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The file URI Scheme

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Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8089.
Author Matthew Kerwin
Last updated 2015-01-11
Replaces draft-kerwin-file-scheme
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state Became RFC 8089 (Proposed Standard)
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Applications Area Working Group                                M. Kerwin
Internet-Draft                                                       QUT
Obsoletes: 1738 (if approved)                           January 12, 2015
Intended status: Standards Track
Expires: July 16, 2015

                          The file URI Scheme


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

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

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

   This draft should be discussed on the IETF Applications Area Working
   Group discussion list <>.

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

   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 July 16, 2015.

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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   ( 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.  History . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Similar Technologies  . . . . . . . . . . . . . . . . . .   3
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Methods on file URIs  . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Translating Local File Path to file URI . . . . . . . . .   7
     3.2.  Translating UNC String to file URI  . . . . . . . . . . .  10
     3.3.  Translating Non-local File Path to file URI . . . . . . .  11
     3.4.  Incompatible File Paths . . . . . . . . . . . . . . . . .  11
       3.4.1.  Namespaces  . . . . . . . . . . . . . . . . . . . . .  11
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

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.  As
   such, there is no well-defined set of methods that can be performed
   on file URIs, nor a media type associated with them.

   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.

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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.draft-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

   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 A.  This
   document describes a means of translating between UNC filespace
   selector strings and file URIs.

1.3.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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", "path-
   absolute", and "query" rules from [RFC3986] (as updated by

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      file-URI       = f-scheme ":" f-hier-part [ "?" query ]

      f-scheme       = "file"

      f-hier-part    = "//" auth-path
                     / local-path

      auth-path      = [ f-auth ] path-absolute
                     / unc-path
                     / windows-path

      f-auth         = [ userinfo "@" ] host

      local-path     = path-absolute
                     / windows-path

      unc-path       = 2*3"/" authority path-absolute

      windows-path   = drive-letter path-absolute
      drive-letter   = ALPHA [ drive-marker ]
      drive-marker   = ":" / "|"

   Note well: the "drive-marker" rule intentionally includes a bar
   character "|" even though that character is not part of either the
   unreserved or reserved character sets in [RFC3986], and thus would
   normally have to be percent-encoded to be included in a URI.  This
   specification explicitly supports the parsing of otherwise invalid
   URIs - those with an unencoded bar character forming part of a DOS or
   Windows drive letter identifier - to facilitate parsing extant
   historical URIs, but new URIs of this form MUST NOT be generated.

   The query field contains non-hierarchical data that, along with data
   in the path components (path-absolute, unc-path, or windows-path)
   serves to identify a resource.  This is not commonly used in
   practice, but could be used to refer to a specific version of a file
   in a versioning file system, for example.

   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
   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.

   The syntax definition above is necessarily different from those given
   in [RFC1630] and [RFC1738] because it depends on the generic syntax
   from [RFC3986] that post-dates all previous specifications.

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   It 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,
         despite being technically incompatible with the definition in

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

         The traditional representation of a local file in a DOS- or
         Windows-based environment.

   o  "file:/path/to/file"

         A "modern" minimal representation of a local file in a UNIX-
         like environment, 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"

         Representations of a local file in a DOS- or Windows-based
         environment, using alternative representations of drive
         letters.  These are supported for compatibility with historical
         implementationsm, but deprecated by this specification.

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

         A representation of a local file in a DOS- or Windows-based
         environment, with no authority field and a slash preceding the
         drive letter.  This representation is less common than those
         above, and is deprecated by this specification.

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   Non-local files:

   o  "file://"

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

   o  "file:////"

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

   o  "file://///"

         As above, with an extra slash between the empty authority and
         the transformed UNC string, conformant with the definition from
         [RFC1738]; see: exceptions in Section 3.2.  This representation
         is deprecated by this specification.  It is notably used by the
         Firefox web browser.

   Dubious encodings:

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

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

         An encoding that includes a Windows drive letter as the
         authority field.  This encoding exists in some extant
         implementations, and is supported by the grammar for historical
         reasons.  New URIs of this form SHOULD NOT be generated.

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

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

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

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

         Various generally invalid URIs that include a disallowed bar
         character "|" in the drive letter identifier.  These encodings
         are supported by the grammar for historical reasons.  As noted
         above, new URIs of this form MUST NOT be generated.

   It also intentionally excludes URIs of the form:

   o  "file://"

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         An encoding that includes both a non-local authority, and a UNC
         string.  The traditional implication was that the shared object
         described by the UNC string may only be accessed from the
         machine "".

3.  Methods on file URIs

   In the strictest terms, the only operations that can be performed on
   a file URI are translating it to and from a file path; subsequent
   methods are performed on the resulting file path, and depend entirely
   on the file system's APIs.

   For example, consider the POSIX "open()", "read()", and "close()"
   methods [POSIX] for reading a file's contents into memory.

   Some APIs allow file system methods to be invoked directly on file
   URIs, while others provide mappings to other similar methods, such as
   GET and PUT from the Hyptertext Transfer Protocol (HTTP) [RFC7231].

   The local file system API can only be used if the file URI has a
   blank (or absent) authority and the path, when transformed to the
   local system's conventions, is not a UNC string.  Note that this
   differs from the definition in [RFC1738] in that previously an
   authority containing the text "localhost" was used to refer to the
   local file system, but in this specification it translates to a UNC
   string referring to the host "localhost".

   This specification does not define a mechanism for accessing files
   stored on non-local file systems.

3.1.  Translating Local File Path to file URI

   Below is an algorithmic description of the process used to convert a
   file path to an Internationalized Resource Identifier (IRI)
   [RFC3987], which can then be translated to a URI as per Section 3.1
   of [RFC3987]; 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.

