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RFC 8089

Proposed STD
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The "file" URI Scheme

Updates:    1738


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Internet Engineering Task Force (IETF)                         M. Kerwin
Request for Comments: 8089                                           QUT
Updates: 1738                                              February 2017
Category: Standards Track
ISSN: 2070-1721


                         The "file" URI Scheme

Abstract

   This document provides a more complete specification of the "file"
   Uniform Resource Identifier (URI) scheme and replaces the very brief
   definition in Section 3.10 of RFC 1738.

   It defines a common syntax that 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8089.

Copyright Notice

   Copyright (c) 2017 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
   (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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Operations Involving <file> URIs  . . . . . . . . . . . . . .   5
   4.  File System Name Encoding . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Differences from Previous Specifications . . . . . .  10
   Appendix B.  Example URIs . . . . . . . . . . . . . . . . . . . .  10
   Appendix C.  Similar Technologies . . . . . . . . . . . . . . . .  10
   Appendix D.  System-Specific Operations . . . . . . . . . . . . .  11
     D.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  11
     D.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  11
     D.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  12
     D.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  12
   Appendix E.  Nonstandard Syntax Variations  . . . . . . . . . . .  12
     E.1.  User Information  . . . . . . . . . . . . . . . . . . . .  12
     E.2.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  13
       E.2.1.  Relative Resolution . . . . . . . . . . . . . . . . .  13
       E.2.2.  Vertical Line Character . . . . . . . . . . . . . . .  14
     E.3.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  15
       E.3.1.  <file> URI with Authority . . . . . . . . . . . . . .  15
       E.3.2.  <file> URI with UNC Path  . . . . . . . . . . . . . .  16
     E.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  17
   Appendix F.  Collected Nonstandard Rules  . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

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
   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.  Where
   incompatibilities arise, they are usually in parts of the scheme that
   were underspecified in earlier definitions and have been tightened up
   by more recent specifications.  Appendix A lists significant changes
   to syntax.

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   Extensions to the syntax that might be encountered in practice are
   listed in Appendix E; these extensions are listed for informational
   purposes and are not a requirement of implementation.

   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] when they
   appear in all upper case.  They may also appear in lower or mixed
   case as English words, without normative meaning.

   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 that have no
   "authority" component 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 postdates 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 that it needs
   to use some other local mechanism to access the file.

   As a special case, the "file-auth" rule can match the string
   "localhost" that 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 maximize compatibility with
   previous specifications, users MAY choose to include an "auth-path"
   with no "file-auth" when creating a URI.

   The path component represents the absolute path to the file in the
   file system.  See Appendix D for some discussion of system-specific
   concerns including absolute file paths and file system roots.

   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 solely the responsibility of the
   implementation processing the file URI on behalf of the referenced
   file system.

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

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3.  Operations Involving <file> URIs

   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 System Name Encoding

   File systems use various encoding schemes to store file and directory
   names.  Many modern file systems store 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].

   When a file URI is produced that represents textual data consisting
   of characters from the Unicode Standard coded character set
   [UNICODE], the data SHOULD be encoded as octets according to the
   UTF-8 character encoding scheme [STD63] before percent-encoding is
   applied (as per Section 2.5 of [RFC3986]).

   A decision not to use percent-encoded UTF-8 is outside the scope of
   this specification.  It will typically require the use of heuristics
   or explicit knowledge about the way the string will be processed.

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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   Treating a non-local file URI as local, or otherwise attempting to
   perform local operations on a non-local URI, can result in security
   problems.

   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 not be possible for this
   specification to list all such significant characters and device
   names.  Implementers should research the reserved names and
   characters for the types of storage devices 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 or from mismatched
   encodings or Unicode equivalences [UAX15] (see Section 4).

6.  IANA Considerations

   This document defines the following permanent URI scheme.  The
   "Uniform Resource Identifier (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.

      Used in development libraries, such as:

      *  Windows Shell (PathCreateFromUrl, UrlCreateFromPath)

      *  libwww-perl - The World-Wide Web library for Perl

   Contact:
      Applications and Real-Time Area <art@ietf.org>

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   Change Controller:
      IETF <ietf@ietf.org>

   References:
      This RFC

7.  References

7.1.  Normative References

   [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>.

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

   [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>.

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

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7.2.  Informative References

   [Bash-Tilde]
              Free Software Foundation, Inc, "Bash Reference Manual:
              Tilde Expansion", September 2016,
              <http://www.gnu.org/software/bash/manual/html_node/
              Tilde-Expansion.html>.

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

   [Bug107540]
              Bugzilla@Mozilla, "Bug 107540", October 2001,
              <https://bugzilla.mozilla.org/show_bug.cgi?id=107540>.

   [MS-DTYP]  Microsoft, "Windows Data Types: 2.2.57 UNC", October 2015,
              <http://msdn.microsoft.com/en-us/library/gg465305.aspx>.

   [POSIX]    IEEE, "IEEE Std 1003.1, 2013 Edition - Standard for
              Information Technology-- Portable Operating System
              Interface (POSIX(R)) Base Specifications, Issue 7",
              DOI 10.1109/IEEESTD.2013.6506091, April 2013.

   [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>.

   [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>.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              DOI 10.17487/RFC2396, August 1998,
              <http://www.rfc-editor.org/info/rfc2396>.

   [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>.

   [UAX15]    Davis, M., Ed. and K. Whistler, Ed., "Unicode Standard
              Annex #15: Unicode Normalization Forms", February 2016,
              <http://www.unicode.org/reports/tr15/tr15-44.html>.

