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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)                              July 23, 2015
Intended status: Standards Track
Expires: January 24, 2016


                          The file URI Scheme
                   draft-ietf-appsawg-file-scheme-03

Abstract

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

   It attempts to define a common core which is intended to interoperate
   across the broad spectrum of existing implementations, while at the
   same time documenting other current practices.

   *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 January 24, 2016.

Copyright Notice

   Copyright (c) 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  History . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Similar Technologies  . . . . . . . . . . . . . . . . . .   3
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Operations on file URIs . . . . . . . . . . . . . . . . . . .   5
     3.1.  Translating Local File Path to file URI . . . . . . . . .   5
     3.2.  Translating Non-local File Path to file URI . . . . . . .   6
     3.3.  Incompatible File Paths . . . . . . . . . . . . . . . . .   6
       3.3.1.  Win32 Namespaces  . . . . . . . . . . . . . . . . . .   7
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Example URIs . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  System-specific Operations . . . . . . . . . . . . .  12
     B.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  12
     B.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  12
     B.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  12
     B.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  12
   Appendix C.  Nonstandard Syntax Variations  . . . . . . . . . . .  12
     C.1.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  13
       C.1.1.  Relative Paths  . . . . . . . . . . . . . . . . . . .  13
       C.1.2.  Vertical Bar Character  . . . . . . . . . . . . . . .  14
     C.2.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  14
     C.3.  UNC Paths . . . . . . . . . . . . . . . . . . . . . . . .  15
     C.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  16
   Appendix D.  Example of IRI vs Percent-Encoded URI  . . . . . . .  16
   Appendix E.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  17
   Appendix F.  Collected Rules  . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20







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1.  Introduction

   A file URI identifies a file on a particular file system.  It can be
   used in discussions about the file, and if other conditions are met
   it can be dereferenced to directly access the file.

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

   This document defines a syntax that is compatible with most extant
   implementations, while attempting to push towards a stricter subset
   of "ideal" constructs.  In many cases it simultaneously acknowledges
   and deprecates some less common or outdated constructs.

1.1.  History

   The file URI scheme was first defined in [RFC1630], which, being an
   informational RFC, does not specify an Internet standard.  The
   definition was standardised in [RFC1738], and the scheme was
   registered with the Internet Assigned Numbers Authority (IANA);
   however that definition omitted certain language included by former
   that clarified aspects such as:

   o  the use of slashes to denote boundaries between directory levels
      of a hierarchical file system; and

   o  the requirement that client software convert the file URI into a
      file name in the local file name conventions.

   The Internet draft [I-D.hoffman-file-uri] was written in an effort to
   keep the file URI scheme on standards track when [RFC1738] was made
   obsolete, but that draft expired in 2005.  It enumerated concerns
   arising from the various, often conflicting implementations of the
   scheme.  It serves as the spiritual predecessor of this document.

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

1.2.  Similar Technologies

   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.  A UNC filespace selector string
   has three parts: host, share, and path; see Appendix E.  This
   document describes but does not specify a means of translating
   between UNC filespace selector strings and file URIs in Appendix C.2.




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1.3.  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" is used to describe files
   that can be accessed directly through the local file system.  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.

2.  Syntax

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

   Please note Appendix C that lists some other commonly seen but
   nonstandard variations.

      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      = [ userinfo "@" ] host

   The syntax definition above is different from those given in
   [RFC1630] and [RFC1738] as it is derived from the generic syntax of
   [RFC3986], which post-dates all previous specifications.

   As a special case, the "auth-path" rule can match the string
   "localhost" or the empty string in the URI's authority component;
   this is interpreted as "the machine from which the URI is being
   interpreted," exactly as if no authority was present.

   Systems exhibit different levels of case-sensitivity.  Unless the
   file system is known to be case-insensitive, implementations MUST
   maintain the case of file and directory names when translating file



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   URIs to and from the local system's representation of file paths, and
   any systems or devices that transport file URIs MUST NOT alter the
   case of file URIs they transport.

3.  Operations on file URIs

   Implementations 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.

   File URIs can also be translated to and from other, similar
   constructs, such as local file paths or UNC strings.

   A file URI can only be dereferenced or translated to a local file
   path if it is local.  A file URI is considered "local" if it has a
   blank or no authority, or the authority is the special string
   "localhost".

