Internet DRAFT - draft-dnoveck-nfsv4-security-needs
draft-dnoveck-nfsv4-security-needs
Network File System Version 4 D. Noveck
Internet-Draft NetApp
Intended status: Informational C. Lever, Ed.
Expires: 25 July 2021 Oracle
21 January 2021
Security Needs for the NFSv4 Protocols
draft-dnoveck-nfsv4-security-needs-02
Abstract
This document discusses the inadequate approach to security within
the family of NFSv4 protocol specifications and proposes steps to
correct the situation. Because the security architecture is similar
for all NFSv4 minor versions, we recommend a single new standards-
track document to encapsulate NFSv4 security fundamentals, and
propose the introduction of several additional security-related
documents.
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-security-needs. 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/.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on 25 July 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2.2. Requirements Language as Used in This Document . . . . . 4
2.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Use of this Document . . . . . . . . . . . . . . . . . . . . 6
3.1. Current Use of this Document . . . . . . . . . . . . . . 6
3.2. Future Use of this Document . . . . . . . . . . . . . . . 7
4. Situation to be Addressed . . . . . . . . . . . . . . . . . . 8
4.1. NFSv4 Use Environments to be Addressed . . . . . . . . . 8
4.2. Emergence and Correction of Security Issues . . . . . . . 9
5. Major Problems to Address . . . . . . . . . . . . . . . . . . 12
5.1. Problems with Security Presentation/Organization . . . . 12
5.1.1. Problems with Presentation of Security
Architecture . . . . . . . . . . . . . . . . . . . . 13
5.1.2. Problems with Security Evaluation . . . . . . . . . . 15
5.2. The Treatment of AUTH_SYS . . . . . . . . . . . . . . . . 15
5.2.1. Current AUTH_SYS Security Policies . . . . . . . . . 16
5.2.2. Working Group Actions . . . . . . . . . . . . . . . . 18
5.3. Problems with Confidentiality . . . . . . . . . . . . . . 20
5.4. File Access Control . . . . . . . . . . . . . . . . . . . 22
5.4.1. File Content Integrity and Provenance . . . . . . . . 23
6. Framework for Correcting Problems . . . . . . . . . . . . . . 23
6.1. Correcting Problems with Regard to Threat Analyses . . . 24
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6.2. Correcting Problems with Regard to Use of Normative
Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7. Opportunities for Improvement Provided by Recent Work within
the RPC Layer . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Opportunities for Improvement in Encryption . . . . . . . 26
7.2. Opportunities for Improvement in Authentication . . . . . 27
7.3. Opportunities for Improvement when Using RDMA
Transports . . . . . . . . . . . . . . . . . . . . . . . 27
8. Issues that Need to be Addressed . . . . . . . . . . . . . . 28
8.1. Threat Analysis Goals . . . . . . . . . . . . . . . . . . 28
8.1.1. Background . . . . . . . . . . . . . . . . . . . . . 28
8.1.2. Issues to be Addressed . . . . . . . . . . . . . . . 30
8.2. NFSv4 Extension Policies . . . . . . . . . . . . . . . . 30
8.2.1. Background . . . . . . . . . . . . . . . . . . . . . 30
8.2.2. Addressing Extension Issues . . . . . . . . . . . . . 31
8.3. TLS Encryption Policies . . . . . . . . . . . . . . . . . 32
8.3.1. Background . . . . . . . . . . . . . . . . . . . . . 32
8.3.2. Issues to Address . . . . . . . . . . . . . . . . . . 33
8.4. Handling of AUTH_SYS . . . . . . . . . . . . . . . . . . 36
8.4.1. Historical Background . . . . . . . . . . . . . . . . 36
8.4.2. Background for Existing AUTH_SYS in NFSv4 . . . . . . 37
8.4.3. Core Issue to Resolve for AUTH_SYS . . . . . . . . . 38
8.4.4. Need to Better Document and Explain Issues with
AUTH_SYS . . . . . . . . . . . . . . . . . . . . . . 38
8.4.5. Issues to Address for Existing Use of AUTH_SYS . . . 40
8.4.6. Issues to Resolve for Revised Approach to AUTH_SYS . 43
8.5. Handling of State Protection . . . . . . . . . . . . . . 47
8.5.1. Background . . . . . . . . . . . . . . . . . . . . . 47
8.5.2. Issues to be Addressed . . . . . . . . . . . . . . . 48
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
10. Security Considerations . . . . . . . . . . . . . . . . . . . 49
10.1. Security Considerations Section for Eventual NFSv4-wide
Standards-track Security Document . . . . . . . . . . . 50
10.2. Security Considerations Section for Eventual
Rfc5661bis . . . . . . . . . . . . . . . . . . . . . . . 50
10.3. Security Considerations Section for Potential Revised RPC
Specification Document . . . . . . . . . . . . . . . . . 51
10.4. Security Considerations Section for Potential
Standards-track Document Dealing with AUTH_SYS . . . . . 52
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 52
11.1. Normative References . . . . . . . . . . . . . . . . . . 52
11.2. Informative References . . . . . . . . . . . . . . . . . 53
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
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1. Introduction
This document proposes specification changes that modernize NFSv4
security. These changes are necessary because the original goal for
the NFSv4 protocol, to enable secure file exchange on the open
internet [RFC2624], has not been effectively realized. Moreover,
existing approaches to NFSv4 security do not meet the security needs
of many contemporary restricted-access environments.
Restricting physical access to the network on which a client and
server interact is a common NFSv4 deployment technique. This
restriction limits access to users associated with a particular
organization but does not eliminate the need for protocol support
that enables completely secure operation. Section 4.1 contains a
taxonomy of specific environments and their corresponding security
needs.
As an example, the use of transport-layer security, including the
encryption of network traffic and the use of client host
authentication, can improve security when AUTH_SYS [RFC5531] is in
use, replacing the security approaches specified in [RFC7530] and
[RFC8881]. For a discussion of issues connected to shifting the
security approach of mature protocols, see Section 4.2.
The current document serves as a resource for the nfsv4 working group
as it updates current standards-track documents and proposes new
standards-track documents to address security issues within the NFSv4
protocol. For further details on the expected use of this document,
see Section 3.
2. Terminology
2.1. 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.
2.2. Requirements Language as Used in This Document
The current document is Informational. Therefore it cannot make
normative statements using the keywords defined in Section 2.1.
However, in discussing the treatment of security within the set of
NFSv4 specifications, there are occasions in which the current
document includes quotations from existing standards-track documents
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that use these keywords. In such cases, the definitions within
BCP 14 [RFC2119] [RFC8174] are to be adhered to when the terms appear
in all-capitals.
The current document also uses these keywords when exploring future
standards-track documents. Although these terms do not have a
normative effect in this setting, their meaning remains the same.
These statements can differ from and may contradict existing
normative statements for one or more of the following reasons:
* Earlier standards-track documents did not handle a security issue
appropriately.
* The community did not recognize a security issue when a standards-
track document was initially approved.
* An actual or potential security feature was not available at the
time standards-track documents were written and approved.
Changes in normative statements applying to implementations of a
particular NFSv4 minor version require special care to avoid creating
later difficulties. Problems might arise because:
* Implementers might choose to leave unimplemented changes that are
not mandatory-to-implement.
* A client implementation assumes that a particular server behavior
is foreclosed (e.g., by a "MUST NOT" or "SHOULD NOT"), but a
future change allows that behavior.
2.3. Glossary
More complete definitions to be filled in later. The security
definitions are based on [RFC3552] and [RFC4949].
Authentication Flavor
As defined in Section 8.2 of [RFC5531].
Confidentiality
Data is kept secret from unintended listeners. RPCSEC_GSS
sometimes uses the term "privacy" to mean the same thing. As
defined in Section 2.1.1 of [RFC3552].
Host authentication
The remote endpoint in the communication is the one we intended.
Also referred to as "peer authentication". As defined in
Section 2.1.3 of [RFC3552].
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Identity
A local trusted label for an object or resource, such as a network
address or a UID.
Integrity
Data that is received (or read) is the same data that was sent (or
written). As defined in Section 2.1.2 of [RFC3552].
Non-repudiation
Demonstration to a third party (or the same parties at a later
time) that both a sender's identity is certain and the data
received from that sender is the same as the data that was sent.
As defined in Section 2.2 of [RFC3552].
Trust
As defined in Section 4 of [RFC4949].
User authentication
Establish the truth that a user is who (s)he claims to be. As
discussed in Section 4.1 of [RFC3552].
3. Use of this Document
3.1. Current Use of this Document
This document is a means to facilitate discussion of the specific
inadequacies that need to be addressed within a new approach to NFSv4
security that we believe is required. For each specfic inadequacy,
as discussion proceeds, the treatment is expected evolve downward in
the following list to enable the working group to formulate text for
new standards-track documents to address the problem:
* The statement that a specific inadequacy exists and needs to be
dealt with.
* A listing of potential ways to deal with a specific inadequacy.
* A proposal by the authors that a specfic adequacy is best dealt
with a particular way, possibly accompanied by a list of other
reasonable aproaches.
* A statement that the working is expected to address a specfic
inadequacy in a particular way.
* A statement that the working is addressing a specfic inadequacy in
a particular way.
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While it is expected that, if the working group concurs, this
document will be adopted as a working group document, that transition
is not expected to affect the document's pattern of use. When
deciding on that issue the working group will not be considering the
specific propoals to address inadequacies, which can be addressed
later. The working group will be considering whether there are
serous inadequacies requiring new documents and whether such a
document will be helpful in addressing them.
The working group discussion anticipated, expected to be documented
in future versions of this document, will guide the writing of a
number of expected standards-track documents:
* A new standards-track document discussing security for all NFSv4
minor versions, updating [RFC7530] and [RFC8881]. It is intended
that this document be referred to by rfc5661bis when it is ready
for publication but it should be able to be published, on its own,
before that, since it would update [RFC8881].
* To address NFSv4.1-specific security issues, new text will be
needed in an anticipated rfc5661bis, updating [RFC8881].
draft-dnoveck-nfsv4-rfc5661bis [I-D.dnoveck-nfsv4-rfc5661bis] is
anticipated to be a precursor document whose drafting will be
affected.
In addition, the working group discussion anticipated might guide the
writing of a number of potential working group documents:
* A new standards-track document discussing AUTH_SYS, including
discussion of the potential use of client-host authentication.
* A new standards-track document updating [RFC5531], focusing on a
more complete and accurate treatment of AUTH_SYS.
3.2. Future Use of this Document
The pattern of use discussed above is expected to continue until the
decisions to be made are finalized. While the documents whose
drafting is to be guided, will be farly far into development, it is
not necessary for the working group to wait for publication of those
documents (or even WGLC) before scheduling WGLC for this one.
The question of possible publication of this document as an
informational RFC remains open. It should probably be discussed
shortly before WGLC. Given this uncertainty, if there is to be a
milestone associated with this document, the target should be WGLC.
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4. Situation to be Addressed
4.1. NFSv4 Use Environments to be Addressed
We will consider a range of network environments and how the type of
environment affects the need for the features being introduced to
provide for secure operation.
1. Within a data center local area network, there may well be
sufficient administrative controls over the software running on
machines with physical access to the network as to make
additional security features within NFS unnecessary.
If such a network is physically isolated or has all access to it
controlled using an appropriately configured firewall, it may be
acceptable, from a security point of view, to use AUTH_SYS
without client host authentication, and without encryption,
assuming all attackers have been excluded from access.
