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Network File System Version 4 T. Myklebust
Internet-Draft Hammerspace
Updates: 5531 (if approved) C. Lever, Ed.
Intended status: Standards Track Oracle
Expires: November 1, 2020 April 30, 2020
Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-07
Abstract
This document describes a mechanism that, through the use of
opportunistic Transport Layer Security (TLS), enables encryption of
in-transit Remote Procedure Call (RPC) transactions while
interoperating 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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 1, 2020.
Copyright Notice
Copyright (c) 2020 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 Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. RPC-Over-TLS in Operation . . . . . . . . . . . . . . . . . . 5
4.1. Discovering Server-side TLS Support . . . . . . . . . . . 6
4.2. Authentication . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Using TLS with RPCSEC GSS . . . . . . . . . . . . . . 8
5. TLS Requirements . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Base Transport Considerations . . . . . . . . . . . . . . 9
5.1.1. Protected Operation on TCP . . . . . . . . . . . . . 9
5.1.2. Protected Operation on UDP . . . . . . . . . . . . . 9
5.1.3. Protected Operation on Other Transports . . . . . . . 10
5.2. TLS Peer Authentication . . . . . . . . . . . . . . . . . 11
5.2.1. X.509 Certificates Using PKIX trust . . . . . . . . . 11
5.2.2. X.509 Certificates Using Fingerprints . . . . . . . . 12
5.2.3. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . 12
5.2.4. Token Binding . . . . . . . . . . . . . . . . . . . . 13
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 13
6.1. DESY NFS server . . . . . . . . . . . . . . . . . . . . . 13
6.2. Hammerspace NFS server . . . . . . . . . . . . . . . . . 14
6.3. Linux NFS server and client . . . . . . . . . . . . . . . 14
6.4. FreeBSD NFS server and client . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7.1. Limitations of an Opportunistic Approach . . . . . . . . 15
7.1.1. STRIPTLS Attacks . . . . . . . . . . . . . . . . . . 15
7.1.2. Privacy Leakage Before Session Establishment . . . . 16
7.2. TLS Identity Management on Clients . . . . . . . . . . . 16
7.3. Security Considerations for AUTH_SYS on TLS . . . . . . . 17
7.4. Best Security Policy Practices . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8.1. RPC Authentication Flavor . . . . . . . . . . . . . . . . 18
8.2. ALPN Identifier for SUNRPC . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix A. Known Weaknesses of the AUTH_SYS Authentication
Flavor . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
RFC Editor: Please remove this Editor's Note and the following
paragraph before this document is published.
The source for this draft is maintained in GitHub. Suggested changes
should be submitted as pull requests at
https://github.com/chucklever/i-d-rpc-tls [1]. Instructions are on
that page as well. Editorial changes can be managed in GitHub, but
any substantive change should be discussed on the nfsv4@ietf.org
mailing list.
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 should
make a genuine 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 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:
o Parts of each RPC header remain in clear-text, constituting a
significant security exposure.
o 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:
o Per-client deployment and administrative costs are not scalable.
Administrators must provide keying material for each RPC client,
including transient clients.
o Host identity management and user identity management must be
enforced in the same security realm. In certain environments,
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different authorities might be responsible for provisioning client
systems versus provisioning new users.
The alternative described in the current document is to employ a
transport layer security mechanism that can protect the
confidentiality 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 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, generating additional keying material, or
provisioning a secure network tunnel.
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.
The current document specifies the implementation of RPC on an
encrypted transport in a manner that is transparent to upper-layer
protocols based on RPC. The imposition of encryption at the
transport layer protects any upper-layer protocol that employs RPC,
without alteration of that protocol.
Further, Section 7 of the current document defines policies in line
with [RFC7435] which enable RPC-on-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. Enforcing the use of RPC-on-TLS is of particular
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importance for existing upper-layer protocols whose security
infrastructure is weak.
The protocol specification in the current document assumes that
support for RPC, TLS, PKI, GSS-API, and DNSSEC is already available
in an RPC implementation where 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
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4.1. Discovering Server-side TLS Support
The mechanism described in the current document interoperates fully
with RPC implementations that do not support TLS. Policy settings on
the RPC-on-TLS-enabled peer determine whether RPC operation continues
without the use of TLS or RPC operation is not permitted.
