Network File System Version 4                               T. Myklebust
Internet-Draft                                               Hammerspace
Updates: 5531 (if approved)                                C. Lever, Ed.
Intended status: Standards Track                                  Oracle
Expires: 22 March 4 May 2021                                 18 September                                      31 October 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
   Remote Procedure Call (RPC) transactions while they are in-transit.
   The proposed mechanism interoperates with ONC RPC implementations
   that do not support it.  This document updates RFC 5531.


   Discussion of this draft takes place on the NFSv4 working group
   mailing list (, which is archived at Working Group
   information can be found at

   This note is to be removed before publishing as an RFC.

   The source for this draft is maintained in GitHub.  Suggested changes
   should be submitted as pull requests at  Instructions are on that
   page as well.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 22 March 4 May 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  RPC-Over-TLS in Operation . . . . . . . . . . . . . . . . . .   6   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  . . . . . . . . . . . . . . . . . . . . . .   9   8
     5.1.  Base Transport Considerations . . . . . . . . . . . . . .   9
       5.1.1.  Protected Operation on TCP  . . . . . . . . . . . . .  10
       5.1.2.  Protected Operation on UDP  . . . . . . . . . . . . .  10
       5.1.3.  Protected Operation on Other Transports . . . . . . .  11
     5.2.  TLS Peer Authentication . . . . . . . . . . . . . . . . .  12
       5.2.1.  X.509 Certificates Using PKIX Trust . . . . . . . . .  12
       5.2.2.  Pre-Shared Keys . . . . . . . . . . . . . . . . . . .  14
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
     6.1.  DESY NFS server . . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Hammerspace NFS server  . . . . . . . . . . . . . . . . .  15
     6.3.  Linux NFS server and client . . . . . . . . . . . . . . .  15
     6.4.  FreeBSD NFS server and client . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     7.1.  The Limitations of Opportunistic Security . . . . . . . .  16
       7.1.1.  STRIPTLS Attacks  . . . . . . . . . . . . . . . . . .  17
       7.1.2.  Privacy Leakage Before Session Establishment  . . . .  17
     7.2.  TLS Identity Management on Clients  . . . . . . . . . . .  18
     7.3.  Security Considerations for AUTH_SYS on TLS . . . . . . .  18
     7.4.  Best Security Policy Practices  . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  RPC Authentication Flavor . . . . . . . . . . . . . . . .  19
     8.2.  ALPN Identifier for SUNRPC  . . . . . . . . . . . . . . .  20
     8.3.  Object Identifier for PKIX Extended Key Usage . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Known Weaknesses of the AUTH_SYS Authentication
           Flavor  . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  25
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   In 2014 the IETF published a document entitled "Pervasive Monitoring
   Is an Attack" [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 includes 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:

   *  Parts of each RPC header remain in clear-text, constituting a loss
      of metadata confidentiality.

   *  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:

   *  Per-client deployment and administrative costs for the only well-
      defined alternative to AUTH_SYS are expensive at scale.  For
      instance, administrators must provide keying material for each RPC
      client, including transient clients.

   *  GSS host identity management and user identity management must
      typically be enforced in the same security realm.  In certain environments,
      different authorities  However, cloud
      providers, for instance, might be responsible prefer to remain authoritative for provisioning client
      systems versus provisioning new users.
      host identity but allow tenants to manage user identities within
      their private networks.

   In view of the challenges with the currently available mechanisms for
   authenticating and protecting the confidentiality of RPC
   transactions, this document specifies a transport-layer security
   mechanism that complements the existing ones.  The Transport Layer
   Security [RFC8446] (TLS) and Datagram Transport Layer Security
   [I-D.ietf-tls-dtls13] (DTLS) protocols are a well-established
   Internet building blocks that protect 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 and providing each with keying material.

   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

   Compatibility:  The imposition of encryption at the transport layer
      protects any upper-layer protocol that employs RPC, without
      alteration of the upper-layer protocol.

   Further, Section 7 of the current document defines policies in line
   with [RFC7435] which enable RPC-over-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 to mitigate against pervasive monitoring attacks

   [RFC7258].  Enforcing the use of RPC-over-TLS is of particular
   importance for existing upper-layer protocols whose security
   infrastructure is weak.

