Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-10
Network File System Version 4 T. Myklebust
Internet-Draft Hammerspace
Updates: 5531 (if approved) C. Lever, Ed.
Intended status: Standards Track Oracle
Expires: 4 May 2021 31 October 2020
Towards Remote Procedure Call Encryption By Default
draft-ietf-nfsv4-rpc-tls-10
Abstract
This document describes a mechanism that, through the use of
opportunistic Transport Layer Security (TLS), enables encryption of
Remote Procedure Call (RPC) transactions while they are in-transit.
The proposed mechanism interoperates with ONC RPC implementations
that do not support it. This document updates RFC 5531.
Note
Discussion of this draft takes place on the NFSv4 working group
mailing list (nfsv4@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/nfsv4/. Working Group
information can be found at https://datatracker.ietf.org/wg/nfsv4/
about/.
This note is to be removed before publishing as an RFC.
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. Instructions are on that
page as well.
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."
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This Internet-Draft will expire on 4 May 2021.
Copyright Notice
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . 10
5.1.2. Protected Operation on UDP . . . . . . . . . . . . . 10
5.1.3. Protected Operation on Other Transports . . . . . . . 11
5.2. TLS Peer Authentication . . . . . . . . . . . . . . . . . 12
5.2.1. X.509 Certificates Using PKIX Trust . . . . . . . . . 12
5.2.2. Pre-Shared Keys . . . . . . . . . . . . . . . . . . . 14
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 14
6.1. DESY NFS server . . . . . . . . . . . . . . . . . . . . . 14
6.2. Hammerspace NFS server . . . . . . . . . . . . . . . . . 15
6.3. Linux NFS server and client . . . . . . . . . . . . . . . 15
6.4. FreeBSD NFS server and client . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7.1. The Limitations of Opportunistic Security . . . . . . . . 16
7.1.1. STRIPTLS Attacks . . . . . . . . . . . . . . . . . . 17
7.1.2. Privacy Leakage Before Session Establishment . . . . 17
7.2. TLS Identity Management on Clients . . . . . . . . . . . 18
7.3. Security Considerations for AUTH_SYS on TLS . . . . . . . 18
7.4. Best Security Policy Practices . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8.1. RPC Authentication Flavor . . . . . . . . . . . . . . . . 19
8.2. ALPN Identifier for SUNRPC . . . . . . . . . . . . . . . 20
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8.3. Object Identifier for PKIX Extended Key Usage . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Known Weaknesses of the AUTH_SYS Authentication
Flavor . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 25
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
In 2014 the IETF published a document entitled "Pervasive Monitoring
Is an Attack" [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 includes 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:
* Parts of each RPC header remain in clear-text, constituting a loss
of metadata confidentiality.
* 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:
* Per-client deployment and administrative costs for the only well-
defined alternative to AUTH_SYS are expensive at scale. For
instance, administrators must provide keying material for each RPC
client, including transient clients.
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* GSS host identity management and user identity management must
typically be enforced in the same security realm. However, cloud
providers, for instance, might prefer to remain authoritative for
host identity but allow tenants to manage user identities within
their private networks.
In view of the challenges with the currently available mechanisms for
authenticating and protecting the confidentiality of RPC
transactions, this document specifies a transport-layer security
mechanism that complements the existing ones. The Transport Layer
Security [RFC8446] (TLS) and Datagram Transport Layer Security
[I-D.ietf-tls-dtls13] (DTLS) protocols are a well-established
Internet building blocks that protect 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 and providing each with keying material.
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.
Compatibility: The imposition of encryption at the transport layer
protects any upper-layer protocol that employs RPC, without
alteration of the upper-layer protocol.
Further, Section 7 of the current document defines policies in line
with [RFC7435] which enable RPC-over-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 to mitigate against pervasive monitoring attacks
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[RFC7258]. Enforcing the use of RPC-over-TLS is of particular
importance for existing upper-layer protocols whose security
infrastructure is weak.
The protocol specification in the current document assumes that
support for ONC RPC [RFC5531], TLS [RFC8446], PKIX [RFC5280], DNSSEC/
DANE [RFC6698], and optionally RPCSEC_GSS [RFC2203] is available
within the platform where RPC-over-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 RPC-over-TLS. When an
RPC-over-TLS-enabled peer encounters a peer that does not support
RPC-over-TLS, policy settings on the RPC-over-TLS-enabled peer
determine whether RPC operation continues without the use of TLS, or
RPC operation is not permitted.
