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Applications Area Working Group                                M. Kerwin
Internet-Draft                                                       QUT
Obsoletes: 1738 (if approved)                             April 12, 2016
Intended status: Standards Track
Expires: October 14, 2016

The file URI Scheme
draft-ietf-appsawg-file-scheme-06

Abstract

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

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

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

This draft should be discussed on the IETF Applications Area Working
Group discussion list <apps-discuss@ietf.org>.

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 http://datatracker.ietf.org/drafts/current/.

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 October 14, 2016.

Copyright (c) 2016 IETF Trust and the persons identified as the

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents

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(http://trustee.ietf.org/license-info) 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.

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
1.1.  History . . . . . . . . . . . . . . . . . . . . . . . . .   3
1.2.  Similar Technologies  . . . . . . . . . . . . . . . . . .   3
1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
3.  Operations on file URIs . . . . . . . . . . . . . . . . . . .   5
3.1.  Translating Local File Path to file URI . . . . . . . . .   5
3.2.  Translating Non-local File Path to file URI . . . . . . .   6
3.3.  Incompatible File Paths . . . . . . . . . . . . . . . . .   7
3.3.1.  Win32 Namespaces  . . . . . . . . . . . . . . . . . .   7
4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
5.  Origins . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
Appendix A.  Example URIs . . . . . . . . . . . . . . . . . . . .  12
Appendix B.  System-specific Operations . . . . . . . . . . . . .  12
B.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  12
B.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  13
B.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  13
B.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  13
Appendix C.  Nonstandard Syntax Variations  . . . . . . . . . . .  13
C.1.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  13
C.1.1.  Relative Paths  . . . . . . . . . . . . . . . . . . .  14
C.1.2.  Vertical Bar Character  . . . . . . . . . . . . . . .  14
C.2.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  15
C.3.  UNC Paths . . . . . . . . . . . . . . . . . . . . . . . .  16
C.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  17
Appendix D.  Example of IRI vs Percent-Encoded URI  . . . . . . .  17
Appendix E.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  18
Appendix F.  Collected Rules  . . . . . . . . . . . . . . . . . .  19
Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

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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, nor with
a specific media type.  See Section 3 for a discussion of operations
that can be performed on a file URI.

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.

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 the
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.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 document.

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 E.  This
document describes but does not specify a means of translating
between UNC filespace selector strings and file URIs in Appendix C.2.

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1.3.  Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

Throughout this document the term "local" is used to describe files
that can be accessed directly through the local file system.  It is
important to note that a local file may not be physically located on
the local machine, for example if a networked file system is
transparently mounted into the local file system.

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" and
"path-absolute" rules from [RFC3986] (as updated by [RFC6874].)

The core syntax in [RFC3986] includes "path" and "authority"
components, for each of which only a subset is used in the definition
of the file URI scheme.  The relevant subset of "path" is "path-
absolute", and the subset of "authority" is "file-auth", given below.

Please note Appendix C that lists some other commonly seen but
nonstandard variations.

file-URI       = file-scheme ":" file-hier-part

file-scheme    = "file"

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

auth-path      = [ file-auth ] path-absolute

local-path     = path-absolute

file-auth      = [ userinfo "@" ] host

The syntax definition above is different from those given in
[RFC1630] and [RFC1738] as it is derived from the generic syntax of
[RFC3986], which post-dates all previous specifications.

As a special case, the "file-auth" rule can match the string
"localhost" or the empty string; either value is interpreted as "the
machine from which the URI is being interpreted," exactly as if no
authority was present.  To maximise compatibility with previous

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specifications, implementations MAY choose to include an empty "file-
auth".

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.

3.  Operations on file URIs

Implementations that provide dereferencing operations on file URIs
SHOULD, at a minimum, provide a read-like operation to return the
contents of a file located by a file URI.  Additional operations MAY
be provided, such as writing to, creating, and deleting files.  See
the POSIX file and directory operations [POSIX] for examples of
standardized operations that can be performed on files.

