Internationalization for the NFSv4 Protocols
draft-dnoveck-nfsv4-internationalization-00

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NFSv4                                                          D. Noveck
Internet-Draft                                                    NetApp
Updates: 5661, 7530 (if approved)                      December 14, 2019
Intended status: Standards Track
Expires: June 16, 2020

              Internationalization for the NFSv4 Protocols
              draft-dnoveck-nfsv4-internationalization-00

Abstract

   This document describes the handling of internationalization for all
   NFSv4 protocols, including NFSv4.0, NFSv4.1, NFSv4.2 and extensions
   thereof, and future minor versions.

   It updates RFC7530 (formally) and RFC5661 (substantively).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 16, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  History . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Limitations on Internationalization-Related Processing in the
       NFSv4 Context . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Summary of Server Behavior Types  . . . . . . . . . . . . . .   9
   6.  String Encoding . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Normalization . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  String Types with Processing Defined by Other Internet Areas   11
   9.  Errors Related to UTF-8 . . . . . . . . . . . . . . . . . . .  13
   10. Servers That Accept File Component Names That Are Not Valid
       UTF-8 Strings . . . . . . . . . . . . . . . . . . . . . . . .  13
   11. Future Minor Versions and Extensions  . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     14.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Internationalization is a complex topic with its own set of
   terminology (see [RFC6365]).  The topic is made more complex for the
   NFSv4 protocols by the tangled history described in Section 3.  This
   document is based on the actual behavior of NFSv4 client and server
   implementations (for all existing minor versions) and is intended to
   serve as a basis for further implementations to be developed that can
   interact with existing implementations as well as those to be
   developed in the future.

   Note that the behaviors on which this document are based are each
   demonstrated by a combination of an NFSv4 server implementation
   proper and a server-side physical file system.  It is common for
   servers and physical file systems to be configurable as to the
   behavior shown.  In the discussion below, each configuration that
   shows different behavior is considered separately.

   As a consequence of this choice, normative terms defined in [RFC2119]
   are derived from implementation behavior, rather than the other way
   around.  The specifics are discussed in Section 2.

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   With regard to the question of interoperability with existing
   specifications for NFSv4 minor versions, different minor versions
   pose different issues.

      With regard to NFSv4.0 [RFC7530], no interoperability issues are
      expected to arise because the internationalization in that
      specification, which is the basis for this one, was also based on
      the behavior of existing implementations.  Although, in a formal
      sense, the treatment of internationalization here supersedes that
      in [RFC7530], the treatments are intended to be the same in order
      to eliminate interoperability issues.

      With regard to NFSv4.1 [RFC5661], the situation is quite
      different.  The approach to internationalization specified in that
      document was never implemented, and implementers were either
      unaware of the troublesome implications of that approach or chose
      to ignore the existing specification as essentially
      unimplementable.  An internationalization approach compatible with
      that specified in [RFC7530] tended to be followed, despite the
      fact that, in other respects, NFSv4.1 was treated as a separate
      protocol.

      If there were NFSv4 servers who obeyed the internationalization
      dictates within [RFC5661], or clients that expected servers to do
      so, they would fail to interoperate with typical clients and
      servers when dealing with non-UTF8 file names, which are quite
      common.  As no such implementation have come to our attention, it
      has to be assumed that they do not exist and interoperability with
      existing implementations is an appropriate basis for this
      document.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as BCP 14 [RFC2119] [RFC8174] when,
   and only when, they appear in all capitals, as shown here.

   Although the key words "MUST", "SHOULD", and "MAY" retain their
   normal meanings, as described above, the fact this specification was
   derived from existing implementation patterns requires that we
   explain how the normative terms used derive from the behavior of
   existing implementations, in those situations in which existing
   implementation behavior patterns can be determined.

   o  Behavior implemented by all existing clients or servers is
      described using "MUST", since new implementations need to follow
      existing ones to be assured of interoperability.  While it is

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      possible that different behavior might be workable, we have found
      no case where this seems reasonable.