To achieve this interoperability, we introduce a new RPC
authentication flavor called AUTH_TLS. The AUTH_TLS authentication
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. The value of AUTH_TLS is defined in
Section 8.1.
An RPC client begins its communication with an RPC server by
selecting a transport and destination port. The choice of transport
and port is typically based on the RPC program that is to be used.
The RPC client might query the RPC server's RPCBIND service to make
this selection (The RPCBIND service is described in [RFC1833]). The
mechanism described in the current document does not support RPC
transports other than TCP and UDP. In all cases, an RPC server MUST
listen on the same ports for (D)TLS-protected RPC programs as the
ports used when (D)TLS is not available.
To protect RPC traffic to a TCP port, the RPC client opens a TCP
connection to that port and sends a NULL RPC procedure with an
auth_flavor of AUTH_TLS on that connection. To protect RPC traffic
to a UDP port, the RPC client sends a UDP datagram to that port
containing a NULL RPC procedure with an auth_flavor of AUTH_TLS. The
client constructs this RPC procedure as follows:
* 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.
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The RPC server signals its corresponding support for RPC-over-TLS by
replying with a reply_stat of MSG_ACCEPTED and an AUTH_NONE verifier
containing the "STARTTLS" token. The client SHOULD proceed with TLS
session establishment, even if the Reply's accept_stat is not
SUCCESS. If the AUTH_TLS probe was done via TCP, the RPC client MUST
send the "ClientHello" message on the same connection. If the
AUTH_TLS probe was done via UDP, the RPC client MUST send the
"ClientHello" message to the same UDP destination port.
Conversely, if the Reply's reply_stat is not MSG_ACCEPTED, if its
verifier flavor is not AUTH_NONE, or if its verifier does not contain
the "STARTTLS" token, the RPC client MUST NOT send a "ClientHello"
message. RPC operation may continue, depending on local policy, but
without confidentiality, integrity, or peer authentication protection
from (D)TLS.
If, after a successful RPC AUTH_TLS probe, the subsequent (D)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
RPC AUTH_TLS probe within an existing (D)TLS session, the RPC server
MUST reject that RPC Call by returning a reply_stat of MSG_DENIED
with a reject_stat of AUTH_ERROR and an auth_stat of AUTH_BADCRED.
Once the TLS session handshake is complete, the RPC client and server
have established a secure channel for exchanging RPC transactions. A
successful AUTH_TLS probe on one particular port/transport tuple does
not imply that RPC-over-TLS is available on that same server using a
different port/transport tuple, nor does it imply that RPC-over-TLS
will be available in the future using the successfully probed port.
4.2. Authentication
There is some overlap between the authentication capabilities of 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.
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:
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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 cannot
utilize the remote TLS peer identity to authenticate RPC users.
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 RPCSEC 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 largely redundant. An RPC implementation capable of
concurrently using TLS and RPCSEC GSS MUST use GSS-API channel
binding, as defined in [RFC5056], to determine when an underlying
transport provides a sufficient degree of confidentiality. RPC-over-
TLS implementations MUST provide the "tls-exporter" channel binding
type, as defined in [I-D.ietf-kitten-tls-channel-bindings-for-tls13].
5. TLS Requirements
When peers negotiate a TLS session that is to transport RPC, the
following restrictions apply:
* 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.
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* Implementations MUST conform to the recommendations for TLS usage
specified in BCP 195 [RFC7525]. Although RFC 7525 permits the use
of TLS v1.2, the requirement to use TLS v1.3 or later for RPC-
over-TLS takes precedence. Further, because TLS v1.3 ciphers are
qualitatively different than cipher suites in previous versions of
TLS and RFC 7525 predates TLS v1.3, the cipher suite
recommendations in RFC 7525 do not apply to RPC-over-(D)TLS. A
strict TLS mode for RPC-over-TLS that protects against STRIPTLS
attacks is discussed in detail in Section 7.1.1.
* Implementations MUST support certificate-based mutual
authentication. Support for PSK mutual authentication is
OPTIONAL; see Section 5.2.2 for further details.
* 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.
