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Internet Engineering Task Force (IETF) T. Myklebust Request for Comments: 9289 Hammerspace Updates: 5531 C. Lever, Ed. Category: Standards Track Oracle ISSN: 2070-1721 September 2022

        Towards Remote Procedure Call Encryption by Default

Abstract

 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 Open Network Computing
 (ONC) RPC implementations that do not support it.  This document
 updates RFC 5531.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9289.

Copyright Notice

 Copyright (c) 2022 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1.  Introduction
 2.  Requirements Language
 3.  Terminology
 4.  RPC-with-TLS in Operation
   4.1.  Discovering Server-Side TLS Support
   4.2.  Authentication
     4.2.1.  Using TLS with RPCSEC_GSS
 5.  TLS Requirements
   5.1.  Base Transport Considerations
     5.1.1.  Protected Operation on TCP
     5.1.2.  Protected Operation on UDP
     5.1.3.  Protected Operation on Other Transports
   5.2.  TLS Peer Authentication
     5.2.1.  X.509 Certificates Using PKIX Trust
       5.2.1.1.  Extended Key Usage Values
     5.2.2.  Pre-shared Keys
 6.  Security Considerations
   6.1.  The Limitations of Opportunistic Security
     6.1.1.  STRIPTLS Attacks
     6.1.2.  Privacy Leakage before Session Establishment
   6.2.  TLS Identity Management on Clients
   6.3.  Security Considerations for AUTH_SYS on TLS
   6.4.  Best Security Policy Practices
 7.  IANA Considerations
   7.1.  RPC Authentication Flavor
   7.2.  ALPN Identifier for SunRPC
   7.3.  Object Identifier for PKIX Extended Key Usage
   7.4.  Object Identifier for ASN.1 Module
 8.  References
   8.1.  Normative References
   8.2.  Informative References
 Appendix A.  Known Weaknesses of the AUTH_SYS Authentication Flavor
 Appendix B.  ASN.1 Module
 Acknowledgments
 Authors' Addresses

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 due to the following:

Parts of each RPC header remain in cleartext, constituting a loss

of metadata confidentiality.

Offloading the Generic Security Service (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 User Identifiers (UIDs) and Group
 Identifiers (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

typically must be enforced in the same security realm. However,

    cloud providers, for instance, might prefer to remain
    authoritative for 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 TLS [RFC8446] and
 Datagram Transport Layer Security (DTLS) [RFC9147] protocols are
 well-established Internet building blocks that protect many standard
 Internet protocols such as the Hypertext Transfer Protocol (HTTP)
 [RFC9110].

 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 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
    authentication.

 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 6 of the current document defines policies in line
 with [RFC7435] that enable RPC-with-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 are used for all
 RPC transactions to mitigate against pervasive monitoring attacks
 [RFC7258].  Enforcing the use of RPC-with-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/
 DNS-Based Authentication of Named Entities (DANE) [RFC6698], and
 optionally RPCSEC_GSS [RFC2203] is available within the platform
 where RPC-with-TLS support is to be added.

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 RPC version 2
 protocol [RFC5531] and the 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-with-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-with-TLS.  When an
 RPC-with-TLS-enabled peer encounters a peer that does not support
 RPC-with-TLS, policy settings on the RPC-with-TLS-enabled peer
 determine whether RPC operation continues without the use of TLS or
 is discontinued altogether.

 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 7.1.

 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-with-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-with-TLS is available on that same server using a
 different port/transport tuple, nor does it imply that RPC-with-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-with-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-with-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 that
    is backed by a trusted entity (e.g., a pre-shared key or a
    certificate validated with a certification path).  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.  Further
    explanation appears in Section 5.2.

 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 Generic Security
 Service Application Program Interface (GSS-API) channel binding, as
 defined in [RFC5056], to determine when an underlying transport
 provides a sufficient degree of confidentiality.  RPC-with-TLS
 implementations MUST provide the "tls-exporter" channel binding type,
 as defined in [RFC9266].

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 1.3 (for

TLS [RFC8446] or DTLS [RFC9147], respectively). Support for

    mandatory-to-implement cipher suites 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 1.2, the requirement to use TLS 1.3 or later for RPC-with-
    TLS takes precedence.  Further, because TLS 1.3 ciphers are
    qualitatively different than cipher suites in previous versions of
    TLS, and RFC 7525 predates TLS 1.3, the cipher suite
    recommendations in RFC 7525 do not apply to RPC-with-(D)TLS.  A
    strict TLS mode for RPC-with-TLS that protects against STRIPTLS
    attacks is discussed in detail in Section 6.1.1.