Although such networks typically do have firewalls, they are
generally configured to allow significant access, including NFSv4
access, to traffic originating outside the data center, although
not outside the owning organization. In such cases, we
effectively have a within-organization network, dealt with in
item 2 below.
2. Within a network devoted to a single organization or a single
site within an organization. Such networks often have associated
firewalls configured to exclude access from outside the
organization, and restrict it inside the organization.
It had until recently been assumed that, by excluding access to
those outside the organization, it could be assumed that
attackers would also be excluded. However, this approach ignores
the possibility of insider threats. Since we feel these threats
cannot be ignored, the security of NFSv4 needs to be upgraded to
prevent attacks by insiders, making the security needs in this
case more like those on the internet.
3. Use on the internet is the most challenging. While secure use on
the internet was a goal of NFSv4 and steps were taken to achieve
that goal, the approach required a number of steps, including a
significant performance penalty and a significantly different
approach to systems administration. Given that those
administering such systems were unwilling to adapt in this way,
we now need to meet the goal of secure use on the internet in a
new way.
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Secure use on the internet requires not only protection of user
data in-flight from unauthorized disclosure or modification, as
in case 2 above, but also effort to deal with the possibility of
extensive external monitoring and denial-of-service attacks.
4. Use of NFSv4 to access data located in the cloud poses many of
issues discussed in item 3 above. However, in general, the cloud
service provider is responsible for protecting multiple tenants
from one another, often involving use of tenant-specific
credentials. If the distribution of such credentials is quite
tightly limited, we can have a situation like that in item 1.
However, if those credentials are available to a larger group,
then the situation become like that in item 2, with insider
threats being a point of concern.
5. The use of NFSv4 within cloud-located data centers presents
different issues. If access to the NFSv4 service is suitably
restricted, the situation is quite close to that described in
case 1, with the physical location of the data center irrelevant.
However, if there means of external access that are not tightly
restricted, the situation becomes like that of case 2 above, with
the possibility of insider threats requiring a security model
with many similarities to that needed for the environments
described in item 3.
As will be discussed below, NFSv4 security, as currently defined, has
significant security issues in many of the above environments,
although not in all. No special provisions will be made for the more
restricted environments, except where necessary to avoid
incompatibilities when interacting with existing implementations.
By focusing on a single set of requirements for all of these
environments, there will necessarily be occasions in which some
security-related work is required that is not, strictly speaking,
needed for the particular environment in which it is used. However,
we have endeavored to keep the required effort small, eliminating
requirements for major administrative changes or compute-intensive
additions to the processing path.
4.2. Emergence and Correction of Security Issues
The fact that serious security issues exist in many of the
environment discussed above has made it necessary to consider taking
the drastic step of modifying security-related recommendations for
existing protocols. This situation seems to call for an explanation
and we will attempt to provide one.
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One possible form this might take would be to explain why [RFC7530]
and [RFC5661] made erroneous choices, with accompanying commentary
explaining the choices that should have been made instead.
Such an explanation does not appear to be possible, since it would
not have been possible for the authors and approvers of these
documents to reach the conclusions we have adopted without an
implausible level of foresight on their part. This is so even though
we would not adopt the choices made in [RFC7530] and [RFC5661] today.
Knowing what was known then, we might well have made the same
choices, although it is hard not to fault the lack of attention, in
the relevant documents, to the security consequences of the decisions
made.
Although we will call attention in sections below to things which
might have been done differently, this does indicate that we believe
that the authors and approvers of these documents, could have arrived
at conclusions similar to the ones we expect to choose now. Our
choices are so different from the ones made previously because they
take into account the needs of NFSv4 today and take advantage of
security facilities not available earlier.
In order to better understand this divergence, we have to look at the
history of NFS and take note of the context in which assumptions were
established with regard to security or changes in security guidance
occurred. In understanding this history and the choices of
RFC2119-defined keywords, the constraints on the working group need
to be understood, as described in Section 6.2
* The basic assumptions about security for NFS were established over
thirty years ago. At that time, it was generally taken for
granted that a co-operating set of UNIX kernels would provide a
uniform infrastructure to govern internode sharing within a
network. Within this framework, security concerns focused on
issues related to the appropriateness of granting or withholding
of privileges by UNIX kernels and there was little concern for
network security as understood today. As a result, NFS
administrators came to take for granted the use of AUTH_SYS making
it difficult to restrict its use, despite the security weaknesses
we perceive in it today. The assumptions underlying AUTH_SYS fit
so well with the way UNIX systems were managed in general led to
the use of AUTH_SYS becoming so well-entrenched that concrete
steps to eliminate it or to substantially restrict its use were
never seriously considered.
* When NFSv4 was developed, around twenty years ago, serious steps
were taken to improve the security situation by requiring the
implementation of RPCSEC_GSS. This included support for
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encryption, necessary to support use on the internet. AUTH_SYS
was retained as an additional optional means of authentication,
which would have allowed implementations to drop support at a
later point, although, as things developed, that became less and
less likely as use of AUTH_SYS remained quite common.
While it is possible to interpret the treatment of the matter in
[RFC3530] as implying that these two (i.e. RPCSEC_GSS and
AUTH_SYS) were equivalent and to find fault with that implication,
it is not clear how different choices would have led to a better
result. In particular, attempts to deprecate AUTH_SYS (e.g. by
using the phrase "SHOULD NOT") would have been problematic in that
they might have retarded adoption of NFSv4 rather than enhancing
adoption of RPCSEC_GSS.
* When NFSv4.1 was developed, there appeared to be no reason to
change the security decisions made for NFSv4.0. Although it might
have been possible to change the status of AUTH_SYS in NFSv4.1,
there was no reason to tie the session architecture to changes in
security that many implementers would have been unwilling to make.
The only significant security-related extension was the provision
made for state protection, which required the server to protect
clients from one another, apart from the users on whose behalf
requests were being made.
* In the ten years since [RFC5661] was published, there was no way
to address new security needs without creating an essentially new
protocol, on the order of NFSv4.1, and there was little interest
in doing that. Within the framework established by [RFC8178],
there was provision for the addition of OPTIONAL features, while
addressing lingering security issues for existing minor versions
did not fit in that framework.
With the above history in mind, we will look at why NFSv4 security
needs are different today and take advantage of some options that
were not available on earlier occasions in which NFSv4 security
decisions were made.
* We have the option of providing better security for AUTH_SYS use
by implementing client host authentication.
* We have the option of providing confidentiality and integrity even
when RPCSEC_GSS is not used.
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* We have the option of providing confidentiality and integrity
universally, without special configuration effort or interfering
with performance by requiring non-offloadable encryption/
decryption on the client and server.
* We have the option of providing state protection (to prevent
clients interfering with one another) without the special
implementation work specified in [RFC8881].
These options have been made available by the work at the RPC layer
described in Section 7 while the specifics of having NFSv4 take
advantage of these options are described in Section 8.
Providing a new security approach for multiple existing protocols is
potentially disruptive and due care will need to be taken to avoid
damaging implementation incompatibilities. However, as will be
discussed below, the existing situation, in which serious security
inadequacies need to be addressed, requires that significant changes
be made. It is our expectation that the situation will be resolved
by the eventual publication of a standards-track document updating
[RFC7530] and [RFC8881] and much of this document will concern itself
with determining how the needed changes can resolve existing security
issues without undue disruption.
5. Major Problems to Address
The problems to be addressed concern the way that security
information has been conveyed in earlier specifications (discussed in
Section 5.1) as well as two sets of substantive security weaknesses
discussed in Sections 5.2 and 5.3.
Although the inadequacies in the presentation of security issues have
contributed to prolongation of the substantive weaknesses and will be
discussed in the latter two sections, there is no reason to believe
that correction of the presentation problems, will, by itself,
improve the security situation, since the substantive problems still
need to be addressed. Nevertheless, correction of the presentation
issues is necessary so that the proposed solutions to the substantive
security issues receive the proper attention and analysis, both now
in connection with the changes to be proposed, and also going
forward, as needs change.
5.1. Problems with Security Presentation/Organization
While attention has previously focused on the deficiencies of the
Security Considerations sections (e.g. Lack of a threat analysis),
this is not the only presentation issue that needs to be addressed.
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Problems with the overall presentation of NFSv4 security,
particularly the discussion of the security architecture will be
dealt with in Section 5.1.1.
Problems with the evaluation of NFSv4 security including the lack of
a threat analysis and the contents of the Security considerations
sections will be dealt with in Section 5.1.2.
5.1.1. Problems with Presentation of Security Architecture
The presentation of the NFSv4 security architecture (for both minor
versions) focuses on the work done to make RPCSEC_GSS usable in view
of the basic architectural decisions made in going from NFSv3 to
NFSv4. These include the use of NFS4ERR_WRONGSEC and SECINFO to
allow the server namespace to be carved into regions with the use of
particular security flavors and services (associated with particular
quality-of- protection values) required in each one.
Unfortunately, features added later in the NFSv4.0 development cycle
were not integrated into the description of the security
architecture. A list of such noteworthy features follows.
Another consequence of the approach taken to writing Security
Considerations sections has been that these sections may have not
appropriately highlighted the security implications of new features
being added to the protocols.
* The addition of callbacks to NFS was not accompanied by an
explanation of how security for callbacks was to be dealt with, It
was never made clear whether the existing mandate to provide
support for RPCSEC_GSS applied to reverse-direction operation.
The fact that there is no reverse-direction complement to
NFS4ERR_WRONGSEC and SECINFO means that there is no way that the
client, as a responder, could force use of another flavor if it
did not provide support for RPCSEC_GSS authentication, in the role
of a responder.
The fact that callbacks are normally issued by the server itself
and not on behalf a specific user, means that the typical function
of RPCSEC_GSS authentication is not relevant to the application.
However, if RPCSEC_GSS is used in the forward direction, then the
mutual authentication of client and server should be adequate to
provide authentication of the server to the client, although the
possibility that an attacker might inject a spurious callback to
the reverse-direction request stream remains a concern.
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In any case, the absence of mention of reverse-direction
authentication in a description of the NFSv4 security approach is
troubling. The lack of mention in the Security Considerations
section is noteworthy because this creates a large set of requests
and responses for which there is no available facility to mandate
encryption, and that lack could be exploited by monitoring
attacks.
* The addition of pNFS to NFSv4.1 [RFC5661] required that there be a
means of providing that the proper quality of protection by
provided by the a pNFS data server implementing the files mapping
type. Given that SECINFO is namespace-oriented, it was necessary
to define SECINFO_NONAME, implementable by the data server, to
provide this functionality.
This security-related change, while explained adequately in
[RFC8881], is not mentioned in the description of the security
architecture or the Security Considerations section. As this
feature does create new threats, it should have been mentioned in
the Security Considerations section. Also, this change while
compatible with the existing NFSv4 security architecture, does
represent a considerable change to it. It seems to call for
special mention in a standards-track document devoted to NFSv4
security issues. It makes sense to include it in an introductory
section providing an overview of the NFSv4 security architecture.
* The implementation of sessions in NFSv4.1 [RFC5661] created a
requirement to protect clients from one another, even when they
were making requests on behalf of the same user. Although
adequate means of dealing with this issue were described in
[RFC5661], they were OPTIONAL features and implementation has been
very limited.
In this case, the additional exposure makes the absence of mention
of the issue in the Security Considerations section troublesome,
although its role in the limited implementation of the
corresponding security features is hard to evaluate at this point.
In any case, this was a major architectural change to the security
architecture which requires more attention. Over the years, there
has been considerable discussion on the NFSv4 mailing list of the
lack of authentication of the client, as opposed to the client
users on whose behalf it is acting, although no work has been
undertaken to make this a basic element of NFSv4 security.