Similarly, 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 (for instance, if the
"ServerHello" message does not conform to the above requirements),
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.
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.
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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-over-TLS support for that RPC program on
that connection. Until an AUTH_TLS probe is done on a connection,
the RPC server treats all traffic as RPC messages. 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.
Reverse-direction operation occurs only on connected transports such
as TCP (see Section 2 of [RFC8167]). To protect reverse-direction
RPC operations, the RPC server does not establish a separate TLS
session on the TCP connection, but instead uses the existing TLS
session on that connection to protect these operations.
When operation is complete, an RPC peer terminates a TLS session by
sending a TLS Closure Alert. It 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
[I-D.ietf-tls-dtls-connection-id]. And, please remove this Editor's
Note before this document is published.
The use of the Datagram Transport Layer Security (DTLS) protocol
[I-D.ietf-tls-dtls13] protects RPC carried in UDP datagrams. As soon
as a client initializes a UDP socket for use with an RPC service, it
uses the mechanism described in Section 4.1 to discover RPC-over-DTLS
support for that RPC program on that port. If spurious traffic
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appears on a 5-tuple between the initial clear-text AUTH_TLS probe
and the DTLS association handshake, receivers MUST discard that
traffic without response.
Using DTLS does not introduce reliable or in-order semantics to RPC
on UDP. The use of DTLS record replay protection is REQUIRED when
transporting RPC traffic.
Each RPC message MUST fit in a single DTLS record. DTLS
encapsulation has overhead, which reduces the Packetization Layer
Path MTU (PLPMTU) and thus the maximum RPC payload size. A possible
PLPMTU discovery mechanism is offered in [RFC8899].
The current document does not specify a mechanism that enables a
server to distinguish between DTLS traffic and unprotected RPC
traffic directed to the same port. To make this distinction, each
peer matches ingress datagrams that appear to be DTLS traffic to
existing DTLS session state. A peer treats any datagram that fails
the matching process as an RPC message.
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-
over-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. Because
neither DTLS nor UDP provide in-order delivery, after session closure
there can be ambiguity as to whether a datagram should be interpreted
as DTLS protected or not. Therefore receivers MUST discard datagrams
exchanged using the same 5-tuple that just terminated the DTLS
session for a sufficient length of time to ensure that
retransmissions have ceased and packets already in the network have
been delivered. In the absence of more specific data, a period of 60
seconds is expected to suffice.
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.
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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.
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
X.509 certificates are specified in [X.509]. [RFC5280] provides a
profile of Internet PKI X.509 public key infrastructure. RPC-over-
TLS implementations are REQUIRED to support the PKIX mechanism
described in [RFC5280].
The rules and guidelines defined in [RFC6125] apply to RPC-over-TLS
certificates with the following considerations:
* Support for the DNS-ID identifier type [RFC6125] is REQUIRED in
RPC-over-TLS client and server implementations. Certification
authorities that issue such certificates MUST support the DNS-ID
identifier type.
* DNS domain names in RPC-over-TLS certificates MUST NOT contain the
wildcard character '*' within the identifier.
When validating a server certificate, an RPC-over-TLS client
implementation takes the following into account:
* Certificate validation MUST include the verification rules as per
Section 6 of [RFC5280] and Section 6 of [RFC6125].
* Server certificate validation MUST include a check on whether the
locally configured expected DNS-ID or iPAddress subjectAltName of
the server that is contacted matches its presented certificate.
* For RPC services accessed by their network identifiers (netids)
and universal network addresses (uaddr), the iPAddress
subjectAltName MUST be present in the certificate and MUST exactly
match the address represented by the universal network address.
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An RPC client's domain name and IP address are often assigned
dynamically, thus RPC servers cannot rely on those to verify client
certificates. Therefore, when an RPC-over-TLS client presents a
certificate to an RPC-over-TLS server, the server takes the following
into account:
* The server MUST use a procedure conformant to Section 6 of
[RFC5280]) to validate the client certificate's certification
path.
* The tuple (serial number of the presented certificate; Issuer)
uniquely identifies the RPC client. The meaning and syntax of
these fields is defined in Section 4 of [RFC5280]).
RPC-over-TLS 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, a set of allowed X.509v3 Certificate Policies, or
a set of extended key usages).
