Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Expires: July 2, 2005                                       January 2005


                          The file URI Scheme
                     draft-hoffman-file-uri-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies the file Uniform Resource Identifier (URI)
   scheme that was originally specified in RFC 1738.  The purpose of
   this document is to allow RFC 1738 to be made obsolete while keeping
   the information about the scheme on standards track.

.  Introduction

   URIs were previously defined in RFC 2396 [RFC2396], which was updated



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   by draft-fielding-uri-rfc2396bis [2396bis].  Those documents also
   specify how to define schemes for URIs.

   The first definition for many URI schemes appeared in RFC 1738
   [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.

.  Scheme Definition

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

   The file URL 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 does not try to resolve those
   problems, only to show what has been commonly seen in use on the
   Internet.

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

   A file URL takes the form:

   file://<host>/<path>

   where <host> is the fully qualified domain name of the system on
   which the <path> is accessible, and <path> is a hierarchical
   directory path of the form <directory>/<directory>/.../<name>.

   As a special case, <host> can be the string "localhost" or the empty
   string; this is interpreted as "the machine from which the URL is
   being interpreted".  However, this part of the syntax has been
   ignored on many systems.  That is, for some systems, the following
   are considered equal, while on others they are not:

      file://localhost/path/to/file.txt
      file:///path/to/file.txt

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

   file://usr/local/bin/

   On systems running some versions of Microsoft Windows, the local



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   drive specification is sometimes preceded by a "/" character.  Thus,
   for a file called "example.ini" in the "windows" directory on the
   "c:" drive, the URL might be:

   file:///c:/windows/example.ini

   For Windows shares, there is an additional "/" prepended to the name.
   Thus, the file "example.doc" on the shared directory "department"
   would have the URL:

   file:////department/example.doc

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

.  Implementation Notes

3.1  Hierarchical Structure

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

   For example, on 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
   paths and native path names.

3.2  Drives, drive letters, mount points, file system root

   There is 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, 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".




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   Drive letters are mapped into the top of a file URI in various ways,
   depending on the implementation; some applications substitute
   vertical bar ("|") for the colon after the drive letter, yielding
   "file:///c|/tmp/test.txt".  In some cases, the colon is left
   unchanged, as in "file:///c:/tmp/test.txt".  In other cases, the
   colon is simply omitted, as in "file:///c/tmp/test.txt".

3.3  Use of hostname and host name checking

   The file URI specification calls for using the actual host name as
   the name authority and allowing it to be omitted.  This practice is
   rarely followed, and frequently is not checked.  Some applications
   generate URIs with no authority component at all, such as
   "file:/this/is/the/path".

3.4  Character sets and encodings

   Local file systems sometimes use many different encodings for
   representing file names.  For interoperability sake, it would be
   preferable for file: URI libraries to translate the native character
   encoding for file names to and from Unicode.

.  Security Considerations

   There are many security considerations for URI schemes discussed in
   [2396bis].

   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.

  Informative References

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

   [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [2396bis]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", work in
              progress, draft-fielding-uri-rfc2396bis-nn.txt.






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Author's Address

   Paul Hoffman
   VPN Consortium
   127 Segre Place
   Santa Cruz, CA  95060
   US

   EMail: paul.hoffman@vpnc.org










































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