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  RPC Authentication Flavor . . . . . . . . . . . . . . . .  18
     8.2.  ALPN Identifier for SUNRPC  . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Appendix A.  Known Weaknesses of the AUTH_SYS Authentication
                Flavor . . . . . . . . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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1.  Introduction

   RFC Editor: Please remove this Editor's Note and the following
   paragraph before this document is published.

   The source for this draft is maintained in GitHub.  Suggested changes
   should be submitted as pull requests at [1].  Instructions are on
   that page as well.  Editorial changes can be managed in GitHub, but
   any substantive change should be discussed on the
   mailing list.

   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 should
   make a genuine 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 antecedents).  Over
   twenty years ago, Eisler et al. first introduced RPCSEC GSS as an in-
   transit encryption mechanism for RPC [RFC2203].  However, experience
   has shown that RPCSEC GSS with in-transit encryption can be
   challenging to use in practice:

   o  Parts of each RPC header remain in clear-text, constituting a
      significant security exposure.

   o  Offloading the GSS privacy service is not practical in large
      multi-user deployments since each message is encrypted using a key
      based on the issuing RPC user.

   However strong GSS-provided confidentiality is, it cannot provide any
   security if the challenges of using it result in choosing not to
   deploy it at all.

   Moreover, the use of AUTH_SYS remains common despite the adverse
   effects that acceptance of UIDs and GIDs from unauthenticated clients
   brings with it.  Continued use is in part because:

   o  Per-client deployment and administrative costs are not scalable.
      Administrators must provide keying material for each RPC client,
      including transient clients.

   o  Host identity management and user identity management must be
      enforced in the same security realm.  In certain environments,

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      different authorities might be responsible for provisioning client
      systems versus provisioning new users.

   The alternative described in the current document is to employ a
   transport layer security mechanism that can protect the
   confidentiality 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 standard Internet protocols such as the Hypertext
   Transport Protocol (HTTP) [RFC2818].

   Encrypting at the RPC transport layer accords several significant

   Encryption By Default:  Transport encryption can be enabled without
      additional administrative tasks such as identifying client systems
      to a trust authority, generating additional keying material, or
      provisioning a secure network tunnel.

   Encryption Offload:  Hardware support for the GSS privacy service has
      not appeared in the marketplace.  However, the use of a well-
      established transport encryption mechanism that is employed by
      other ubiquitous network protocols makes it more likely that
      encryption offload for RPC is practicable.

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

   Decoupled User and Host Identities:  TLS can be used to authenticate
      peer hosts while other security mechanisms can handle user

   The current document specifies the implementation of RPC on an
   encrypted transport in a manner that is transparent to upper-layer
   protocols based on RPC.  The imposition of encryption at the
   transport layer protects any upper-layer protocol that employs RPC,
   without alteration of that protocol.

   Further, Section 7 of the current document defines policies in line
   with [RFC7435] which enable RPC-on-TLS to be deployed
   opportunistically in environments that contain RPC implementations
   that do not support TLS.  However, specifications for RPC-based
   upper-layer protocols should choose to require even stricter policies
   that guarantee encryption and host authentication is used for all RPC
   transactions.  Enforcing the use of RPC-on-TLS is of particular

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   importance for existing upper-layer protocols whose security
   infrastructure is weak.

   The protocol specification in the current document assumes that
   support for RPC, TLS, PKI, GSS-API, and DNSSEC is already available
   in an RPC implementation where TLS support is to be added.

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 long ago adopted the use of the term
   "privacy" from documents such as [RFC2203].  In the current document,
   the authors use the term "privacy" only when referring specifically
   to the historic GSS privacy service defined in [RFC2203].  Otherwise,
   the authors use the term "confidentiality", following the practices
   of contemporary security communities.

   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
   the current document there is little distinction between these terms.

   The term "user authentication" in the current 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

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4.1.  Discovering Server-side TLS Support

   The mechanism described in the current document interoperates fully
   with RPC implementations that do not support TLS.  Policy settings on
   the RPC-on-TLS-enabled peer determine whether RPC operation continues
   without the use of TLS or RPC operation is not permitted.

   To achieve this, we introduce a new RPC authentication flavor called
   AUTH_TLS.  This new flavor signals 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 unaware
   of security encapsulation at the transport layer.

   When an RPC client is ready to begin a TLS session, it sends a NULL
   RPC procedure with an auth_flavor of AUTH_TLS.  The value of AUTH_TLS
   is defined in Section 8.1.  The NULL request is made to the same port
   as if TLS were not in use.