   The protocol specification in the current document assumes that
   support for ONC RPC [RFC5531], TLS [RFC8446], PKIX [RFC5280], DNSSEC/
   DANE [RFC6698], and optionally RPCSEC_GSS [RFC2203] is available
   within the platform where RPC-over-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
4.1.  Discovering Server-side TLS Support

   The mechanism described in the current document interoperates fully
   with RPC implementations that do not support RPC-over-TLS.  When an
   RPC-over-TLS-enabled peer encounters a peer that does not support
   RPC-over-TLS, policy settings on the RPC-over-TLS-enabled peer
   determine whether RPC operation continues without the use of TLS, or
   RPC operation is not permitted.

   To achieve this interoperability, we introduce a new RPC
   authentication flavor called AUTH_TLS.  The AUTH_TLS authentication
   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.  The value of AUTH_TLS is defined in
   Section 8.1.

   An RPC client begins its communication with an RPC server by
   selecting a transport and destination port.  The choice of transport
   and port is typically based on the RPC program that is to be used.
   The RPC client might query the RPC server's RPCBIND service to make
   this selection (The RPCBIND service is described in [RFC1833]).  The
   mechanism described in the current document does not support RPC
   transports other than TCP and UDP.  In all cases, an RPC server MUST
   listen on the same ports for (D)TLS-protected RPC programs as the
   ports used when (D)TLS is not available.

   To protect RPC traffic to a TCP port, the RPC client opens a TCP
   connection to that port and sends a NULL RPC procedure with an
   auth_flavor of AUTH_TLS on that connection.  To protect RPC traffic
   to a UDP port, the RPC client sends a UDP datagram to that port
   containing a NULL RPC procedure with an auth_flavor of AUTH_TLS.  The
   client constructs this RPC procedure as follows:

   *  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.

   The RPC server signals its corresponding support for RPC-over-TLS by
   replying with a reply_stat of MSG_ACCEPTED and an AUTH_NONE verifier
   containing the "STARTTLS" token.  The client SHOULD proceed with TLS
   session establishment, even if the Reply's accept_stat is not
   SUCCESS.  If the AUTH_TLS probe was done via TCP, the RPC client MUST
   send the "ClientHello" message on the same connection.  If the
   AUTH_TLS probe was done via UDP, the RPC client MUST send the
   "ClientHello" message to the same UDP destination port.

   Conversely, if the Reply's reply_stat is not MSG_ACCEPTED, if its
   verifier flavor is not AUTH_NONE, or if its verifier does not contain
   the "STARTTLS" token, the RPC client MUST NOT send a "ClientHello"
   message.  RPC operation may continue, depending on local policy, but
   without confidentiality, integrity, or peer authentication protection
   from (D)TLS.

   If, after a successful RPC AUTH_TLS probe, the subsequent (D)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
   RPC AUTH_TLS probe within an existing (D)TLS session, the RPC server
   MUST reject that RPC Call by returning a reply_stat of MSG_DENIED
   with a reject_stat of AUTH_ERROR and an auth_stat of AUTH_BADCRED.

   Once the TLS session handshake is complete, the RPC client and server
   have established a secure channel for exchanging RPC transactions.  A
   successful AUTH_TLS probe on one particular port/transport tuple does
   not imply that RPC-over-TLS is available on that same server using a
   different port/transport tuple, nor does it imply that RPC-over-TLS
   will be available in the future using the successfully probed port.

4.2.  Authentication

   There is some overlap between the authentication capabilities of 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.

   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 cannot
   utilize the remote TLS peer identity to authenticate RPC users.

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 RPCSEC 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 largely redundant.  An RPC implementation capable of
   concurrently using TLS and RPCSEC GSS MUST use GSS-API channel
   binding, as defined in [RFC5056], to determine when an underlying
   transport provides a sufficient degree of confidentiality.  RPC-over-
   TLS implementations MUST provide the "tls-exporter" channel binding
   type, as defined in [I-D.ietf-kitten-tls-channel-bindings-for-tls13].

5.  TLS Requirements

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

   *  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.

   *  Implementations MUST conform to the recommendations for TLS usage
      specified in BCP 195 [RFC7525].  Although RFC 7525 permits the use
      of TLS v1.2, the requirement to use TLS v1.3 or later for RPC-
      over-TLS takes precedence.  Further, because TLS v1.3 ciphers are
      qualitatively different than cipher suites in previous versions of
      TLS and RFC 7525 predates TLS v1.3, the cipher suite
      recommendations in RFC 7525 do not apply to RPC-over-(D)TLS.  A
      strict TLS mode for RPC-over-TLS that protects against STRIPTLS
      attacks is discussed in detail in Section 7.1.1.

