Network File System Version 4 T. Myklebust
Internet-Draft Hammerspace
Updates: 5531 (if approved) C. Lever, Ed.
Intended status: Standards Track Oracle
Expires: October 17, 2019 April 15, 2019

Remote Procedure Call Encryption By Default


This document describes a mechanism that opportunistically enables encryption of in-transit Remote Procedure Call (RPC) transactions with minimal administrative overhead and full interoperation with ONC RPC implementations that do not support this mechanism. This document updates RFC 5531.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on October 17, 2019.

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Table of Contents

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 a Proposed Standard for three decades (see [RFC5531] and its antecedants). Eisler et al. first introduced an in-transit encryption mechanism for RPC with RPCSEC GSS over twenty years ago [RFC2203]. However, experience has shown that RPCSEC GSS can be difficult to deploy:

However strong a privacy service is, it cannot provide any security if the challenges of using it result in it not being used at all.

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 the Hypertext Transport Protocol (http) [RFC2818].

Encrypting at the RPC transport layer enables several significant benefits.

Encryption By Default

In-transit encryption by itself may be enabled without additional administrative actions such as identifying client systems 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

TLS can be used to authenticate peer hosts while other security mechanisms can handle user authentictation. Cryptographic authentication of hosts can be provided while still using simpler user authentication flavors such as AUTH_SYS.
Encryption Offload

Whereas hardware support for GSS privacy has not appeared in the marketplace, the use of a well-established transport encryption mechanism that is also employed by other very common network protocols makes it likely that a hardware encryption implementation will be available to offload encryption and decryption. A single key protects all messages associated with one TLS session.
Securing AUTH_SYS

Most critically, several security issues inherent in the current widespread use of AUTH_SYS (i.e., acceptance of UIDs and GIDs generated by an unauthenticated client) can be significantly ameliorated.

2. Requirements Language

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

3. Terminology

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

Note also that the NFS community uses the term "privacy" where other Internet communities use "confidentiality". In this document the two terms are synonymous.

We cleave 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". TLS documentation refers to the same as "peer authentication". In this document there is little distinction.

The term "user authentication" in this document refers specifically to RPC users; i.e., the process owner of the application which is using RPC.

4. RPC-Over-TLS in Operation

4.1. Discovering Server-side TLS Support

The mechanism described in this document interoperates fully with RPC implementations that do not support TLS. The use of TLS is automatically disabled in these cases.


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 */


To achieve this, we introduce a new RPC authentication flavor called AUTH_TLS. This new flavor is used to signal 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 largely unaware of security encapsulation.

The flavor value of the verifier received in the reply message from the server MUST be AUTH_NONE. The bytes of the verifier's string encode the fixed ASCII characters "STARTTLS".

When an RPC client is ready to begin sending traffic to a server, it starts with a NULL RPC request with an auth_flavor of AUTH_TLS. The NULL request is made to the same port as if TLS were not in use.

The RPC server can respond in one of three ways:

Once the TLS handshake is complete, the RPC client and server will have established a secure channel for communicating. The client MUST switch to a security flavor other than AUTH_TLS within that channel, presumably after negotiating down redundant RPCSEC_GSS privacy and integrity services and applying channel binding [RFC7861].

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

If an RPC client attempts to use AUTH_TLS for anything other than the NULL RPC procedure, the RPC server MUST respond with a reject_stat of AUTH_ERROR. If the client sends a STARTTLS after it has sent other non-encrypted RPC traffic or after a TLS session has already been negotiated, the server MUST silently discard it.

4.2. Authentication

Both RPC and TLS have their own variants of authentication, and there is some overlap in capability. The goal of interoperability with implementations that do not support TLS requires that we limit the combinations that are allowed and precisely specify the role that each layer plays. We also want to handle TLS such that an RPC implementation can make the use of TLS invisible to existing RPC consumer applications.

Depending on its configuration, an RPC server MAY request a TLS identity from each client upon first contact. This permits two different modes of deployment:

Server-only Host Authentication

A server possesses a unique global identity (e.g., a certificate that is signed by a well-known trust anchor) while its clients are anonymous (i.e., present no identifier). In this situation, the client SHOULD authenticate the server host using the presented TLS identity, but the server cannot authenticate clients.
Mutual Host Authentication

In this type of deployment, both the server and its clients possess unique identities (e.g., certificates). As part of the TLS handshake, both peers SHOULD authenticate using the presented TLS identities. Should authentication of either peer fail, or should authorization based on those identities block access to the server, the client association MAY be rejected.