   The length of the opaque data constituting the credential sent in the
   RPC 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 in the RPC Reply message received
   from the server MUST be AUTH_NONE.  The length of the verifier's body
   field is eight.  The bytes of the verifier's body field encode the
   ASCII characters "STARTTLS" as a fixed-length opaque.

   If the RPC server replies with a reply_stat of MSG_ACCEPTED and an
   AUTH_NONE verifier containing the "STARTTLS" token, the RPC client
   follows with a "ClientHello" message.  The client MAY proceed with
   TLS session establishment even if the Reply's accept_stat is not
   SUCCESS (for example, if the accept_stat is PROG_UNAVAIL).  Once the
   TLS handshake is complete, the RPC client and server have established
   a secure channel for communicating.

   If the Reply's reply_stat is MSG_ACCEPTED but the verifier does not
   contain the "STARTTLS" token, or if the Reply's reply_stat is
   MSG_DENIED, the RPC client MUST NOT send a "ClientHello" message.
   RPC operation can continue, however it will be without any
   confidentiality, integrity or authentication protection from (D)TLS.

   If, after a successful RPC AUTH_TLS probe, the subsequent TLS
   handshake should fail for any reason, the RPC client reports this
   failure to the upper-layer application the same way it reports an
   AUTH_ERROR rejection from the RPC server.

   If an RPC client uses the AUTH_TLS authentication flavor on any
   procedure other than the NULL procedure, or an RPC client sends an

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   RPC AUTH_TLS probe within an existing TLS session, the RPC server
   MUST reject that RPC Call by setting the reply_stat field to
   MSG_DENIED, the reject_stat field to AUTH_ERROR, and the auth_stat
   field to AUTH_BADCRED.

4.2.  Authentication

   Both RPC and TLS have peer and user authentication, with some overlap
   in capability between RPC and TLS.  The goal of interoperability with
   implementations that do not support TLS requires limiting the
   combinations that are allowed and precisely specifying 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, the client can authenticate the server
      host using the presented server peer TLS identity, but the server
      cannot authenticate the client.  In this situation, RPC-over-TLS
      clients are anonymous.  They present no globally unique identifier
      to the server peer.

   Mutual Host Authentication
      In this type of deployment, the client possesses an identity (e.g.
      a certificate) that is backed by a trusted entity.  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 peers MUST reject the association.

   In either of these modes, RPC user authentication is not affected by
   the use of transport layer security.  When a client presents a TLS
   peer identity to an RPC server, the protocol extension described in
   the current document provides no way for the server to know whether
   that identity represents one RPC user on that client, or is shared
   amongst many RPC users.  Therefore, a server implementation must not
   utilize the remote TLS peer identity for RPC user authentication.

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4.2.1.  Using TLS with RPCSEC GSS

   To use GSS, an RPC server has to possess a GSS service principal.  On
   a TLS session, GSS mutual (peer) authentication occurs as usual, but
   only after a TLS session has been established for communication.
   Authentication of GSS users is unchanged by the use of TLS.

   RPCSEC GSS can also perform per-request integrity or confidentiality
   protection.  When operating over a TLS session, these GSS services
   become redundant.  An RPC implementation capable of concurrently
   using TLS and RPCSEC GSS can use GSS channel binding, as defined in
   [RFC5056], to determine when an underlying transport provides a
   sufficient degree of confidentiality.  Channel bindings for the TLS
   channel type are defined in [RFC5929].

5.  TLS Requirements

   When peers negotiate a TLS session that is to transport RPC, the
   following restrictions apply:

   o  Implementations MUST NOT negotiate TLS versions prior to v1.3 (for
      TLS [RFC8446] or DTLS [I-D.ietf-tls-dtls13] respectively).
      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 confidentiality as well as
      integrity protection is REQUIRED.  Support for and negotiation of
      compression is OPTIONAL.

   Client implementations MUST include the
   "application_layer_protocol_negotiation(16)" extension [RFC7301] in
   their "ClientHello" message and MUST include the protocol identifier
   defined in Section 8.2 in that message's ProtocolNameList value.

   Similary, in response to the "ClientHello" message, server
   implementations MUST include the
   "application_layer_protocol_negotiation(16)" extension [RFC7301] in
   their "ServerHello" message and MUST include only the protocol
   identifier defined in Section 8.2 in that message's ProtocolNameList

   If the server responds incorrectly, the client MUST NOT establish a
   TLS session for use with RPC on this connection.  See [RFC7301] for
   further details about how to form these messages properly.

