Network Working Group P. Hoffman
Internet-Draft VPN Consortium
Expires: March 19, 2005 September 18, 2004
The file URI Scheme
draft-hoffman-file-uri-01.txt
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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 moved to historic while
keeping the information about the scheme on standards track.
1. Introduction
URIs are were previously defined in RFC 2396 [RFC2396], which was
updated by draft-fielding-uri-rfc2396bis [2396bis]. Those documents
also specify how to define schemes for URIs.
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The first definition for many URI schemes appeared in RFC 1738
[RFC1738]. Because that document has been moved to Historic status,
this document copies the file URI scheme from it to allow that
material to remain on standards track.
2. 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 syntaxt to use for a
particular filesystem. This docoument does not try to resolve those
problems, only to show what has been commonly seen in use on the
Internet.
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
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
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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.
3. 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
on 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".
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".
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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.
4. Security Considerations
There are many security considerations for URI schemes discussed in
draft-fielding-uri-rfc2396bis [2396bis].
5 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.
Author's Address
Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA 95060
US
EMail: paul.hoffman@vpnc.org
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