Despite the delay, the issue now seems on its way to effective
resolution, using the work done in [I-D.ietf-nfsv4-rpc-tls], as
described in Section 8.5.
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As we have seen, the description of the security architecture of
NFSv4 is quite limited and a new security document will have some
gaps to fill.
5.1.2. Problems with Security Evaluation
The principal difficulty in the overall approach to security taken in
the Security Considerations section of [RFC7530] and [RFC8881]
concerns the absence of a threat analysis within these documents.
The absence of such an analysis has meant that:
* There was no occasion to determine whether the security
improvements made relative to earlier versions of NFS were
adequate to realize any particular reasonable interpretation of
the goal of "secure use on the internet", which goal was never
clearly defined.
* In the absence of a clear goal or a means of testing whether that
goal had been met, a choice of security facilities was made based
on their likely availability. The prevalent assumption that
security against network-based attacks was not important in most
NFS environments made it difficult to consider improvements, or to
forthrightly discuss existing security weaknesses and make plans
to address them.
5.2. The Treatment of AUTH_SYS
Sections 7 and 8.2 of [RFC5531] introduce the RPC auth_flavor
enumerator and a pair of opaque fields (the credential and the
verifier fields). These fields carry material in each RPC message
that can identify and authenticate the RPC client and the user who
initiated the RPC transaction. The enumerator field determines the
structure and content of the credential and verifier.
The simpler auth_flavors (e.g., AUTH_SYS) merely identify the
requesting user and sending host. These flavors do not provide any
cryptographic proof of identity. Therefore they do not provide true
authentication of the conveyed user and host identities.
The RPCSEC_GSS credential and verifier can both identify and
authenticate the requesting user. They can additionally provide
message integrity and confidentiality.
[RFC3530], [RFC7530], and [RFC8881] state that implementation of
RPCSEC_GSS is REQUIRED. However, they do not also mandate its use,
exposing an opportunity for attackers to issue unauthenticated
requests targeting NFSv4 servers in which RPCSEC_GSS is not the only
form of authentication available.
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Among the RPC auth_flavors that can present unauthenticated requests
to a server, AUTH_SYS is the most significant of these for many
reasons:
* Because the AUTH_SYS flavor is simple to implement and administer,
existing NFSv4 servers and client implementations make it almost
universally available.
* Unlike the AUTH_NONE flavor, whose name gives a fair warning that
requests are unauthenticated, [RFC5531] describes AUTH_SYS as a
means of authentication without addressing the question of what is
being authenticated and by whom.
* The substance of server and client implementations is not
described in standards-track documents, leaving much to
implementers' choice.
* There is no clear and complete description of the security
consequences of deploying AUTH_SYS RPC services.
* Even [RFC5531] designates AUTH_SYS as "insecure, all NFSv4 minor
versions continue to allow its use.
The everyday use of AUTH_SYS arises from NFS's early history and
development as part of the RPC authentication function and NFSv2
[RFC1094] and NFSv3 [RFC1813]. That approach, in which security was
made the responsibility of large multi-user clients trusted by
servers, might have made sense when it was initially developed, but
has become less relevant to NFSv4 security needs over time.
The designers of the NFSv4 protocol made RPCSEC_GSS mandatory-to-
implement. AUTH_SYS was left in place as an alternative without
considering the corresponding consequences for security. [RFC5531],
[RFC7530], and [RFC8881] mention several common practices that
servers have typically used to determine whether AUTH_SYS requests
should be accepted, but without explicit dicussion of their obvious
security weaknesses.
5.2.1. Current AUTH_SYS Security Policies
Current NFSv4 client and server implementations commonly provide the
following security policies. These policies attempt to improve the
security offered by AUTH_SYS. Because these policies are not
specified or discussed within standards-track documents, they have
not been subject to threat analysis. There is an opportunity to
describe and analyze security policies for AUTH_SYS and other
authentication flavors used with NFSv4 in an NFSv4-wide standards-
track security document.
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Network Address-based Access Control
A typical server policy, which first appeared with implementations
of earlier versions of NFS, is to limit the acceptance of AUTH_SYS
requests to requests from known clients, as determined by the
client's network address.
The Security Considerations section of [RFC7530] does not address
the possibility of address spoofing, although it does state that
this is "certainly not a safe model", most likely alluding to this
possibility. The text does not follow up to call into question
the use of AUTH_SYS as an OPTIONAL security flavor (stated earlier
in the form "MAY be implemented"). Instead, it is used as
justification for the mandatory nature of RPCSEC_GSS
implementation; the lack of security when only AUTH_SYS is in use
is not mentioned further.
Network Port-based Access Control
Another frequently-used server-side security policy is to restrict
the use of AUTH_SYS based on the associated client source port,
assuming that this ensures that the client's kernel (or a
privileged user space agent) has vetted the request. That
assumption has, over time, become dubious.
Appendix A of [RFC5531] refers to this practice as follows:
"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)."
Identity Squashing
NFS servers can implement a security policy that executes all
AUTH_SYS requests as a suitably unprivileged user ID, such as the
anonymous user. An alternate policy treats only requests from a
particular privileged user ID (e.g., root) this way.
Squashing does not prevent an attacker from masquerading as
another user but only limits the range of user identities that can
be assumed without difficulty.
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5.2.2. Working Group Actions
The Security Considerations section of [RFC5531] states that AUTH_SYS
is "known to be insecure" and further states "AUTH_SYS SHOULD NOT be
used for services that permit clients to modify data." Nevertheless,
AUTH_SYS has been commonly used by NFSv4 clients modifying data since
NFSv4 was first implemented, while the security consequences have
never been addressed.
Given the security difficulties that use of AUTH_SYS gives rise to,
the NFS community faces a choice between a few alternatives:
* Take steps to eliminate or otherwise minimize the use of AUTH_SYS
as a valid authentication flavor, either for NFSv4 as a whole or
for minor versions based on NFSv4.1, whose description was
published as a Proposed Standard after the publication of
[RFC5531], which stated that AUTH_SYS was "known to be insecure".
Given the many clients using AUTH_SYS, such steps might take the
form of recommendations, with the difficulty of switching
authentication models considered a valid reason to continue to use
AUTH_SYS as long as one is aware of the security consequences.
Regardless of the working group's choice of normative terms, it is
crucial that new NFSv4 standards clearly explain the security
consequences of using AUTH_SYS.
* Revise AUTH_SYS to enable secure implementation of an
authentication model in which authenticated client hosts
determine, within appropriate limits, the user IDs used for each
request sent.
To do this, servers would authenticate client hosts issuing
AUTH_SYS requests, taking advantage of the authentication provided
by [I-D.ietf-nfsv4-rpc-tls] as described in Section 7.2. A more
detailed discussion of issues to be addressed in establishing the
rules for this form of authentication appears in Section 8.4.6.
* Continue to pursue alternative RPC security mechanisms that are
simple to deploy and not costly to run. One such mechanism is
RPC-over-TLS, which should be available in time to include in
security documents published together with rfc5661bis. However,
we can easily imagine mechanisms based on other open federated
authentication standards that can be made available within NFSv4
via subsequent extensions.
The nfsv4 working group should consider all of these alternatives.
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The first of these is unlikely to be feasible; otherwise, the NFS
community might have already attempted such a step. The use of
AUTH_SYS has continued even though alternatives are available, and
there is no reason to expect that the use of the words "SHOULD NOT"
or "MUST NOT" would prevent implementers from continuing to support
it or administrators depending on such support, despite the negative
effect on the security of NFSv4 implementations
Because of the current predominant role of AUTH_SYS in existing NFSv4
implementations, providing better security while remaining within the
AUTH_SYS framework will also be difficult because of the issues
discussed earlier in Section 6.2. Nevertheless, we feel it needs to
be attempted as a new security framework cannot address AUTH_SYS
without clearly discussing its security weaknesses. Once that is
done, we need to take advantage of strong client host authentication
to provide a modicum of security while allowing administrators to
delegate credential-checking to trusted clients, as they are now
accustomed to doing.
This approach is far from ideal. It requires a trust relationship
between clients and servers. Adopting it provides a reasonable
approach to support NFSv4 in the near-term while allowing the
development of a better alternative (e.g., a new authentication
flavor) to proceed in parallel. Concerning our immediate need for
better NFSv4 security, the obvious alternatives might not be
realizable:
* Given the weaknesses of AUTH_SYS and the need to present a threat
analysis, we cannot commend AUTH_SYS without client host
authentication as OPTIONAL to use, either explicitly or by
implication.
* Recommending that AUTH_SYS without client host authentication not
be used, with RPCSEC_GSS as the only alternative, will not be
effective and would limit adoption of a better, although far-from-
ideal, alternative.
Therefore, the authors propose that upcoming NFSv4 standards-track
documents mandate or recommend that, when AUTH_SYS is in use, a form
of strong host authentication is always deployed along with it.
Ultimately this is a matter for working group decision, however. For
a further discussion of related issues, see Section 8.4.3. Although
the rest of this document assumes the authors' preference is
selected, Section 8.4.3 explains the changes to an eventual
standards-track document required if the working group concludes that
different choices are necessary.
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5.3. Problems with Confidentiality
Confidentiality is currently provided within NFSv4 by the use of
RPCSEC_GSS, with the server charged with enforcing any administrator-
specified needs to use these facilities on appropriate portions of
the server namespace. Requirements for the use of integrity
protection are handled similarly. This approach is capable of
providing confidentiality when accessing certain directories or file
systems, assuming that files that require such protection can be
isolated in certain regions of the server namespace.
An important difficulty with regard to this approach to
confidentiality is that there is significant non-encrypted data sent
on NFSv4 connections which can allow extensive data to be gathered on
the part of those engaged in monitoring attacks
* Certain parts of RPC requests and response are not encrypted and
can be the basis of traffic analysis. Fortunately, the structure
of NFSv4 requests limits the data exposed since the requested
operation is always COMPOUND (or CB_COMPOUND). Nevertheless, the
size of requests and information determinable from those allows
patterns of reading and writing data by specific clients to be
inferred.
* Because the focus of this approach is on areas of the server
namespace, there is no way to force use of encryption on requests
used to set up connections and sessions.
* Similarly, there is no provision for negotiating/enforcing the use
of integrity or encryption on reverse-direction requests.
Despite the availability of encryption in NFSv4, it is very rarely
used, which makes its formal sufficiency essentially irrelevant. In
understanding why confidentiality is not more generally used, we need
look at the issues below in order to understand how to address the
problem going forward.
* The cost is such that its use has a noticeable effect on
performance, given that the design (by requiring different keys
for different users) makes it difficult or impossible for the work
of encryption to be offloaded.
It is likely that, given increasing network speeds, this factor is
more important today than it was when this approach was settled
upon.
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* In many environments it might not be possible to isolate files
needing such protection in a way consistent with the way file
protections are normally managed. As a result, encryption would
be present for all files or for none, with none being chosen most
often because of performance concerns.
* Implementation of confidentiality requirements may face
difficulties due to uncertainty about whether client encryption
support is available. For example, [RFC7530] specifies that
privacy support is required (although only for krb5), while
[RFC8881] says only that clients "SHOULD" support privacy. The
document give no guidance regarding what might be considered valid
reasons to not support privacy, leaving servers without any
reliable way of demanding confidentiality on certain portions of
the server namespace.
