Remote Procedure Call Version 2 Encryption By Default
draft-cel-nfsv4-rpc-tls-00

Abstract

This document proposes a mechanism that makes it possible to enable in-transit encryption of Remote Procedure Call traffic with little administrative overhead and full compatibility with implementations that do not support it.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 16, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. RPC on TLS in Operation
• 3.1. Discovering Server-side TLS Support
• 3.2. Streams and Datagrams
• 3.3. Authentication
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Acknowledgments
• Authors' Addresses

1. Introduction

In 2014, the IETF published [RFC7258] which recognized that unauthorized observation of network traffic had become widespread, and was a subversive threat to all who make use of the Internet at large. It strongly recommended that newly defined Internet protocols make a real effort to mitigate monitoring attacks. Typically this mitigation is done by encrypting data in transit.

The Remote Procedure Call version 2 protocol has been around for more than a decade [RFC5531]. Support for in-transit encryption of RPC was introduced with RPCSEC GSS [RFC7861]. However, experience has shown that RPCSEC GSS is challenging to deploy, especially in environments where:

• Per-host administrative or deployment costs must be kept to a minimum,
• Parts of the RPC header that remain in clear-text are a security exposure,
• Host CPU resources are at a premium, or
• Host identity management is carried out in a security domain that is distinct from user identity management.

However strong a privacy service is, it is not effective if it cannot be deployed in typical environments.

An alternative approach is to employ a transport layer security mechanism that can protect the privacy of each RPC connection transparently to RPC and Upper Layer protocols. The Transport Layer Security protocol [RFC8446] (TLS) is a well-established Internet building block that protects many common Internet protocols such as https [RFC2818].

Encrypting at the RPC transport layer enables several significant benefits.

Encryption By Default

With the use of pre-shared keys, in-transit encryption can be enabled immediately after installation without additional administrative actions such as identifying the host system to a trust authority, generating additional key material, or provisioning a secure network tunnel.

Protection of Existing Protocols

The imposition of encryption at the transport layer protects any Upper Layer protocol that employs RPC without alteration of that protocol. RPC transport layer encryption can protect recent versions of NFS such as NFS version 4.2 [RFC7862] and indeed legacy NFS versions such as NFS version 3 [RFC1813] and NFS side-band protocols such as the MNT protocol [RFC1813].

Decoupled User and Host Identities

RPCSEC GSS provides a framework for cryptographically protecting user and host identities, but assumes that both are managed by the same security authority.

Encryption Offload

The use of a well-established transport encryption mechanism that is also employed by other very common network protocols makes it possible to use hardware encryption implementations so that the host CPU is not burdened with the work of encrypting and decrypting large RPC arguments and results.

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].

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. RPC on TLS in Operation

3.1. Discovering Server-side TLS Support

<CODE BEGINS>

enum auth_flavor {
        AUTH_NONE       = 0,
        AUTH_SYS        = 1,
        AUTH_SHORT      = 2,
        AUTH_DH         = 3,
        AUTH_KERB       = 4,
        AUTH_RSA        = 5,
        RPCSEC_GSS      = 6,
        AUTH_TLS        = 7,

        /* and more to be defined */
};

<CODE ENDS>

The mechanism described in this document interoperates fully with implementations that do not support it. Encryption (TLS) is automatically disabled in these cases. To achieve this, we introduce a new authentication flavor called AUTH_TLS.

When an RPC client is ready to initiate TLS negotiation, it sends a NULL RPC request with an auth_flavor of AUTH_TLS. The server can respond in one of three ways:

• If the server does not recognise the AUTH_TLS authentication flavor, it responds with a reject_stat of AUTH_ERROR. The client then knows that this server does not support TLS.
• If the server accepts the NULL RPC procedure, but fails to return an AUTH_NONE verifier containing the string "STARTTLS", the client knows that this server does not support TLS.
• If the server accepts the NULL RPC procedure, and returns an AUTH_NONE verifier containing the string "STARTTLS", the client MAY proceed with TLS security negotiation.

If a client attempts to use AUTH_TLS for anything other than the NULL RPC procedure, the server responds with a reject_stat of AUTH_ERROR.

Once TLS security negotiation is complete, the client and server will have established a secure channel for communicating and can proceed to use standard security flavours within that channel, presumably after negotiating down the irrelevant RPCSEC_GSS privacy and integrity services and applying channel binding.

If TLS negotiation fails for any reason (for example the server rejects the certificate presented by the client), the RPC client reports this failure to the calling application the same way it would report an AUTH_ERROR rejection from the server.

3.2. Streams and Datagrams

RPC commonly operates on stream transports and datagram transports. When operating on a stream transport, using TLS [RFC8446] is appropriate. On a datagram transport, RPC should use DTLS [RFC6347].

RPC-over-RDMA [RFC8166] may make use of transport layer security below the RDMA transport layer.

3.3. Authentication

Both RPC and TLS have their own in-built forms of host and user authentication. Each have their strengths and weaknesses. We believe the combination of host authentication via TLS and user authentication via RPC provides optimal security, efficiency, and flexibility, although many combinations are possible.

TLS Encryption-only with AUTH_SYS:

A pre-shared key enables TLS encryption. The RPC client uses AUTH_SYS to identify users with the guarantee that the UID and GID values cannot be observed or altered in transit.

TLS Encryption-only with RPCSEC GSS Kerberos:

A pre-shared key enables TLS encryption in encryption-only mode. The RPC client uses Kerberos to identify the client host and its users, and does not need to additionally require costly GSS integrity or privacy services.

TLS per-client certificate with AUTH_SYS:

During TLS negotiation, the client identifies itself to the server with a unique certificate. The server can use this identity to perform additional authorization of the client's requests.

TLS per-user certificate with AUTH_NONE:

Each user establishes her own TLS context with the server, identified by a unique certficate. There is no need for any additional information at the RPC layer, so the RPC client can use the simplest authentication flavor for RPC transactions.

4. Security Considerations

One purpose of the mechanism described in this document is to protect RPC-based applications against threats to the privacy of RPC transactions and RPC user identities. A taxonomy of these threats appears in Section 5 of [RFC6973]. In addition, Section 6 of [RFC7525] contains a detailed discussion of technologies used in conjunction with TLS. Implementers should familiarize themselves with these materials.

The NFS version 4 protocol permits more than one user to use an NFS client at the same time [RFC7862]. Typically that NFS client will conserve connection resources by routing RPC transactions from all of its users over a few or a single connection. In circumstances where the users on that NFS client belong to multiple distinct security domains, it may be necessary to establish separate TLS-protected connections that do not share the same encryption parameters.

5. IANA Considerations

This document does not require actions by IANA.

6. References

6.1. Normative References

6.2. Informative References

Acknowledgments

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

The authors wish to thank Bill Baker, David Black, Benjamin Kaduk Greg Marsden, David Noveck, and Justin Mazzola Paluska for their input and support of this work.

Special thanks go to Transport Area Director Spencer Dawkins, NFSV4 Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4 Working Group Secretary Thomas Haynes for their guidance and oversight.

Authors' Addresses