      The converse holds for "MUST NOT": if a type of behavior poses
      interoperability problems, it MUST NOT be implemented by any
      existing clients or servers.

   o  Behavior implemented by most existing clients or servers, where
      that behavior is more desirable than any alternative, is described
      using "SHOULD", since new implementations need to follow that
      existing practice unless there are strong reasons to do otherwise.

      The converse holds for "SHOULD NOT".

   o  Behavior implemented by some, but not all, existing clients or
      servers is described using "MAY", indicating that new
      implementations have a choice as to whether they will behave in
      that way.  Thus, new implementations will have the same
      flexibility that existing ones do.

   o  Behavior implemented by all existing clients or servers, so far as
      is known -- but where there remains some uncertainty as to details
      -- is described using "should".  Such cases primarily concern
      details of error returns.  New implementations should follow
      existing practice even though such situations generally do not
      affect interoperability.

   There are also cases in which certain server behaviors, while not
   known to exist, cannot be reliably determined not to exist.  In part,
   this is a consequence of the long period of time that has elapsed
   since the publication of the defining specifications, resulting in a
   situation in which those involved in t implementation work may no
   longer be involved in or aware of working group activities.

   In the case of possible server behavior that is neither known to
   exist nor known not to exist, we use "SHOULD NOT" and "MUST NOT" as
   follows, and similarly for "SHOULD" and "MUST".

   o  In some cases, the potential behavior is not known to exist but is
      of such a nature that, if it were in fact implemented,
      interoperability difficulties would be expected and reported,
      giving us cause to conclude that the potential behavior is not
      implemented.  For such behavior, we use "MUST NOT".  Similarly, we
      use "MUST" to apply to the contrary behavior.

   o  In other cases, potential behavior is not known to exist but the
      behavior, while undesirable, is not of such a nature that we are
      able to draw any conclusions about its potential existence.  In

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      such cases, we use "SHOULD NOT".  Similarly, we use "SHOULD" to
      apply to the contrary behavior.

   In the case of a "MAY", "SHOULD", or "SHOULD NOT" that applies to
   servers, clients need to be aware that there are servers that may or
   may not take the specified action, and they need to be prepared for
   either eventuality.

3.  History

   The history of internationalization within NFSv4 is discussed in this
   section.  Despite the fact that NFSv4.0 and subsequent minor versions
   have differed in many ways, the actual implementations of
   internationalization have remained the same and internationalized
   names have been handled without regard to the minor version being
   used.  As a result, this document treats internationalization for all
   NFSv4 minor versions together.

   During the period from the publication of [RFC3010] until now, two
   different perspectives with regard to internationalization have been
   held and represented, to varying degrees, in specifications for NFSv4
   minor versions.

   o  The perspective held by NFSv4 implementers treated
      internationalization as basically outside the scope of what NFSv4
      client and server implementers could deal with.  This was because
      the POSIX interface treated filenames as uninterpreted strings of
      bytes, because the file systems used by NFSv4 servers treated
      filenames similarly, and because those file systems contained
      files with internationalized names using a number of different
      encoding methods, chosen by the users of the POSIX interface.
      From this perspective, wider support for internationalized names
      and general use of universal encodings was a matter for users and
      applications and not for protocol implementers or designers.

   o  Within the IETF in general and in the IESG, there was a feeling
      that new protocols, such as NFSv4, could not avoid dealing with
      internationalization issues, making it difficult to treat these
      matters, as the implementers' perspective would have it, as
      essentially out of scope.

   As specifications were developed, approved, and at times rewritten,
   this fundamental difference of approach was never fully resolved,
   although, with the publication of [RFC7530], a satisfactory modus
   vivendi may have been arrived at.