* The concurrent use of AUTH_SYS might have a similar inhibiting
effect. While it is possible, within the protocol, to force a
transition to use of RPCSEC_GSS with privacy, RPCSEC GSS access
might not be possible in many cases (e. g. the proper id mapping
facilities might not have been configured).
* The current Security Considerations sections for the NFSv4
protocols may have contributed to the continuation of the problem
by not drawing sufficient attention to the security issues
involved in allowing access without encryption.
Looking at the relevant Security Considerations sections, we find the
following:
* Although the cost of performing encryption is mentioned as a
reason not to perform it, there is no discussion of the security
consequences of not performing it.
* Similarly, there is no discussion of the use of integrity services
in preventing modification of user data in flight and the data
corruption that attackers could cause when these services are not
used.
* There are a number of references to integrity protection for
various sort of operations but none connected with the protection
of user data, possibly suggesting to the reader that this issue is
not important. Ironically, while the operations used to obtain
security requirements for a given portion of the namespace are
among these mentioned, an attacker could not use these to obtain
transmission in the clear but would instead cause denial-of-
service, as would happen the bogus security parameters were
rejected.
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All of the issues discussed above may have contributed to the lack of
use of encryption with NFSv4, but the issues of poor performance due
to the a non-offloadable nature of encryption appears to have had a
critical role in creating an unsatisfactory situation for a protocol
where high performance is often of critical importance.
Once the choice was made to limit encryption to specific file systems
or directories, there was commonly a perceived need to avoid the cost
of encryption and the difficulties that approach to the matter led to
were exacerbated by the lack within the Security Considerations
sections of information about the damage thus done. Because there
was insufficient attention to these issues, effective action to
address them was delayed, until the problem could no longer be
ignored.
Given these weaknesses with regard to encryption, those needing to
use NFSv4 outside local area networks often implemented NFSv4 over
secure tunnels. As this work proceeded, it served as an inspiration
for the work done in [I-D.ietf-nfsv4-rpc-tls] that provided the
opportunity to provide encryption uniformly and potentially with high
performance. An overview of steps necessary to address these issues
appears in Section 7.1.
5.4. File Access Control
Originally, the NFS protocol provided only a basic form of access
control in the form of permission bits (also known as mode bits).
NFSv4 introduced a rich access control list mechanism which
supplements mode bit-based access control [RFC7530]. This is
referred to as a Discretionary Access Control (DAC) mechanism, where
access to the file's content is at the discretion of the file's owner
and is based on identity of the requesting user. A more complete
definition of DAC is available in Section 4 of [RFC4949].
The NFSv4.2 protocol provides a basis for supporting Mandatory Access
Control (MAC) in the form of Security Labels, as described in
Section 9 of [RFC7862]. Mandatory Access Control is generally
mandated from a central authority that constrains access to file
content based on broad trust categories. Because this usage
overloads the abbreviation MAC, which can also refer to Message
Authentication Codes and Media Access Control, we avoid using the
abbreviation unless the meaning is clear. A more detailed definition
of MAC is available in Section 4 of [RFC4949].
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Both of these file access authorization mechanisms need a threat
analysis and a discussion of exposures due to missing access control
facilities. However, without documentation of access semantics in
the case of Security Labels, any threat analysis might not be
complete.
5.4.1. File Content Integrity and Provenance
Ever since RPC has supported an integrity-protected mode of transport
(via RPCSEC_GSS integrity pseudoflavors), NFS has been able to
provide a strong guarantee that NFS messages in transit on a network
are immutable.
Recent work in block storage has extended the immutability of file
content from application to storage and back (for example, SCSI
commands related to Protection Information, as defined by the T10
technical committee). This provides a guarantee that what an
application reads at time T+1 is what an application wrote at time T.
The NFS protocol can and should enable this stronger form of
integrity guarantee.
Provenance is a deeper guarantee. It additionally identifies the
author of a file's content, and guarantees that content is exactly
what the author originally provided. An NFS client might use
verifiable provenance to gate access to a critical executable or
other resources contained in files stored on an NFS server.
Lastly, some systems provide a policy-based access control mechanism
based on whether file data content passes various integrity checks.
These capabilities require the storage and transit of additional
metadata associated with each file, which is currently not a part of
the NFSv4 protocol. Security Considerations sections should explain
how data is put at risk when these capabilities are not present.
6. Framework for Correcting Problems
This section addresses the fundamental issues of the organization and
presentation of a new security framework. The complementary issues
involved in making substantive improvements in NFSv4 security will be
dealt with in Section 7.
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6.1. Correcting Problems with Regard to Threat Analyses
Of prime importance is the inclusion of appropriate threat analyses,
even apart from the requirement (in BCP 72 [RFC3552]) that such
analyses appear in Security Considerations sections. As we have
already seen, without such an analysis, there is no way to determine
whether any security changes adopted are adequate to address NFSv4's
security difficulties. In the case of NFSv4, because of its
complicated history and the conflict between the status of AUTH_SYS
as specified by the NFSv4 specification documents ([RFC7530],
[RFC8881]) and that in [RFC5531], there might be multiple analyses,
located in different places:
* Because the security needs of the various minor versions are so
similar, the bulk of the analysis could appear in an NFSv4-wide
standards-track document updating [RFC7530] and [RFC8881].
* The case of a threat analysis for AUTH_SYS raises a special set of
issues. Currently, AUTH_SYS is discussed and declared "insecure"
within [RFC5531] with no threat analysis appearing within the
document. A new threat analysis could appear in the NFSv4-wide
security document referred to above but the fact that AUTH_SYS
might be considered as part of RPC rather than of NFSv4, might
dictate otherwise. Such an analysis might appear in a document
updating [RFC5531], or in a new document devoted to that specific
purpose. Whichever choice is made, there will be a need for a
document updating [RFC5531].
The appropriate location of a threat analysis for AUTH_SYS as
currently used depends on the question of whether to consider
AUTH_SYS as part of RPC, as it formally is, or to transfer
responsibility to NFSv4, since it was deprecated as insecure in
[RFC5531], while at the same time embedded as a heavily used optional
feature for all minor versions of NFSv4, including a new protocol,
NFSv4.1 published as a Proposed Standard subsequently, in [RFC5661].
Also relevant to the question of the appropriate location for this
threat analysis is the question of whether there might be a need for
a revised treatment of AUTH_SYS including client host authentication.
The possible existence of such a revised treatment, together with
issues related to a threat analysis necessary for its inclusion is
discussed in Section 8.4.4.
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6.2. Correcting Problems with Regard to Use of Normative Terms
Another important issue concerns the potential need for adjustment of
inappropriate use of terms defined by [RFC2119], particularly any
suggestion that use of AUTH_SYS may be validly used without important
security consequences, as suggested by the current understanding that
its use is optional.
However, as desirable as such clarifications might be, it is
necessary that we be realistic about what such changes can
accomplish, even if the particular issues cited in Section 2.2 are
properly attended to. Documents can be written to define certain
implementations as non-conformant, but the ability of any such
designation to affect implementer behavior is strongly affected by
circumstances, particularly when the term is a hard one to pin down
such as "SHOULD" or "SHOULD NOT".
The ability of such term choices to affect implementer behavior is at
its maximum when a new protocol is defined. In this case, the
specification document formalizes a contract between implementations,
so that the consequence of not following these terms is a lack of
interoperability rather the fact that one's implementation may be,
formally speaking, non-conformant. In the case of a protocol such as
NFSv4,intended to build and extend the work of earlier NFS versions,
the ability of the IETF to affect implementer choices is
significantly reduced, as interoperability with the existing protocol
and the existing infrastructure to support it will be of greater
importance to implementers. Further, it needs to be recognized that
when these terms are used to constrain security-related behavior
rather than interoperability per se, their effect on implementer
choice is likely to be similarly reduced.
Although [RFC2119] provides definitions of "SHOULD" and "SHOULD NOT",
it is difficult to determine the proper interpretation unless it is
clear what the "valid reasons" that might supersede the
recommendation or the expected consequences of doing so. For this
reason, when these terms are used in proposed text, we will make such
matters explicit, where they are not otherwise clear, as suggested by
Section 7 of [RFC2119].
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7. Opportunities for Improvement Provided by Recent Work within the RPC
Layer
The work done to provide TLS support for RPC (in
[I-D.ietf-nfsv4-rpc-tls]) presents an important opportunity to
address the substantive security problems described in Sections 5.2
and 5.3. While there will be a need to specify appropriate policies
for its use by NFSv4 as a ULP (see Sections 8.3.2 and 8.4.6 for
details), the following opportunities present themselves and are
likely to be taken advantage of:
* The use of transport-level encryption is a good way of dealing
with the inadequacies discussed in Section 5.3. The use of a
single key per connection makes offloaded implementations possible
allowing use for all requests including those currently not
encrypted. This is discussed in more detail in Section 7.1
* Authentication of the client host, as provided by the
authentication material presented at connection initialization
might be used to authenticate the client host, to avoid acceptance
of AUTH_SYS requests from unauthenticated clients. This is
discussed in more detail in Section 7.2
Although [I-D.ietf-nfsv4-rpc-tls]) does not provide support for RDMA
transports, parallel services to provide encryption and
authentication are expected to be available, as discussed in
Section 7.3.
7.1. Opportunities for Improvement in Encryption
[I-D.ietf-nfsv4-rpc-tls] provides the ability to encrypt all traffic
on the connection used between an NFSv4 client and server. This
requires that that both the client and server provide support for
RPC-over-TLS. Appropriate requirements/recommendations are discussed
in Section 8.3.2.
Because this facility is based on an opportunistic use of TLS, there
is a need for ULP-defined policies to deal with situations in which
the server rejected the request for a TLS connection. These matters
are discussed in Section 8.3.2 as well, as is the policy that servers
are to adopt when dealing with a connection on which RPC-over-TLS is
not requested.
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Because this new form of encryption is being added to an existing
protocol, all of these policies are more complicated than would be
the case for a new protocol, since there might be a need to
accommodate earlier implementations, which might not have had time to
be enhanced, while providing the ability to take advantage of
whatever confidentiality support that might be present for a given
client and server.
7.2. Opportunities for Improvement in Authentication
When establishing a connection, as provided for by
[I-D.ietf-nfsv4-rpc-tls], authentication material is provided by the
client and could be used to authenticate the client host to the
server.
By using this authentication material to authenticate the client
host, it would be possible to correct some of the issues discussed in
Section 5.2. This would involve clearly specifying how that material
is to be used, in contrast to the situation for the existing
AUTH_SYS, for which current documentation (in Appendix A of
[RFC5531]) is quite limited. Any such respecification would need to
be acceptable to those used to using AUTH_SYS while avoiding the
security issues that make its further use, in the old form,
inadvisable.
This could allow the following improvements:
* This would avoid dependence on the use of source request IP
address, which is insecure, given the possibility of address
spoofing.
* Authentication material could be separately defined for user and
kernel-mode clients to avoid dependence on the outdated privileged
port mechanism.
* More options could be provided to allow limitation of the user ids
upon behalf of which requests are made, to address the limitations
of "root squashing".
The details of how the authentication material could be used are
discussed Section 8.4.6.
7.3. Opportunities for Improvement when Using RDMA Transports
As noted above, [I-D.ietf-nfsv4-rpc-tls]) is not supported on RDMA
transports, making it necessary to provide appropriate support for
encryption and client host authentication in other ways.
