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rfc:rfc8089

Internet Engineering Task Force (IETF) M. Kerwin Request for Comments: 8089 QUT Updates: 1738 February 2017 Category: Standards Track ISSN: 2070-1721

                       The "file" URI Scheme

Abstract

 This document provides a more complete specification of the "file"
 Uniform Resource Identifier (URI) scheme and replaces the very brief
 definition in Section 3.10 of RFC 1738.
 It defines a common syntax that 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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8089.

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (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.

Kerwin Standards Track [Page 1] RFC 8089 "file" Scheme February 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
 2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Operations Involving <file> URIs  . . . . . . . . . . . . . .   5
 4.  File System Name Encoding . . . . . . . . . . . . . . . . . .   5
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
   7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
 Appendix A.  Differences from Previous Specifications . . . . . .  10
 Appendix B.  Example URIs . . . . . . . . . . . . . . . . . . . .  10
 Appendix C.  Similar Technologies . . . . . . . . . . . . . . . .  10
 Appendix D.  System-Specific Operations . . . . . . . . . . . . .  11
   D.1.  POSIX Systems . . . . . . . . . . . . . . . . . . . . . .  11
   D.2.  DOS- and Windows-Like Systems . . . . . . . . . . . . . .  11
   D.3.  Mac OS X Systems  . . . . . . . . . . . . . . . . . . . .  12
   D.4.  OpenVMS Files-11 Systems  . . . . . . . . . . . . . . . .  12
 Appendix E.  Nonstandard Syntax Variations  . . . . . . . . . . .  12
   E.1.  User Information  . . . . . . . . . . . . . . . . . . . .  12
   E.2.  DOS and Windows Drive Letters . . . . . . . . . . . . . .  13
     E.2.1.  Relative Resolution . . . . . . . . . . . . . . . . .  13
     E.2.2.  Vertical Line Character . . . . . . . . . . . . . . .  14
   E.3.  UNC Strings . . . . . . . . . . . . . . . . . . . . . . .  15
     E.3.1.  <file> URI with Authority . . . . . . . . . . . . . .  15
     E.3.2.  <file> URI with UNC Path  . . . . . . . . . . . . . .  16
   E.4.  Backslash as Separator  . . . . . . . . . . . . . . . . .  17
 Appendix F.  Collected Nonstandard Rules  . . . . . . . . . . . .  17
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

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
 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.  Where
 incompatibilities arise, they are usually in parts of the scheme that
 were underspecified in earlier definitions and have been tightened up
 by more recent specifications.  Appendix A lists significant changes
 to syntax.

Kerwin Standards Track [Page 2] RFC 8089 "file" Scheme February 2017

 Extensions to the syntax that might be encountered in practice are
 listed in Appendix E; these extensions are listed for informational
 purposes and are not a requirement of implementation.
 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] when they
 appear in all upper case.  They may also appear in lower or mixed
 case as English words, without normative meaning.
 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 that have no
 "authority" component 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 postdates the previous file URI specifications.
 Appendix A enumerates significant differences.

Kerwin Standards Track [Page 3] RFC 8089 "file" Scheme February 2017

    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 that it needs
 to use some other local mechanism to access the file.
 As a special case, the "file-auth" rule can match the string
 "localhost" that 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 maximize compatibility with
 previous specifications, users MAY choose to include an "auth-path"
 with no "file-auth" when creating a URI.
 The path component represents the absolute path to the file in the
 file system.  See Appendix D for some discussion of system-specific
 concerns including absolute file paths and file system roots.
 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 solely the responsibility of the
 implementation processing the file URI on behalf of the referenced
 file system.
 Also see Appendix E, which lists some nonstandard syntax variations
 that can be encountered in practice.

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3. Operations Involving <file> URIs

 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 System Name Encoding

 File systems use various encoding schemes to store file and directory
 names.  Many modern file systems store 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].
 When a file URI is produced that represents textual data consisting
 of characters from the Unicode Standard coded character set
 [UNICODE], the data SHOULD be encoded as octets according to the
 UTF-8 character encoding scheme [STD63] before percent-encoding is
 applied (as per Section 2.5 of [RFC3986]).
 A decision not to use percent-encoded UTF-8 is outside the scope of
 this specification.  It will typically require the use of heuristics
 or explicit knowledge about the way the string will be processed.

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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 Treating a non-local file URI as local, or otherwise attempting to
 perform local operations on a non-local URI, can result in security
 problems.
 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 not be possible for this
 specification to list all such significant characters and device
 names.  Implementers should research the reserved names and
 characters for the types of storage devices 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 or from mismatched
 encodings or Unicode equivalences [UAX15] (see Section 4).

