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Versions: (draft-kerwin-file-scheme) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 8089

Applications Area Working Group                                M. Kerwin
Internet-Draft                                                       QUT
Obsoletes: 1738 (if approved)                               May 15, 2016
Intended status: Standards Track
Expires: November 16, 2016


                          The file URI Scheme
                   draft-ietf-appsawg-file-scheme-09

Abstract

   This document specifies the "file" Uniform Resource Identifier (URI)
   scheme, obsoleting the definition in RFC 1738.

   It defines a common syntax which is intended to interoperate across
   the broad spectrum of existing usages.  At the same time it notes
   some other current practices around the use of file URIs.

Note to Readers (To be removed by the RFC Editor)

   This draft should be discussed on the IETF Applications Area Working
   Group discussion list <apps-discuss@ietf.org>.

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 http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on November 16, 2016.

Copyright Notice

   Copyright (c) 2016 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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Operations Involving file URIs  . . . . . . . . . . . . . . .   5
   4.  File Name Encoding  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Differences from Previous Specifications . . . . . .   9
   Appendix B.  Example URIs . . . . . . . . . . . . . . . . . . . .   9
   Appendix C.  Similar Technologies . . . . . . . . . . . . . . . .  10
   Appendix D.  System-specific Operations . . . . . . . . . . . . .  10
     D.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  11
     D.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  11
     D.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  11
     D.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  11
   Appendix E.  Nonstandard Syntax Variations  . . . . . . . . . . .  11
     E.1.  User Information  . . . . . . . . . . . . . . . . . . . .  11
     E.2.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  12
       E.2.1.  Relative Paths  . . . . . . . . . . . . . . . . . . .  12
       E.2.2.  Vertical Bar Character  . . . . . . . . . . . . . . .  13
     E.3.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  14
       E.3.1.  file URI with Authority . . . . . . . . . . . . . . .  14
       E.3.2.  file URI with UNC Path  . . . . . . . . . . . . . . .  15
     E.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  16
   Appendix F.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  16
   Appendix G.  Collected Rules  . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   A file URI identifies an object (a "file") stored in a structured
   object naming and accessing environment on a host (a "file system.")
   The URI can be used in discussions about the file, and if other




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   conditions are met it can be dereferenced to directly access the
   file.

   This document specifies a syntax based on the generic syntax of
   [RFC3986] that is compatible with most existing usages.  Optional
   extensions to the syntax which might be encountered in practice are
   listed in appendices; these extensions are listed for informational
   purposes only.

   The file URI scheme is not coupled with a specific protocol, nor with
   a specific media type [RFC6838].  See Section 3 for a discussion of
   operations that can be performed on the object identified by a file
   URI.

1.1.  Notational Conventions

   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 described in [RFC2119].

   Throughout this document the term "local file" is used to describe
   files that can be accessed through the local file system API using
   only the information included in the file path, not relying on other
   information such as network addresses.  It is important to note that
   a local file may not be physically located on the local machine, for
   example if a networked file system is transparently mounted into the
   local file system.

   The term "local file URI" is used to describe file URIs which have no
   authority, or where the authority is the special string "localhost"
   or a fully qualified domain name that resolves to the machine from
   which the URI is being interpreted (Section 2).

2.  Syntax

   The file URI syntax is defined here in Augmented Backus-Naur Form
   (ABNF) [RFC5234], importing the "host" and "path-absolute" rules from
   [RFC3986] (as updated by [RFC6874].)

   The generic syntax in [RFC3986] includes "path" and "authority"
   components, for each of which only a subset is used in the definition
   of the file URI scheme.  The relevant subset of "path" is "path-
   absolute", and the subset of "authority" is "file-auth", given below.

   The syntax definition below is different from those given in
   [RFC1630] and [RFC1738] as it is derived from the generic syntax of
   [RFC3986], which post-dates the previous file URI specifications.
   Appendix A enumerates significant differences.



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      file-URI       = file-scheme ":" file-hier-part

      file-scheme    = "file"

      file-hier-part = ( "//" auth-path )
                     / local-path

      auth-path      = [ file-auth ] path-absolute

      local-path     = path-absolute

      file-auth      = "localhost"
                     / host

   The "host" is the fully qualified domain name of the system on which
   the file is accessible.  This allows a client on another system to
   know that it cannot access the file system, or perhaps to use some
   other local mecahnism to access the file.