   This specification neither defines nor forbids a mechanism for
   accessing non-local files.  See SMB [MS-SMB], NFS [RFC7530], NCP
   [NOVELL] for examples of protocols that can be used to access files
   over a network.  Also see Appendix C.2 for a discussion on
   translating non-local file URIs to and from UNC stings.

3.1.  Translating Local File Path to file URI

   Below is an algorithmic description of the process used to convert a
   file path to a URI; see Section 4.

   1.  Resolve the file path to its fully qualified absolute form.

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

   3.  If including an empty authority field, append the "//" sigil to
       the URI.

   4.  Append a slash character "/" to the URI, to signify the path
       root.

   5.  For each directory in the path after the root:

       1.  Transform the directory name to a path segment ([RFC3986],
           Section 3.3) as per Section 2 of [RFC3986].





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

   6.  If the path includes a file name:

       1.  Transform the file name to a path segment as above.

       2.  Append the transformed segment to the URI.

   *Differences from RFC 1738*

   In [RFC1738] a file URL always started with the token "file://",
   followed by an (optionally blank) authority and a "/".  That "/" was
   not considered part of the path.  This implies that the correct
   encoding for a file path in a UNIX-like environment would have been:

        token     + authority + slash + path
      = "file://" + ""        + "/"   + "/path/to/file.txt"
      = "file:////path/to/file.txt"

   However that construct was never observed in practice, and in fact
   would have collided with the eventual encoding of UNC strings in URIs
   described in Appendix C.3.

3.2.  Translating Non-local File Path to file URI

   Translating a non-local file path, including a UNC string, to a file
   URI follows the same basic algorithm as for local files, above,
   except that the authority MUST refer to the network-accesible node
   that hosts the file.

   For example, in a clustered OpenVMS Files-11 system the authority
   would contain the node name.  Where the original node reference
   includes a username and password in an access control string, they
   MAY be transcribed into the userinfo field of the authority
   ([RFC3986], Section 3.2.1), security considerations (Section 5)
   notwithstanding.

   See Appendix C.2 for an explicit handling of UNC strings.

3.3.  Incompatible File Paths

   Some conventional file path formats are known to be incompatible with
   the file URI scheme.







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3.3.1.  Win32 Namespaces

   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.

4.  Encoding

   To avoid ambiguity, a file URI SHOULD be transported as an
   Internationalized Resource Identifier (IRI) [RFC3987], or as a URI
   with non-ASCII characters encoded according to the UTF-8 character
   encoding [STD63] and percent-encoded as needed ([RFC3986],
   Section 2.5).

   The encoding of a file URI depends on the file system that stores the
   identified file.  If the file system uses a known non-Unicode
   character encoding, the path SHOULD be converted to a sequence of
   characters from the Universal Character Set [ISO10646] normalized
   according to Normalization Form C (NFC) [UTR15], before being
   translated to a file URI, and conversely a file URI SHOULD be
   converted back to the file system's native encoding when
   dereferencing or translating to a file path.

      Note that many modern file systems encode directory and file names
      as arbitrary sequences of octets.  In those cases, the
      representation as an encoded string often depends on the user's
      localization settings, or defaults to UTF-8 [STD63].

   When the file system's encoding is not known the file URI SHOULD be
   transported as an Internationalized Resource Identifier (IRI)
   [RFC3987] to avoid ambiguity.  See Appendix D for examples.

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.  Software using file
   URIs MUST NOT grant greater access than would be available for other
   file access methods.



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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.

   Additionally, 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.

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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   [*RFC Editor Note:* Replace XXXX with this RFC's reference.]

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 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>.

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

   [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>.

   [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>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <http://www.rfc-editor.org/info/rfc3987>.



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   [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>.

   [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>.

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.

   [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.






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   [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>.

   [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.

Appendix A.  Example URIs

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

   Local files:

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

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

   o  "file:/path/to/file"

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

   Non-local files:

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





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         The representation of a non-local file, with an explicit
         authority.

Appendix B.  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.

B.1.  POSIX Systems

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

B.2.  DOS- and Windows-Like Systems

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

   See Appendix C.1 for explicit (but non-normative and strictly
   optional) rules for interacting with DOS- or Windows-like file paths
   and URIs.

B.3.  Mac OS X Systems

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

B.4.  OpenVMS Files-11 Systems

   When mapping a VMS file path to a file URI, map the device name 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, use that as the
   authority.  Where the original node reference includes a username and
   password in an access control string, they can be transcribed into
   the userinfo field of the authority ([RFC3986], Section 3.2.1),
   security considerations (Section 5) notwithstanding.