To achieve this, we introduce a new RPC authentication flavor called
AUTH_TLS. This new 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.
When an RPC client is ready to begin a TLS session, it sends a NULL
RPC procedure with an auth_flavor of AUTH_TLS. The value of AUTH_TLS
is defined in Section 8.1. The NULL request is made to the same port
as if TLS were not in use.
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.
If the RPC server replies with a reply_stat of MSG_ACCEPTED and an
AUTH_NONE verifier containing the "STARTTLS" token, the RPC client
follows with a "ClientHello" message. The client MAY proceed with
TLS session establishment even if the Reply's accept_stat is not
SUCCESS (for example, if the accept_stat is PROG_UNAVAIL). Once the
TLS handshake is complete, the RPC client and server have established
a secure channel for communicating.
If the Reply's reply_stat is MSG_ACCEPTED but the verifier does not
contain the "STARTTLS" token, or if the Reply's reply_stat is
MSG_DENIED, the RPC client MUST NOT send a "ClientHello" message.
RPC operation can continue, however it will be without any
confidentiality, integrity or authentication protection from (D)TLS.
If, after a successful RPC AUTH_TLS probe, the subsequent 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
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RPC AUTH_TLS probe within an existing TLS session, the RPC server
MUST reject that RPC Call by setting the reply_stat field to
MSG_DENIED, the reject_stat field to AUTH_ERROR, and the auth_stat
field to AUTH_BADCRED.
4.2. Authentication
Both RPC and TLS have peer and user authentication, with some overlap
in capability between 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. We also want to handle TLS such that an RPC
implementation can make the use of TLS invisible to existing RPC
consumer applications.
Each RPC server that supports RPC-over-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-over-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 (e.g.
a certificate) that is backed by a trusted entity. 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.
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 must not
utilize the remote TLS peer identity for RPC user authentication.
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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 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 redundant. An RPC implementation capable of concurrently
using TLS and RPCSEC GSS can use GSS channel binding, as defined in
[RFC5056], to determine when an underlying transport provides a
sufficient degree of confidentiality. Channel bindings for the TLS
channel type are defined in [RFC5929].
5. TLS Requirements
When peers negotiate a TLS session that is to transport RPC, the
following restrictions apply:
o 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.
o Implementations MUST support certificate-based mutual
authentication. Support for TLS-PSK mutual authentication
[RFC4279] is OPTIONAL. See Section 4.2 for further details.
o Negotiation of a ciphersuite providing confidentiality as well as
integrity protection is REQUIRED. Support for and negotiation of
compression is OPTIONAL.
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.
Similary, 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, 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.
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5.1. Base Transport Considerations
There is traditionally a strong association between an RPC program
and a 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 Transport Layer Security (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-on-TLS support for that connection. If
spurious traffic appears on a TCP connection between the initial
clear-text 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
clear-text 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.
Backchannel operation occurs only on connected transports such as
TCP. To protect backchannel operations, an RPC server uses the
existing TLS session on that connection to send backchannel
operations. The server does not attempt to establish a TLS session
on a TCP connection for backchannel operation.
When operation is complete, an RPC peer terminates a TLS session by
sending a TLS Closure Alert and may then close the TCP connection.
5.1.2. Protected Operation on UDP
RFC Editor: In the following section, please replace TBD with the
connection_id extension number that is to be assigned in
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[I-D.ietf-tls-dtls-connection-id]. And, please remove this Editor's
Note before this document is published.
RPC over UDP is protected using the Datagram Transport Layer Security
(DTLS) protocol [I-D.ietf-tls-dtls13].
Using DTLS does not introduce reliable or in-order semantics to RPC
on UDP. Each RPC message MUST fit in a single DTLS record. DTLS
encapsulation has overhead, which reduces the effective Path MTU
(PMTU) and thus the maximum RPC payload size. The use of DTLS record
replay protection is REQUIRED when transporting RPC traffic.
As soon as a client initializes a UDP socket for use with an RPC
server, it uses the mechanism described in Section 4.1 to discover
DTLS support for an RPC program on a particular port. It then
negotiates a DTLS session.
