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Applications Area Working Group                                M. Kerwin
Internet-Draft                                                       QUT
Obsoletes: 1738 (if approved)                               May 15, 2016
Intended status: Standards Track
Expires: November 16, 2016

The file URI Scheme
draft-ietf-appsawg-file-scheme-09

Abstract

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

It defines a common syntax which is intended to interoperate across
the broad spectrum of existing usages.  At the same time it notes
some other current practices around the use of file URIs.

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 November 16, 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
3.  Operations Involving file URIs  . . . . . . . . . . . . . . .   5
4.  File Name Encoding  . . . . . . . . . . . . . . . . . . . . .   5
5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
Appendix A.  Differences from Previous Specifications . . . . . .   9
Appendix B.  Example URIs . . . . . . . . . . . . . . . . . . . .   9
Appendix C.  Similar Technologies . . . . . . . . . . . . . . . .  10
Appendix D.  System-specific Operations . . . . . . . . . . . . .  10
D.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  11
D.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  11
D.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  11
D.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  11
Appendix E.  Nonstandard Syntax Variations  . . . . . . . . . . .  11
E.1.  User Information  . . . . . . . . . . . . . . . . . . . .  11
E.2.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  12
E.2.1.  Relative Paths  . . . . . . . . . . . . . . . . . . .  12
E.2.2.  Vertical Bar Character  . . . . . . . . . . . . . . .  13
E.3.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  14
E.3.1.  file URI with Authority . . . . . . . . . . . . . . .  14
E.3.2.  file URI with UNC Path  . . . . . . . . . . . . . . .  15
E.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  16
Appendix F.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  16
Appendix G.  Collected Rules  . . . . . . . . . . . . . . . . . .  16
Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

A file URI identifies an object (a "file") stored in a structured
object naming and accessing environment on a host (a "file system.")
The URI can be used in discussions about the file, and if other

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conditions are met it can be dereferenced to directly access the
file.

This document specifies a syntax based on the generic syntax of
[RFC3986] that is compatible with most existing usages.  Optional
extensions to the syntax which might be encountered in practice are
listed in appendices; these extensions are listed for informational
purposes only.

The file URI scheme is not coupled with a specific protocol, nor with
a specific media type [RFC6838].  See Section 3 for a discussion of
operations that can be performed on the object identified by a file
URI.

1.1.  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 file" is used to describe
files that can be accessed through the local file system API using
only the information included in the file path, not relying on other
information such as network addresses.  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.

The term "local file URI" is used to describe file URIs which have no
authority, or where the authority is the special string "localhost"
or a fully qualified domain name that resolves to the machine from
which the URI is being interpreted (Section 2).

2.  Syntax

The file URI syntax is defined here in Augmented Backus-Naur Form
(ABNF) [RFC5234], importing the "host" and "path-absolute" rules from
[RFC3986] (as updated by [RFC6874].)

The generic 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.

The syntax definition below is different from those given in
[RFC1630] and [RFC1738] as it is derived from the generic syntax of
[RFC3986], which post-dates the previous file URI specifications.
Appendix A enumerates significant differences.

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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      = "localhost"
/ host

The "host" is the fully qualified domain name of the system on which
the file is accessible.  This allows a client on another system to
know that it cannot access the file system, or perhaps to use some
other local mecahnism to access the file.

As a special case, the "file-auth" rule can match the string
"localhost" which is interpreted as "the machine from which the URI
is being interpreted," exactly as if no authority were present.  Some
current usages of the scheme incorrectly interpret all values in the
authority of a file URI, including "localhost", as non-local.  Yet
others interpret any value as local, even if the "host" does not
resolve to the local machine.  To maximise compatibility with
previous specifications, users MAY choose to include an "auth-path"
with no "file-auth" when creating a URI.

Some file systems have case-sensitive file naming and some do not.
As such the file URI scheme supports case sensitivity, in order to
retain the case as given.  Any transport-related handling of the file
URI scheme MUST retain the case as given.  Any mapping to or from a
case-insensitive form is soley the responsibility of the
implementation processing the file URI on behalf of the referenced
file system.

Some file systems allow directory objects to be treated as files in
some cases.  This can be reflected in a file URI by omitting the
trailing slash "/" from the path.  Be aware that merging a relative
URI reference to such a base URI as per Section 5.2 of [RFC3986]
could remove the directory name from the resulting target URI.

Also see Appendix E that lists some nonstandard syntax variations
that can be encountered in practice.

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3.  Operations Involving 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.

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 no "file-auth", or the "file-auth" is the special string
"localhost" or a fully qualified domain name that resolves to the
machine from which the URI is being interpreted (Section 2).

This specification neither defines nor forbids any set of operations
that might be performed on a file identified by a non-local file URI.

4.  File Name Encoding

File systems use various encoding schemes to store file and directory
names.  Many modern file systems encode file and directory names as
arbitrary sequences of octets, in which case the representation as an
encoded string often depends on the user's localization settings, or
defaults to UTF-8 [STD63].