Appendix C.  Nonstandard Syntax Variations

   These variations may be encountered for historical reasons, but are
   not supported by the normative syntax of Section 2.




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   This appendix is not normative.

C.1.  DOS and Windows Drive Letters

   On Windows- or DOS-based file systems a 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 ":"

   This is intended to support URIs of the form:

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

         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.

   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 implementations therefore canonicalize drive
   letters in file URIs by converting them to uppercase.

C.1.1.  Relative Paths

   In DOS- or Windows-based file systems, relative paths beginning with
   a slash "/" should be resolved relative to the drive letter, and
   resolution of ".." dot segments (per Section 5.2.4 of [RFC3986])
   should not ever overwrite the drive letter.

   e.g.:

      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") should not be
   accepted as dereferenceable URIs in DOS or Windows systems.




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C.1.2.  Vertical Bar Character

   Historically some implementations have used 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 are 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 URIs of the form:

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

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

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

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

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

C.2.  UNC Strings

   A UNC filespace selector string can be directly translated to a URI;
   see Section 4.  The following is an algorithmic description of the
   process of translating a UNC string to a file URI:

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

   2.  Append the authority:

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




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       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) as per 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 ([RFC3986],
           Section 3.3) as per Section 2 of [RFC3986].

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

   Example:

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

C.3.  UNC Paths

   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 is replaced
   with the following:

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

      unc-authority  = 2*3"/" [ userinfo "@" ] file-host

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

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

   This syntax uses the "userinfo", "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.





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

   Non-local files:

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

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

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

         As above, with an extra slash between the empty authority and
         the transformed UNC string, conformant with the definition from
         [RFC1738].  This representation is notably used by the Firefox
         web browser.  See Bugzilla#107540 [Bug107540].

   It also further limits the set of file URIs that can be translated to
   a local file path to those with a path that does not encode a UNC
   string.

C.4.  Backslash as Separator

   Historically some implementations have copied entire file paths into
   the path components of file URIs.  Where DOS or Windows file paths
   were copied thus, 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 D.  Example of IRI vs Percent-Encoded URI

   The following examples demonstrate the advantage of encoding file
   URIs as IRIs to avoid ambiguity (see Section 4).

   Example: file IRI:











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   | Bytes of file IRI in a UTF-8 document:
   |    66 69 6c 65 3a 43 3a 2f 72 65 c3 a7 75 2e 74 78 74
   |    f  i  l  e  :  c  :  /  r  e  ( c ) u  .  t  x  t
   |
   | Interpretation:
   |    A file named "recu.txt" with a cedilla on the "c", in the
   |    directory "C:\" of a DOS or Windows file system.
   |
   | Character value sequences of file paths, for various file system
   | encodings:
   |
   |  o UTF-16 (e.g. NTFS):
   |       0043 003a 005c 0072 0065 00e7 0075 002e 0074 0078 0074
   |
   |  o Codepage 437 (e.g. MS-DOS):
   |       43   3a   5c   72   65   87   75   2e   74   78   74

   Counter-example: ambiguous file URI:

   | Percent-encoded file URI, in any ASCII-compatible document:
   |    "file:///%E3%81%A1"
   |
   | Possible interpretations of the file name, depending on the
   | encoding of the file system:
   |
   |  o UTF-8:
   |       <HIRAGANA LETTER TI (U+3061)>
   |
   |  o Codepage 437:
   |       <GREEK SMALL LETTER PI (U+03C0)> +
   |       <LATIN SMALL LETTER U WITH DIAERESIS (U+00FC)> +
   |       <LATIN SMALL LETTER I WITH ACUTE (U+00ED)>
   |
   |  o EBCDIC:
   |       "Ta~"
   |
   | etc.

Appendix E.  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].




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      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 F.  Collected Rules

   Here are the collected syntax rules for all optional appendices,
   presented for convenience.

      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      = [ userinfo "@" ] host

      unc-authority  = 2*3"/" [ userinfo "@" ] 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  "file:///path/to/file"

         A traditional file URI for a local file, with an empty
         authority.

   o  "file:/path/to/file"

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

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

         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.

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

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

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

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

   Non-local files:

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

         The representation of a non-local file, with an explicit
         authority.

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

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

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

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




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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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