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5.1.  Base Transport Considerations

   There is traditionally a strong association between an RPC program
   and a destination port number.  The use of TLS or DTLS does not
   change that association.  Thus it is frequently -- though not always
   -- the case that a single TLS session carries traffic for only one
   RPC program.

5.1.1.  Protected Operation on TCP

   The use of the Transport Layer Security (TLS) protocol [RFC8446]
   protects RPC on TCP connections.  Typically, once an RPC client
   completes the TCP handshake, it uses the mechanism described in
   Section 4.1 to discover RPC-on-TLS support for that connection.  If
   spurious traffic appears on a TCP connection between the initial
   clear-text AUTH_TLS probe and the TLS session handshake, receivers
   MUST discard that data without response and then SHOULD drop the

   The protocol convention specified in the current document assumes
   there can be no more than one concurrent TLS session per TCP
   connection.  This is true of current generations of TLS, but might be
   different in a future version of TLS.

   Once a TLS session is established on a TCP connection, no further
   clear-text communication can occur on that connection until the
   session is terminated.  The use of TLS does not alter RPC record
   framing used on TCP transports.

   Furthermore, if an RPC server responds with PROG_UNAVAIL to an RPC
   Call within an established TLS session, that does not imply that RPC
   server will subsequently reject the same RPC program on a different
   TCP connection.

   Backchannel operation occurs only on connected transports such as
   TCP.  To protect backchannel operations, an RPC server uses the
   existing TLS session on that connection to send backchannel
   operations.  The server does not attempt to establish a TLS session
   on a TCP connection for backchannel operation.

   When operation is complete, an RPC peer terminates a TLS session by
   sending a TLS Closure Alert and may then close the TCP connection.

5.1.2.  Protected Operation on UDP

   RFC Editor: In the following section, please replace TBD with the
   connection_id extension number that is to be assigned in

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   [I-D.ietf-tls-dtls-connection-id].  And, please remove this Editor's
   Note before this document is published.

   RPC over UDP is protected using the Datagram Transport Layer Security
   (DTLS) protocol [I-D.ietf-tls-dtls13].

   Using DTLS does not introduce reliable or in-order semantics to RPC
   on UDP.  Each RPC message MUST fit in a single DTLS record.  DTLS
   encapsulation has overhead, which reduces the effective Path MTU
   (PMTU) and thus the maximum RPC payload size.  The use of DTLS record
   replay protection is REQUIRED when transporting RPC traffic.

   As soon as a client initializes a UDP socket for use with an RPC
   server, it uses the mechanism described in Section 4.1 to discover
   DTLS support for an RPC program on a particular port.  It then
   negotiates a DTLS session.

   Multi-homed RPC clients and servers may send protected RPC messages
   via network interfaces that were not involved in the handshake that
   established the DTLS session.  Therefore, when protecting RPC
   traffic, each DTLS handshake MUST include the "connection_id(TBD)"
   extension described in Section 9 of [I-D.ietf-tls-dtls13], and RPC-
   on-DTLS peer endpoints MUST provide a ConnectionID with a non-zero
   length.  Endpoints implementing RPC programs that expect a
   significant number of concurrent clients should employ ConnectionIDs
   of at least 4 bytes in length.

   Sending a TLS Closure Alert terminates a DTLS session.  Subsequent
   RPC messages exchanged between the RPC client and server are no
   longer protected until a new DTLS session is established.

5.1.3.  Protected Operation on Other Transports

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

   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 the current document.  Because there might not be other
   provisions to exchange client and server certificates, authentication
   material exchange needs to be provided by facilities within a future
   version of the RPC-over-RDMA transport protocol.

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5.2.  TLS Peer Authentication

   TLS can perform peer authentication using any of the following

5.2.1.  X.509 Certificates Using PKIX trust

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

   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:iPAddress has
      precedence over CN.  Implementors of this specification are
      advised to read Section 6 of [RFC6125] for more details on DNS
      name validation.

   o  For services accessed by their network identifiers (netids) and
      universal network addresses (uaddr), the iPAddress subjectAltName
      SHOULD be present in the certificate and must exactly match the
      address represented by universal address.

   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.