Without other encoding information, percent-encoded octets in a file
URI ([RFC3986], Section 2.1) MAY be interpreted according to the
preferred or configured encoding of the system on which the URI is
being interpreted.

5.  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.

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 as the origin for
each file URI [RFC6454], which is the most secure option.

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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.

File systems vary in the way they handle case.  Care must (?) be
taken to avoid issues resulting from possibly unexpected aliasing
from case-only differences between file paths or URIs.  Similarly,
care must be taken to avoid issues resulting from aliasing from
mismatched encodings or Unicode equivalences [UTR15] (see Section 4).

6.  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.

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7.  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 early comments and
suggestions.

8.  References

8.1.  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>.

[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>.

[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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8.2.  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.

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

[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.

[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>.

[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>.

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[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>.

[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>.

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

[RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.

[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>.

[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.  Differences from Previous Specifications

According to the definition in [RFC1738] a file URL always started
with the token "file://", followed by an (optionally blank) host name
and a "/".  The syntax given in Section 2 makes the entire authority
component, including the double slashes "//", optional.

Appendix B.  Example URIs

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

Local files:

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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 C.  Similar Technologies

o  The WHATWG defines a living URL standard [WHATWG-URL], which
includes algorithms for interpreting file URIs (as URLs).

o  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, except that files located by UNC
filespace selector strings are typically stored on a remote
machine and accessed using a network protocol.  A UNC filespace
selector string has three parts: host, share, and path; described
for informational purposes in Appendix F.  Appendix E.3 lists some
ways in which UNC filespace selector strings are currently made to
interoperate with the file URI scheme.

o  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.

Appendix D.  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.

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D.1.  POSIX Systems

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

D.2.  DOS- and Windows-Like Systems

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

Appendix E lists some non-standard techniques for interacting with
DOS- or Windows-like file paths and URIs.

D.3.  Mac OS X Systems

The HFS+ file system uses a non-standard normalization form, similar
to Normalization Form D [UTR15].  Take care when transforming HFS+
file paths to and from URIs (Section 4).

D.4.  OpenVMS Files-11 Systems

When mapping a VMS file path to a file URI, the device name is mapped
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, that is used as the
authority.  Where the original node reference includes a username and
password in an access control string, they can be transcribed into
the authority using the non-standard syntax extension in
Appendix E.1.

Appendix E.  Nonstandard Syntax Variations

These variations may be encountered by existing usages of the file
URI scheme, but are not supported by the normative syntax of
Section 2.

This appendix is not normative.

E.1.  User Information

It might be necessary to include user information such as a username
in a file URI, for example when mapping a VMS file path with a node

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To allow user information to be included in a file URI, the "file-
auth" rule in Section 2 can be replaced with the following:

file-auth      = "localhost"
/ [ userinfo "@" ] host

This uses the "userinfo" rule from [RFC3986].

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,
MUST NOT (?) contain a password.

E.2.  DOS and Windows Drive Letters

On Windows- or DOS-based file systems an 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 ":"

The "ALPHA" rule is defined in [RFC5234].

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 usages of file URIs therefore canonicalize
drive letters by converting them to uppercase.

E.2.1.  Relative Paths

To mimic the behaviour of DOS- or Windows-based file systems,
relative paths beginning with a slash "/" can be resolved relative to
the drive letter, when present, and resolution of ".." dot segments

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(per Section 5.2.4 of [RFC3986]) can be modified to not ever
overwrite the drive letter.

For example:

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") might not be
accepted as dereferenceable URIs in DOS or Windows systems.

E.2.2.  Vertical Bar Character

Historically some usages of file URIs have included 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 can be replaced with the
following:

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"

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

E.3.  UNC Strings

Some usages of the file URI scheme allow UNC filespace selector
strings [MS-DTYP] to be translated to and from file URIs, either by
mapping the equivalent segments of the two schemes (hostname to
authority, sharename+objectnames to path), or by mapping the entire
UNC string to the path segment of a URI.

E.3.1.  file URI with Authority

The following is an algorithmic description of the process of
translating a UNC filespace selector string to a file URI by mapping
the equivalent segments of the two schemes:

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) to conform to the encoding rules of Section 2 of
[RFC3986].

2.  Append a delimiting slash character "/" and the transformed
segment to the URI.

4.  For each objectname:

1.  Transform the objectname to a path segment as above.

2.  Append a delimiting slash character "/" and the transformed
segment to the URI.

For example:

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

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E.3.2.  file URI with UNC Path

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 can be
replaced with the following:

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

unc-authority  = 2*3"/" file-host

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

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

"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 B:

Non-local files:

o  The 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, as per the syntax defined in [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 definition of a "local file URI" by
excluding any with a path that encodes a UNC string.

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E.4.  Backslash as Separator

Historically some usages have copied entire file paths into the path
components of file URIs.  Where DOS or Windows file paths were thus
copied the 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 F.  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].

Appendix G.  Collected Rules

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

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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      = "localhost"
/ [ userinfo "@" ] host

unc-authority  = 2*3"/" 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"

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"

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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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