6. IANA Considerations

 This document defines the following permanent URI scheme.  The
 "Uniform Resource Identifier (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.
    Used in development libraries, such as:
  • Windows Shell (PathCreateFromUrl, UrlCreateFromPath)
  • libwww-perl - The World-Wide Web library for Perl
 Contact:
    Applications and Real-Time Area <art@ietf.org>

Kerwin Standards Track [Page 6] RFC 8089 "file" Scheme February 2017

 Change Controller:
    IETF <ietf@ietf.org>
 References:
    This RFC

7. References

7.1. Normative References

 [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>.
 [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
            DOI 10.17487/RFC6454, December 2011,
            <http://www.rfc-editor.org/info/rfc6454>.
 [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>.
 [STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003,
            <http://www.rfc-editor.org/info/std63>.

Kerwin Standards Track [Page 7] RFC 8089 "file" Scheme February 2017

7.2. Informative References

 [Bash-Tilde]
            Free Software Foundation, Inc, "Bash Reference Manual:
            Tilde Expansion", September 2016,
            <http://www.gnu.org/software/bash/manual/html_node/
            Tilde-Expansion.html>.
 [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
            and Registration Procedures for URI Schemes", BCP 35,
            RFC 7595, June 2015,
            <http://www.rfc-editor.org/info/bcp35>.
 [Bug107540]
            Bugzilla@Mozilla, "Bug 107540", October 2001,
            <https://bugzilla.mozilla.org/show_bug.cgi?id=107540>.
 [MS-DTYP]  Microsoft, "Windows Data Types: 2.2.57 UNC", October 2015,
            <http://msdn.microsoft.com/en-us/library/gg465305.aspx>.
 [POSIX]    IEEE, "IEEE Std 1003.1, 2013 Edition - Standard for
            Information Technology-- Portable Operating System
            Interface (POSIX(R)) Base Specifications, Issue 7",
            DOI 10.1109/IEEESTD.2013.6506091, April 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>.
 [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax", RFC 2396,
            DOI 10.17487/RFC2396, August 1998,
            <http://www.rfc-editor.org/info/rfc2396>.
 [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>.
 [UAX15]    Davis, M., Ed. and K. Whistler, Ed., "Unicode Standard
            Annex #15: Unicode Normalization Forms", February 2016,
            <http://www.unicode.org/reports/tr15/tr15-44.html>.

Kerwin Standards Track [Page 8] RFC 8089 "file" Scheme February 2017

 [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
            9.0.0", ISBN 978-1-936213-13-9, June 2016,
            <http://www.unicode.org/versions/Unicode9.0.0/>.
 [WHATWG-URL]
            WHATWG, "URL Living Standard", January 2017,
            <https://url.spec.whatwg.org/>.
 [Win32-Namespaces]
            Microsoft Developer Network Blogs, "Naming Files, Paths,
            and Namespaces", June 2013, <https://msdn.microsoft.com/
            en-us/library/windows/desktop/aa365247(v=vs.85).aspx>.
 [Zsh-Tilde]
            "The Z Shell Manual: 14.7 Filename Expansion", December
            2015, <http://zsh.sourceforge.net/Doc/Release/
            Expansion.html#Filename-Expansion>.

Kerwin Standards Track [Page 9] RFC 8089 "file" Scheme February 2017

Appendix A. Differences from Previous Specifications

 The syntax definition in Section 2 inherits incremental differences
 from the general syntax of [RFC1738], as described by Appendix G of
 [RFC2396] and Appendix D of [RFC3986].
 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:
 o  A traditional file URI for a local file with an empty authority.
    This is the most common format in use today.  For example:
  • "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 "/". For example: * "file:/path/to/file" Non-local files: o A non-local file with an explicit authority. For example: * "file:host.example.com/path/to/file"

Appendix C. Similar Technologies

 o  The WHATWG URL specification [WHATWG-URL] defines browser behavior
    for a variety of inputs, including file URIs.  As a living
    document, it changes to reflect updates in browser behavior.  As a
    result, its algorithms and syntax definitions may or may not be
    consistent with this specification.  Implementors should be aware
    of this possible discrepancy if they expect to share file URIs
    with browsers that follow the WHATWG specification.
 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

Kerwin Standards Track [Page 10] RFC 8089 "file" Scheme February 2017

    machine and accessed using a network protocol.  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
 behaviors and provides system-specific guidance for interacting with
 file URIs and paths.  This is not an exhaustive list of operating or
 file systems; rather, it is intended to illustrate certain types of
 interactions that might be encountered.

D.1. POSIX Systems

 In a POSIX file system, the root of the file system is represented as
 a directory with a zero-length name, usually written as "/"; the
 presence of this root in a file URI can be taken as given by the
 initial slash in the "path-absolute" rule.
 Common UNIX shells such as the Bourne-Again SHell (bash) and Z SHell
 (zsh) provide a function known as "tilde expansion" [Bash-Tilde] or
 "filename expansion" [Zsh-Tilde], where a path that begins with a
 tilde character "~" can be expanded out to a special directory name.
 No such facility exists using the file URI scheme; a tilde in a file
 URI is always just a tilde.