Multi-homed 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(TBD)"
extension described in Section 9 of [I-D.ietf-tls-dtls13], and RPC-
on-DTLS peer endpoints MUST provide a ConnectionID with a non-zero
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. Subsequent
RPC messages exchanged between the RPC client and server are no
longer protected until a new DTLS session is established.
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.
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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
Implementations are REQUIRED to support this mechanism. In this
mode, the tuple (serial number of the presented certificate; Issuer)
uniquely identifies the RPC peer.
o Implementations MUST allow the configuration of a list of trusted
Certification Authorities for incoming connections.
o Certificate validation MUST include the verification rules as per
[RFC5280].
o Implementations SHOULD indicate their trusted Certification
Authorities (CAs).
o Peer validation always includes a check on whether the locally
configured expected DNS name or IP address of the server that is
contacted matches its presented certificate. DNS names and IP
addresses can be contained in the Common Name (CN) or
subjectAltName entries. For verification, only one of these
entries is to be considered. The following precedence applies:
for DNS name validation, subjectAltName:DNS has precedence over
CN; for IP address validation, subjectAltName:iPAddress has
precedence over CN. Implementors of this specification are
advised to read Section 6 of [RFC6125] for more details on DNS
name validation.
o For services accessed by their network identifiers (netids) and
universal network addresses (uaddr), the iPAddress subjectAltName
SHOULD be present in the certificate and must exactly match the
address represented by universal address.
o 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 or a set of allowed X509v3 Certificate
Policies).
o When the configured trust base changes (e.g., removal of a CA from
the list of trusted CAs; issuance of a new CRL for a given CA),
implementations MAY renegotiate the TLS session to reassess the
connecting peer's continued authorization.
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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 administrator can implement the
trust model. As a suggestion, at least the following parameters of
the X.509 client certificate SHOULD be exposed:
o Originating IP address
o Certificate Fingerprint
o Issuer
o Subject
o all X509v3 Extended Key Usage
o all X509v3 Subject Alternative Name
o all X509v3 Certificate Policies
5.2.2. X.509 Certificates Using Fingerprints
This mechanism is OPTIONAL to implement. In this mode, the
fingerprint of the presented certificate uniquely identifies the RPC
peer.
Implementations SHOULD allow the configuration of a list of trusted
certificates, identified via fingerprint of the DER-encoded
certificate octets. Implementations MUST support SHA-256
[FIPS.180-4] or stronger as the hash algorithm for the fingerprint.
5.2.3. Pre-Shared Keys
This mechanism is OPTIONAL to implement. In this mode, the RPC peer
is uniquely identified by keying material that has been shared out-
of-band or by a previous TLS-protected connection (see Section 2.2 of
[RFC8446]). At least the following parameters of the TLS connection
SHOULD be exposed:
o Originating IP address
o TLS Identifier
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5.2.4. Token Binding
This mechanism is OPTIONAL to implement. In this mode, a token
uniquely identifies the RPC peer.
Versions of TLS after TLS 1.2 contain a token binding mechanism that
is more secure than using certificates. This mechanism is detailed
in [RFC8471].
6. Implementation Status
RFC Editor: Please remove this section and the reference to RFC 7942
before this document is published.
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. DESY NFS server
Organization: DESY
URL: https://desy.de [2]
Maturity: Implementation will be based on mature versions of the
current document.
Coverage: The bulk of this specification is implemented including
DTLS.
Licensing: LGPL
Implementation experience: The implementer has read and commented on
the current document.
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6.2. Hammerspace NFS server
Organization: Hammerspace
URL: https://hammerspace.com [3]
Maturity: Prototype software based on early versions of the current
document.
Coverage: The bulk of this specification is implemented. The use of
DTLS functionality is not implemented.
Licensing: Proprietary
Implementation experience: No comments from implementors.
6.3. Linux NFS server and client
Organization: The Linux Foundation
URL: https://www.kernel.org [4]
Maturity: Prototype software based on early versions of the current
document.
Coverage: The bulk of this specification has yet to be implemented.
The use of DTLS functionality is not planned.
Licensing: GPLv2
Implementation experience: No comments from the implementor.
6.4. FreeBSD NFS server and client
Organization: The FreeBSD Project
URL: https://www.freebsd.org [5]
Maturity: Prototype software based on early versions of the current
document.
Coverage: The bulk of this specification is implemented. The use of
DTLS functionality is not planned.