   As a special case, the "file-auth" rule can match the string
   "localhost" which is interpreted as "the machine from which the URI
   is being interpreted," exactly as if no authority were present.  Some
   current usages of the scheme incorrectly interpret all values in the
   authority of a file URI, including "localhost", as non-local.  Yet
   others interpret any value as local, even if the "host" does not
   resolve to the local machine.  To maximise compatibility with
   previous specifications, users MAY choose to include an "auth-path"
   with no "file-auth" when creating a URI.

   Some file systems have case-sensitive file naming and some do not.
   As such the file URI scheme supports case sensitivity, in order to
   retain the case as given.  Any transport-related handling of the file
   URI scheme MUST retain the case as given.  Any mapping to or from a
   case-insensitive form is soley the responsibility of the
   implementation processing the file URI on behalf of the referenced
   file system.

   Some file systems allow directory objects to be treated as files in
   some cases.  This can be reflected in a file URI by omitting the
   trailing slash "/" from the path.  Be aware that merging a relative
   URI reference to such a base URI as per Section 5.2 of [RFC3986]
   could remove the directory name from the resulting target URI.

   Also see Appendix E that lists some nonstandard syntax variations
   that can be encountered in practice.






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3.  Operations Involving file URIs

   Implementations that provide dereferencing operations on file URIs
   SHOULD, at a minimum, provide a read-like operation to return the
   contents of a file located by a file URI.  Additional operations MAY
   be provided, such as writing to, creating, and deleting files.  See
   the POSIX file and directory operations [POSIX] for examples of
   standardized operations that can be performed on files.

   A file URI can be dependably dereferenced or translated to a local
   file path only if it is local.  A file URI is considered "local" if
   it has no "file-auth", or the "file-auth" is the special string
   "localhost" or a fully qualified domain name that resolves to the
   machine from which the URI is being interpreted (Section 2).

   This specification neither defines nor forbids any set of operations
   that might be performed on a file identified by a non-local file URI.

4.  File Name Encoding

   File systems use various encoding schemes to store file and directory
   names.  Many modern file systems encode file and directory names as
   arbitrary sequences of octets, in which case the representation as an
   encoded string often depends on the user's localization settings, or
   defaults to UTF-8 [STD63].

   Without other encoding information, percent-encoded octets in a file
   URI ([RFC3986], Section 2.1) MAY be interpreted according to the
   preferred or configured encoding of the system on which the URI is
   being interpreted.

5.  Security Considerations

   There are many security considerations for URI schemes discussed in
   [RFC3986].

   File access and the granting of privileges for specific operations
   are complex topics, and the use of file URIs can complicate the
   security model in effect for file privileges.

   Historically, user agents have granted content from the file URI
   scheme a tremendous amount of privilege.  However, granting all local
   files such wide privileges can lead to privilege escalation attacks.
   Some user agents have had success granting local files directory-
   based privileges, but this approach has not been widely adopted.
   Other user agents use globally unique identifiers as the origin for
   each file URI [RFC6454], which is the most secure option.




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   File systems typically assign an operational meaning to special
   characters, such as the "/", "\", ":", "[", and "]" characters, and
   to special device names like ".", "..", "...", "aux", "lpt", etc.  In
   some cases, merely testing for the existence of such a name will
   cause the operating system to pause or invoke unrelated system calls,
   leading to significant security concerns regarding denial of service
   and unintended data transfer.  It would be impossible for this
   specification to list all such significant characters and device
   names.  Implementers MUST research the reserved names and characters
   for the types of storage device that may be attached to their
   application and restrict the use of data obtained from URI components
   accordingly.

   File systems vary in the way they handle case.  Care must (?) be
   taken to avoid issues resulting from possibly unexpected aliasing
   from case-only differences between file paths or URIs.  Similarly,
   care must be taken to avoid issues resulting from aliasing from
   mismatched encodings or Unicode equivalences [UTR15] (see Section 4).

6.  IANA Considerations

   This document defines the following URI scheme, so the "Permanent URI
   Schemes" registry has been updated accordingly.  This registration
   complies with [BCP35].