   *  Implementations MUST support certificate-based mutual
      authentication.  Support for PSK mutual authentication is
      OPTIONAL; see Section 5.2.2 for further details.

   *  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.

   Similarly, 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 (for instance, if the
   "ServerHello" message does not conform to the above requirements),
   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.

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-over-TLS support for that RPC program on
   that connection.  Until an AUTH_TLS probe is done on a connection,
   the RPC server treats all traffic as RPC messages.  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 connection.

   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.

   Reverse-direction operation occurs only on connected transports such
   as TCP (see Section 2 of [RFC8167]).  To protect reverse-direction
   RPC operations, the RPC server does not establish a separate TLS
   session on the TCP connection, but instead uses the existing TLS
   session on that connection to protect these operations.

   When operation is complete, an RPC peer terminates a TLS session by
   sending a TLS Closure Alert.  It 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
   [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 use of the Datagram Transport Layer Security (DTLS) protocol [I-D.ietf-tls-dtls13].
   [I-D.ietf-tls-dtls13] protects RPC carried in UDP datagrams.  As soon
   as a client initializes a UDP socket for use with an RPC service, it
   uses the mechanism described in Section 4.1 to discover RPC-over-DTLS
   support for that RPC program on that port.  If spurious traffic
   appears on a 5-tuple between the initial clear-text AUTH_TLS probe
   and the DTLS association handshake, receivers MUST discard that
   traffic without response.

   Using DTLS does not introduce reliable or in-order semantics to RPC
   on UDP.  The use of DTLS record replay protection is REQUIRED when
   transporting RPC traffic.

   Each RPC message MUST fit in a single DTLS record.  DTLS
   encapsulation has overhead, which reduces the Packetization Layer
   Path MTU (PLPMTU) and thus the maximum RPC payload size.  A possible
   PLPMTU discovery mechanism is offered in [RFC8899].

   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.

   The current document does not specify a mechanism that enables a
   server to distinguish between DTLS traffic and unprotected RPC
   traffic directed to the same port.  To make this distinction, each
   peer matches ingress datagrams that appear to be DTLS traffic to
   existing DTLS session state.  A peer treats any datagram that fails
   the matching process as an RPC message.

   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-
   over-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.  Because
   neither DTLS nor UDP provide in-order delivery, after session closure
   there can be ambiguity as to whether a datagram should be interpreted
   as DTLS protected or not.  Therefore receivers MUST discard datagrams
   exchanged using the same 5-tuple that just terminated the DTLS
   session for a sufficient length of time to ensure that
   retransmissions have ceased and packets already in the network have
   been delivered.  In the absence of more specific data, a period of 60 seconds.
   seconds is expected to suffice.

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.

5.2.  TLS Peer Authentication

   TLS can perform peer authentication using any of the following

5.2.1.  X.509 Certificates Using PKIX Trust

   X.509 certificates are specified in [X.509].  [RFC5280] provides a
   profile of Internet PKI X.509 public key infrastructure.  RPC-over-
   TLS implementations are REQUIRED to support the PKIX mechanism
   described in [RFC5280].

   The rules and guidelines defined in [RFC6125] apply to RPC-over-TLS
   certificates with the following considerations:

   *  Support for the DNS-ID identifier type [RFC6125] is REQUIRED in
      RPC-over-TLS client and server implementations.  Certification
      authorities that issue such certificates MUST support the DNS-ID
      identifier type.

   *  DNS domain names in RPC-over-TLS certificates MUST NOT contain the
      wildcard character '*' within the identifier.

   When validating a server certificate, an RPC-over-TLS client
   implementation takes the following into account:

   *  Certificate validation MUST include the verification rules as per
      Section 6 of [RFC5280] and Section 6 of [RFC6125].

   *  Server certificate validation MUST include a check on whether the
      locally configured expected DNS-ID or iPAddress subjectAltName of
      the server that is contacted matches its presented certificate.

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

   An RPC client's domain name and IP address are often assigned
   dynamically, thus RPC servers cannot rely on those to verify client
   certificates.  Therefore, when an RPC-over-TLS client presents a
   certificate to an RPC-over-TLS server, the server takes the following
   into account:

   *  The server MUST use a procedure conformant to Section 6 of
      [RFC5280]) to validate the client certificate's certification

   *  The tuple (serial number of the presented certificate; Issuer)
      uniquely identifies the RPC client.  The meaning and syntax of
      these fields is defined in Section 4 of [RFC5280]).

   RPC-over-TLS 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, a set of allowed X.509v3 Certificate Policies, or
   a set of extended key usages).

   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 SHOULD reevaluate the certificate originally
   presented in the context of the new configuration and terminate the
   TLS session if the certificate is no longer trustworthy.  Extended Key Usage Values

   Section of [RFC5280] specifies the extended key usage X.509
   certificate extension.  This extension, which may appear in end-
   entity certificates, indicates one or more purposes for which the
   certified public key may be used in addition to or in place of the
   basic purposes indicated in the key usage extension.

   The current document defines two new KeyPurposeId values: one that
   identifies the RPC-over-TLS peer as an RPC client, and one that
   identifies the RPC-over-TLS peer as an RPC server.  Additional
   KeyPurposeId values related to RPC-over-TLS may be specified in
   subsequent Standards Track documents.

   The inclusion of the RPC server value (id-kp-rpcTLSServer) indicates
   that the certificate has been issued for allowing the holder to
   process RPC transactions.  Such a certificate is a Resource
   Certificate and therefore MUST conform to the constraints specified
   in [RFC6487].

   The inclusion of the RPC client value (id-kp-rpcTLSClient) indicates
   that the certificate has been issued for allowing the holder to
   request RPC transactions.

5.2.2.  Pre-Shared Keys

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

   *  PSK Identifier

6.  Implementation Status

   This section is to be removed before publishing as an RFC.

   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


   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.

6.2.  Hammerspace NFS server

   Organization:  Hammerspace


   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


   Maturity:  Not complete.

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

   Licensing:  GPLv2

   Implementation experience:  A Linux in-kernel prototype is underway,
              but implementation delays have resulted from the
              challenges of handling a TLS handshake in a kernel
              environment.  Those issues stem from the architecture of
              TLS and the kernel, not from the design of the RPC-over-
              TLS protocol.

6.4.  FreeBSD NFS server and client

   Organization:  The FreeBSD Project


   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.

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.  Section 8 of [RFC5280] covers important
   considerations about handling certificate material securely.
   Implementers should familiarize themselves with these materials.

   Once a TLS session is established, the RPC payload carried on TLS
   version 1.3 is forward-secure.  However, implementers need to be
   aware that replay attacks can occur during session establishment.
   Remedies for such attacks are discussed in detail in Section 8 of
   [RFC8446].  Further, the current document does not provide a profile
   that defines the use of 0-RTT data (see Appendix E.5 of [RFC8446]).
   Therefore, RPC-over-TLS implementations MUST NOT use 0-RTT data.

7.1.  The Limitations of Opportunistic Security

   Readers can find the definition of Opportunistic Security in
   [RFC7435].  A discussion of its underlying principals appears in
   Section 3 of that document.

   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.  Client
   implementers can choose from the following to mitigate STRIPTLS

   *  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.  When an opportunistic
      security policy is in place, a client SHOULD check for the
      existence of a TLSA record for the target server before initiating
      an RPC-over-TLS association.

   *  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.  This
      policy prevents an attacker from causing the client to silently
      fall back to plaintext.  If TLSA records are not available, this
      approach is strongly encouraged.

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 connection handshake information and
   TLS certificate material that might enable attackers to track the RPC
   client.  Note that when PSK authentication is used, the PSK
   identifier is exposed during the TLS handshake, and can be used to
   track the RPC client.

   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, so
   there is negligible privacy impact from this exception.

7.2.  TLS Identity Management on Clients

   The goal of the RPC-over-TLS protocol extension is to hide the
   content of RPC requests while they are in transit.  The RPC-over-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 credentials are 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
   credentials 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 in
   AUTH_SYS (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 in-transit insertion and deletion of RPC messages,
   thus ensuring the integrity of the message stream between RPC client
   and server.  DTLS does not provide full message stream protection,
   but it does enable receivers to reject non-participant messages.  In
   particular, transport layer encryption plus peer authentication
   protects receiving XDR decoders from deserializing untrusted data, a
   common coding vulnerability.  However, these decoders would still be
   exposed to untrusted input in the case of the compromise of a trusted
   peer or Certificate Authority.

