| Network File System Version 4 | T. Myklebust |
| Internet-Draft | Hammerspace |
| Updates: 5531 (if approved) | C. Lever, Ed. |
| Intended status: Standards Track | Oracle |
| Expires: August 15, 2019 | February 11, 2019 |
Remote Procedure Call Encryption By Default
draft-cel-nfsv4-rpc-tls-02
This document describes a mechanism that enables encryption of in-transit Remote Procedure Call (RPC) transactions with minimal administrative overhead and full interoperation with RPC implementations that do not support this mechanism. This document updates RFC 5531.
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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 is difficult to deploy:
However strong a privacy service is, it can not provide any security if the difficulties of deploying and 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.
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.
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 might use "confidentiality". In this document the two terms are synonymous.
In this section we cleave to the convention that a "client" is the peer host that actively initiates a connection, and a "server" is the peer host that passively accepts a connection request.
<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. The use of TLS is automatically disabled in these cases. To achieve this, we introduce a new 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.
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 initiate a TLS handshake, it sends 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:
If an RPC client attempts to use AUTH_TLS for anything other than the NULL RPC procedure, the RPC server responds with a reject_stat of AUTH_ERROR.
Once the TLS handshake is complete, the RPC client and server will have established a secure channel for communicating and can proceed to use standard security flavors within that channel, presumably after negotiating down the irrelevant 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.
RPC operates on several different types of transports. RPC on a stream transport is protected by using TLS [RFC8446]; on a datagram transport, RPC must use DTLS [RFC6347].
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.
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.
Toward these ends, there are two main deployment modes.
In a basic deployment, a server possesses a certificate that is self-signed or signed by a well-known trust anchor, while its clients might not possess a certificate. In this situation, the client MAY authenticate the server host, but the server cannot authenticate connecting clients. Here, encryption of the transport connection is established and the RPC requests in transit carry user and group identities according to the conventions of the ONC RPC protocol.
In this type of deployment, both the server and its clients possess valid certificates. As part of the TLS handshake, both peers MAY authenticate. Should authentication of either peer fail, or should authorization based on those identities block access to the server, the connection can be rejected. However, once encryption of the transport connection is established, the server MUST NOT utilize TLS identity for the purpose of authorizing RPC requests.
In some cases, a client might choose to present a certificate that represents a user rather than one that is bound to the client host. As above, the server MUST NOT utilize this identity for the purpose of authorizing RPC requests.
RPCSEC GSS can provide integrity or privacy (also known as confidentiality) services. When operating over an encrypted 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 authentication of server and client still occurs after the TLS session is established.
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]. When such versions of TLS are used to encrypted RPC traffic, token binding may replace the use of certificates, but the restrictions specified earlier in this section still apply.
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.
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 over AUTH_SYS. For various reasons, unfortunately 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 both servers and clients have certificate material available so that servers can have a degree of trust that clients are acting responsibly.
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.
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 are grateful to Bill Baker, David Black, Lars Eggert, Benjamin Kaduk Greg Marsden, Alex McDonald, David Noveck, Justin Mazzola Paluska, and Tom Talpey 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.