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 nonstandard techniques for interacting with
 DOS- or Windows-like file paths and URIs.

Kerwin Standards Track [Page 11] RFC 8089 "file" Scheme February 2017

D.3. Mac OS X Systems

 The Hierarchical File System Plus (HFS+) uses a nonstandard
 normalization form, similar to Normalization Form D [UAX15].  Take
 care when transforming HFS+ file paths to and from URIs (Section 4).

D.4. OpenVMS Files-11 Systems

 When mapping a Virtual Memory System (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
 Section 2.2 of [RFC3986], so it should be percent-encoded if present
 in the device name.
 If the VMS file path includes a node reference, that reference is
 used as the authority.  Where the original node reference includes a
 user name and password in an access control string, they can be
 transcribed into the authority using the nonstandard 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 user name
 in a file URI, for example, when mapping a VMS file path with a node
 reference that includes an access control string.
 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

Kerwin Standards Track [Page 12] RFC 8089 "file" Scheme February 2017

 password in a "user:password" userinfo field is deprecated by
 [RFC3986].  Take care when dealing with information that can be used
 to identify a user or grant access to a system.

E.2. DOS and Windows Drive Letters

 On Windows- or DOS-like 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-like environment, with no authority field
 and an absolute path that begins with a drive letter.  For example:
 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.  In some usages of file URIs, drive letters are
 canonicalized by converting them to uppercase; other usages treat
 URIs that differ only in the case of the drive letter as identical.

E.2.1. Relative Resolution

 To mimic the behavior of DOS- or Windows-like file systems, relative
 references beginning with a slash "/" can be resolved relative to the
 drive letter when present; resolution of ".." dot segments (per
 Section 5.2.4 of [RFC3986]) can be modified to not ever overwrite the
 drive letter.
 For example:
    base URI:   file:///c:/path/to/file.txt
    rel. ref.:  /some/other/thing.bmp
    resolved:   file:///c:/some/other/thing.bmp
    base URI:   file:///c:/foo.txt
    rel. ref.:  ../bar.txt
    resolved:   file:///c:/bar.txt

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 A relative reference starting with a drive letter would be
 interpreted by a generic URI parser as a URI with the drive letter as
 its scheme.  Instead, such a reference ought to be constructed with a
 leading slash "/" character (e.g., "/c:/foo.txt").
 Relative references 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-like systems.

E.2.2. Vertical Line 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.  For example:
 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
 ":".

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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.  Initialize the URI with the "file:" scheme identifier.
 2.  Append the authority:
     1.  Append the "//" authority sigil to the URI.
     2.  Append the host-name field of the UNC string to the URI.
 3.  Append the share-name:
     1.  Transform the share-name to a path segment (see Section 3.3
         of [RFC3986]) 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 object-name:
     1.  Transform the objectname to a path segment as above.
         The colon character ":" is allowed as a delimiter before
         stream-name and stream-type in the file-name, if present.
     2.  Append a delimiting slash character "/" and the transformed
         segment to the URI.
 For example, the UNC String:
    UNC String:   \\host.example.com\Share\path\to\file.txt
 would be transformed into the URI:
    URI:          file://host.example.com/Share/path/to/file.txt

Kerwin Standards Track [Page 15] RFC 8089 "file" Scheme February 2017

 The inverse algorithm for translating a file URI to a UNC filespace
 selector string is left as an exercise for the reader.

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"
 This syntax uses the "IPv4address", "IPv6address", "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 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.  For example:
  • "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].
    For example:
  • "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 file URI with a path that encodes a UNC string.

Kerwin Standards Track [Page 16] RFC 8089 "file" Scheme February 2017

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. Collected Nonstandard 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      = "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 "|"

Kerwin Standards Track [Page 17] RFC 8089 "file" Scheme February 2017

 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.
    For example:
  • "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 "/". For example: * "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. For example: * "file:c:/path/to/file" o Regular DOS or Windows file URIs with vertical line characters in the drive letter construct. For example: * "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.
    For example:
  • "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. For example: * "file:host.example.com/path/to/file" o As above, with an extra slash between the empty authority and the transformed UNC string. For example: * "file:/host.example.com/path/to/file" Kerwin Standards Track [Page 18] RFC 8089 "file" Scheme February 2017 Acknowledgements Contributions from many members of the IETF and W3C communities – notably Dave Crocker, Graham Klyne, Tom Petch, and John Klensin – are greatly appreciated. Additional thanks to Dave Risney, author of the informative IEBlog 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; and to Paul Hoffman, whose earlier work served as an inspiration for this undertaking. Author's Address Matthew Kerwin Queensland University of Technology Victoria Park Road Kelvin Grove, QLD 4059 Australia Email: matthew.kerwin@qut.edu.au Kerwin Standards Track [Page 19]
/data/webs/external/dokuwiki/data/pages/rfc/rfc8089.txt · Last modified: 2017/02/18 04:31 by 127.0.0.1

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