Implementations MUST support certificate-based mutual

authentication. Support for Pre-Shared Key (PSK) mutual

    authentication is OPTIONAL; see Section 5.2.2 for further details.

Negotiation of a cipher suite providing confidentiality as well as

integrity protection is REQUIRED.

 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 7.2 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 7.2 in that message's ProtocolNameList
 value.

 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 frequently a strong association between an RPC program and a
 particular 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 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-with-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 cleartext 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
 cleartext 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

 The use of the DTLS protocol [RFC9147] 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-with-DTLS support for that RPC program on that port.  If
 spurious traffic appears on a 5-tuple between the initial cleartext
 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].

 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.

 Multihomed 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(54)"
 extension described in Section 9 of [RFC9147], and RPC-with-DTLS peer
 endpoints MUST provide a ConnectionID with a nonzero 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 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
 UDP.

 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
 mechanisms.

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-with-
 TLS implementations are REQUIRED to support the PKIX mechanism
 described in [RFC5280].

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

The DNS-ID identifier type is a subjectAltName extension that

contains a dNSName, as defined in Section 4.2.1.6 of [RFC5280].

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

To specify the identity of an RPC peer as a domain name, the

certificate MUST contain a subjectAltName extension that contains

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

To specify the identity of an RPC peer as a network identifier

(netid) or a universal network address (uaddr), the certificate

    MUST contain a subjectAltName extension that contains an
    iPAddress.

 When validating a server certificate, an RPC-with-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 netids and uaddrs, 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-with-TLS client presents a
 certificate to an RPC-with-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 path.

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-with-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 set of trust anchors changes (e.g., removal of a
 Certification Authority (CA) from the list of trusted CAs; issuance
 of a new Certificate Revocation List (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.

5.2.1.1. Extended Key Usage Values

 Section 4.2.1.12 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-with-TLS peer as an RPC client, and one that
 identifies the RPC-with-TLS peer as an RPC server.

 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.

 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]).  The PSK Identifier
 SHOULD be exposed at the RPC layer.

6. 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-with-TLS implementations MUST NOT use 0-RTT data.

6.1. The Limitations of Opportunistic Security

 Readers can find the definition of Opportunistic Security in
 [RFC7435].  A discussion of its underlying principles 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-
 with-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-with-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.

6.1.1. STRIPTLS Attacks

 The initial AUTH_TLS probe occurs in cleartext.  An on-path attacker
 can alter a cleartext 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 attacks:

A TLSA record [RFC6698] can alert clients that TLS is expected to

work, and provide a binding of a hostname to the 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-with-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 association to fall
    back to cleartext silently.  If TLSA records are not available,
    this approach is strongly encouraged.

6.1.2. 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 cleartext 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.

6.2. TLS Identity Management on Clients

 The goal of RPC-with-TLS is to hide the content of RPC requests while
 they are in transit.  RPC-with-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.

6.3. 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 nonparticipant messages.  In
 particular, transport-layer encryption plus peer authentication
 protects receiving eXternal Data Representation (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 Certification
 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.

6.4. Best Security Policy Practices

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

largely redundant when TLS is in use. These services SHOULD be

    disabled in that case.

7. IANA Considerations

7.1. RPC Authentication Flavor

 Following Appendix B of [RFC5531], an entry has been added to 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 have been assigned as follows:

 Identifier String:  AUTH_TLS

 Flavor Name:  TLS

 Value:  7

 Description:  Indicates support for RPC-with-TLS

 Reference:  RFC 9289

7.2. ALPN Identifier for SunRPC

 Following Section 6 of [RFC7301], the following value has been
 allocated in the "TLS 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 9289

7.3. Object Identifier for PKIX Extended Key Usage

 Per the Specification Required policy defined in Section 4.6 of
 [RFC8126], the following new values have been registered in the "SMI
 Security for PKIX Extended Key Purpose" registry (1.3.6.1.5.5.7.3)
 (see Section 5.2.1.1 and Appendix B).