File URIs can also be translated to and from other, similar
constructs, such as local file paths or UNC strings.

A file URI can be dependably dereferenced or translated to a local
file path only if it is local.  A file URI is considered "local" if
it has a blank or no authority, or the authority is the special
string "localhost".

This specification neither defines nor forbids a mechanism for
accessing non-local files.  See SMB [MS-SMB], NFS [RFC7530], NCP
[NOVELL] for examples of protocols that can be used to access files
over a network.  Also see Appendix C.2 for a non-normative discussion
on translating non-local file URIs to and from UNC strings.

3.1.  Translating Local File Path to file URI

Below is an algorithmic description of the process used to convert a
file path to a URI; see Section 4 for encoding considerations.

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.

4.  Append a slash character "/" to the URI, to signify the path
root.

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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.

This algorithm is intentionally general, and may not cover some
system-specific edge cases.  See Appendix B for brief discussions on
system-specific considerations for some systems.

*Differences from RFC 1738*

In [RFC1738] a file URL always started with the token "file://",
followed by an (optionally blank) authority and a "/".  That "/" was
not considered part of the path.  This implies that the correct
encoding for a 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 observed in practice, and in fact
would have collided with the eventual encoding of UNC strings in URIs
described non-normatively in Appendix C.3.

3.2.  Translating Non-local File Path to file URI

Translating a non-local file path, including 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
MAY be transcribed into the userinfo field of the authority
([RFC3986], Section 3.2.1), security considerations (Section 6)
notwithstanding.

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See Appendix C.2 for an explicit (but non-normative and strictly
optional) handling of UNC strings.

3.3.  Incompatible File Paths

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

3.3.1.  Win32 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

To avoid ambiguity, a file URI SHOULD be transported as an
Internationalized Resource Identifier (IRI) [RFC3987], or as a URI
with non-ASCII characters encoded according to the UTF-8 character
encoding [STD63] and percent-encoded as needed ([RFC3986],
Section 2.5).

The encoding of a file URI depends on the file system that stores the
identified file.  If the file system uses a known non-Unicode
character encoding, the path SHOULD 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
dereferencing or 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)
[RFC3987] to avoid ambiguity.  See Appendix D for examples.

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5.  Origins

As per [RFC6454], Section 4, when determining the origin of a file
URI implementations MAY return an implementation-defined value.

Historically, user agents have granted content from the file URI
scheme a tremendous amount of privilege.  However, granting all local
files such wide privileges can lead to privilege escalation attacks.
Some user agents have had success granting local files directory-
based privileges, but this approach has not been widely adopted.
Other user agents use globally unique identifiers for each file URI,
which is the most secure option.

6.  Security Considerations

There are many security considerations for URI schemes discussed in
[RFC3986].

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.

File systems typically assign an operational meaning to special
characters, such as the "/", "\", ":", "[", and "]" characters, and
to special device names like ".", "..", "...", "aux", "lpt", etc.  In
some cases, merely testing for the existence of such a name will
cause the operating system to pause or invoke unrelated system calls,
leading to significant security concerns regarding denial of service
and unintended data transfer.  It would be impossible for this
specification to list all such significant characters and device
names.  Implementers MUST research the reserved names and characters
for the types of storage device that may be attached to their
application and restrict the use of data obtained from URI components
accordingly.

Additionally, as discussed in the HP OpenVMS Systems Documentation
<http://h71000.www7.hp.com/doc/84final/ba554_90015/ch03s09.html>
"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
[RFC3986].  As such, the userinfo field of a file URI, if present,

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7.  IANA Considerations

This document defines the following URI scheme, so the "Permanent URI
Schemes" registry has been updated accordingly.  This registration
complies with [BCP35].

Scheme name:
file

Status:
permanent

Applications/protocols that use this scheme name:
Commonly used in hypertext documents to refer to files without
depending on network access.  Supported by major browsers.

