Towards Remote Procedure Call Encryption By Default

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Table of Contents

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 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 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 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 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Terminology

This document adopts the terminology introduced in Section 3 of [RFC6973] and assumes a working knowledge of the Remote Procedure Call (RPC) version 2 protocol [RFC5531] and the Transport Layer Security (TLS) version 1.3 protocol [RFC8446].¶

Note also that the NFS community 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

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.¶

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 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.¶

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 RFCs.¶

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

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.¶

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.¶

Appendix A. Known Weaknesses of the AUTH_SYS Authentication Flavor

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

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

A string identifies peers (hosts) in each RPC request. [RFC5531] does not specify any requirements for this string other than that is no longer than 255 octets. It does not have to be the same from request to request. Also, it does not have to match the DNS hostname of the sending host. For these reasons, even though most implementations fill in their hostname in this field, receivers typically ignore its content.¶

Appendix A of [RFC5531] contains a brief explanation of security considerations:¶

It should be noted that use of this flavor of authentication does not guarantee any security for the users or providers of a service, in itself. The authentication provided by this scheme can be considered legitimate only when applications using this 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

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.¶

<CODE BEGINS>

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


<CODE ENDS>

Acknowledgments

Special mention goes to Charles Fisher, author of "Encrypting NFSv4 with 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 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. 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