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   4.  Append the root directory:

       *  On a DOS- or Windows-based system, assign the drive letter
          (e.g. "c:") as the first path segment, and append it to the
          URI, followed by a slash character "/".

          +  If an empty authority was included at step 3, a slash "/"
             is prepended to the drive letter (e.g. "/c:") to
             distinguish it from the authority.

       *  On an OpenVMS Files-11 system, append a slash "/" to the URI,
          and encode the device name as the first segment as per step 5,
          below, except that the dollars sign character "$" is not
          treated as a reserved character in this segment.

       *  On a UNIX-like system, append a slash "/" to the URI, to
          denote the root directory.

   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].

       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.

   7.  If any non-hierarchical data is required to identify the file
       (for example a version number in a versioning file system):

       1.  Append a question mark character "?" to the URI.

       2.  Transform the non-hierarchical data to a query field
           ([RFC3986], Section 3.4) as per Section 2 of [RFC3986].

       3.  Append the transformed query field to the URI.

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      File Path                      | URIs (minimal, traditional)
      UNIX-like:                     |
        /path/to/file                | file:/path/to/file
                                     | file:///path/to/file
        /path/to/dir/                | file:/path/to/dir/
                                     | file:///path/to/dir/
      DOS- or Windows-based:         |
        c:\path\to\file.txt          | file:c:/path/to/file.txt
                                     | file:///c:/path/to/file.txt
        c:\path\to\dir\              | file:c:/path/to/dir/
                                     | file:///c:/path/to/dir/
      VMS Files-11:                  |
        ::DISK1:[PATH.TO]FILE.TXT;2  | file:/DISK1/PATH/TO/FILE.TXT?2
                                     | file:///DISK1/PATH/TO/FILE.TXT?2

Differences from RFC 1738

   In [RFC1738] a file URL always started with the token "file://",
   followed by an authority and a "/".  That "/" was not considered part
   of the path.  This implies that the correct encoding for the above
   example 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 used in practice, and in fact would
   have collided with the eventual encoding of UNC strings in URIs.


      Some implementations leave the leading slash off before the drive
      letter when authority is blank, e.g. "file://c:/..."

      Some implementations replace ":" with "|", and others leave it off
      completely. e.g. "file:///c|/..." or "file:///c/..."

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3.2.  Translating UNC String to file URI

   A UNC filespace selector string can be directly translated to an
   Internationalized Resource Identifier (IRI) [RFC3987], which can then
   be translated to a URI as per Section 3.1 of [RFC3987]; see:
   Section 4.

   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) 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.


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


   Many implementations accept the full UNC string in the URI path (with
   all backslashes "\" converted to slashes "/").  Additionally, because
   [RFC1738] said that the first "/" after "file://[authority]" wasn't
   part of the path, some implementations (including Firefox) require an
   additional slash before the UNC string.

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   For example:

          \_____/\__________________________ /
          Scheme     Transformed UNC string

          \_____/|\__________________________ /
          Scheme |    Transformed UNC string
                 Extra slash

3.3.  Translating Non-local File Path to file URI

   Translating a non-local file path other than 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)

3.4.  Incompatible File Paths

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

3.4.1.  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

   The encoding of a file URI depends on the file system.  If the file
   system uses a known non-Unicode character encoding, the path SHOULD

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   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 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)

   Example: file IRI:

   | 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:

   | File URI, in any ASCII-compatible document:
   |    "file:///%E3%81%A1"
   | Possible interpretations of the file name, depending on the
   | (unknown) encoding of the file system:
   |  o UTF-8:
   |       <HIRAGANA LETTER TI (U+3061)>

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   |  o Codepage 437:
   |       <GREEK SMALL LETTER PI (U+03C0)> +
   |  o EBCDIC:
   |       "Ta~"
   |  o US-ASCII:
   |       "%E3%81%A1"
   | etc.

5.  Security Considerations

   There are many security considerations for URI schemes discussed in

   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.

   Additionally, as discussed in the HP OpenVMS Systems Documentation
   "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

   IANA maintains the registry of URI Schemes [BCP115] at .

   This document defines the following URI scheme, so the "Permanent URI
   Schemes" registry has been updated accordingly.

           | URI Scheme | Description              | Reference |
           | file       | Host-specific file names | RFC XXXX  |

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

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7.  Acknowledgements

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

   Additional thanks to Dave Risney, author of the informative IE Blog
   windows.aspx , and Dave Thaler for their comments and suggestions.

8.  References

8.1.  Normative References

   [BCP115]   Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35, RFC
              4395, February 2006.

              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,

   [MS-NBTE]  Microsoft Open Specifications, "NetBIOS over TCP (NBT)
              Extensions", May 2014,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

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   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, February 2013.

   [UTR15]    Davis, M. and K. Whistler, "Unicode Normalization Forms",
              August 2012.

8.2.  Informative References

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

   [MS-SMB]   Microsoft Open Specifications, "Server Message Block (SMB)
              Protocol", January 2013,

   [NOVELL]   Novell, "NetWare Core Protocols", 2013,

   [POSIX]    IEEE, "IEEE Std 1003.1, 2013 Edition", 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, June 1994.

   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, December 1994.

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

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              WHATWG, "URL Living Standard", May 2013,

              Microsoft Developer Network, "Naming Files, Paths, and
              Namespaces", June 2013.

Appendix A.  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:

      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

   The precise format of "sharename" depends on the protocol; see: SMB
   [MS-SMB], NFS [RFC3530], NCP [NOVELL].

Author's Address

   Matthew Kerwin


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