   Scheme name:
      file

   Status:
      permanent

   Applications/protocols that use this scheme name:
      Commonly used in hypertext documents to refer to files without
      depending on network access.  Supported by major browsers.

      Windows API (PathCreateFromUrl, UrlCreateFromPath).

      Perl LWP.

   Contact:
      Matthew Kerwin <matthew.kerwin@qut.edu.au>

   Change Controller:
      This scheme is registered under the IETF tree.  As such, the IETF
      maintains change control.






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7.  Acknowledgements

   This specification is derived from [RFC1738], [RFC3986], and
   [I-D.hoffman-file-uri] (expired); the acknowledgements in those
   documents still apply.

   Additional thanks to Dave Risney, author of the informative IE Blog
   article <http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-
   windows.aspx>, and Dave Thaler for their early comments and
   suggestions.

8.  References

8.1.  Normative References

   [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <http://www.rfc-editor.org/info/bcp35>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <http://www.rfc-editor.org/info/rfc6874>.

   [UTR15]    Davis, M. and K. Whistler, "Unicode Normalization Forms",
              August 2012,
              <http://unicode.org/reports/tr15/tr15-18.html>.








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8.2.  Informative References

   [Bug107540]
              Bugzilla@Mozilla, "Bug 107540", October 2007,
              <https://bugzilla.mozilla.org/show_bug.cgi?id=107540>.

   [I-D.hoffman-file-uri]
              Hoffman, P., "The file URI Scheme", draft-hoffman-file-
              uri-03 (work in progress), January 2005.

   [ISO10646]
              International Organization for Standardization,
              "Information Technology - Universal Multiple-Octet Coded
              Character Set (UCS)", ISO/IEC 10646:2003, December 2003.

   [MS-DTYP]  Microsoft Open Specifications, "Windows Data Types, 2.2.56
              UNC", January 2013,
              <http://msdn.microsoft.com/en-us/library/gg465305.aspx>.

   [MS-NBTE]  Microsoft Open Specifications, "NetBIOS over TCP (NBT)
              Extensions", May 2014,
              <http://msdn.microsoft.com/en-us/library/dd891412.aspx>.

   [MS-SMB]   Microsoft Open Specifications, "Server Message Block (SMB)
              Protocol", January 2013,
              <http://msdn.microsoft.com/en-us/library/cc246231.aspx>.

   [NOVELL]   Novell, "NetWare Core Protocols", 2013,
              <http://www.novell.com/developer/ndk/
              netware_core_protocols.html>.

   [POSIX]    IEEE, "IEEE Std 1003.1, 2013 Edition", 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <http://www.rfc-editor.org/info/rfc1123>.

   [RFC1630]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
              Unifying Syntax for the Expression of Names and Addresses
              of Objects on the Network as used in the World-Wide Web",
              RFC 1630, DOI 10.17487/RFC1630, June 1994,
              <http://www.rfc-editor.org/info/rfc1630>.




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   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
              December 1994, <http://www.rfc-editor.org/info/rfc1738>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <http://www.rfc-editor.org/info/rfc7530>.

   [STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/std63>.

   [WHATWG-URL]
              WHATWG, "URL Living Standard", May 2013,
              <http://url.spec.whatwg.org/>.

   [Win32-Namespaces]
              Microsoft Developer Network, "Naming Files, Paths, and
              Namespaces", June 2013, <https://msdn.microsoft.com/en-
              au/library/windows/desktop/aa365247(v=vs.85).aspx>.

Appendix A.  Differences from Previous Specifications

   According to the definition in [RFC1738] a file URL always started
   with the token "file://", followed by an (optionally blank) host name
   and a "/".  The syntax given in Section 2 makes the entire authority
   component, including the double slashes "//", optional.