   Although many specifications were published dealing with NFSv4
   internationalization, all minor versions used the same implementation

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   approach, even when the current specification for that minor version
   specified an entirely different approach.  As a result, we need to
   treat the history of NFSv4 internationalization below as an
   integrated whole, rather than treating individual minor versions
   separately.

   o  The approach to internationalization specified in [RFC3010]
      sidestepped the conflict of approaches cited above by discussing
      the reasons that UTF-8 encoding was desirable while leaving
      filenames as uninterpreted strings of bytes.  The issue of string
      normalization was avoided by saying "The NFS version 4 protocol
      does not mandate the use of a particular normalization form at
      this time."

      Despite this approach's inconsistency with general IETF
      expectations regarding internationalization, RFC3010 was published
      as a Proposed Standard.  NFSv4.0 implementation related to
      internationalization followed the same paradigm used by NFSv3,
      assuring interoperability with files created using that protocol,
      as well as with those created using local means of file creation.

   o  When it became necessary, because of issues with byte-range
      locking, to create an rfc3010bis, no change to the previously
      approved approach seemed indicated and the drafts submitted up
      until [I-D.ietf-nfsv4-rfc3010bis] closely followed RFC3010 as far
      as internationalization.  The IESG then decided that a different
      approach to internationalization was required, to be based on
      stringprep [RFC3454] and rfc3010bis was accordingly revised,
      replacing all of the Internationalization section, before being
      published as [RFC3530].

      These changes required the rejection of file names that were not
      valid UTF-8, file names that included code points not, at the time
      of publication, assigned a Unicode character (e.g. capital eszett)
      or that were not allowed by stringprep (e.g.  Zero-width joiner
      and non-joiner characters).  Because these restrictions would have
      caused the set of valid file names to be different on NFS-mounted
      and local file systems there was no chance of them ever being
      implemented.

      Because these changes were made without working group involvement,
      most implementers were unaware of them while those who were aware
      of the changes ignored them and maintained the
      internationalization approach specified in RFC3010.

   o  When NFsv4.1 was being developed, it seemed that no changes in
      internationalization would be required.  Many people were unaware
      of the stringprep-based requirements which made the NFSv4.0

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      internationalization specified in RFC3530 unimplementable.  As a
      result, the internationalization specified in [RFC5661] was the
      same as that in RFC3530.

      As a result, even though NFSv4.1 was a separate protocol and could
      have had a different approach to internationalization, for a
      considerable time, internationalization for both protocols was
      specified to be the same (in RFC3530 and RFC5661) while the actual
      implementations of the two minor versions both followed the
      approach specified in RFC3010, despite its obsoleted status.

   o  When work started on rfc3530bis it was clear that issues related
      to internationalization had to be addressed.  When the
      implications of the stringprep references in RFC3530 were
      discussed with implementers it became clear that mandating that
      NFSv4.0 filenames conform to stringprep was not appropriate.
      While some working group members articulated the view that,
      because of the need to maintain compatibility with the POSIX
      interface and existing file systems, internationalization for
      NFSv4 could not be successfully addressed by the IETF, the
      rfc3530bis draft submitted to the IESG did not explicitly embrace
      the implementers' perspective set forth above.

      The draft submitted to the IESG and [RFC7530] as published
      provided an explanation (see Section 4) as to why restrictions on
      character encodings were not viable.  It allowed non-UTF-8
      encodings to be used while defining UTF-8 as the preferred
      encoding and allowing servers to reject non-UTF-8 string as
      invalid.  Other stringprep-based string restrictions were
      eliminated.  With regard to normalization, it continued to defer
      the matter, leaving open the possibility that one might be chosen
      later.

      This approach is compatible, in implementation terms, with that
      specified in [RFC3010], allowing it to be used compatibly with
      existing implementation for all existing minor versions.  This is
      despite the fact that [RFC5661] specifies an entirely different
      approach.

