The file URI Scheme

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Author Matthew Kerwin 
Last updated 2013-06-26
Replaced by RFC 8089, RFC 8089
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Network Working Group                                          M. Kerwin
Intended status: Standards Track                           June 27, 2013
Expires: December 29, 2013

                          The file URI Scheme


   This document specifies the file Uniform Resource Identifier (URI)
   scheme that was originally specified in [RFC1738].  The purpose of
   this document is to keep the information about the scheme on
   standards track, since [RFC1738] has been made obsolete.

Note to Readers

   This draft should be discussed on its github project page [github].

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on December 29, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   2
   2.  History . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Scheme Definition . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Implementation Notes  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Hierarchical Structure  . . . . . . . . . . . . . . . . .   5
     4.2.  Relative file paths . . . . . . . . . . . . . . . . . . .   5
     4.3.  Drives, drive letters, mount points, file system root . .   6
     4.4.  UNC File Paths  . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Namespaces  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.6.  Character sets and encodings  . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9

1.  Introduction

   URIs were previously defined in [RFC1738], which was updated by
   [RFC3986].  Those documents also specify how to define schemes for

   The first definition for many URI schemes appeared in [RFC1738].
   Because that document has been made obsolete, this document copies
   the file URI scheme from it to allow that material to remain on
   standards track.

1.1.  Conventions and Terminology

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

2.  History

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

   o  the use of slashes to donate boundaries between directory levels
      of a hierarchical file system

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   o  the requirement that client software convert the file: URL into
      a file name in the local file name conventions

   o  a justification for defining the scheme.

   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.

   The file URI scheme defined in [RFC1738] is referenced three times in
   the current URI Generic Syntax standard [RFC3986], despite the
   former's obsoletion:

   1.  Section 1.1 uses "file:///etc/hosts" as an example

   2.  Section 1.2.3 mentions the "file" scheme regarding relative

   3.  Section 3.2.2 says that '...the "file" URI scheme is defined so
       that no authority, an empty host, and "localhost" all mean the
       end-user's machine...'.

   Finally the WHATWG defines a living URL standard [WHATWG-URL], which
   includes algorithms for interpreting file URIs.

3.  Scheme Definition

   The file URI scheme is used to designate files accessible on a
   particular host computer.  This scheme, unlike most other URI
   schemes, does not designate a resource that is universally accessible
   over the Internet.

   The file URI scheme has historically had little or no
   interoperability between platforms.  Further, implementers on a
   single platform have often disagreed on the syntax to use for a
   particular filesystem.  This document attempts to resolve those
   problems, and define a standard scheme which is interoperable between
   different extant and future implementations.  Additionally, it aims
   to ease implementation by conforming to a general syntax that allows
   existing machinery to parse file: URIs.

   Note that file: and ftp: URIs are not the same, even when the
   target of the ftp: URI is the local host.

   The syntax of a file: URI conforms with the generic syntax
   presented in [RFC3986], with the following components:

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   scheme name
      The literal value "file"

      If present, either the fully qualified domain name of the
      system on which the file is accessible; or one of the special
      values "localhost" or the empty string (""), in which case it is
      interpreted as "the machine from which the URI is being
      interpreted".  An absent authority component SHOULD be interpreted
      as if it were present and had the value "localhost".

      A host name, if supplied, is intended to inform a client on a
      remote machine that it cannot access the file system, or perhaps
      to use some other mechanism to access the file.  It does not imply
      that the file should be accessible over a network connection.

      The hierarchical directory path to the file, using the slash
      character ("/") to separate directories.  Implementations SHOULD
      translate between the URI syntax and the local system's
      conventions for specifying file paths, where they differ.  (See:
      Section 4.1)

      Some systems allow file: URIs to point to directories.  In this
      case, there is usually (but not always) a terminating slash
      character, such as in:


   Because the file URI scheme does not define a retrieval mechanism for
   dereferencing a file: URI, the semantics of a query or fragment
   component are considered unknown and are effectively unconstrained.
   A protocol or system that utilises the file URI scheme MAY define its
   own semantics for query and/or fragment components for file: URIs
   it uses.

   Systems exhibit different levels of case-sensitivity.
   Implementations SHOULD attempt to 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 SHOULD NOT alter the case of file: URIs
   they transport.

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4.  Implementation Notes

4.1.  Hierarchical Structure

   Most implementations of the file URI scheme do a reasonable job of
   mapping the hierarchical part of a directory structure into the slash
   ("/") delimited hierarchy of the URI syntax, independent of the
   native platform's delimiter.

   For example, on Microsoft Windows platforms, it is typical that the
   file system presents backslash ("\") as the file delimeter for file
   names, yet the URI's forward slash ("/") can be used in file: URIs.
   Similarly, on (some) Macintosh OS versions, at least in some
   contexts, the colon (":") is used as the delimiter in the native
   presentation of file path names.  Unix systems natively use the same
   forward slash ("/") delimiter for hierarchy, so there is a closer
   mapping between file: URI paths and native path names.

   In accordance with Section 3.3 of [RFC3986], the path segments
   . and .., also known as dot-segments, are only interpreted within
   the URI path hierarchy and are removed as part of the resolution
   process ([RFC3986], Section 5.2).  Implementations operating on
   or interacting with systems that allow dot-segments in their resolved
   native path representation may be required to escape those segments
   using some other means.