Appendix B.  Example URIs

   The syntax in Section 2 is intended to support file URIs that take
   the following forms:

   Local files:




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   o  A traditional file URI for a local file, with an empty authority.
      This is the most common format in use today.  E.g.:

      *  "file:///path/to/file"

   o  The minimal representation of a local file, with no authority
      field and an absolute path that begins with a slash "/".  E.g.:

      *  "file:/path/to/file"

   Non-local files:

   o  A non-local file, with an explicit authority.  E.g.:

      *  "file://host.example.com/path/to/file"

Appendix C.  Similar Technologies

   o  The WHATWG defines a living URL standard [WHATWG-URL], which
      includes algorithms for interpreting file URIs (as URLs).

   o  The Universal Naming Convention (UNC) [MS-DTYP] defines a string
      format that can perform a similar role to the file URI scheme in
      describing the location of files, except that files located by UNC
      filespace selector strings are typically stored on a remote
      machine and accessed using a network protocol.  A UNC filespace
      selector string has three parts: host, share, and path; described
      for informational purposes in Appendix F.  Appendix E.3 lists some
      ways in which UNC filespace selector strings are currently made to
      interoperate with the file URI scheme.

   o  The Microsoft Windows API defines Win32 Namespaces
      [Win32-Namespaces] for interacting with files and devices using
      Windows API functions.  These namespaced paths are prefixed by
      "\\?\" for Win32 File Namespaces and "\\.\" for Win32 Device
      Namespaces.  There is also a special case for UNC file paths in
      Win32 File Namespaces, referred to as "Long UNC", using the prefix
      "\\?\UNC\".  This specification does not define a mechanism for
      translating namespaced paths to or from file URIs.

Appendix D.  System-specific Operations

   This appendix is not normative; it highlights some observed
   behaviours and provides system-specific guidance for interacting with
   file URIs and paths.






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D.1.  POSIX Systems

   There is little to say about POSIX file systems; the file URI
   structure already closely resembles POSIX file paths.

D.2.  DOS- and Windows-Like Systems

   When mapping a DOS- or Windows-like file path to a file URI, the
   drive letter (e.g. "c:") is typically mapped into the first path
   segment.

   Appendix E lists some non-standard techniques for interacting with
   DOS- or Windows-like file paths and URIs.

D.3.  Mac OS X Systems

   The HFS+ file system uses a non-standard normalization form, similar
   to Normalization Form D [UTR15].  Take care when transforming HFS+
   file paths to and from URIs (Section 4).

D.4.  OpenVMS Files-11 Systems

   When mapping a VMS file path to a file URI, the device name is mapped
   into the first path segment.  Note that the dollars sign "$" is a
   reserved character per the definition in [RFC3986], Section 2.2, so
   should be percent-encoded if present in the device name.

   If the VMS file path includes a node reference, that is used as the
   authority.  Where the original node reference includes a username and
   password in an access control string, they can be transcribed into
   the authority using the non-standard syntax extension in
   Appendix E.1.

Appendix E.  Nonstandard Syntax Variations

   These variations may be encountered by existing usages of the file
   URI scheme, but are not supported by the normative syntax of
   Section 2.

   This appendix is not normative.

E.1.  User Information

   It might be necessary to include user information such as a username
   in a file URI, for example when mapping a VMS file path with a node
   reference that includes a username.





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   To allow user information to be included in a file URI, the "file-
   auth" rule in Section 2 can be replaced with the following:

      file-auth      = "localhost"
                     / [ userinfo "@" ] host

   This uses the "userinfo" rule from [RFC3986].

   As discussed in the HP OpenVMS Systems Documentation
   <http://h71000.www7.hp.com/doc/84final/ba554_90015/ch03s09.html>
   "access control strings include sufficient information to allow
   someone to break in to the remote account, [therefore] they create
   serious security exposure."  In a similar vein, the presence of a
   password in a "user:password" userinfo field is deprecated by
   [RFC3986].  As such, the userinfo field of a file URI, if present,
   MUST NOT (?) contain a password.

E.2.  DOS and Windows Drive Letters

   On Windows- or DOS-based file systems an absolute file path can begin
   with a drive letter.  To facilitate this, the "local-path" rule in
   Section 2 can be replaced with the following:

      local-path     = [ drive-letter ] path-absolute

      drive-letter   = ALPHA ":"

   The "ALPHA" rule is defined in [RFC5234].

   This is intended to support the minimal representation of a local
   file in a DOS- or Windows-based environment, with no authority field
   and an absolute path that begins with a drive letter.  E.g.:

   o  "file:c:/path/to/file"

   URIs of the form "file:///c:/path/to/file" are already supported by
   the "path-absolute" rule.