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   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              9.0.0", ISBN 978-1-936213-13-9, June 2016,
              <http://www.unicode.org/versions/Unicode9.0.0/>.

   [WHATWG-URL]
              WHATWG, "URL Living Standard", January 2017,
              <https://url.spec.whatwg.org/>.

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

   [Zsh-Tilde]
              "The Z Shell Manual: 14.7 Filename Expansion", December
              2015, <http://zsh.sourceforge.net/Doc/Release/
              Expansion.html#Filename-Expansion>.

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Appendix A.  Differences from Previous Specifications

   The syntax definition in Section 2 inherits incremental differences
   from the general syntax of [RFC1738], as described by Appendix G of
   [RFC2396] and Appendix D of [RFC3986].

   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:

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

      *  "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 "/".  For example:

      *  "file:/path/to/file"

   Non-local files:

   o  A non-local file with an explicit authority.  For example:

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

Appendix C.  Similar Technologies

   o  The WHATWG URL specification [WHATWG-URL] defines browser behavior
      for a variety of inputs, including file URIs.  As a living
      document, it changes to reflect updates in browser behavior.  As a
      result, its algorithms and syntax definitions may or may not be
      consistent with this specification.  Implementors should be aware
      of this possible discrepancy if they expect to share file URIs
      with browsers that follow the WHATWG specification.

   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

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      machine and accessed using a network protocol.  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
   behaviors and provides system-specific guidance for interacting with
   file URIs and paths.  This is not an exhaustive list of operating or
   file systems; rather, it is intended to illustrate certain types of
   interactions that might be encountered.

D.1.  POSIX Systems

   In a POSIX file system, the root of the file system is represented as
   a directory with a zero-length name, usually written as "/"; the
   presence of this root in a file URI can be taken as given by the
   initial slash in the "path-absolute" rule.

   Common UNIX shells such as the Bourne-Again SHell (bash) and Z SHell
   (zsh) provide a function known as "tilde expansion" [Bash-Tilde] or
   "filename expansion" [Zsh-Tilde], where a path that begins with a
   tilde character "~" can be expanded out to a special directory name.
   No such facility exists using the file URI scheme; a tilde in a file
   URI is always just a tilde.

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 nonstandard techniques for interacting with
   DOS- or Windows-like file paths and URIs.

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D.3.  Mac OS X Systems

   The Hierarchical File System Plus (HFS+) uses a nonstandard
   normalization form, similar to Normalization Form D [UAX15].  Take
   care when transforming HFS+ file paths to and from URIs (Section 4).

D.4.  OpenVMS Files-11 Systems

   When mapping a Virtual Memory System (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
   Section 2.2 of [RFC3986], so it should be percent-encoded if present
   in the device name.

   If the VMS file path includes a node reference, that reference is
   used as the authority.  Where the original node reference includes a
   user name and password in an access control string, they can be
   transcribed into the authority using the nonstandard 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 user name
   in a file URI, for example, when mapping a VMS file path with a node
   reference that includes an access control string.

   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

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   password in a "user:password" userinfo field is deprecated by
   [RFC3986].  Take care when dealing with information that can be used
   to identify a user or grant access to a system.

E.2.  DOS and Windows Drive Letters

   On Windows- or DOS-like 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-like environment, with no authority field
   and an absolute path that begins with a drive letter.  For example:

   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.  In some usages of file URIs, drive letters are
   canonicalized by converting them to uppercase; other usages treat
   URIs that differ only in the case of the drive letter as identical.

E.2.1.  Relative Resolution

   To mimic the behavior of DOS- or Windows-like file systems, relative
   references beginning with a slash "/" can be resolved relative to the
   drive letter when present; resolution of ".." dot segments (per
   Section 5.2.4 of [RFC3986]) can be modified to not ever overwrite the
   drive letter.

   For example:

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

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

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   A relative reference starting with a drive letter would be
   interpreted by a generic URI parser as a URI with the drive letter as
   its scheme.  Instead, such a reference ought to be constructed with a
   leading slash "/" character (e.g., "/c:/foo.txt").

   Relative references 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-like systems.

E.2.2.  Vertical Line 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.  For example:

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

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

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

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

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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.  Initialize the URI with the "file:" scheme identifier.

   2.  Append the authority:

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

       2.  Append the host-name field of the UNC string to the URI.

   3.  Append the share-name:

       1.  Transform the share-name to a path segment (see Section 3.3
           of [RFC3986]) 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 object-name:

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

           The colon character ":" is allowed as a delimiter before
           stream-name and stream-type in the file-name, if present.

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

   For example, the UNC String:

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

   would be transformed into the URI:

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

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   The inverse algorithm for translating a file URI to a UNC filespace
   selector string is left as an exercise for the reader.

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.  For example:

      *  "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].
      For example:

      *  "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 file URI 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.  Collected Nonstandard Rules

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

      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 "|"

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   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.
      For example:

      *  "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 "/".  For example:

      *  "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.  For example:

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

   o  Regular DOS or Windows file URIs with vertical line characters in
      the drive letter construct.  For example:

      *  "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.
      For example:

      *  "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.
      For example:

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

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

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

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Acknowledgements

   Contributions from many members of the IETF and W3C communities --
   notably Dave Crocker, Graham Klyne, Tom Petch, and John Klensin --
   are greatly appreciated.

   Additional thanks to Dave Risney, author of the informative IEBlog
   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; and to Paul Hoffman, whose earlier work served as an
   inspiration for this undertaking.

Author's Address

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

   Email: matthew.kerwin@qut.edu.au