4.2.  Relative file paths

   As relative references are resolved into their respective (absolute)
   target URIs, according to Section 5 of [RFC3986], before any
   dereferencing can take place, this document does not describe that
   resolution.  However, a fully resolved file: URI may contain a non-
   absolute file path.  For example, the URI:


   would be interpreted as file "c", in directory "b", in directory "a",
   on the machine on which the URI is being interpreted (i.e.
   localhost); however there is no apparent indication of the location
   of the directory "a" on that machine.  By convention an absolute file
   path would begin with a slash character ("/") on a Unix-based system,
   or a drive letter (e.g. "c:\") on a Microsoft Windows system, etc.

   Resolution of relative file paths is left undefined by this

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4.3.  Drives, drive letters, mount points, file system root

   Historically there has been considerable difference, in practice, for
   handling of the syntax for the "top" of the hierarchy.  The file:
   URI syntax provides one simple place for designating the root of the
   file hierachy, and implementations have diverged, even on the same
   platform, sometimes even within a single application.

   For example, Microsoft DOS- and Windows-based systems support the
   notion of a "drive letter", a single character which represents a
   (virtual) drive, mount point, or device.  Native representations of
   file paths start with the drive letter, a colon, and then the path;
   e.g., "c:\TMP\test.txt".

   Drive letters are mapped into the top of a file: URI in various
   ways.  On systems running some versions of Microsoft Windows, the
   drive letter may be specified with a colon character (":"), however
   sometimes the colon is replaced with a pipe character ("|"), and in
   some implementations the colon is omitted entirely.  The three
   representations MAY be considered equivalent, and any implementation
   which could interact with a Microsoft Windows environment SHOULD
   interpret a single letter, optionally followed by a colon or pipe
   character, in the first segment of the path as a drive letter.  For
   example, the following URIs:


   when interpreted on the same machine, would refer to the same file:


   Implementations SHOULD use a colon character (":") to specify drive
   letters when generating URIs.

   Note that some systems running some versions of Microsoft Windows are
   known to omit the slash before the drive letter, effectively
   replacing the authority component with the drive specification.  In
   line with Postel's robustness principle ("an implementation must be
   conservative in its sending behavior, and liberal in its receiving
   behavior" [RFC791]) implementations that are likely to encounter such
   a URI MAY interpret it as a drive letter, but SHOULD NOT generate
   such URIs.

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4.4.  UNC File Paths

   File names encoded using the Universal Naming Convention (UNC)
   [MS-DTYP], for example Windows shares accessed via the SMB/CIFS
   protocol [MS-SMB2], SHOULD be translated entirely into the path
   segment of a file: URI, including both leading slashes.  For
   example, the UNC path


   would become

                        translated UNC path

   According to [RFC3986], a URI that does not contain an authority
   component cannot begin with two slash characters ("//").  Therefore a
   file: URI that includes a UNC path MUST include an authority

   Note that the "hostname" part of a UNC path refers to the server or
   domain hosting the shared resource, and is usually different from the
   "host" part of the file: URI, which describes the machine from
   which the UNC hostname can be resolved.

   The file URI scheme is unusual in that it does not specify an
   Internet protocol or access method for shared files; as such, its
   utility in network protocols between hosts is limited.

4.5.  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 [MS-DTYP] in Win32 File Namespaces,
   using the prefix _\\?\UNC\_.

   This document does not define a mechanism for encoding namespaced
   file paths into file: URIs.

4.6.  Character sets and encodings

   Local file systems sometimes use many different encodings for
   representing file names.  The URI syntax defined in [RFC3986]
   provides a method of encoding data, presumably for the sake of
   identifying a resource, as a sequence of characters.  The URI

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   characters are, in turn, frequently encoded as octets for transport
   or presentation.  This specification does not mandate any particular
   character encoding for mapping between URI characters and the octets
   used to store or transmit those characters, however for the sake of
   interoperability, file: URI libraries MAY translate the native
   character encoding for file names to and from their equivalent
   Unicode representation [UNICODE] encoded as UTF-8 [RFC3629] and then
   percent-encoded into valid ASCII [RFC20].

   A protocol or system that utilises the file URI scheme MAY restrict
   the encoding of file: URIs it uses, and SHOULD declare such
   restrictions.  If no such declaration is given, implementations
   SHOULD expect percent-encoded UTF-8 Unicode, as described above.

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.  Under no circumstance
   should software using file: URIs grant greater access than would be
   available for other file access methods.

6.  IANA Considerations

   This document does not modify the existing entry in the URI Schemes
   registry [IANA-URI-Schemes], except by updating its reference RFC.

7.  Acknowledgements

   This specification is derived from RFC 1738 [RFC1738], RFC 3986
   [RFC3986], and I-D draft-hoffman-file-uri (expired)
   [I-D.draft-hoffman-file-uri]; the acknowledgements in those documents
   still apply.

8.  References

8.1.  Normative References

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

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

8.2.  Informative References

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

   [RFC20]    Cerf, V., "ASCII format for Network Interchange", RFC 20,
              October 1969.

   [RFC791]   Postel, J., "Internet Protocol - DARPA Internet Program,
              Protocol Specification", RFC 791, September 1981.

   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

              WHATWG, "URL Living Standard", May 2013,

   [MS-DTYP]  Microsoft Open Specifications, "Windows Data Types,
              Section 2.2.56 UNC", January 2013,

   [MS-SMB2]  Microsoft Open Specifications, "Server Message Block (SMB)
              Protocol Versions 2 and 3", January 2013,

              Internet Assigned Numbers Authority, "Uniform Resource
              Identifier (URI) Schemes registry", June 2013, <http://

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              6.1", 2012,

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

   [github]   Kerwin, M., "file-uri-scheme GitHub repository", n.d.,

Author's Address

   Matthew Kerwin


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