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   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 administrator can implement the
   trust model.  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

   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, the
   fingerprint of the presented certificate uniquely identifies the RPC

   Implementations SHOULD allow the configuration of a list of trusted
   certificates, identified via fingerprint of the DER-encoded
   certificate octets.  Implementations MUST support SHA-256
   [FIPS.180-4] or stronger as the hash algorithm for the fingerprint.

5.2.3.  Pre-Shared Keys

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

   o  Originating IP address

   o  TLS Identifier

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5.2.4.  Token Binding

   This mechanism is OPTIONAL to implement.  In this mode, a token
   uniquely identifies the RPC peer.

   Versions of TLS after TLS 1.2 contain a token binding mechanism that
   is more secure than using certificates.  This mechanism is detailed
   in [RFC8471].

6.  Implementation Status

   RFC Editor: Please remove this section and the reference to RFC 7942
   before this document is published.

   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
   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

   URL: [2]

   Maturity:  Implementation will be based on mature versions of the
              current document.

   Coverage:  The bulk of this specification is implemented including

   Licensing: LGPL

   Implementation experience:  The implementer has read and commented on
              the current document.

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6.2.  Hammerspace NFS server

   Organization:  Hammerspace

   URL: [3]

   Maturity:  Prototype software based on early versions of the current

   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

   URL: [4]

   Maturity:  Prototype software based on early versions of the current

   Coverage:  The bulk of this specification has yet to be implemented.
              The use of DTLS functionality is not planned.

   Licensing: GPLv2

   Implementation experience:  No comments from the implementor.

6.4.  FreeBSD NFS server and client

   Organization:  The FreeBSD Project

   URL: [5]

   Maturity:  Prototype software based on early versions of the current

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

   Licensing: BSD

   Implementation experience:  Implementers have read and commented on
              the current document.

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7.  Security Considerations

   One purpose of the mechanism described in the current document is to
   protect RPC-based applications against threats to the confidentiality
   of RPC transactions and RPC user identities.  A taxonomy of these
   threats appears in Section 5 of [RFC6973].  Also, 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

   The purpose of using an explicitly opportunistic approach is to
   enable interoperation with implementations that do not support RPC-
   over-TLS.  A range of options is allowed by this approach, from "no
   peer authentication or encryption" to "server-only authentication
   with encryption" to "mutual authentication with encryption".  The
   actual security level may indeed be selected based on policy and
   without user intervention.

   In environments where interoperability is a priority, the security
   benefits of TLS are partially or entirely waived.  Implementations of
   the mechanism described in the current document must take care to
   accurately represent to all RPC consumers the level of security that
   is actually in effect, and are REQUIRED to provide an audit log of
   RPC-over-TLS security mode selection.

   In all other cases, the adoption, implementation, and deployment of
   RPC-based upper-layer protocols that enforce the use of TLS
   authentication and encryption (when similar RPCSEC GSS services are
   not in use) is strongly encouraged.

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 provide 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 require that a TLS session is
      established on every connection.  If an attacker spoofs the

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      handshake, the client disconnects and reports the problem.  If
      TLSA records are not available, this approach is strongly

7.1.2.  Privacy Leakage Before Session Establishment

   As mentioned earlier, communication between an RPC client and server
   appears in the clear on the network prior to the establishment of a
   TLS session.  This clear-text information usually includes transport
   connection handshake exchanges, the RPC NULL procedure probing
   support for TLS, and the initial parts of TLS session establishment.
   Appendix C of [RFC8446] discusses precautions that can mitigate
   exposure during the exchange of connnection handshake information and
   TLS certificate material that might enable attackers to track the RPC

   Any RPC traffic that appears on the network before a TLS session has
   been established is vulnerable to monitoring or undetected
   modification.  A secure client implementation limits or prevents any
   RPC exchanges that are not protected.

   The exception to this edict is the initial RPC NULL procedure that
   acts as a STARTTLS message, which cannot be protected.  This RPC NULL
   procedure contains no arguments or results, and the AUTH_TLS
   authentication flavor it uses does not contain user information.

7.2.  TLS Identity Management on Clients

   The goal of the RPC-on-TLS protocol extension is to hide the content
   of RPC requests while they are in transit.  The RPC-on-TLS protocol
   by itself cannot protect against exposure of a user's RPC requests to
   other users on the same client.

   Moreover, client implementations are free to transmit RPC requests
   for more than one RPC user using the same TLS session.  Depending on
   the details of the client RPC implementation, this means that the
   client's TLS identity material is potentially visible to every RPC
   user that shares a TLS session.  Privileged users may also be able to
   access this TLS identity.