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 SHOULD reevaluate the certificate originally
presented in the context of the new configuration and terminate the
TLS session if the certificate is no longer trustworthy.
5.2.1.1. Extended Key Usage Values
Section 4.2.1.12 of [RFC5280] specifies the extended key usage X.509
certificate extension. This extension, which may appear in end-
entity certificates, indicates one or more purposes for which the
certified public key may be used in addition to or in place of the
basic purposes indicated in the key usage extension.
The current document defines two new KeyPurposeId values: one that
identifies the RPC-over-TLS peer as an RPC client, and one that
identifies the RPC-over-TLS peer as an RPC server.
The inclusion of the RPC server value (id-kp-rpcTLSServer) indicates
that the certificate has been issued for allowing the holder to
process RPC transactions.
The inclusion of the RPC client value (id-kp-rpcTLSClient) indicates
that the certificate has been issued for allowing the holder to
request RPC transactions.
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5.2.2. Pre-Shared Keys
This mechanism is OPTIONAL to implement. In this mode, the RPC peer
can be uniquely identified by keying material that has been shared
out-of-band (see Section 2.2 of [RFC8446]). At least the following
parameter of the TLS connection SHOULD be exposed at the RPC layer:
* PSK Identifier
6. Implementation Status
This section is to be removed before publishing as an RFC.
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 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
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
Maturity: Not complete.
Coverage: The bulk of this specification has yet to be implemented.
The use of DTLS functionality is not planned.
Licensing: GPLv2
Implementation experience: A Linux in-kernel prototype is underway,
but implementation delays have resulted from the
challenges of handling a TLS handshake in a kernel
environment. Those issues stem from the architecture of
TLS and the kernel, not from the design of the RPC-over-
TLS protocol.
6.4. FreeBSD NFS server and client
Organization: The FreeBSD Project
URL: https://www.freebsd.org
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.
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Licensing: BSD
Implementation experience: Implementers have read and commented on
the current document.
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. Section 8 of [RFC5280] covers important
considerations about handling certificate material securely.
Implementers should familiarize themselves with these materials.
Once a TLS session is established, the RPC payload carried on TLS
version 1.3 is forward-secure. However, implementers need to be
aware that replay attacks can occur during session establishment.
Remedies for such attacks are discussed in detail in Section 8 of
[RFC8446]. Further, the current document does not provide a profile
that defines the use of 0-RTT data (see Appendix E.5 of [RFC8446]).
Therefore, RPC-over-TLS implementations MUST NOT use 0-RTT data.
7.1. The Limitations of Opportunistic Security
Readers can find the definition of Opportunistic Security in
[RFC7435]. A discussion of its underlying principals appears in
Section 3 of that document.
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.
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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. Client
implementers can choose from the following to mitigate STRIPTLS
attacks:
* 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. When an opportunistic
security policy is in place, a client SHOULD check for the
existence of a TLSA record for the target server before initiating
an RPC-over-TLS association.
* 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. This
policy prevents an attacker from causing the client to silently
fall back to plaintext. 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 connection handshake information and
TLS certificate material that might enable attackers to track the RPC
client. Note that when PSK authentication is used, the PSK
identifier is exposed during the TLS handshake, and can be used 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.
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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, so
there is negligible privacy impact from this exception.
7.2. TLS Identity Management on Clients
The goal of the RPC-over-TLS protocol extension is to hide the
content of RPC requests while they are in transit. The RPC-over-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 credentials are 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
credentials 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 in
AUTH_SYS (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 in-transit insertion and deletion of RPC messages,
thus ensuring the integrity of the message stream between RPC client
and server. DTLS does not provide full message stream protection,
but it does enable receivers to reject non-participant messages. In
particular, transport layer encryption plus peer authentication
protects receiving XDR decoders from deserializing untrusted data, a
common coding vulnerability. However, these decoders would still be
exposed to untrusted input in the case of the compromise of a trusted
peer or Certificate Authority.
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
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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, when AUTH_SYS is used, the use of a TLS mutual
authentication mechanism is RECOMMENDED to prove that the RPC client
is known to the RPC server. The server can then determine whether
the UIDs and GIDs in AUTH_SYS requests from that client can be
accepted, based on the authenticated identity of the client.
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.