Windows API (PathCreateFromUrl, UrlCreateFromPath).

Perl LWP.

Contact:
Matthew Kerwin <matthew.kerwin@qut.edu.au>

Change Controller:
This scheme is registered under the IETF tree.  As such, the IETF
maintains change control.

8.  Acknowledgements

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

Additional thanks to Dave Risney, author of the informative IE Blog
article <http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-
windows.aspx>, and Dave Thaler for their comments and suggestions.

9.  References

9.1.  Normative References

[BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<http://www.rfc-editor.org/info/bcp35>.

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[ISO10646]
International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded
Character Set (UCS)", ISO/IEC 10646:2003, December 2003.

[RFC1035]  Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.

[RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.

[RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <http://www.rfc-editor.org/info/rfc3987>.

[RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.

[RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <http://www.rfc-editor.org/info/rfc6874>.

[UTR15]    Davis, M. and K. Whistler, "Unicode Normalization Forms",
August 2012,
<http://unicode.org/reports/tr15/tr15-18.html>.

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9.2.  Informative References

[Bug107540]
Bugzilla@Mozilla, "Bug 107540", October 2007,
<https://bugzilla.mozilla.org/show_bug.cgi?id=107540>.

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

[MS-DTYP]  Microsoft Open Specifications, "Windows Data Types, 2.2.56
UNC", January 2013,
<http://msdn.microsoft.com/en-us/library/gg465305.aspx>.

[MS-NBTE]  Microsoft Open Specifications, "NetBIOS over TCP (NBT)
Extensions", May 2014,
<http://msdn.microsoft.com/en-us/library/dd891412.aspx>.

[MS-SMB]   Microsoft Open Specifications, "Server Message Block (SMB)
Protocol", January 2013,
<http://msdn.microsoft.com/en-us/library/cc246231.aspx>.

[NOVELL]   Novell, "NetWare Core Protocols", 2013,
<http://www.novell.com/developer/ndk/
netware_core_protocols.html>.

[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, DOI 10.17487/RFC1630, June 1994,
<http://www.rfc-editor.org/info/rfc1630>.

[RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
December 1994, <http://www.rfc-editor.org/info/rfc1738>.

[RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.

[RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>.

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[STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/std63>.

[WHATWG-URL]
WHATWG, "URL Living Standard", May 2013,
<http://url.spec.whatwg.org/>.

[Win32-Namespaces]
Microsoft Developer Network, "Naming Files, Paths, and
Namespaces", June 2013, <https://msdn.microsoft.com/en-
au/library/windows/desktop/aa365247(v=vs.85).aspx>.

Appendix A.  Example URIs

The syntax in Section 2 is intended to support file URIs that take
the following forms:

Local files:

o  A traditional file URI for a local file, with an empty authority.
This is the most common format in use today.  E.g.:

*  "file:///path/to/file"

o  The minimal representation of a local file, with no authority
field and an absolute path that begins with a slash "/".  E.g.:

*  "file:/path/to/file"

Non-local files:

o  A non-local file, with an explicit authority.  E.g.:

*  "file://host.example.com/path/to/file"

Appendix B.  System-specific Operations

This appendix is not normative; it highlights some observed
behaviours and provides system-specific guidance for interacting with
file URIs and paths.

B.1.  POSIX Systems

There is little to say about POSIX file systems; the file URI
structure already closely resembles POSIX file paths.

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B.2.  DOS- and Windows-Like Systems

When mapping a DOS- or Windows-like file path to a file URI,
implementations typically map the drive letter (e.g. "c:") into the
first path segment.

See Appendix C for some explicit (but non-normative and strictly
optional) rules for interacting with DOS- or Windows-like file paths
and URIs.

B.3.  Mac OS X Systems

The HFS+ file system uses a non-standard normalization form, similar
to Normalization Form D.  Take care when transforming HFS+ file paths
to and from URIs using Normalization Form C (Section 4).

B.4.  OpenVMS Files-11 Systems

When mapping a VMS file path to a file URI, map the device name into
the first path segment.  Note that the dollars sign "\$" is a reserved
character per the definition in [RFC3986], Section 2.2, so should be
percent-encoded if present in the device name.

If the VMS file path includes a node reference, use that as the
authority.  Where the original node reference includes a username and
password in an access control string, they can be transcribed into
the userinfo field of the authority ([RFC3986], Section 3.2.1),
security considerations (Section 6) notwithstanding.

Appendix C.  Nonstandard Syntax Variations

These variations may be encountered for historical reasons, but are
not supported by the normative syntax of Section 2.

This appendix is not normative.

C.1.  DOS and Windows Drive Letters

On Windows- or DOS-based file systems a absolute file path can begin
with a drive letter.  To facilitate this, the "local-path" rule in
Section 2 can be replaced with the following:

local-path     = [ drive-letter ] path-absolute

drive-letter   = ALPHA ":"

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This is intended to support 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.  E.g.:

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

URIs of the form "file:///c:/path/to/file" are already supported by
the "path-absolute" rule.

Note that comparison of drive letters in DOS or Windows file paths is
case-insensitive.  Some implementations therefore canonicalize drive
letters in file URIs by converting them to uppercase.

C.1.1.  Relative Paths

In DOS- or Windows-based file systems, relative paths beginning with
a slash "/" should be resolved relative to the drive letter, and
resolution of ".." dot segments (per Section 5.2.4 of [RFC3986])
should not ever overwrite the drive letter.

e.g.:

base:       file:///c:/path/to/file.txt
rel. URI:   /some/other/thing.bmp
resolved:   file:///c:/some/other/thing.bmp

base:       file:///c:/foo.txt
rel. URI:   ../../bar.txt
resolved:   file:///c:/bar.txt

Relative paths with a drive letter followed by a character other than
a slash (e.g. "c:bar/baz.txt" or "c:../foo.txt") should not be
accepted as dereferenceable URIs in DOS or Windows systems.

C.1.2.  Vertical Bar Character

Historically some implementations have used a vertical line character
"|" instead of a colon ":" in the drive letter construct.  [RFC3986]
forbids the use of the vertical line, however it may be necessary to
interpret or update old URIs.

For interpreting such URIs, the "auth-path" and "local-path" rules in
Section 2 and the "drive-letter" rule above are replaced with the
following:

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auth-path      = [ file-auth ] path-absolute
/ [ file-auth ] file-absolute

local-path     = [ drive-letter ] path-absolute
/ file-absolute

file-absolute  = "/" drive-letter path-absolute

drive-letter   = ALPHA ":"
/ ALPHA "|"

This is intended to support regular DOS or Windows file URIs with
vertical line characters in the drive letter construct.  E.g.:

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

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

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

To update such an old URI, replace the vertical line "|" with a colon
":".

C.2.  UNC Strings

A UNC filespace selector string can be directly translated to a URI;
see Section 4.  The following is an algorithmic description of the
process of translating a UNC string to a file URI:

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:

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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.

Example:

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

C.3.  UNC Paths

It is common to encounter file URIs that encode entire UNC strings in
the path, usually with all backslash "\" characters replaced with
slashes "/".

To interpret such URIs, the "auth-path" rule in Section 2 is replaced
with the following:

auth-path      = [ file-auth ] path-absolute
/ unc-authority path-absolute

unc-authority  = 2*3"/" [ userinfo "@" ] file-host

file-host      = inline-IP / IPv4address / reg-name

inline-IP      = "%5B" ( IPv6address / IPvFuture ) "%5D"

"IPvFuture", and "reg-name rules from [RFC3986].

Note that the "file-host" rule is the same as "host" but with
percent-encoding applied to "[" and "]" characters.