      As a result of discussions leading up to the publishing of
      RFC7530, it was discovered that some local file systems used with
      NFSv4 were configured to be both normalization-aware and
      normalization-preserving, mapping all canonically equivalent file
      names to the same file while preserving the form actually used to
      create the file, of whatever form, normalized or not.  This
      behavior, which is legal according to RFC3010, which says little
      about name mapping is probably illegal according to stringprep.
      Nevertheless, it was expressly pointed out in RFC7530 as a valid

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      choice to deal with normalization issues, since it allows
      normalization-aware processing without the difficulties that arise
      in imposing a particular normalization form, as described in
      Section 7.

   o  NFSv4.2 made no changes to internationalization.  As a result,
      [RFC7862] which made no mention of internationalization,
      implicitly aligned internationalization in NFSv4.2 with that in
      NFSv4.1, as specified by [RFC5661].

      As a result of this implicit alignment, there is no need for this
      document to specifically address NFSv4.2 or be marked as updating
      RFC7862.  It is sufficient that it updates RFC5661, which
      specifies the internationalization for NFSv4.1, inherited by
      NFSv4.2.

   The above history, can, for the purposes of the rest of this document
   be summarized in the following two statements:

   o  The actual treatment of internationalization within NFSv4 has not
      been affected by the particular minor version used, despite the
      fact that the specifications for the minor versions have often
      differed in their treatment of internationalization.

   o  Implementations have followed the internationalization approach
      specified in RFC3010, which is compatible with the treatment in
      RFC7530.

   In order to deal with all NFSv4 minor versions, this document follows
   the internationalization approach defined in RFC7530, in order to
   maintain compatibility with all existing NFSv4 minor versions.
   Issues relating to potential future minor versions and protocol
   extensions are dealt with in Section 11.

4.  Limitations on Internationalization-Related Processing in the NFSv4
    Context

   There are a number of noteworthy circumstances that limit the degree
   to which internationalization-related encoding and normalization-
   related restrictions can be universal with regard to NFSv4 clients
   and servers:

   o  The NFSv4 client is part of an extensive set of client-side
      software components whose design and internal interfaces are not
      within the IETF's purview, limiting the degree to which a
      particular character encoding might be made standard.

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   o  Server-side handling of file component names is typically
      implemented within a server-side physical file system, whose
      handling of character encoding and normalization is not
      specifiable by the IETF.

   o  Typical implementation patterns in UNIX systems result in the
      NFSv4 client having no knowledge of the character encoding being
      used, which might even vary between processes on the same client
      system.

   o  Users may need access to files stored previously with non-UTF-8
      encodings, or with UTF-8 encodings that are not in accord with any
      particular normalization form.

5.  Summary of Server Behavior Types

   As mentioned in Section 8, servers MAY reject component name strings
   that are not valid UTF-8.  This leads to a number of types of valid
   server behavior, as outlined below.  When these are combined with the
   valid normalization-related behaviors as described in Section 6, this
   leads to the combined behaviors outlined below.

   o  Servers that limit file component names within a given file system
      to UTF-8 strings exist with normalization-related handling as
      described in Section 6.  These are best described as behaving as
      "UTF-8-only servers".

   o  Servers that do not limit file component names on particular file
      systems to UTF-8 strings are very common and are necessary to deal
      with clients/applications not oriented to the use of UTF-8.  Such
      servers ignore normalization-related issues, and there is no way
      for them to implement either normalization or representation-
      independent lookups.  These are best described as behaving as
      "UTF-8-unaware servers" for such file systems, since they treat
      file component names as uninterpreted strings of bytes and have no
      knowledge of the characters represented.  See Section 9 for
      details.

   o  It is possible for a server to allow component names that are not
      valid UTF-8, while still being aware of the structure of UTF-8
      strings.  Such servers could implement either normalization or
      representation-independent lookups but apply those techniques only
      to valid UTF-8 strings.  Such servers are not common, but it is
      possible to configure at least one known server to have this
      behavior.  This behavior SHOULD NOT be used due to the possibility
      that a filename using one encoding may, by coincidence, have the
      appearance of a UTF-8 filename; the results of UTF-8 normalization
      or representation-independent lookups are unlikely to be correct

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      in all cases, when considered from the viewpoint of the other
      encoding.

