Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-06
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Trond Myklebust
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Chuck Lever
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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: August 6, 2020 February 3, 2020
Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-06
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 August 6, 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.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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 . . . . . . . . . . . 5
4.2. Authentication . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Using TLS with RPCSEC GSS . . . . . . . . . . . . . . 8
5. TLS Requirements . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Base Transport Considerations . . . . . . . . . . . . . . 8
5.1.1. Operation on TCP . . . . . . . . . . . . . . . . . . 8
5.1.2. Operation on UDP . . . . . . . . . . . . . . . . . . 9
5.1.3. Operation on Other Transports . . . . . . . . . . . . 9
5.2. TLS Peer Authentication . . . . . . . . . . . . . . . . . 10
5.2.1. X.509 Certificates Using PKIX trust . . . . . . . . . 10
5.2.2. X.509 Certificates Using Fingerprints . . . . . . . . 11
5.2.3. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . 11
5.2.4. Token Binding . . . . . . . . . . . . . . . . . . . . 11
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 12
6.1. DESY NFS server . . . . . . . . . . . . . . . . . . . . . 12
6.2. Hammerspace NFS server . . . . . . . . . . . . . . . . . 12
6.3. Linux NFS server and client . . . . . . . . . . . . . . . 13
6.4. FreeBSD NFS server and client . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Limitations of an Opportunistic Approach . . . . . . . . 14
7.1.1. STRIPTLS Attacks . . . . . . . . . . . . . . . . . . 14
7.2. TLS Identity Management on Clients . . . . . . . . . . . 14
7.3. Security Considerations for AUTH_SYS on TLS . . . . . . . 15
7.4. Best Security Policy Practices . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
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9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Known Weaknesses of the AUTH_SYS Authentication
Flavor . . . . . . . . . . . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 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 GSS privacy is not practical in large multi-user
deployments since each message is encrypted using a key based on
the issuing RPC user.
However strong a privacy service 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,
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 privacy of
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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 GSS privacy 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 fashion 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, the current document defines policies in line with [RFC7435]
which enable RPC-on-TLS to be deployed opportunistically in
environments with RPC implementations that do not support TLS.
Specifications for RPC-based upper-layer protocols are free to
require stricter policies to guarantee that encryption or host
authentication is in use on every connection.
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.
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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 the current document
the two terms are synonymous.
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 this 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
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.
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.
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<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 length of the opaque data constituting the credential sent in the
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 received in the reply message 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.
When an RPC client is ready to begin sending encrypted 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:
o If the RPC server does not recognize the AUTH_TLS authentication
flavor, it responds with a reject_stat of AUTH_ERROR. The RPC
client then knows that this server does not support TLS.
o If the RPC server accepts the NULL RPC procedure but fails to
return an AUTH_NONE verifier containing the "STARTTLS" token in
the NULL Reply, the RPC client knows that this server does not
support TLS.
o If the RPC server accepts the NULL RPC procedure and returns an
AUTH_NONE verifier containing the "STARTTLS" token in the NULL
Reply, the RPC client immediately initiates a TLS session. This
NULL Reply signals that the RPC server is prepared for the client
to begin TLS session negotiation.
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Once the TLS handshake is complete, the RPC client and server 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, the RPC client reports this
failure to the upper-layer 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.
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 a unique global
identity (e.g., a certificate). 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
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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.
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, the
GSS services become redundant. A TLS-capable RPC implementation uses
GSS channel binding to determine when GSS integrity or privacy is
unnecessary. See Section 2.5 of [RFC7861] for details.
When using GSS on a TLS session, the RPC server is still required to
possess a GSS service principal. GSS mutual authentication still
occurs after a TLS session has been established.
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
[RFC8446]. 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.
5.1. Base Transport Considerations
5.1.1. Operation on TCP
The use of TLS [RFC8446] protects RPC on TCP connections. Typically,
once a client completes the TCP handshake and performs RPC service
discovery via NULL RPC operations, it uses the mechanism described in
Section 4.1 to discover TLS support. It can then negotiate a TLS
session on that connection.
After establishing a TLS session, an RPC server MUST reject with a
reject_stat of AUTH_ERROR any subsequent RPC requests over a TLS-
protected connection that are outside of a TLS session. Likewise, an
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RPC client MUST silently discard any subsequent RPC replies over the
connection that are outside of a TLS session.
This restriction includes reverse-direction RPC operations (i.e., RPC
calls initiated on the server-end of the connection). An RPC client
receiving a reverse-direction call on a connection outside of an
existing TLS session MUST reject the request with a reject_stat of
AUTH_ERROR.
An RPC peer terminates a TLS session by sending a TLS closure alert,
or by closing the TLS-protected TCP connection.
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. Each RPC message MUST fit 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. Sending a TLS closure alert
terminates a DTLS session. Subsequent RPC messages passing between
the client and server are no longer protected until a new TLS session
is established.
5.1.3. Operation on Other Transports
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. Because there might not be other provisions
to exchange client and server certificates, authentication material
exchange would need to be provided by facilities within a future RPC-
over-RDMA transport.
Transports that provide intrinsic TLS-level security (e.g., QUIC)
would need to be addressed separately from the current document. In
such cases, the use of TLS would not be opportunistic as it is for
TCP or UDP.
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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:iPAddr has
precedence over CN. Implementors of this specification are
advised to read Section 6 of [RFC6125] for more details on DNS
name validation.
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.
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
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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
5.2.4. Token Binding
This mechanism is OPTIONAL to implement. In this mode, a token
uniquely identifies the RPC peer.
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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
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
Maturity: Implementation will be based on mature versions of the
current document.
Coverage: The implementation is under way. The use of DTLS
functionality is not implemented.
Licensing: LGPL
Implementation experience: The implementer has read and commented on
the current document.
6.2. Hammerspace NFS server
Organization: Hammerspace
URL: https://hammerspace.com
Maturity: Prototype software based on early versions of this
document.
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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
Maturity: Prototype software based on early versions of this
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
Maturity: Prototype software based on early versions of this
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
this document.
7. Security Considerations
One purpose of the mechanism described in the current 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]. Also, Section 6 of [RFC7525]
contains a detailed discussion of technologies used in conjunction
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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 cases 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. Implementations are REQUIRED to provide an
audit log of RPC-over-TLS security mode selection.
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
handshake, the client disconnects and reports the problem. If
TLSA records are not available, this approach is strongly
encouraged.
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.
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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.
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 a privacy service 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.
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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
authentication fails. GSS integrity and privacy services,
therefore, can be disabled in favor of TLS encryption with peer
authentication.
8. IANA Considerations
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
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.
[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>.
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[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>.
[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>.
[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>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[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>.
[RFC7861] Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
November 2016, <https://www.rfc-editor.org/info/rfc7861>.
[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>.
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[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>.
[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>.
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9.3. URIs
[1] 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
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 offers little
to no communication security:
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1. It does not protect the privacy 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.
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" [1]. His article inspired the mechanism described
in this document.
Many thanks to Tigran Mkrtchyan for his work on the DESY prototype
and his 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.
The authors are 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.
Lastly, special thanks to document shepherd David Noveck, 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: trond.myklebust@hammerspace.com
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Charles Lever (editor)
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
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