In either of these modes, RPC user authentication is not affected by the use of transport layer security. Once a TLS session is established, the server MUST NOT utilize the client peer's TLS identity for the purpose of authorizing individual RPC requests.

4.2.1. Using TLS with RPCSEC GSS

RPCSEC GSS can provide per-request integrity or privacy (also known as confidentiality) services. When operating over a TLS session, these services become redundant. Each RPC implementation is responsible for using channel binding for detecting when GSS integrity or privacy is unnecessary and can therefore be disabled. See Section 2.5 of [RFC7861] for details.

Note that a GSS service principal is still required on the server, and mutual GSS authentication of server and client still occurs after the TLS session is established.

5. TLS Requirements

When a TLS session is negotiated for the purpose of transporting RPC, the following restrictions apply:

5.1. Connection Types

5.1.1. Operation on TCP

RPC over TCP is protected by using TLS [RFC8446]. As soon as a client completes the TCP handshake, it uses the mechanism described in Section 4.1 to discover TLS support and then negotiate a TLS session.

An RPC client terminates a TLS session by sending a TLS closure alert, or by closing the underlying TCP socket. After TLS session termination, any subsequent RPC request over the same socket MUST fail with a reject_stat of AUTH_ERROR.

5.1.2. Operation on UDP

RPC over UDP is protected using DTLS [RFC6347]. As soon as a client initializes a socket for use with an unfamiliar server, it uses the mechanism described in Section 4.1 to discover DTLS support and then negotiate a DTLS session. Connected operation is RECOMMENDED.

Using a DTLS transport does not introduce reliable or in-order semantics to RPC on UDP. Also, DTLS does not support fragmentation of RPC messages. One RPC message fits in a single DTLS datagram. DTLS encapsulation has overhead which reduces the effective Path MTU (PMTU) and thus the maximum RPC payload size.

DTLS does not detect STARTTLS replay. A DTLS session can be terminated by sending a TLS closure alert. Subsequent RPC messages passing between the client and server will no longer be protected until a new TLS session is established.

5.1.3. Operation on an RDMA Transport

RPC-over-RDMA can make use of Transport Layer Security below the RDMA transport layer [RFC8166]. The exact mechanism is not within the scope of this document.

5.2. TLS Peer Authentication

Peer authentication can be performed by TLS using any of the following mechanisms:

5.2.1. X.509 Certificates Using PKIX trust

Implementations are REQUIRED to support this mechanism. In this mode, an RPC peer is uniquely identified by the tuple (serial number of presented certificate;Issuer).

Authenticating a connecting entity does not mean the RPC server necessarily wants to communicate with that client. For example, if the Issuer is not in a trusted set of Issuers, the RPC server may decline to perform RPC transactions with this client. Implementations that want to support a wide variety of trust models should expose as many details of the presented certificate to the administrator as possible so that the trust model can be implemented by the administrator. As a suggestion, at least the following parameters of the X.509 client certificate should be exposed:

5.2.2. X.509 Certificates Using Fingerprints

This mechanism is OPTIONAL to implement. In this mode, an RPC peer is uniquely identified by the fingerprint of the presented certificate.

Implementations SHOULD allow the configuration of a list of trusted certificates, identified via fingerprint of the DER encoded certificate octets. Implementations MUST support SHA-1 as the hash algorithm for the fingerprint. To prevent attacks based on hash collisions, support for a more contemporary hash function, such as SHA-256, is RECOMMENDED.

5.2.3. Pre-Shared Keys

This mechanism is OPTIONAL to implement. In this mode, an RPC peer is uniquely identified by key material that has been shared out-of-band or by a previous TLS-protected connection (see [RFC8446] Section 2.2). At least the following parameters of the TLS connection should be exposed:

5.2.4. Token Binding

This mechanism is OPTIONAL to implement. In this mode, an RPC peer is uniquely identified by a token.

Versions of TLS subsequent to TLS 1.2 feature a token binding mechanism which is nominally more secure than using certificates. This is discussed in further detail in [RFC8471].

6. Implementation Status

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs.