Licensing: BSD
Implementation experience: Implementers have read and commented on
the current document.
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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. Implementers should familiarize themselves
with these materials.
7.1. Limitations of an Opportunistic Approach
The purpose of using an explicitly opportunistic approach is to
enable interoperation with implementations that do not support RPC-
over-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-over-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.
7.1.1. 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:
o A TLSA record [RFC6698] can alert clients that TLS is expected to
work, and provide a binding of hostname to x.509 identity. If TLS
cannot be negotiated or authentication fails, the client
disconnects and reports the problem.
o Client security policy can require that a TLS session is
established on every connection. If an attacker spoofs the
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handshake, the client disconnects and reports the problem. If
TLSA records are not available, this approach is strongly
encouraged.
7.1.2. 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 clear-text 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 connnection handshake information and
TLS certificate material that might enable attackers 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.
7.2. TLS Identity Management on Clients
The goal of the RPC-on-TLS protocol extension is to hide the content
of RPC requests while they are in transit. The RPC-on-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 identity material is 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
identity material so that local access to it is restricted to only
the local users that are authorized to perform operations on the
remote RPC server.
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7.3. 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 (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 the
insertion or deletion of messages, thus also ensuring the integrity
of the message stream between RPC client and server. Lastly,
transport layer encryption plus peer authentication protects
receiving XDR decoders from deserializing untrusted data, a common
coding vulnerability.
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, it is RECOMMENDED that when AUTH_SYS is used,
every RPC client should present host authentication material to RPC
servers to prove that the client is a known one. The server can then
determine whether the UIDs and GIDs in AUTH_SYS requests from that
client can be accepted.
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.
7.4. Best Security Policy Practices
RPC-over-TLS implementations and deployments are strongly encouraged
to adhere to the following policies to achieve the strongest possible
security with RPC-over-TLS.
o When using AUTH_NULL or AUTH_SYS, both peers are required to have
DNS TLSA records and certificate material, and a policy that
requires mutual peer authentication and rejection of a connection
when host authentication fails.
o When using RPCSEC_GSS, GSS/Kerberos provides adequate host
authentication and a policy that requires GSS mutual
authentication and rejection of a connection when host
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authentication fails. GSS integrity and privacy services,
therefore, can be disabled in favor of TLS encryption with peer
authentication.
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.
8.1. RPC Authentication Flavor
Following Appendix B of [RFC5531], the authors request a single new
entry in 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 are assigned as follows:
Identifier String: AUTH_TLS
Flavor Name: TLS
Value: 7
Description: Signals the use of TLS to protect RPC messages on
socket-based transports
Reference: RFC-TBD
8.2. ALPN Identifier for SUNRPC
Following Section 6 of [RFC7301], the authors request the allocation
of the following value in the "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-TBD
9. References
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9.1. Normative References
[FIPS.180-4]
National Institute of Standards and Technology, "Secure
Hash Standard, Federal Information Processing Standards
Publication FIPS PUB 180-4", FIPS PUB 180-4, August 2015.
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
id-07 (work in progress), October 2019.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-37 (work in progress), March
2020.
[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>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.
[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>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
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[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>.
[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>.
[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>.
[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,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
9.2. Informative References
[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>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[RFC8471] Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
"The Token Binding Protocol Version 1.0", RFC 8471,
DOI 10.17487/RFC8471, October 2018,
<https://www.rfc-editor.org/info/rfc8471>.
9.3. URIs
[1] https://github.com/chucklever/i-d-rpc-tls
[2] https://desy.de
[3] https://hammerspace.com
[4] https://www.kernel.org
[5] https://www.freebsd.org
[6] https://www.linuxjournal.com/content/encrypting-nfsv4-stunnel-tls
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
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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.
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4. Because the user and group ID fields are not integrity-protected,
AUTH_SYS does not provide non-repudiation.
Acknowledgments
Special mention goes to Charles Fisher, author of "Encrypting NFSv4
with Stunnel TLS" [6]. 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.
The authors are additionally grateful to Bill Baker, David Black,
Alan DeKok, Lars Eggert, Benjamin Kaduk, Olga Kornievskaia, Greg
Marsden, Alex McDonald, Justin Mazzola Paluska, Tom Talpey, and
Martin Thomson 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 Ste 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
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