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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.
1. 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.
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 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.
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
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.
4. 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.
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.
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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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