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

6.1. Linux NFS server and client

The Linux Foundation
Prototype software based on early versions of this document.
The bulk of this specification is implemented. The use of DTLS functionality is not implemented.
Implementation experience:
No comments from implementors.

6.2. DESY NFS server

Prototype software based on early versions of this document.
The bulk of this specification is implemented. The use of DTLS functionality is not implemented.
Freely distributable with acknowledgment.
Implementation experience:
No comments from implementors.

7. 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 implementation conserves connection resources by routing RPC transactions from all of its users over a small number of connections. In circumstances where the users on that NFS client belong to multiple distinct security domains, the client MUST establish independent TLS sessions for each distinct security domain.

7.1. Implications for AUTH_SYS

Ever since the IETF NFSV4 Working Group took over the maintenance of the NFSv4 family of protocols (currently specified in [RFC7530], [RFC5661], and [RFC7863], among others), it has encouraged the use of RPCSEC GSS rather than AUTH_SYS. For various reasons, AUTH_SYS continues to be the primary authentication mechanism deployed by NFS administrators. As a result, NFS security remains in an unsatisfactory state.

A deeper purpose of this document is to attempt to address some of the shortcomings of AUTH_SYS so that, where it has been impractical to deploy RPCSEC GSS, better NFSv4 security can nevertheless be achieved.

When AUTH_SYS is used with TLS and no client certificate is available, the RPC server is still acting on RPC requests for which there is no trustworthy authentication. In-transit traffic is protected, but the client itself can still misrepresent user identity without detection. This is an improvement from AUTH_SYS without encryption, but it leaves a critical security exposure.

Therefore, the RECOMMENDED deployment mode is that clients have certificate material configured and used so that servers can have a degree of trust that clients are acting responsibly.

7.2. STRIPTLS Attacks

A classic form of attack on network protocols that initiate an association in plain-text to discover support for TLS is a man-in-the-middle that alters the plain-text handshake to make it appear as though TLS support is not available on one or both peers. Clients implementers can choose from the following to mitigate STRIPTLS attacks:

8. IANA Considerations

In accordance with Section 6 of [RFC7301], the authors request that IANA allocate the following value in the "Application-Layer Protocol Negotiation (ALPN) Protocol IDs" registry. The "sunrpc" string identifies SunRPC when used over TLS.


Identification Sequence:

0x73 0x75 0x6e 0x72 0x70 0x63 ("sunrpc")


9. References

9.1. 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.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, DOI 10.17487/RFC4279, December 2005.
[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.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[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.
[RFC7861] Adamson, A. and N. Williams, "Remote Procedure Call (RPC) Security Version 3", RFC 7861, DOI 10.17487/RFC7861, November 2016.
[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.
[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.

9.2. Informative References

[RFC1813] Callaghan, B., Pawlowski, B. and P. Staubach, "NFS Version 3 Protocol Specification", RFC 1813, DOI 10.17487/RFC1813, June 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.
[RFC5661] Shepler, S., Eisler, M. and D. Noveck, "Network File System (NFS) Version 4 Minor Version 1 Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010.
[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.
[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.
[RFC7530] Haynes, T. and D. Noveck, "Network File System (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, March 2015.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862, November 2016.
[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor Version 2 External Data Representation Standard (XDR) Description", RFC 7863, DOI 10.17487/RFC7863, November 2016.
[RFC8166] Lever, C., Simpson, W. and T. Talpey, "Remote Direct Memory Access Transport for Remote Procedure Call Version 1", RFC 8166, DOI 10.17487/RFC8166, June 2017.
[RFC8471] Popov, A., Nystroem, M., Balfanz, D. and J. Hodges, "The Token Binding Protocol Version 1.0", RFC 8471, DOI 10.17487/RFC8471, October 2018.


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

Many thanks to Tigran Mkrtchyan for his work on the DESY prototype and resulting feedback to this document.

The authors are grateful to Bill Baker, David Black, Alan DeKok, Lars Eggert, Benjamin Kaduk, Olga Kornievskaia, Greg Marsden, Alex McDonald, David Noveck, Justin Mazzola Paluska, Tom Talpey, and Martin Thomson for their input and support of this work.

Lastly, special thanks go to Transport Area Director Magnus Westerlund, NFSV4 Working Group Chairs 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 EMail:
Charles Lever (editor) Oracle Corporation United States of America EMail: