Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-05

Document Type Active Internet-Draft (nfsv4 WG)
Last updated 2020-01-11 (latest revision 2020-01-10)
Replaces draft-cel-nfsv4-rpc-tls
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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: July 13, 2020                                  January 10, 2020

          Towards Remote Procedure Call Encryption By Default
                      draft-ietf-nfsv4-rpc-tls-05

Abstract

   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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on July 13, 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
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   publication of this document.  Please review these documents
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   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
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   Without obtaining an adequate license from the person(s) controlling
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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.  TLS Identity Management on Clients  . . . . . . . . . . .  14
     7.3.  Security Considerations for AUTH_SYS on TLS . . . . . . .  14
     7.4.  Best Security Policy Practices  . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17

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   Appendix A.  Known Weaknesses of the AUTH_SYS Authentication
                Flavor . . . . . . . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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

   However strong a privacy service 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,
      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 privacy of
   each RPC connection transparently to RPC and upper-layer protocols.

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   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
   benefits:

   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 GSS privacy 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
      authentication.

   The current document specifies the implementation of RPC on an
   encrypted transport in a fashion 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, the current document defines policies in line with [RFC7435]
   which 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 encryption or host
   authentication is in use on every connection.

   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.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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 the current 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
   the current 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 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.

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   <CODE BEGINS>

   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 */
   };

   <CODE ENDS>

   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 recognize 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 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, the RPC client reports this
   failure to the upper-layer 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 already in place,
   the server MUST silently discard it.

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 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 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

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

   When using GSS on a TLS session, the RPC server is still required to
   possess a GSS service principal.  GSS mutual authentication still
   occurs after a TLS session has been established.

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
      [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 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

   The use of TLS [RFC8446] protects RPC on TCP connections.  As soon as
   a client completes the TCP handshake, it uses the mechanism described
   in [RFC8446]. to discover TLS support and then negotiate a TLS
   session.

   After establishing a TLS session, an RPC server MUST reject with a
   reject_stat of AUTH_ERROR any subsequent RPC requests over the
   connection that are outside of a TLS session.  Likewise, an RPC
   client MUST silently discard any subsequent RPC replies over the
   connection that are outside of a TLS session.

   This restriction includes reverse-direction operations (i.e., RPC
   calls initiated on the server-end of the connection).  An RPC client
   receiving a reverse-direction call on a connection outside of an

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   existing TLS session MUST reject the request with a reject_stat of
   AUTH_ERROR.

   An RPC peer terminates a TLS session by sending a TLS closure alert,
   or by closing the underlying TCP socket.

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.  Each RPC message MUST fit 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.  Sending a TLS closure alert
   terminates a DTLS session.  Subsequent RPC messages passing between
   the client and server are no longer 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 other provisions
   to exchange client and server certificates, authentication material
   exchange would need to be provided by facilities within a future RPC-
   over-RDMA transport.

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

5.2.  TLS Peer Authentication

   TLS can perform peer authentication 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, the tuple (serial number of the presented certificate; Issuer)
   uniquely identifies the RPC peer.

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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
      [RFC5280].

   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
      Policies).

   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 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

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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, the
   fingerprint of the presented certificate uniquely identifies the RPC
   peer.

   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

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

   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
   RFCs.

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   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
   exist.

6.1.  DESY NFS server

   Organization:  DESY

   URL:       https://desy.de

   Maturity:  Prototype software based on early versions of this
              document.

   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

   URL:       https://hammerspace.com

   Maturity:  Prototype software based on early versions of this
              document.

   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:       https://www.kernel.org

   Maturity:  Prototype software based on early versions of this
              document.

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   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 implementors.

7.  Security Considerations

   One purpose of the mechanism described in the current 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].  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 cases 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.  Implementations are REQUIRED to provide an
   audit log of RPC-over-TLS security mode selection.

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
   attacks:

   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.

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   o  Client security policy can require that a TLS session is
      established on every connection.  If an attacker spoofs the
      handshake, the client disconnects and reports the problem.  If
      TLSA records are not available, this approach is strongly
      encouraged.

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.

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 a privacy service 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.

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

8.  IANA Considerations

   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

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9.  References

9.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc4279>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
              May 2009, <https://www.rfc-editor.org/info/rfc5531>.

   [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, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [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, <https://www.rfc-editor.org/info/rfc7301>.

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   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <https://www.rfc-editor.org/info/rfc7861>.

   [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,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

9.2.  Informative References

   [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
              Specification", RFC 2203, DOI 10.17487/RFC2203, September
              1997, <https://www.rfc-editor.org/info/rfc2203>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [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, <https://www.rfc-editor.org/info/rfc6698>.

   [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,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [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, <https://www.rfc-editor.org/info/rfc7525>.

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   [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,
              <https://www.rfc-editor.org/info/rfc8166>.

   [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,
              <https://www.rfc-editor.org/info/rfc8471>.

9.3.  URIs

   [1] https://www.linuxjournal.com/content/encrypting-nfsv4-stunnel-tls

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
   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
   considerations:

      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

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      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.

   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.

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

Acknowledgments

   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 his 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
   discussion.

   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 David Noveck, Spencer Shepler,
   and Brian Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes
   for their guidance and oversight.

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Authors' Addresses

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

   Email: trond.myklebust@hammerspace.com

   Charles Lever (editor)
   Oracle Corporation
   United States of America

   Email: chuck.lever@oracle.com

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