6.  String Encoding

   Strings that potentially contain characters outside the ASCII range
   [RFC20] are generally represented in NFSv4 using the UTF-8 encoding
   [RFC3629] of Unicode [UNICODE].  See [RFC3629] for precise encoding
   and decoding rules.

   Some details of the protocol treatment depend on the type of string:

   o  For strings that are component names, the preferred encoding for
      any non-ASCII characters is the UTF-8 representation of Unicode.

      In many cases, clients have no knowledge of the encoding being
      used, with the encoding done at the user level under the control
      of a per-process locale specification.  As a result, it may be
      impossible for the NFSv4 client to enforce the use of UTF-8.  The
      use of non-UTF-8 encodings can be problematic, since it may
      interfere with access to files stored using other forms of name
      encoding.  Also, normalization-related processing (see Section 7)
      of a string not encoded in UTF-8 could result in inappropriate
      name modification or aliasing.  In cases in which one has a non-
      UTF-8 encoded name that accidentally conforms to UTF-8 rules,
      substitution of canonically equivalent strings can change the non-
      UTF-8 encoded name drastically.

      The kinds of modification and aliasing mentioned here can lead to
      both false negatives and false positives, depending on the strings
      in question, which can result in security issues such as elevation
      of privilege and denial of service (see [RFC6943] for further
      discussion).

   o  For strings based on domain names, non-ASCII characters MUST be
      represented using the UTF-8 encoding of Unicode, and additional
      string format restrictions apply.  See Section 8 for details.

   o  The contents of symbolic links (of type linktext4 in the XDR) MUST
      be treated as opaque data by NFSv4 servers.  Although UTF-8
      encoding is often used, it need not be.  In this respect, the
      contents of symbolic links are like the contents of regular files
      in that their encoding is not within the scope of this
      specification.

   o  For other sorts of strings, any non-ASCII characters SHOULD be
      represented using the UTF-8 encoding of Unicode.

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7.  Normalization

   The client and server operating environments may differ in their
   policies and operational methods with respect to character
   normalization (see [UNICODE] for a discussion of normalization
   forms).  This difference may also exist between applications on the
   same client.  This adds to the difficulty of providing a single
   normalization policy for the protocol that allows for maximal
   interoperability.  This issue is similar to the issues of character
   case where the server may or may not support case-insensitive
   filename matching and may or may not preserve the character case when
   storing filenames.  The protocol does not mandate a particular
   behavior but allows for a range of useful behaviors.

   The NFSv4 protocol does not mandate the use of a particular
   normalization form.  A subsequent minor version of the NFSv4 protocol
   might specify a particular normalization form, although there would
   be difficulties in doing so (see Section 11 for details).  In any
   case, the server and client can expect that they may receive
   unnormalized characters within protocol requests and responses.  If
   the operating environment requires normalization, then the
   implementation will need to normalize the various UTF-8 encoded
   strings within the protocol before presenting the information to an
   application (at the client) or local file system (at the server).

   Server implementations MAY normalize filenames to conform to a
   particular normalization form before using the resulting string when
   looking up or creating a file.  Servers MAY also perform
   normalization-insensitive string comparisons without modifying the
   names to match a particular normalization form.  Except in cases in
   which component names are excluded from normalization-related
   handling because they are not valid UTF-8 strings, a server MUST make
   the same choice (as to whether to normalize or not, the target form
   of normalization, and whether to do normalization-insensitive string
   comparisons) in the same way for all accesses to a particular file
   system.  Servers SHOULD NOT reject a filename because it does not
   conform to a particular normalization form, as this would deny access
   to clients that use a different normalization form.