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* Often RNICs used to implement RDMA transports will often implement
encryption to secure inter-RNIC traffic. When, as is often the
case, the inter-RNIC protocols are not standardized, the judgment
as to the adequacy of the encryption is out-of-scope, from our
point of view. However, the client and server could be configured
to take advantage of this encryption, when it is judged
appropriate to do so by those responsible. In addition, the
transport characteristics mechanism defined in
[I-D.ietf-nfsv4-rpcrdma-version-two] could allow those configuring
the client and server to make independent judgments on encryption
adequacy, with RPCSEC_GSS integrity and encryption only
superseded, when both agree. (See Section 8.3.2 for details).
* Authentication material to support client host authentication can
be provided by using the transport properties mechanism provided
for in [I-D.ietf-nfsv4-rpcrdma-version-two].
It should be noted that our need to normatively reference
[I-D.ietf-nfsv4-rpcrdma-version-two] would affect the schedule of a
standards-track document dealing with NFSv4 security. Given that the
current milestone for requesting publication is December 2020, this
not expected to pose an obstacle.
8. Issues that Need to be Addressed
These sections discuss the issues that need to be addressed by new
standards-track documents including both issues of the presentation/
evaluation of NFSv4 (discussed in Section 5.1) and the substantive
security weaknesses discussed in Sections 5.2 and 5.3. For each
issue there is a section providing background followed by information
suggesting how the issue would be addressed or issues that would need
to be resolved before the issue could be addressed.
8.1. Threat Analysis Goals
8.1.1. Background
For reasons that remain unclear, none of the specification documents
for the existing NFSv4 minor versions contain a threat analysis. As
a result, for the reasons discussed in Section 5.1, we need to
provide an appropriate threat analysis for all NFSv4 minor versions.
Because of the high degree of overlap between pairs of NFSv4 minor
versions, most of the analysis will be common to all versions.
However, there are some areas where features in later minor versions
have significant security implications:
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* In NFSv4.1 [RFC5661], SECINFO_NO_NAME was added, in large part
because it would be necessary to allow the data server, when the
pNFS file mapping type is used, to communicate to the client that
either integrity or encryption would be required when access to
file data was provided through the data server (as opposed to the
metadata server).
The use of TLS-based encryption, which we expect to be recommended
(see Section 8.3.2, may limit the need for this feature, but it
will remain REQUIRED, when file-based pNFS is implemented.
* NFSv4.1 [RFC5661] facilities were added to protect locking and
session-related state from modification by other servers (i.e.
SP4_MACHCRED and SP4_SSV).
These features have had very limited implementation work, so it
would be desirable to provide alternative means to achieve the
goal, as described in Section 8.5.
While the bulk of the threat analysis would be minor-version-
independent, differences between minor version will have to be noted,
as will differences that reflect uses of the facilities discussed in
Section 7.
Beyond the lack of NFsv4 threat analyses, there is a further problem
in that there is no threat analysis dealing with the AUTH_SYS case in
the existing RPC specification [RFC5531]. In this case, the gap is
easier to understand since RPC deals with security on a per-flavor
basis and declares AUTH_SYS as "insecure". It may well be that the
authors, the working group and the IESG thought this was adequate,
and it would have been if this had resulted in its not being used
subsequently. As things happened however, AUTH_SYS continued to be
used extensively, including for NFsv4, so that there still remains a
gap in this regard.
The following facts are relevant to the continued use of AUTH_SYS
despite its security problems:
* It was stated that AUTH_SYS "SHOULD NOT" be used for services
capable of modifying data. Given that its further use was still
anticipated, in some circumstances, a security analysis would
still be required. In addition, use within the purview of the
"SHOULD NOT" would make such analysis even more important, since
that recommendation would allow use presumably based on balancing
of possible benefits and corresponding problems. Without an
understanding of the security problems (i.e. the substance behind
the designation of AUTH_SYS as insecure), such balancing would not
be possible.
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* Despite the recommendation that AUTH_SYS not be used for services,
such as NFSv4, that are capable of modifying data, NFSv4 was
specified allowing such use with very limited attention to the
security issues that uses of AUTH_SYS gives rise to. As a result,
there remains a needs for an appropriate threat analysis including
for AUTH_SYS, since no matter what the working group decides to do
in this area, use of AUTH_SYS, in connection with all NFS
protocols (and possible other ONCRPC protocols) will continue for
some time.
* The reason stated for considering AUTH_SYS insecure, the lack of a
verifier, while worthwhile to note, does not touch the substance
of the problem, that unauthenticated requests are accepted and
potentially acted upon. The difficulty is further compounded by
the fact that the client validation checks made by actual
implementations are not fully described within RFC (They are
described above in Section 5.2).
8.1.2. Issues to be Addressed
It is expected that a threat analysis dealing with all NFSv4 minor
will be dealt with in new standards-track document dealing with NFSv4
security. Because this document will address security for all minor
versions, it will update [RFC7530], [RFC8881], and [RFC7862].
There will, in addition, be a need for a threat analysis dealing with
AUTH_SYS, which might or might not appear in the NFSv4-wide security
document for reasons explained in Section 6.1. The question of where
this best be done is discussed in Section 8.4.4.
8.2. NFSv4 Extension Policies
8.2.1. Background
The basic framework for the extension of NFSv4 was established by
[RFC8178]. Given that the anticipated standards-track document
dealing with NFSv4 security will, in a sense, extend NFSv4, there
might be some uncertainty about whether the changes anticipated are
in line with that extension framework and whether there might be a
need for that document to update [RFC8178].
The following questions need to be addressed:
* Whether the changes in policies anticipated here, with regard to
encryption and use of AUTH_SYS, are consistent with [RFC8178].
* How the use of new security-related facilities provided at the RPC
level relates to the extension framework established by [RFC8178].
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* How further extensions might relate to use of such new RPC-level
facilities.
* How to deal with and document extensions that provide new
security-related features.
8.2.2. Addressing Extension Issues
We anticipate that the material would be in a section entitled,
"NFSv4 Security Changes and the Existing NFSv4 Extension Model"
The following introductory material seems appropriate:
* The security-related changes recommended in this document are
outside the scope of the extension model presented in [RFC8178].
However, they do not contradict it and there is no need for this
document to update that one.
* Normally, changes requiring coordinated work on the client and
server are signaled using a new minor version. In that case, the
new minor version is essentially treated as a new protocol and
there is no need to update [RFC8178] or specifications for
existing minor versions.
The following paragraphs discuss the core issues:
* That approach is not possible in this case since the basic need is
to change the handling of existing minor versions to improve
security. This create the possibility of interoperability issues
since there is neither a new minor version nor a means of
distinguishing a consistent set of OPTIONAL features supported by
the server and known to the client.
* In order to limit the possibility of interoperability issues, the
security changes are made in the form of recommendations, but,
because there exist implementations built before these
recommendations were in effect, clients and server have to be
aware of the possibility that these recommendations might not be
followed. As a result, the effect of these recommendations, made
to limit security issues, is, from an interoperability point of
view, similar to those that might arise from OPTIONAL features.
With regard to the NFSv4 extension framework,
- The use of the new RPC-level facilities does not raise any
extension issues, since the existing specification do not limit
the transport used, except to require reliable and in-order
delivery. For this reason, these facilities can be used
without updating the existing minor version specifications.
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Use of these facilities does not raise additional
interoperability issues since [I-D.ietf-nfsv4-rpc-tls], by
using the opportunistic TLS model, provided for
interoperability between those that do and do not support these
extensions.
- The policies to use these facilities, made the responsibility
of the ULP by [I-D.ietf-nfsv4-rpc-tls] are significant
additions to existing minor versions, requiring that the new
standards-track document dealing with NFSv4 security update
[RFC7530] and [RFC8881].
These changes do not raise interoperability issues since the
policies apply to all uses of these transport-level facilities
for NFSV4. That addresses the case in which client and server
both provide support while other cases are dealt with as
indicated in the previous bullet.
- The new recommendations regarding use of existing features
(e.g. encryption, AUTH_SYS) often contradict existing guidance
and so also require that new standards-track document dealing
with NFSv4 security update [RFC7530] and [RFC8881].
These change do potentially raise interoperability issues which
need to be dealt with by giving sufficient scope, within the
framework of new recommendations, to accommodate existing
behavior. As a result, from an interoperability standpoint,
these changes are compatible with the previous designation of
OPTIONAL use, even though the security consequences make it
necessary that the use of these previously OPTIONAL facilities
(e.g. transferring data in the clear, use of AUTH_SYS without
client host authentication) be something to be warned against.
8.3. TLS Encryption Policies
8.3.1. Background
As discussed in Section 5.3, NFSv4 as currently defined, has serious
problems providing the appropriate level of confidentiality for its
transmissions. Providing encryption at the transport layer, whether
as described in Section 7.1 or otherwise, can be expected to resolve
this issue by providing encryption in a potentially offloadable way,
making use of RPC-level privacy and integrity services unnecessary.
If we were defining a new protocol, such transport-level encryption
could be REQUIRED without difficulty. However, given that we are
upgrading the security for an existing protocol, it might not be
possible to designate this encryption as "REQUIRED", since this might
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give rise to extensive interoperability problems between new and old
implementations. In any case, existing implementers of the existing
protocol would not have the same interest in specification compliance
as in the corresponding situation with a new protocol, leading to a
situation in which one group of implementations essentially ignored
the issue of specification-compliance with regard to the latest
protocol specifications.
We need to be aware that,
* In implementation of a new protocol, any requirements on the
client the server such as we have been discussing act
synergistically to marginalize non-compliant implementations.
Since non-compliant server will not interoperate with a compliant
client and vice versa.
* The corresponding case of a requirement providing for a security
upgrade, as in this case, is different. Implementers, and those
charged with allocating resources to their work will naturally
focus on improvement/maintenance of the working protocol and limit
the effort devoted to being the first to do an implementation that
would not be used until others do their part. In such cases, if
there are parties who are not convinced of the necessity for the
upgrade, it takes strong input from users/customers to ensure that
the needed upgrade is given sufficient attention.
* In some cases, a new protocol containing an important security
upgrade can result in a situation more like the latter case than
the former.
As we consider how to discuss the need for encryption in
Section 8.3.2, we will be mindful of the above. Since it not
possible to mandate the use of encryption, we will make it
RECOMMENDED. However, in discussing the valid reasons that
encryption might not be provided, we will try to restrict them to the
degree we can reasonably do so. Further, we will try to clearly
state the security consequences which implementers and those
configuring clients and server need to be aware of when choosing to
not provide the RECOMMENDED encryption.
8.3.2. Issues to Address
There is a need to recommend the implementation and use of
appropriate encryption by the server and client, as is done, for
example, by the following paragraphs.
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* Data sent over connections used to connect an NFSv4 client and
server SHOULD be encrypted, whether this uses TLS encryption, as
described in [I-D.ietf-nfsv4-rpc-tls] (to be used for TCP
connections) or another form of encryption. For example,
[I-D.ietf-nfsv4-rpc-tls] does not address encryption for RPC-over-
RDMA ([RFC8166], [I-D.ietf-nfsv4-rpcrdma-version-two]) but RNICs
used to implement RDMA transports will often implement encryption
to secure inter-RNIC traffic.
* Valid reasons for not implementing the encryption recommendation
include insufficient time and resources to code, test, and deploy
an appropriate implementation. Implementers and those deploying
NFSv4 servers and clients and those deploying NFSv4 services need
to remain aware of the consequences of not providing encryption.
These consequences are of concern regardless of the validity of
the reasons for not doing so, which may be expected to become more
restricted as time goes on.
* Given that it is necessary for both the client and the server to
provide implementations capable of transport-level encryption,
those deploying NFSv4 implementations need to be aware of possible
steps to provide remediation, the difficulties involved in
providing it and the consequences of not doing do.
* The lack of encryption for the connection as a whole can be
addressed by encryption of individual requests using the
facilities within RPCSEC_GSS specified in [RFC7530] and [RFC8881]
as providing "privacy". When doing so, the following complicating
issues need to be taken account of:
- Such facilities are not likely to be offloadable and often will
reduce performance dramatically.
- Support for confidentiality via encryption is not required by
[RFC8881] but only recommended, saying it "SHOULD be
supported". Unfortunately, the document does not specify what
might be valid reasons not to provide such support. Also,
although [RFC2119] states the one choosing not to follow such a
recommendation should be aware of the consequences, given the
general reticence of NFSv4 specification Security
Considerations sections about such matters, it is not clear
that implementers have been able to properly consider the issue
in the past. In any case, implementations without such support
probably exist, making encryption unavailable for some existing
client-server pairs.
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* When encryption is not provided, or, because of its effect on
performance, not used, all of the user data read or written is
transmitted in the clear, subject to interception or modification
in flight. While this is clearly unacceptable for use on the
internet, it should not be assumed that it is acceptable in more
restricted environments.
* Where this problem exists, it will often be necessary to restrict
such traffic to specific network segments, and make it impossible,
via administrative measures, to limit access to such network
segments or monitor them extensively.
Although, as indicated in [I-D.ietf-nfsv4-rpc-tls], that there should
be no need to negotiate security flavors to be used to provide
integrity and confidentiality, the exact effect on existing clients
and servers needs to be made clearer.
The text below is a suggested way of doing this.
* Although the pseudo-flavors created to indicate use of integrity
or encryption continue to be a part of NFSv4, when transport-level
encryption is provided, their use is unnecessary, although server
responses to SECINFO and SECINFO_NONAME may continue to include
them.
* When TLS is used to provide encryption, as specified in
[I-D.ietf-nfsv4-rpc-tls], the client and server will, as indicated
by that document, be aware that encryption, is present. In the
case of RPC-over-RDMA, when the RNICs provide encryption
transparently, the server and client have the opportunity to each
make their own judgment about the adequacy of the encryption
provided and communicate this to their peer as a transport
property as provided for by [I-D.ietf-nfsv4-rpcrdma-version-two]).
In either case, the client and server will only act, as described
below, on knowledge about the existence of encryption shared by
both parties:
- The server treats all requests as being made requesting
encryption, even if a different pseudo-flavor is used. As a
result, the server will never return, NFS4ERR_WRONGSEC because
of a quality-of-protection issue, despite a possible mismatch
between the pseudo-flavor specified in SECINFO response and the
one actually used.
The server will still return NFS4ERR_WRONGSEC when an
inappropriate security flavor or oid is used and clients need
to be prepared to use SECINFO to determine the security to be
used.
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- The client is free to ignore the quality-of-protection issues
in a SECINFO response, making use of authentication-only
variant of RPCSEC_GSS most efficient.
- Although use of integrity or privacy pseudo-flavors is
unnecessary on an encrypted channel, the server still needs to
accept such requests.
- Within NFSv4.1 [RFC8881], support for SECINFO_NONAME is still
REQUIRED when pNFS is supported, even if the server only
accepts connection on which encryption is used, making
generation of NFS4ERR_WRONGSEC on a connection to the metadata
server impossible.
8.4. Handling of AUTH_SYS
8.4.1. Historical Background
During the development of NFSv2 [RFC1094], little attention was
directed to network security and no attention was directed to the
possibility that a machine on the network might represent itself as a
client, with the ability to make requests on behalf of non-existent
client processes it identified by user id. During this time, NFS
clients were multi-user machines and it made sense to many for the
client kernel to have the same responsibilities to verify user
credentials and keep track of process user ids as it had successfully
been doing with local file systems. This was the origin of AUTH_UNIX
(later renamed AUTH_SYS) which was based on a model of co-operating
UNIX kernels and provided no protection against an attacker with
kernel privileges. Despite the later name change, actual
implementations were strongly focused on UNIX, deriving the necessary
implementation requirements from shared code and from informal inter-
developer communications, rather than standards documents. As a
result, as discussed in Section 5.2, when it was necessary to discuss
the security of AUTH_SYS in connection with RPC (in [RFC5531], there
was insufficient information on which to base a threat analysis,
although it was correctly concluded that the result of use was a lack
of security.
With regard to the following aspects of AUTH_SYS, this UNIX
orientation was significant. In many cases as the computing
environment changed, it was not possible to change the details to
keep up with evolving needs.
* As the assumption of kernel trustworthiness became increasingly
unviable as a basis for security, no action was taken to address
the issue. While the check for a privileged port retained some
residual value, it was used to exclude requests issued by non-
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kernel clients as inherently untrustworthy. The more significant
and difficult problem of distinguishing trustworthy and
untrustworthy kerenel-level access was never addressed, leaving
AUTH_SYS of little value as a means of authentication, despite its
continuing use.
* AUTH_SYS was focused on the use of the non-hierarchical 32-bit
user id space typical of UNIX. Although later attempts were made
to generalize this within NFSv4, the fact that this approach is
embedded in many server filesystems meant that this aspect of
AUTH_SYS, does not seriously obstruct access to such servers.
* The particular user id of zero (denoting "root" in UNIX) was often
treated specially, by treating the request as issued by the
predefined user id "nobody". While this was undoubtedly helpful
when first defined, it is now the case that other non-root users
might have significant privileges and that the ability of an
attacker to assume any chosen user id allows much important
information to be obtained and corrupted.
Later, with the development of NFSv3 [RFC1813], some of these
assumptions started to seem less justifiable and alternate
authentication models were developed. However, because AUTH_SYS was
so suited to the Unix environment, making NFS administration so
similar to local file system administration, its use remained
predominant despite its obvious weaknesses from the security
standpoint.
8.4.2. Background for Existing AUTH_SYS in NFSv4
When NFSv4 was first defined (in [RFC3530]), RPCSEC_GSS was added as
a mandatory-to-implement means of authentication which was optional
to use, presumably under the assumption that other optional-to-
implement authentication flavors, such as AUTH_SYS would be used by
some clients
The continued use of AUTH_SYS is troubling and hard to explain if one
is focused on network security, but the following possibilities are
worthy of note:
* It might well have been felt that the problems with AUTH_SYS,
although troubling, did not really apply to the most common use
case for NFS, on local area networks.
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* The familiarity of AUTH_SYS to Unix administrators, and general
consistency with existing UNIX methods of system administration
may have made it difficult to contemplate its abrupt removal in
NFSv4.0, with the expectation that doing so might have
substantially impeded NFSv4 adoption.
Regardless of why this happened, the important fact is that it did
happen, despite the serious security problems that it gave rise to.
Later subsections of Section 8.4 will discuss ways to address the
resulting situation.
8.4.3. Core Issue to Resolve for AUTH_SYS
Given the serious security weaknesses described in Section 5.2, there
is clearly a need to discourage its further use in its existing form,
since the difficulties with it will not be eliminated by the adoption
of encryption of NFSv4 connections alone
There are two basic forms that such an effort might take:
* Seek to discourage the use of AUTH_SYS in the expectation that it
will be eventually replaced by RPCSEC_GSS or a new security
flavor.
* Seek to eliminate or mitigate its security problems and to
discourage the use AUTH_SYS variants that have not addressed the
underlying problem, in favor of a revised approach to AUTH_SYS
with better security, based on client host authentication.
While the authors are strongly of the opinion that the second choice
is more likely to be successful, and this document reflects that
view, there is no intention to foreclose the first option, should the
working group choose it. If that option is chosen, then the material
in Section 8.4.6 becomes irrelevant and can be ignored while much of
that in Section 8.4.5 can be simply modified to expand its scope, as
described below in that section.
8.4.4. Need to Better Document and Explain Issues with AUTH_SYS
As described in Section 8.1 there is a need for an appropriate threat
analysis for NFSv4. For a number of reasons, issues with AUTH_SYS
that would make it difficult to refer to in an NFSv4-wide security
document without some additional work preparatory work:
* Unlike the case of RPCSEC_GSS, there is no discussion of possible
security threats.
* There is no complete documentation how AUTH_SYS is to be used.
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* The reason for deciding that AUTH_SYS is "insecure", while
probably valid, adds considerable confusion since it makes it
harder to address the issue complained of.
Under other circumstances, it might have been possible to dispense
with further analysis of AUTH_SYS because of the following facts:
* AUTH_SYS is declared "insecure", which might be considered as
making a further security analysis beside the point.
* There are a number of statements with [RFC5531] limiting the use
of AUTH_SYS, which might lead a reader to conclude that it was, in
essence, a historical artifact.
Despite the above, we will need a more complete of treatment of
AUTH_SYS security for a number of reasons.
* Despite the recognition of its insecurity and restrictions on its
use, AUTH_SYS has been at least on a par with RPCSEC_GSS in terms
of NFSv4 use, for over a decade following.
* Given that the ongoing security issues with AUTH_SYS might give
rise to efforts to address them, an accurate understanding of
these issues is even more important, regardless of how the working
group chooses to resolve the issue discussed in Section 8.4.3.
If the decision is made to improve the security characteristics of
AUTH_SYS, it is important to understand existing security issues
and how they might be best addressed.
If the decision is made to eliminate the use of AUTH_SYS, an
adequate understanding of the security issues that made that
decision necessary would seem to be required. In that context,
the current statement in [RFC5531], regarding the insecurity of
AUTH_SYS needs to be clarified.
* It is likely that attempts to eliminate or restrict use of
AUTH_SYS will take the form of a recommendation that it not be
used or only be used in certain ways or under certain
circumstances. Such recommendations normally require that the
user be made aware of the consequences of doing something other
than what is recommended. In this case that would be either to
use AUTH_SYS or to use it without client host authentication.
An important question is where such a treatment should appear. In
deciding this question, the important question is whether, at this
point, it is best addressed as a feature associated with NFSv4 or
with RPC.
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* While structurally, AUTH_SYS, presented as one of security
supported security flavors associated with RPC would seem to
require treatment within the framework of RPC, its recent
treatment raises questions about continuing to deal with it solely
within this framework. This is not due solely because of the
attempt (in [RFC5531]) to discourage its use, as was done for
AUTH_DH. In that case it remains appropriate to discuss it as
part of RPC, without a threat analysis, since there is no
continuing use justifying another course.
* The continued use of AUTH_SYS by all NFSv4 minor versions might
well be considered to justify a transfer of responsibility to
NFSv4, with the authentication flavor being mentioned, in RPC
specifications, only as a historical artifact.
The potential continued use of AUTH_SYS with client-host
authentication strengthens the NFSv4 connection. Since
[I-D.ietf-nfsv4-rpc-tls] assigns the specification of policies
related to authentication to the ULP, it seems that AUTH_SYS needs
to be discussed by NFSv4 documents in a manner more substantial
than simply listed as another authentication flavor.
* AUTH_SYS, both with and without client host authentication could
be discussed in its own document, including an appropriate threat
analysis.
That document could be referenced by a new document updating or
obsoleting [RFC5531]. This would provide an opportunity to
address existing text which was essentially ignored by NFSv4.
That document could also be referenced by a new standards-track
document dealing with NFSv4 security. The threat analysis would
provide a basis for the whatever approach the working group
decides to take with regard to the discussion of AUTH_SYS with
client-host authentication. See Section 8.4.6 for further
details.
The last of these is most likely to be adopted regardless of how the
working group decides the issue described in Section 8.4.3. However,
if the working group decides to strongly discourage the use of
AUTH_SYS even with client-host authentication, the first is a
possibility that should be considered.
8.4.5. Issues to Address for Existing Use of AUTH_SYS
This section presents material that might be included in a standards-
track document in an effort to suitably discourage use of AUTH_SYS in
its current (insecure) form.
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While it is sometimes the case that such a task can be accomplished
by substituting one of the terms defined in [RFC2119] by another,
that is not the case here While the designation of AUTH_SYS as
optional (to implement) with [RFC7530] and [RFC8881] using the
language "MAY be implemented", is wrong and probably needs to be
fixed, the situation is more complex than generally recognized. It
should be kept in mind that these terms are, in many cases, ignored
or followed while ignoring the underlying intent. As a result, such
changes, without supporting explanations might not be effective in
changing implementer behavior.
To summarize the confusing situation with AUTH_SYS and NFS and the
role of the RPC specification, [RFC5531].
* At the time of publication of [RFC5531], AUTH_SYS was extensively
used by NFSv2 [RFC1094], NFSv3 [RFC1813], and NFSv4.0 [RFC3530].
* The Security Considerations Section of [RFC5531], states "AUTH_SYS
SHOULD NOT be used for services that permit clients to modify
data", which all versions of NFS clearly are.
* Section 10 of [RFC5531] states "Implementors MAY include AUTH_SYS
in their implementations to support existing applications."
Although this statement was probably intended to authorize
continued use of AUTH_SYS, the contradiction between this
statement and the Security Consideration section is not noted so
there is no way to determine how this contradiction is to be
resolved or precisely what "an application" is.
* The case of NFsv4.1, later defined in [RFC5661], is different
since, being a separate protocol, with additional vulnerabilities
when AUTH_SYS is used, it would presumably not be an "existing
application".
* Turning from [RFC5531] to [RFC8881] and [RFC7530], we find
AUTH_SYS described as an OPTIONAL means of authentication, with
its security weaknesses not being discussed or treated as a
significant concern.
While there may well be sufficient loopholes within [RFC5531] to
avoid an outright contradiction of the rules established there, it is
certainly troubling that the security implications of implementing
and using AUTH_SYS are not properly recommended against.
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While we may need to adjust the normative language, the effect of
doing so is limited for reasons that have already been discussed. As
a result, we need to put substantial emphasis on clearly stating the
consequences of doing what "SHOULD NOT" (or "should not") be done, as
is suggested by [RFC2119], at least for the former case.
The following text is one way to present the situation:
* The use of AUTH_SYS, as described (incompletely) in Appendix A of
[RFC5531] is quite detrimental to security and so SHOULD NOT be
used.
* This is so because it involves accepting requests from clients on
behalf of specific users without authentication of the clients
themselves. In essence, the task of authentication is delegated
to NFSv4 client implementations, with the server accepting such
authentication decisions. Normally, such an arrangement would
require the establishment of a trust relationship between client
and server.
* Typically, when using AUTH_SYS, use of a privileged port
represents the client kernel's judgment (by granting root access)
that the client proper is to be trusted but there is no good
reason to trust the client kernels in this regard.
* Although [RFC8881] and [RFC7530] previously treated implementation
of AUTH_SYS as optional, it appears that this treatment was in
error in that it focused on the relevant interoperability
constraints without attention to the security consequences of use
(discussed in Section 7 of [RFC2119]). While use of AUTH_SYS was
not characterized using any of the keywords defined in [RFC2119],
it was reasonable for implementers to assume this was valid given
the lack of attention to network security issues and the fact that
there would be no point in implementing support for AUTH_SYS in
servers if it could not be used.
* Although, as a result of these previous approaches to AUTH_SYS
without client host authentication, it might be used extensively
despite the security issues that cause us to specify that it
"SHOULD NOT" be used are sufficient to justify this shift.
However, due to the difficulty of making such a shift, the need to
maintain continuity of support is a valid reason to continue to
use it in this way, as long as this decision is made with
awareness of the security consequences:
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- That it is possible for an attacker to impersonate a trusted
client by using its IP address in source IP address of packet
sent, enabling to send its own requests to read or modify data,
as any particular user.
- That compromising any client whose AUTH_SYS requests are
accepted allows it to create a connection from which its
unauthenticated requests will be executed by the server.
- That it is possible for attackers to work around "root
squashing" since it can assume any identity it intends to,
other than that of root.
8.4.6. Issues to Resolve for Revised Approach to AUTH_SYS
The availability of client-host authentication makes possible a
revised approach to AUTH_SYS addressing some of its security
vulnerabilities. A revised approach to AUTH_SYS might provide:
1. The ability to prevent, using client host-authentication, another
machine masquerading as the expected client.
The authentication features described in
[I-D.ietf-nfsv4-rpc-tls], would be assure that you could identify
trusted client machines, eliminating a major security issue with
AUTH_SYS. On the other hand, it is not clear whether servers
would be wise to trust clients to this degree, especially if the
items below (#2 and #3) are not satisfactorily dealt with.
2. It would be possible to include, as part of the client
authentication material, information known only to the client and
the server. The secret value would be chosen by the client, and
saved so it is usable by later client instances. If this were
done, it would prevent any process with root privileges on a
trusted machine from impersonating the trusted client. If this
were not done, then any compromise of a single trusted client
would compromise any server which trusted it to present AUTH_SYS
requests.
To make this approach viable, servers would have to maintain, in
persistent storage, a record of the secret value assigned to each
authenticated client. When an authenticated client connected
with a different value it would indicate that a root process on
the client-host was attempting to impersonate the NFSv4 client.
3. The power granted to the authenticated client-host, to make a
request on behalf of user or group has troubling consequences.
Although most implementations have the ability to exclude
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requests from the root user, that may not, as has been noted
previously, be sufficient to satisfactorily deal with the
problem, since many deployments contain security-critical files
that can be accessed and modified but users other than root.
It would be desirable to exclude a wider range of users such as
all users within a given group (or alternatively include only
users within a given group). However, neither of these is viable
since servers receiving AUTH_SYS requests typically have no
knowledge of what users belong to what groups, relying on the
AUTH_SYS client to provide this information within the request.
Given this lack of knowledge on the part of the servers, any
restrictions on the ability of AUTH_SYS requests to access or
modify files designate a set of protected files in some way.
Since designating a list of files is complicated and hard to
maintain, it might be possible to describe the files whose access
and/or modification by means of an ACL. When a request is made
to access a file, that request is suppressed (i.e. treated as a
request by "nobody") iff it would be allowed to access by the
designated ACL and similarly in the case of modifying requests.
How to address the question of the potential use of AUTH_SYS with
client-host authentication would depend on the working group's
judgement about how likely such an arrangement would be likely to be
in providing security when AUTH_SYS is used. This in turn would
depend on the working group's judgment as to whether solutions for #2
and #3 could be defined and effectively deployed, as well as the
results of the threat analysis
We assume that AUTH_SYS with client-host authentication needs to be
considered inferior to RPCSEC_GSS but sufficiently superior to
AUTH_SYS without client authentication to make it desirable to
encourage its use, given that it is impossible to eliminate use of
AUTH_SYS, even with a "MUST NOT". A number of potential
introductions to the topic, of varying tone are explored below:
* If the working group wants to focus on the improvement relative to
AUTH_SYS without client-host authentication, the following
paragraphs might be an appropriate introduction:
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When AUTH_SYS is used with client-host authentication, then the
server, by allowing use of AUTH_SYS for that specific client, is
essentially delegating the user authentication task to that client
who is trusted by the server to perform that task locally, just as
typically done when authenticating users for the purpose of local
file access, even though the scope of damage that could be enabled
by any false authentication is likely to be much wider in this
case.
By accepting AUTH_SYS requests from a particular client, a server
is relying on the security of the client for the security of all
data stored on the server. This is in contrast to RPCSC_GSS, in
which authentication is provided for each request issued. Because
of this, the use of AUTH_SYS should only be allowed from client
hosts that can reasonably be trusted.
* If the working group wants to adopt a more cautionary tone
regarding use of AUTH_SYS with client-host authentication, the
following paragraphs might be an appropriate introduction:
By accepting AUTH_SYS requests from a particular client, a server
is relying on the security of the client for the security of all
data stored on the server. This is in contrast to RPCSC_GSS, in
which authentication is provided for each request issued. Because
of this, the use of AUTH_SYS should only be allowed from client
hosts that can reasonably be trusted and whose internal security
is sufficiently robust that the additional risk to server data can
be considered reasonable.
* If the working group wants to focus more on encouraging use of
RPCSEC_GSS, then on improving the security of those choosing to
use AUTH_SYS, a paragraph like the following might be appropriate.
While the use of AUTH_SYS with client-host authentication
eliminates some of the major security issues that arise from use
of AUTH_SYS, it is still inferior, from a security point of view,
to RPCSEC_GSS. As a result, its use should be limited to
situations in which, use of RPCSEC_GSS is not a practical
possibility.
The following paragraph might be added if item #2 is not successfully
addressed:
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* It should be noted that any security weaknesses in the client host
kernel which might allow an attacker to execute in privileged
mode, are likely to allow that that attacker to present AUTH_SYS
requests to the server and have them executed without any
authentication beyond verifying that they were issued by a trusted
host.
The following paragraph might be added if item #2 is successfully
addressed:
* In order to assure that it is not possible for a process running
as root to successfully masquerade as a previously active kernel-
based client on that machine, user-level clients even privileged
ones, will present distinct authentication material from a kernel-
based client and from each other. Servers SHOULD maintain,
persistently, sufficient information so to be able to detect a
situation in which process with root privileges on a trusted
server masquerades as a client previously recognized on that and
judged secure.
The following paragraph might be added if item #3 is not successfully
addressed:
* There is no standardized way the server can effectively limit the
range of client user identities and group memberships assumed by
AUTH_SYS requests except for the commonly implemented practice of
"root squashing".
The following paragraph might be added if item #3 is successfully
addressed:
* Server SHOULD provide a means by which specific sets of sensitive
files are made inaccessible or unmodifiable by use of AUTH_SYS
requests, in order to limit the damage that could occur in the
event that a client compromise allows unauthenticated requests to
be generated and acted upon. One desirable way of providing such
a facility would be to allow an ACL to be associated with each
authenticated client host (e.g. by designating a file whose ACL is
to be used). If this is done any request whose user together with
a group set would allow it to be accepted by the specified ACL
would be rejected.
The following closing paragraphs are proposed for an eventual
introduction to AUTH_SYS, as revised. In these paragraphs, material
to be added only if a particular item above is addressed or not
addressed will appear in brackets with the prefix "if-#x:" or "if-
not-#x:".
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* The ability to make AUTH_SYS requests of a server SHOULD be
limited to those cases in which the client host making those
requests can be trusted to appropriately verify a user's
credentials (e.g. passwords) before issuing RPC requests on that
user's behalf. [if-#3: This limitation should be applied even when
the set of users or owning groups is restricted to specially
protect files deemed sensitive.] [if-not-#3: When not so limited,
requests purportedly from any user except possibly root, can be
synthesized and acted upon, without effective authentication.]
* For that reason, accepting AUTH_SYS requests from a large set of
client hosts is a practice to be avoided, with individual hosts
included where their security has been verified. The only
possible exception is where a set of policies is in place which
effectively limits the kernel software allowed to run on those
hosts to ensure that those hosts appropriately providing user
authentication [if-not-#2: and capable of excluding untrusted code
from masquerading as an NFSv4 server by assuming root privileges.]
8.5. Handling of State Protection
8.5.1. Background
The inclusion of state management within NFSv4 created new security
issues in which the objects requiring protection were not files
associated with particular users but locks, opens, and delegations,
associated with particular clients.
In NFSv4.0, some protection could be provided when RPCSEC_GSS was
used by limiting modification of state objects to users having access
to the associated file. Nevertheless, the fundamental issue remained
unresolved.
When sessions were introduced into NFSv4 (in NFSv4.1) there were
additional vulnerabilities that needed to be protected against.
There needed to be protection to prevent a session established by one
client being bound to a connection established by another client,
leading to the following possibilities:
* The existing session could be destroyed.
* The associated clientid could be destroyed.
* The existing session slots could be used resulting in the
legitimate owner of the session having his requests rejected
because his slot sequence values had been rendered invalid because
of the interloper's requests using the same slots.
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Facilities were defined in [RFC5661] to allow such client-based
protection to be implemented: SP4_MACHCRED and SP4_SSV but
implementation has been quite limited. Client-host authentication
makes it possible to apply such protection more generally, as
described below.
8.5.2. Issues to be Addressed
While transport-level encryption would make it harder for attackers
to mount attacks based on these vulnerabilities, the authentication
material provided when a TLS connection is established could enable
the vulnerabilities discussed above to be eliminated without the work
of implementing SP4_MACHCRED or SP4_SSV, and irrespective of whether
the client is using AUTH_SYS or RPCSEC_GSS.
If one has a TLS connection including suitable authentication
material, then the server is in a position to reject attempts to bind
to sessions established under this connection by other connections
that are not TLS connections with client-host authentication or are
not established by the same client, unless SP4_MACHCRED or SP4_SSV is
in effect. This would make state protection available to those using
SP4_NONE, i.e. the vast majority, whether they used RPCSEC_GSS or
AUTH_SYS with client-host authentication.
The fact that this is an NFSv4.1-only security feature would make it
difficult to fully it address solely in the NFSv4-wide security
document which will be written. The following proposal is intended
to allow the security document to be published before proceeding to
publish the anticipated rfc5661bis:
* In the anticipated standards-track NFSv4 security document, it can
be stated that when SP4_NONE is specified and the client owning
the session is authenticated, then the session is protected from
being bound-to or destroyed by an unauthenticated client or a
different authenticated client, then the effect is the same as if
SP4_MACHCRED or SP4_SSV were used by the owning client and a non-
matching client was found. Since the new standards-track document
would update [RFC8881], that provides adequate notice of this
change
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* Later, when rfc5661bis is ready to be published, there would an
opportunity to provide introductory material explaining the
various ways clients can be authenticated for the purpose of state
protection, including use of SP4_MACHCRED or SP4_SSV and
authentication as part of use of RPC-TLS. This would allow
defining the circumstances under which one connection can access/
modify state owned by an another client in terms of whether the
connections involved are part of the same client. References to
such abstract relationships would replace most current mentions of
SP4_NONE, SP4_MACHCRED, and SP4_SSV in [RFC8881].
It should be noted that this use of client-host authentication
provides applicable protection even when AUTH_SYS is used. This is
so however the issues discussed in Section 8.4.6 are addressed, and
independent of the decision made regarding the choice discussed in
Section 8.4.3 unless it is decided that AUTH_SYS MUST NOT be used.
Issues regarding the impersonation of a large set of users are not
relevant because user ids are not referenced. Furthermore, the
possibility of a privileged user-level client implementing a denial-
of-service attach (if issue #2 in Section 8.4.6 is not addressed), is
not of concern since such a privileged process would find it far
easier to deny service directly on the client host.
9. IANA Considerations
The current document does not require any actions by IANA.
10. Security Considerations
The convention of requiring a Security Considerations Section within
an I-D encounters difficulties when applied to a document dealing
solely with security, making it most unclear what such a section
might contain for such documents.
In addition, the nature of this particular document poses further
issues regarding the possible content of a Security Consideration
Section:
* This document does not define a protocol or protocol feature that
can be implemented, making it unclear how a threat analysis could
be performed whose security considerations would be described.
* Despite this document's informational nature, it does attempt to
address security issues for an existing set of protocols
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In light of the above, this section will summarize the security-
related message of earlier sections and will, in subsections, discuss
the appropriate contents of Security Considerations Sections for a
eventual standards-track documents to be written
The message of this document with regard to security issues can be
summarized as follows
* The current security approach for the NFSv4 protocols has some
significant weaknesses, requiring a major reworking of the
security approach.
* This reworking can be accomplished without change to the NFS
protocols per se, by taking advantage of recent security
improvements defined within the RPC layer.
* A threat analysis, not provided in previous NFSv4 specification
documents, will be the provided within a new standards-track
document addressing security for all minor versions of NFSv4.
* Because there is no threat analysis of AUTH_SYS in existence, one
needs to be provided, preferably in a new standard-track document
dealing with AUTH_SYS. This is despite the designation of
AUTH_SYS (in [RFC5531] as "insecure", which was never, at least in
the case of NFSv4, acted upon. Such a document is needed to
clarify the security weaknesses of AUTH_SYS and the degree such
weakness could be rectified by the implementation of TLS-based
encryption and/or client host authentication.
10.1. Security Considerations Section for Eventual NFSv4-wide
Standards-track Security Document
Although it is clear that such a document would need to include a
threat analysis, as provided for by [RFC3552]. However, the length
of such an analysis might well be so large that it cannot be
reasonably contained within a Security Considerations Section. In
that case, the Security Considerations Section will summarize the
results of the threat analysis, referring to those parts that appear
in other sections of the document.
10.2. Security Considerations Section for Eventual Rfc5661bis
This section would have to refer to the Security Considerations
section of the new NFSv4-wide secrity document. In addition, to deal
with NFSv4.1-specific security issues (i.e. pNFS and state
protection), there would be additional material, most likely within
the Security Considerations section or a subsection thereof.
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10.3. Security Considerations Section for Potential Revised RPC
Specification Document
The general form of this section in [RFC5531] in which references are
made to documents for each security flavor, seem appropriate.
However, the following changes are likely to be required:
* The treatment of AUTH_SYS might need to reference a new document
describing AUTH_SYS, rather than the incomplete description in
Appendix A, which needs to be removed.
In addition, the last two sentences of the initial paragraph of
section 14 need to be reworked to respond to the fact that while,
there was no clear violation of them, this insecure authentication
flavor was used by NFSv4 for over a decade subsequently. Although
the following attempt at loophole removal might not survive the
editing and review process, the issues it raises are worth
considering.
- As discussed in the referenced document, the use of AUTH_SYS in
the clear without client host-authentication introduces
significant security vulnerabilities in that it allows
unauthenticated requests to be created by an unauthenticated
client. This make its use highly inappropriate for any RPC
service, particularly those that allow clients to modify data.
- Similarly, in specifications of standards-track RPC services,
it is inappropriate to make such use REQUIRED or RECOMMENDED.
Where such use is allowed at all (whether by "MAY" or "SHOULD
NOT" is used), viable non-disruptive alternatives need to
provided and the associated Security Consideration sections
need to clearly explain the security consequences of use.
* The case of AUTH_DH is similar to that of AUTH_SYS in that there
is a simple declaration of insecurity. However, there is a
document referenced and there is no indication that the loopholes
in the treatment were bypassed in this case, as they were for
AUTH_SYS. Still, it might worth revising the final two sentences
as were done with AUTH_SYS.
* Given that the final paragraph of the Security Considerations
section was drafted without consideration of reverse direction
operation and without consideration the possibility that either
TLs encryption or client host authentication might be available
(and potentially REQUIRED), redrafting it to be more appropriate
for the environment made possible by [I-D.ietf-nfsv4-rpc-tls] is
likely.
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The following replacement text is suggested to deal with these
issues:
- Standards-track RPC services need to mandate server and client
support for RPCSEC_GSS when used in the forward direction. The
treatment of authentication for reverse-direction operation
needs to provide for authentication of the identity of the
requester (i.e. on the server) to the responder (i.e. on the
client). When reverse direction requests are not made on
behalf of a specific user, the mutual authentication provided
by RPC-TLS can be relied on but authentication of users on
reverse-direction requests will need to use RPCSEC_GSS.
- For either direction of operation, when user identities must be
authenticated, such services need to mandate support for an
authentication pseudo-flavor. In situations in which it is
possible/allowed for RPC services to be used without TLS
encryption there is also a need to mandate pseudo-flavors with
additional appropriate levels of security, depending on the
need for authentication only, integrity (a.k.a. non-
repudiation), or encryption to provide data confidentiality.
10.4. Security Considerations Section for Potential Standards-track
Document Dealing with AUTH_SYS
As mentioned above, a prime goal of a general threat analysis for
AUTH_SYS would be to clarify the degree to which the security
weaknesses of AUTH_SYS can be successfully addressed using transport-
level security facilities. Those pieces of the threat analysis might
well appear outside the Security Consideration section proper.
The Security Considerations would need to summarize these and provide
a basis for ULPs that allow AUTH_SYS to specify the appropriate
conditions for use and the consequences of not enforcing these.
11. References
11.1. Normative References
[I-D.ietf-nfsv4-rpc-tls]
Myklebust, T. and C. Lever, "Towards Remote Procedure Call
Encryption By Default", Work in Progress, Internet-Draft,
draft-ietf-nfsv4-rpc-tls-11, 23 November 2020,
<https://tools.ietf.org/html/draft-ietf-nfsv4-rpc-tls-11>.
[I-D.ietf-nfsv4-rpcrdma-version-two]
Lever, C. and D. Noveck, "RPC-over-RDMA Version 2
Protocol", Work in Progress, Internet-Draft, draft-ietf-
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nfsv4-rpcrdma-version-two-03, 10 August 2020,
<https://tools.ietf.org/html/draft-ietf-nfsv4-rpcrdma-
version-two-03>.
[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>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[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>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
[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>.
[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>.
[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.
11.2. Informative References
[I-D.dnoveck-nfsv4-rfc5661bis]
Noveck, D., "Network File System (NFS) Version 4 Minor
Version 1 Protocol", Work in Progress, Internet-Draft,
draft-dnoveck-nfsv4-rfc5661bis-00, 31 December 2020,
<https://tools.ietf.org/html/draft-dnoveck-nfsv4-
rfc5661bis-00>.
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[RFC1094] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <https://www.rfc-editor.org/info/rfc1094>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<https://www.rfc-editor.org/info/rfc1813>.
[RFC2624] Shepler, S., "NFS Version 4 Design Considerations",
RFC 2624, DOI 10.17487/RFC2624, June 1999,
<https://www.rfc-editor.org/info/rfc2624>.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, DOI 10.17487/RFC3530,
April 2003, <https://www.rfc-editor.org/info/rfc3530>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<https://www.rfc-editor.org/info/rfc5661>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
Acknowledgements
The authors would like to thank all who contributed to
[I-D.ietf-nfsv4-rpc-tls] for their important work providing security
at the RPC layer, enabling us to break the NFSv4 security logjam.
The authors would like to thank Tom Haynes (of Hammerspace) for
introducing the idea of dealing with security for all minor versions
in a single document.
Authors' Addresses
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David Noveck
NetApp
1601 Trapelo Road
Waltham, MA 02451
United States of America
Phone: +1 781 572 8038
Email: davenoveck@gmail.com
Charles Lever (editor)
Oracle Corporation
United States of America
Email: chuck.lever@oracle.com
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