* When using AUTH_NULL or AUTH_SYS, both peers are RECOMMENDED to
have DNSSEC TLSA records, keys with which to perform mutual peer
authentication using one of the methods described in Section 5.2,
and a security policy that requires mutual peer authentication and
rejection of a connection when host authentication fails.
* RPCSEC GSS provides integrity and privacy services which are
largely redundant when TLS is in use. These services SHOULD be
disabled in that case.
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
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Flavor Name: TLS
Value: 7 (to be confirmed by IANA)
Description: Indicates support for RPC-over-TLS.
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
8.3. Object Identifier for PKIX Extended Key Usage
RFC Editor: In the following subsection, please replace XXX and YYY
with the IANA numbers assigned to these new entries. And, please
remove this Editor's Note before this document is published.
Per the Specification Required policy as defined in Section 4.6 of
[RFC8126], the authors request the reservation of the following new
values:
* The RPC-over-TLS ASN.1 module in the "SMI Security for PKIX
Extended Key Purpose" registry (1.3.6.1.5.5.7.3) (see
Section 5.2.1.1 and Appendix B.
* The RPC-over-TLS client certificate extended key usage
(1.3.6.1.5.5.7.3.XXX). The description of this new entry should
be "id-kp-rpcTLSClient".
* The RPC-over-TLS server certificate extended key usage
(1.3.6.1.5.5.7.3.YYY). The description of this new entry should
be "id-kp-rpcTLSServer".
IANA should use the current document (RFC-TBD) as the reference for
the new entries.
9. References
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9.1. Normative References
[I-D.ietf-kitten-tls-channel-bindings-for-tls13]
Whited, S., "Channel Bindings for TLS 1.3", Work in
Progress, Internet-Draft, draft-ietf-kitten-tls-channel-
bindings-for-tls13-00, 11 June 2020,
<https://tools.ietf.org/html/draft-ietf-kitten-tls-
channel-bindings-for-tls13-00>.
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", Work in Progress, Internet-
Draft, draft-ietf-tls-dtls-connection-id-07, 21 October
2019, <https://tools.ietf.org/html/draft-ietf-tls-dtls-
connection-id-07>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-38, 29 May 2020,
<https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[X.509] International Telephone and Telegraph Consultative
Committee, "ITU-T X.509 - Information technology - The
Directory: Public-key and attribute certificate
frameworks.", ISO/IEC 9594-8, CCITT Recommendation X.509,
October 2019.
9.2. Informative References
[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC Version 2",
RFC 1833, DOI 10.17487/RFC1833, August 1995,
<https://www.rfc-editor.org/info/rfc1833>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[RFC8167] Lever, C., "Bidirectional Remote Procedure Call on RPC-
over-RDMA Transports", RFC 8167, DOI 10.17487/RFC8167,
June 2017, <https://www.rfc-editor.org/info/rfc8167>.
[RFC8899] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
Völker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
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
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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. In addition, the
mapping of these integers to users and groups has to be
consistent amongst a server and its cohort of clients.
4. Because the user and group ID fields are not integrity-protected,
AUTH_SYS does not provide non-repudiation.
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Appendix B. ASN.1 Module
RFC Editor: In the following section, please replace XXX and YYY with
the IANA numbers assigned to these new entries. And, please remove
this Editor's Note before this document is published.
<CODE BEGINS>
-- OID Arc
id-kp OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) kp(3) }
-- Extended Key Usage Values
id-kp-rpcTLSClient OBJECT IDENTIFIER ::= { id-kp XXX }
id-kp-rpcTLSServer OBJECT IDENTIFIER ::= { id-kp YYY }
<CODE ENDS>
Acknowledgments
Special mention goes to Charles Fisher, author of "Encrypting NFSv4
with Stunnel TLS" (https://www.linuxjournal.com/content/encrypting-
nfsv4-stunnel-tls). 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. Also, thanks to Benjamin Kaduk for his expert guidance
on the use of PKIX and TLS. In addition, the authors thank the other
members of the IESG for their astute review comments. These
contributors made this a significantly better document.
The authors are additionally grateful to Bill Baker, David Black,
Alan DeKok, Lars Eggert, Olga Kornievskaia, Greg Marsden, Alex
McDonald, Justin Mazzola Paluska, Tom Talpey, Martin Thomson, and
Nico Williams, for their input and support of this work.
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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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