8.  String Types with Processing Defined by Other Internet Areas

   There are two types of strings that NFSv4 deals with that are based
   on domain names.  Processing of such strings is defined by other
   Internet standards, and hence the processing behavior for such
   strings should be consistent across all server operating systems and
   server file systems.

   These are as follows:

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   o  Server names as they appear in the fs_locations and
      fs_locations_info attribute.  Notes that for most purposes, such
      server names will only be sent by the server to the client.  The
      exception is the use of these attributes in a VERIFY or NVERIFY
      operation.

   o  Principal suffixes that are used to denote sets of users and
      groups, and are in the form of domain names.

   The general rules for handling all of these domain-related strings
   are similar and independent of the role of the sender or receiver as
   client or server, although the consequences of failure to obey these
   rules may be different for client or server.  The server can report
   errors when it is sent invalid strings, whereas the client will
   simply ignore invalid string or use a default value in their place.

   The string sent SHOULD be in the form of one or more U-labels as
   defined by [RFC5890].  If that is impractical, it can instead be in
   the form of one or more LDH labels [RFC5890] or a UTF-8 domain name
   that contains labels that are not properly formatted U-labels.  The
   receiver needs to be able to accept domain and server names in any of
   the formats allowed.  The server MUST reject, using the error
   NFS4ERR_INVAL, a string that is not valid UTF-8, or that contains an
   ASCII label that is not a valid LDH label, or that contains an
   XN-label (begins with "xn--") for which the characters after "xn--"
   are not valid output of the Punycode algorithm [RFC3492].

   When a domain string is part of id@domain or group@domain, there are
   two possible approaches:

   1.  The server treats the domain string as a series of U-labels.  In
       cases where the domain string is a series of A-labels or
       Non-Reserved LDH (NR-LDH) labels, it converts them to U-labels
       using the Punycode algorithm [RFC3492].  In cases where the
       domain string is a series of other sorts of LDH labels, the
       server can use the ToUnicode function defined in [RFC3490] to
       convert the string to a series of labels that generally conform
       to the U-label syntax.  In cases where the domain string is a
       UTF-8 string that contains non-U-labels, the server can attempt
       to use the ToASCII function defined in [RFC3490] and then the
       ToUnicode function on the string to convert it to a series of
       labels that generally conform to the U-label syntax.  As a
       result, the domain string returned within a user id on a GETATTR
       may not match that sent when the user id is set using SETATTR,
       although when this happens, the domain will be in the form that
       generally conforms to the U-label syntax.

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   2.  The server does not attempt to treat the domain string as a
       series of U-labels; specifically, it does not map a domain string
       that is not a U-label into a U-label using the methods described
       above.  As a result, the domain string returned on a GETATTR of
       the user id MUST be the same as that used when setting the
       user id by the SETATTR.

   A server SHOULD use the first method.

   For VERIFY and NVERIFY, additional string processing requirements
   apply to verification of the owner and owner_group attributes; see
   the section entitled "Interpreting owner and owner_group" for the
   document specifying the minor version in question ([RFC7530],
   [RFC5661])

9.  Errors Related to UTF-8

   Where the client sends an invalid UTF-8 string, the server MAY return
   an NFS4ERR_INVAL error.  This includes cases in which inappropriate
   prefixes are detected and where the count includes trailing bytes
   that do not constitute a full Multiple-Octet Coded Universal
   Character Set (UCS) character.

   Requirements for server handling of component names that are not
   valid UTF-8, when a server does not return NFS4ERR_INVAL in response
   to receiving them, are described in Section 10.

   Where the string supplied by the client is not rejected with
   NFS4ERR_INVAL but contains characters that are not supported by the
   server as a value for that string (e.g., names containing slashes, or
   characters that do not fit into 16 bits when converted from UTF-8 to
   a Unicode codepoint), the server should return an NFS4ERR_BADCHAR
   error.