             +=========+====================+===========+
             | Decimal | Description        | Reference |
             +=========+====================+===========+
             | 33      | id-kp-rpcTLSClient | RFC 9289  |
             +---------+--------------------+-----------+
             | 34      | id-kp-rpcTLSServer | RFC 9289  |
             +---------+--------------------+-----------+

                               Table 1

7.4. Object Identifier for ASN.1 Module

 Per the Specification Required policy defined in Section 4.6 of
 [RFC8126], the following new value has been registered in the "SMI
 Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0) (see
 Appendix B).

           +=========+========================+===========+
           | Decimal | Description            | Reference |
           +=========+========================+===========+
           | 105     | id-mod-rpcWithTLS-2021 | RFC 9289  |
           +---------+------------------------+-----------+

                               Table 2

8. References

8.1. Normative References

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

 [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
            Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
            <https://www.rfc-editor.org/info/rfc5056>.

 [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>.

 [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>.

 [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>.

 [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,
            <https://www.rfc-editor.org/info/rfc8126>.

 [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>.

 [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
            Datagram Transport Layer Security (DTLS) Protocol Version
            1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
            <https://www.rfc-editor.org/info/rfc9147>.

 [RFC9266]  Whited, S., "Channel Bindings for TLS 1.3", RFC 9266,
            DOI 10.17487/RFC9266, July 2022,
            <https://www.rfc-editor.org/info/rfc9266>.

 [X.509]    International Telecommunication Union, "Information
            technology - Open Systems Interconnection - The Directory:
            Public-key and attribute certificate frameworks", ISO/
            IEC 9594-8, ITU-T Recommendation X.509, October 2019.

 [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
            One (ASN.1): Specification of basic notation", ITU-T
            Recommendation X.680, February 2021,
            <https://www.itu.int/rec/T-REC-X.680>.

 [X.690]    ITU-T, "Information technology - ASN.1 encoding rules:
            Specification of Basic Encoding Rules (BER), Canonical
            Encoding Rules (CER) and Distinguished Encoding Rules
            (DER)", ITU-T Recommendation X.690, February 2021,
            <https://www.itu.int/rec/T-REC-X.690>.

8.2. Informative References

 [RFC1833]  Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
            RFC 1833, DOI 10.17487/RFC1833, August 1995,
            <https://www.rfc-editor.org/info/rfc1833>.

 [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>.

 [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>.

 [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>.

 [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>.

 [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>.

 [RFC8167]  Lever, C., "Bidirectional Remote Procedure Call on RPC-
            over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
            June 2017, <https://www.rfc-editor.org/info/rfc8167>.

 [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, <https://www.rfc-editor.org/info/rfc8899>.

 [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "HTTP Semantics", STD 97, RFC 9110,
            DOI 10.17487/RFC9110, June 2022,
            <https://www.rfc-editor.org/info/rfc9110>.

Appendix A. Known Weaknesses of the AUTH_SYS Authentication Flavor

 The ONC RPC protocol, as specified in [RFC5531], provides several
 modes of security, commonly 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 it
 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
 |  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
 security:

 1.  It does not protect the confidentiality 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.  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

 The following module adheres to ASN.1 specifications [X.680] and
 [X.690].

 <CODE BEGINS>
 RPCwithTLS-2021
   { iso(1) identified-organization(3) dod(6) internet(1)
   security(5) mechanisms(5) pkix(7) id-mod(0)
   id-mod-rpcWithTLS-2021(105) }

 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN

- OID Arc

 id-kp OBJECT IDENTIFIER ::=
   { 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 33 }
 id-kp-rpcTLSServer OBJECT IDENTIFIER ::= { id-kp 34 }

 END
 <CODE ENDS>

Acknowledgments

 Special mention goes to Charles Fisher, author of "Encrypting NFSv4
 with Stunnel TLS" <https://www.linuxjournal.com/content/encrypting-
 nfsv4-stunnel-tls>.  His article inspired the mechanism described in
 the current document.

 Many thanks to Benjamin Coddington, Tigran Mkrtchyan, and Rick
 Macklem for their work on prototype implementations and feedback on
 the current document.  Also, thanks to Benjamin Kaduk for his expert
 guidance on the use of PKIX and TLS and to Russ Housley for his ASN.1
 expertise and for providing other proper finishing touches.  In
 addition, the authors thank the other members of the IESG for their
 astute review comments.  These contributors made this a significantly
 better document.

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

 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.

 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, Suite 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