   Note that comparison of drive letters in DOS or Windows file paths is
   case-insensitive, some usages of file URIs therefore canonicalize
   drive letters by converting them to uppercase.

E.2.1.  Relative Paths

   To mimic the behaviour of DOS- or Windows-based file systems,
   relative paths beginning with a slash "/" can be resolved relative to
   the drive letter, when present, and resolution of ".." dot segments




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   (per Section 5.2.4 of [RFC3986]) can be modified to not ever
   overwrite the drive letter.

   For example:

      base:       file:///c:/path/to/file.txt
      rel. URI:   /some/other/thing.bmp
      resolved:   file:///c:/some/other/thing.bmp

      base:       file:///c:/foo.txt
      rel. URI:   ../../bar.txt
      resolved:   file:///c:/bar.txt

   Relative paths with a drive letter followed by a character other than
   a slash (e.g. "c:bar/baz.txt" or "c:../foo.txt") might not be
   accepted as dereferenceable URIs in DOS or Windows systems.

E.2.2.  Vertical Bar Character

   Historically some usages of file URIs have included a vertical line
   character "|" instead of a colon ":" in the drive letter construct.
   [RFC3986] forbids the use of the vertical line, however it may be
   necessary to interpret or update old URIs.

   For interpreting such URIs, the "auth-path" and "local-path" rules in
   Section 2 and the "drive-letter" rule above can be replaced with the
   following:

      auth-path      = [ file-auth ] path-absolute
                     / [ file-auth ] file-absolute

      local-path     = [ drive-letter ] path-absolute
                     / file-absolute

      file-absolute  = "/" drive-letter path-absolute

      drive-letter   = ALPHA ":"
                     / ALPHA "|"

   This is intended to support regular DOS or Windows file URIs with
   vertical line characters in the drive letter construct.  E.g.:

   o  "file:///c|/path/to/file"

   o  "file:/c|/path/to/file"

   o  "file:c|/path/to/file"




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   To update such an old URI, replace the vertical line "|" with a colon
   ":".

E.3.  UNC Strings

   Some usages of the file URI scheme allow UNC filespace selector
   strings [MS-DTYP] to be translated to and from file URIs, either by
   mapping the equivalent segments of the two schemes (hostname to
   authority, sharename+objectnames to path), or by mapping the entire
   UNC string to the path segment of a URI.

E.3.1.  file URI with Authority

   The following is an algorithmic description of the process of
   translating a UNC filespace selector string to a file URI by mapping
   the equivalent segments of the two schemes:

   1.  Initialise the URI with the "file:" scheme identifier.

   2.  Append the authority:

       1.  Append the "//" authority sigil to the URI.

       2.  Append the hostname field of the UNC string to the URI.

   3.  Append the sharename:

       1.  Transform the sharename to a path segment ([RFC3986],
           Section 3.3) to conform to the encoding rules of Section 2 of
           [RFC3986].

       2.  Append a delimiting slash character "/" and the transformed
           segment to the URI.

   4.  For each objectname:

       1.  Transform the objectname to a path segment as above.

       2.  Append a delimiting slash character "/" and the transformed
           segment to the URI.

   For example:

      UNC String:   \\host.example.com\Share\path\to\file.txt
      URI:          file://host.example.com/Share/path/to/file.txt






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E.3.2.  file URI with UNC Path

   It is common to encounter file URIs that encode entire UNC strings in
   the path, usually with all backslash "\" characters replaced with
   slashes "/".

   To interpret such URIs, the "auth-path" rule in Section 2 can be
   replaced with the following:

      auth-path      = [ file-auth ] path-absolute
                     / unc-authority path-absolute

      unc-authority  = 2*3"/" file-host

      file-host      = inline-IP / IPv4address / reg-name

      inline-IP      = "%5B" ( IPv6address / IPvFuture ) "%5D"

   This syntax uses the "IPv4address", "IPv6address", "IPvFuture", and
   "reg-name" rules from [RFC3986].

      Note that the "file-host" rule is the same as "host" but with
      percent-encoding applied to "[" and "]" characters.