   Where a UTF-8 string is used as a filename, and the file system,
   while supporting all of the characters within the name, does not
   allow that particular name to be used, the server should return the
   error NFS4ERR_BADNAME.  This includes such situations as file system
   prohibitions of "." and ".." as filenames for certain operations, and
   similar constraints.

10.  Servers That Accept File Component Names That Are Not Valid UTF-8
     Strings

   As stated previously, servers MAY accept, on all or on some subset of
   the physical file systems exported, component names that are not
   valid UTF-8 strings.  A typical pattern is for a server to use
   UTF-8-unaware physical file systems that treat component names as

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   uninterpreted strings of bytes, rather than having any awareness of
   the character set being used.

   Such servers SHOULD NOT change the stored representation of component
   names from those received on the wire and SHOULD use an octet-by-
   octet comparison of component name strings to determine equivalence
   (as opposed to any broader notion of string comparison).  This is
   because the server has no knowledge of the character encoding being
   used.

   Nonetheless, when such a server uses a broader notion of string
   equivalence than what is recommended in the preceding paragraph, the
   following considerations apply:

   o  Outside of 7-bit ASCII, string processing that changes string
      contents is usually specific to a character set and hence is
      generally unsafe when the character set is unknown.  This
      processing could change the filename in an unexpected fashion,
      rendering the file inaccessible to the application or client that
      created or renamed the file and to others expecting the original
      filename.  Hence, such processing should not be performed, because
      doing so is likely to result in incorrect string modification or
      aliasing.

   o  Unicode normalization is particularly dangerous, as such
      processing assumes that the string is UTF-8.  When that assumption
      is false because a different character set was used to create the
      filename, normalization may corrupt the filename with respect to
      that character set, rendering the file inaccessible to the
      application that created it and others expecting the original
      filename.  Hence, Unicode normalization SHOULD NOT be performed,
      because it may cause incorrect string modification or aliasing.

   When the above recommendations are not followed, the resulting string
   modification and aliasing can lead to both false negatives and false
   positives, depending on the strings in question, which can result in
   security issues such as elevation of privilege and denial of service
   (see [RFC6943] for further discussion).

11.  Future Minor Versions and Extensions

   As stated above, all current NFSv4 minor versions allow use of non-
   UTF-8 encodings, allow servers a choice of whether to be aware of
   normalization issues or not, and allows servers a number of choices
   about how to address normalization issues.  This range of choices
   reflects the need to accommodate existing file systems and user
   expectations about character handling which in turn reflect the
   assumptions of the POSIX model of handling file names.

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   While it is theoretically possible for a subsequent minor version to
   change these aspects of the protocol (see [RFC8178]), this section
   will explain why any such change is highly unlikely, making it
   expected that these aspects of NFSv4 internationalization handling
   will be retained indefinitely.  As a result, any new minor version
   specification document that made such a change would have to be
   marked as updating or obsoleting this document

   No such change could be done as an extension to an existing minor
   version or in a new minor version consisting only of OPTIONAL
   features.  Such a change could only be done in a new minor version,
   which like minor version one, was prepared to be incompatible to some
   degree with the previous minor versions.  While it appears unlikely
   that such minor versions will be adopted, the possibility cannot be
   excluded, so we need to explore the difficulties of changing the
   aspects of internationalization handling mentioned above.

   o  Establishing UTF-8 as the sole means of encoding for
      internationalized characters, would make inaccessible existing
      files stored with other encodings.  Further, unless there were a
      corresponding change in the UNIX file interface model, it would
      cause the set of valid names for local and remote files to
      diverge.

   o  Imposing a particular normalization form, in the sense of refusing
      to create to allow access to files whose UTF-8-encoded names are
      not of the selected normalization form would give rise to similar
      difficulties.

   o  Defining a preferred normalization form to be returned as the
      names of all internationalized files, would result in applications
      having to deal with sudden unexplained changes of file names for
      existing files.