   As a result, client implementations need to carefully segregate TLS
   identity material so that local access to it is restricted to only
   the local users that are authorized to perform operations on the
   remote RPC server.

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7.3.  Security Considerations for AUTH_SYS on TLS

   Using a TLS-protected transport when the AUTH_SYS authentication
   flavor is in use addresses several longstanding weaknesses (as
   detailed in Appendix A).  TLS augments AUTH_SYS by providing both
   integrity protection and confidentiality that AUTH_SYS lacks.  TLS
   protects data payloads, RPC headers, and user identities against
   monitoring and 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.  Lastly,
   transport layer encryption plus peer authentication protects
   receiving XDR decoders from deserializing untrusted data, a common
   coding vulnerability.

   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 still
   acts 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.
   TLS without authentication 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,
   every RPC client should present host authentication material to RPC
   servers to prove that the client is a known one.  The server can then
   determine whether the UIDs and GIDs in AUTH_SYS requests from that
   client can be accepted.

   The use of TLS does not enable RPC clients to detect compromise that
   leads to the impersonation of RPC users.  Also, 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.

7.4.  Best Security Policy Practices

   RPC-over-TLS implementations and deployments are strongly encouraged
   to adhere to the following policies to achieve the strongest possible
   security with RPC-over-TLS.

   o  When using AUTH_NULL or AUTH_SYS, both peers are required to have
      DNS TLSA records and certificate material, and a policy that
      requires mutual peer authentication and rejection of a connection
      when host authentication fails.

   o  When using RPCSEC_GSS, GSS/Kerberos provides adequate host
      authentication and a policy that requires GSS mutual
      authentication and rejection of a connection when host

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      authentication fails.  GSS integrity and privacy services,
      therefore, can be disabled in favor of TLS encryption with peer

8.  IANA Considerations

   RFC Editor: In the following subsections, please replace RFC-TBD with
   the RFC number assigned to this document.  And, please remove this
   Editor's Note before this document is published.

8.1.  RPC Authentication Flavor

   Following Appendix B of [RFC5531], the authors request a single new
   entry in the RPC Authentication Flavor Numbers registry.  The purpose
   of the new authentication flavor is to signal the use of TLS with
   RPC.  This new flavor is not a pseudo-flavor.

   The fields in the new entry are assigned as follows:

   Identifier String:  AUTH_TLS

   Flavor Name:  TLS

   Value:  7

   Description:  Signals the use of TLS to protect RPC messages on
      socket-based transports

   Reference:  RFC-TBD

8.2.  ALPN Identifier for SUNRPC

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

   Protocol:  SunRPC

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

   Reference:  RFC-TBD

9.  References

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9.1.  Normative References

              National Institute of Standards and Technology, "Secure
              Hash Standard, Federal Information Processing Standards
              Publication FIPS PUB 180-4", FIPS PUB 180-4, August 2015.

              Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
              id-07 (work in progress), October 2019.

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-37 (work in progress), March

   [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,

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

   [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, <>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,

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   [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, <>.

   [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, <>.

   [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

   [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,

   [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, <>.

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   [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, <>.

   [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,

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".  Some of these flavors provide much more than an
   authentication service.  We refer to these as authentication flavors,
   security flavors, or simply, flavors.  One of the earliest and most

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   basic flavors is AUTH_SYS, also known as AUTH_UNIX.  Appendix A of
   [RFC5531] specifies AUTH_SYS.

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

   A string identifies peers (hosts) in each RPC request.  [RFC5531]
   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.  Also, it does not have to match the DNS hostname
   of the sending host.  For these reasons, even though most
   implementations fill in their hostname in this field, receivers
   typically ignore its content.

   Appendix A of [RFC5531] 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 (i.e., without
   strong client authentication) offers little to no communication

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

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

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

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


   Special mention goes to Charles Fisher, author of "Encrypting NFSv4
   with Stunnel TLS" [6].  His article inspired the mechanism described
   in the current document.

   Many thanks to Tigran Mkrtchyan and Rick Macklem for their work on
   prototype implementations and feedback on the current document.

   Thanks to Derrell Piper for numerous suggestions that improved both
   this simple mechanism and the current document's security-related

   Many thanks to Transport Area Director Magnus Westerlund for his
   sharp questions and careful reading of the final revisions of the
   current document.  The text of Section 5.1.2 is mostly his

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

   Finally, special thanks to NFSV4 Working Group Chair and document
   shepherd David Noveck, 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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