   This extended syntax is intended to support URIs that take the
   following forms, in addition to those in Appendix B:

   Non-local files:

   o  The representation of a non-local file, with an empty authority
      and a complete (transformed) UNC string in the path.  E.g.:

      *  "file:////host.example.com/path/to/file"

   o  As above, with an extra slash between the empty authority and the
      transformed UNC string, as per the syntax defined in [RFC1738].
      E.g.:

      *  "file://///host.example.com/path/to/file"

      This representation is notably used by the Firefox web browser.
      See Bugzilla#107540 [Bug107540].

   It also further limits the definition of a "local file URI" by
   excluding any with a path that encodes a UNC string.






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E.4.  Backslash as Separator

   Historically some usages have copied entire file paths into the path
   components of file URIs.  Where DOS or Windows file paths were thus
   copied the resulting URI strings contained unencoded backslash "\"
   characters, which are forbidden by both [RFC1738] and [RFC3986].

   It may be possible to translate or update such an invalid file URI by
   replacing all backslashes "\" with slashes "/", if it can be
   determined with reasonable certainty that the backslashes are
   intended as path separators.

Appendix F.  UNC Syntax

   The UNC filespace selector string is a null-terminated sequence of
   characters from the Universal Character Set [ISO10646].

   The syntax of a UNC filespace selector string, as defined by
   [MS-DTYP], is given here in Augmented Backus-Naur Form (ABNF)
   [RFC5234] for convenience.  Note that this definition is informative
   only; the normative description is in [MS-DTYP].

      UNC = "\\" hostname "\" sharename *( "\" objectname )
      hostname   = netbios-name / fqdn / ip-address
      sharename  = <name of share or resource to be accessed>
      objectname = <depends on resource being accessed>

   o  "netbios-name" from [MS-NBTE], Section 2.2.1.

   o  "fqdn" from [RFC1035] or [RFC1123]

   o  "ip-address" from Section 2.1 of [RFC1123], or Section 2.2 of
      [RFC4291].

   The precise format of "sharename" depends on the protocol; see: SMB
   [MS-SMB], NFS [RFC7530], NCP [NOVELL].

Appendix G.  Collected Rules

   Here are the collected syntax rules for all optional appendices,
   presented for convenience.  This collected syntax is not normative.










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      file-URI       = file-scheme ":" file-hier-part

      file-scheme    = "file"

      file-hier-part = ( "//" auth-path )
                     / local-path

      auth-path      = [ file-auth ] path-absolute
                     / [ file-auth ] file-absolute
                     / unc-authority path-absolute

      local-path     = [ drive-letter ] path-absolute
                     / file-absolute

      file-auth      = "localhost"
                     / [ userinfo "@" ] host

      unc-authority  = 2*3"/" file-host

      file-host      = inline-IP / IPv4address / reg-name

      inline-IP      = "%5B" ( IPv6address / IPvFuture ) "%5D"

      file-absolute  = "/" drive-letter path-absolute

      drive-letter   = ALPHA ":"
                     / ALPHA "|"

   This collected syntax is intended to support file URIs that take the
   following forms:

   Local files:

   o  A traditional file URI for a local file, with an empty authority.
      E.g.:

      *  "file:///path/to/file"

   o  The minimal representation of a local file, with no authority
      field and an absolute path that begins with a slash "/".  E.g.:

      *  "file:/path/to/file"

   o  The minimal representation of a local file in a DOS- or Windows-
      based environment, with no authority field and an absolute path
      that begins with a drive letter.  E.g.:

      *  "file:c:/path/to/file"



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   o  Regular DOS or Windows file URIs, with vertical line characters in
      the drive letter construct.  E.g.:

      *  "file:///c|/path/to/file"

      *  "file:/c|/path/to/file"

      *  "file:c|/path/to/file"

   Non-local files:

   o  The representation of a non-local file, with an explicit
      authority.  E.g.:

      *  "file://host.example.com/path/to/file"

   o  The "traditional" representation of a non-local file, with an
      empty authority and a complete (transformed) UNC string in the
      path.  E.g.:

      *  "file:////host.example.com/path/to/file"

   o  As above, with an extra slash between the empty authority and the
      transformed UNC string.  E.g.:

      *  "file://///host.example.com/path/to/file"

Author's Address

   Matthew Kerwin
   Queensland University of Technology
   Victoria Park Road
   Kelvin Grove, QLD  4059
   Australia

   Email: matthew.kerwin@qut.edu.au















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