   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, when AUTH_SYS is used, the use of a TLS mutual
   authentication mechanism is RECOMMENDED to prove that the RPC client
   is known to the RPC server.  The server can then determine whether
   the UIDs and GIDs in AUTH_SYS requests from that client can be
   accepted, based on the authenticated identity of the client.

   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.

   *  When using AUTH_NULL or AUTH_SYS, both peers are RECOMMENDED to
      have DNSSEC TLSA records, keys with which to perform mutual peer
      authentication using one of the methods described in Section 5.2,
      and a security policy that requires mutual peer authentication and
      rejection of a connection when host authentication fails.

   *  RPCSEC GSS provides integrity and privacy services which are
      largely redundant when TLS is in use.  These services SHOULD be
      disabled in that case.

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 (to be confirmed by IANA)

   Description:  Indicates support for RPC-over-TLS.

   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

8.3.  Object Identifier for PKIX Extended Key Usage

   RFC Editor: In the following subsection, please replace XXX and YYY
   with the IANA numbers assigned to these new entries.  And, please
   remove this Editor's Note before this document is published.

   Per the Specification Required policy as defined in Section 4.6 of
   [RFC8126], the authors request the reservation of the following new

   *  The RPC-over-TLS ASN.1 module in the "SMI Security for PKIX
      Extended Key Purpose" registry ( (see
      Section and Appendix B.

   *  The RPC-over-TLS client certificate extended key usage
      (  The description of this new entry should
      be "id-kp-rpcTLSClient".

   *  The RPC-over-TLS server certificate extended key usage
      (  The description of this new entry should
      be "id-kp-rpcTLSServer".

   IANA should use the current document (RFC-TBD) as the reference for
   the new entries.

9.  References
9.1.  Normative References

              Whited, S., "Channel Bindings for TLS 1.3", Work in
              Progress, Internet-Draft, draft-ietf-kitten-tls-channel-
              bindings-for-tls13-00, 11 June 2020,

              Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", Work in Progress, Internet-
              Draft, draft-ietf-tls-dtls-connection-id-07, 21 October
              2019, <

              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-38, 29 May 2020,

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

   [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, <>.

   [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, <>.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,

   [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, <>.

   [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, <>.

   [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,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [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,

   [X.509]    International Telephone and Telegraph Consultative
              Committee, "ITU-T X.509 - Information technology - The
              Directory: Public-key and attribute certificate
              frameworks.", ISO/IEC 9594-8, CCITT Recommendation X.509,
              October 2019.

9.2.  Informative References

   [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
              RFC 1833, DOI 10.17487/RFC1833, August 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,

   [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,

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

   [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,

   [RFC8167]  Lever, C., "Bidirectional Remote Procedure Call on RPC-
              over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
              June 2017, <>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <>.

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

   |  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.  In addition, the
       mapping of these integers to users and groups has to be
       consistent amongst a server and its cohort of clients.

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

Appendix B.  ASN.1 Module

   RFC Editor: In the following section, please replace XXX and YYY with
   the IANA numbers assigned to these new entries.  And, please remove
   this Editor's Note before this document is published.

   -- OID Arc

   { iso(1) identified-organization(3) dod(6) internet(1)
   security(5) mechanisms(5) pkix(7) kp(3) }

   -- Extended Key Usage Values

   id-kp-rpcTLSClient OBJECT IDENTIFIER ::= { id-kp XXX }
   id-kp-rpcTLSServer OBJECT IDENTIFIER ::= { id-kp YYY }


   Special mention goes to Charles Fisher, author of "Encrypting NFSv4
   with Stunnel TLS" (
   nfsv4-stunnel-tls).  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
   contribution.  Also, thanks to Benjamin Kaduk for his expert guidance
   on the use of PKIX and TLS.  In addition, the authors thank the other
   members of the IESG for their astute review comments.  These
   contributors made this a significantly better document.

   The authors are additionally grateful to Bill Baker, David Black,
   Alan DeKok, Lars Eggert, Olga Kornievskaia, Greg Marsden, Alex
   McDonald, Justin Mazzola Paluska, Tom Talpey, Martin Thomson, and
   Nico Williams, 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