   None of the above appears likely since there does not seem to be any
   corresponding benefits to justify the difficulties that they would
   create.

   There would also be difficulties in otherwise reducing the set of
   three acceptable normalization handling options, without reducing it
   to a single option by imposing a specific normalization form.

   o  Eliminating the possibility of a single possible normalization
      form, would pose similar difficulties to imposing the other one,
      even if representation-independent comparisons were also allowed.

      In either case, a specific normalization form would be disfavored,
      with no corresponding benefit.

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   o  Allowing only representation-independent lookups would not impose
      difficulties for clients, but there are reasons to doubt it could
      be unversally implemented, since such name comparisons would have
      to be done within the file system itself.

      Such a change could only be made once support file system support
      for representation-independent file lookups would become commonly
      available.  As long as the POSIX file naming model continues its
      sway, that would be unlikely to happen.

   One possible internationalization-related extension that the working
   could adopt would be definition of an OPTIONAL per-fs attribute
   defining the internationalization-related handling for that file
   system.  That would allow clients to be aware of server choices in
   this area and could be adopted without disrupting existing clients
   and servers.

12.  IANA Considerations

   The current document does not require any actions by IANA.

13.  Security Considerations

   Unicode in the form of UTF-8 is generally is used for file component
   names (i.e., both directory and file components).  However, other
   character sets may also be allowed for these names.  For the owner
   and owner_group attributes and other sorts strings whose form is
   affected by standard outside NFSv4 (see Section 8.) are always
   encoded as UTF-8.  String processing (e.g., Unicode normalization)
   raises security concerns for string comparison.  See Sections 8 and 7
   as well as the respective Sections 5.9 of [RFC7530] and [RFC5661] for
   further discussion.  See [RFC6943] for related identifier comparison
   security considerations.  File component names are identifiers with
   respect to the identifier comparison discussion in [RFC6943] because
   they are used to identify the objects to which ACLs are applied (See
   the respective Sections 6 of [RFC7530] and [RFC5661]).

14.  References

14.1.  Normative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.

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   [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>.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, DOI 10.17487/RFC3490, March 2003,
              <https://www.rfc-editor.org/info/rfc3490>.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
              <https://www.rfc-editor.org/info/rfc3492>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [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>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [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>.

   [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>.

   [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>.

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   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              7.0.0", (Mountain View, CA: The Unicode Consortium,
              2014 ISBN 978-1-936213-09-2), June 2014,
              <http://www.unicode.org/versions/latest/>.

14.2.  Informative References

   [I-D.ietf-nfsv4-rfc3010bis]
              Shepler, S., "NFS version 4 Protocol", draft-ietf-
              nfsv4-rfc3010bis-04 (work in progress), October 2002.

   [RFC3010]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "NFS version 4
              Protocol", RFC 3010, DOI 10.17487/RFC3010, December 2000,
              <https://www.rfc-editor.org/info/rfc3010>.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              DOI 10.17487/RFC3454, December 2002,
              <https://www.rfc-editor.org/info/rfc3454>.

   [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>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <https://www.rfc-editor.org/info/rfc6943>.

Appendix A.  Acknowledgements

   This document is based, in large part, on Section 12 of [RFC7530] and
   all the people who contributed to that work, have helped make this
   document possible, including David Black, Peter Staubach, Nico
   Williams, Mike Eisler, Trond Myklebust, James Lentini, Mike Kupfer
   and Peter Saint-Andre.

   The author wishes to thank Tom Haynes for his timely suggestion to
   pursue the task of dealing with internationalization on an NFSv4-wide
   basis.

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Author's Address

   David Noveck
   NetApp
   1601 Trapelo Road
   Waltham, MA  02451
   United States of America

   Phone: +1 781 572 8038
   Email: davenoveck@gmail.com

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