This extended syntax is intended to support URIs that take the
following forms, in addition to those in Appendix A:

Non-local files:

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

*  "file:////host.example.com/path/to/file"

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o  As above, with an extra slash between the empty authority and the
transformed UNC string, conformant with the definition from
[RFC1738].  E.g.:

*  "file://///host.example.com/path/to/file"

This representation is notably used by the Firefox web browser.
See Bugzilla#107540 [Bug107540].

It also further limits the set of file URIs that can be translated to
a local file path to those with a path that does not encode a UNC
string.

C.4.  Backslash as Separator

Historically some implementations have copied entire file paths into
the path components of file URIs.  Where DOS or Windows file paths
were copied thus, resulting URI strings contained unencoded backslash
"\" characters, which are forbidden by both [RFC1738] and [RFC3986].

It may be possible to translate or update such an invalid file URI by
replacing all backslashes "\" with slashes "/", if it can be
determined with reasonable certainty that the backslashes are
intended as path separators.

Appendix D.  Example of IRI vs Percent-Encoded URI

The following examples demonstrate the advantage of encoding file
URIs as IRIs to avoid ambiguity (see Section 4).

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

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Counter-example: ambiguous file URI:

| Percent-encoded file URI, in any ASCII-compatible document:
|    "file:///%E3%81%A1"
|
| Possible interpretations of the file name, depending on the
| encoding of the file system:
|
|  o UTF-8:
|       <HIRAGANA LETTER TI (U+3061)>
|
|  o Codepage 437:
|       <GREEK SMALL LETTER PI (U+03C0)> +
|       <LATIN SMALL LETTER U WITH DIAERESIS (U+00FC)> +
|       <LATIN SMALL LETTER I WITH ACUTE (U+00ED)>
|
|  o EBCDIC:
|       "Ta~"
|
| etc.

Appendix E.  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.  Note that this definition is informative
only; the normative description is in [MS-DTYP].

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
[RFC4291].

The precise format of "sharename" depends on the protocol; see: SMB
[MS-SMB], NFS [RFC7530], NCP [NOVELL].

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Appendix F.  Collected Rules

Here are the collected syntax rules for all optional appendices,
presented for convenience.  This collected syntax is not normative.

file-URI       = file-scheme ":" file-hier-part

file-scheme    = "file"

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

auth-path      = [ file-auth ] path-absolute
/ [ file-auth ] file-absolute
/ unc-authority path-absolute

local-path     = [ drive-letter ] path-absolute
/ file-absolute

file-auth      = [ userinfo "@" ] host

unc-authority  = 2*3"/" [ userinfo "@" ] file-host

file-host      = inline-IP / IPv4address / reg-name

inline-IP      = "%5B" ( IPv6address / IPvFuture ) "%5D"

file-absolute  = "/" drive-letter path-absolute

drive-letter   = ALPHA ":"
/ ALPHA "|"

This collected syntax is intended to support file URIs that take the
following forms:

Local files:

o  A traditional file URI for a local file, with an empty authority.
E.g.:

*  "file:///path/to/file"

o  The minimal representation of a local file, with no authority
field and an absolute path that begins with a slash "/".  E.g.:

*  "file:/path/to/file"

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o  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.  E.g.:

*  "file:c:/path/to/file"

o  Regular DOS or Windows file URIs, with vertical line characters in
the drive letter construct.  E.g.:

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

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

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

Non-local files:

o  The representation of a non-local file, with an explicit
authority.  E.g.:

*  "file://host.example.com/path/to/file"

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

*  "file:////host.example.com/path/to/file"

o  As above, with an extra slash between the empty authority and the
transformed UNC string.  E.g.:

*  "file://///host.example.com/path/to/file"

Matthew Kerwin
Queensland University of Technology
Kelvin Grove, QLD  4059
Australia

Email: matthew.kerwin@qut.edu.au

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