Applications Area Working Group M. Kerwin
Internet-Draft QUT
Obsoletes: 1738 (if approved) April 14, 2015
Intended status: Standards Track
Expires: October 16, 2015

The file URI Scheme


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

It attemps 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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on October 16, 2015.

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Table of Contents

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. As such, there is no well-defined set of operations that can be performed on file URIs, nor a specific media type associated with them.

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 In many cases it simultaneously acknowledges and deprecates some less common or outdated constructs.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 former that clarified aspects such as:

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 a means of translating between UNC filespace selector strings and file URIs.

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

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.

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 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, 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 local file paths or UNC strings.

A file URI can only be translated to a local file path if it has a blank or no authority. Note that this differs from the previous specification in [RFC1738], in that previously an authority of “localhost” was used to refer to the local file system, but in this specification it equates to a UNC string with the host “localhost”.

This specification neither defines nor forbids a mechanism for accessing non-local files. See SMB [MS-SMB], NFS [RFC3530], NCP [NOVELL] for examples of protocols that can be used to access files over a network.

3.1. Translating Local File Path to file URI

Below is an algorithmic description of the process used to convert a file path to an Internationalized Resource Identifier (IRI) [RFC3987], which can then be translated to a URI as per Section 3.1 of [RFC3987]; see: Section 4.

  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.
  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.

Differences from RFC 1738

In [RFC1738] a file URL always started with the token “file://”, followed by an 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 used in practice, and in fact would have collided with the eventual encoding of UNC strings in URIs described in Appendix C.2.

3.2. Translating UNC String to file URI

A UNC filespace selector string can be directly translated to an Internationalized Resource Identifier (IRI) [RFC3987], which can then be translated to a URI as per Section 3.1 of [RFC3987]; see: Section 4.

  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:
    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.


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

Differences from RFC 1738

In [RFC1738] a file URL an authority of “localhost” was used to refer to the local file system, but in this specification it equates to a UNC string with the host “localhost”.

3.3. Translating Non-local File Path to file URI

Translating a non-local file path other than 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 includes a username and password in an access control string, they MAY be transcribed into the userinfo field of the authority ([RFC3986], Section 3.2.1), security considerations (Section 5) notwithstanding.

3.4. Incompatible File Paths

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

3.4.1. 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

The encoding of a file URI depends on the file system. 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 translating to a file path.

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.

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. 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 securt 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 “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 password in a “user:password” userinfo field is deprecated by [RFC3986]. As such, the userinfo field of a file URI, if present, MUST NOT contain a password.

6. IANA Considerations

IANA maintains the registry of URI Schemes [BCP115] at

This document defines the following URI scheme, so the “Permanent URI Schemes” registry has been updated accordingly.

URI Scheme Description Reference
file Host-specific file names RFC XXXX

[RFC Editor Note: Replace XXXX with this RFC’s reference.]

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, and Dave Thaler for their comments and suggestions.

8. References

8.1. Normative References

[BCP115] Hansen, T., Hardie, T. and L. Masinter, "Guidelines and Registration Procedures for New URI Schemes", BCP 35, RFC 4395, February 2006.
[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, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6874] Carpenter, B., Cheshire, S. and R. Hinden, "Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers", RFC 6874, February 2013.
[UTR15] Davis, M. and K. Whistler, "Unicode Normalization Forms", August 2012.

8.2. Informative References

[I-D.hoffman-file-uri] Hoffman, P., "The file URI Scheme", Internet-Draft draft-hoffman-file-uri-03, January 2005.
[MS-DTYP] Microsoft Open Specifications, "Windows Data Types, 2.2.56 UNC", January 2013.
[MS-NBTE] Microsoft Open Specifications, "NetBIOS over TCP (NBT) Extensions", May 2014.
[MS-SMB] Microsoft Open Specifications, "Server Message Block (SMB) Protocol", January 2013.
[NOVELL] Novell, "NetWare Core Protocols", 2013.
[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, June 1994.
[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource Locators (URL)", RFC 1738, December 1994.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame, C., Eisler, M. and D. Noveck, "Network File System (NFS) version 4 Protocol", RFC 3530, April 2003.
[STD63] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[WHATWG-URL] WHATWG, "URL Living Standard", May 2013.
[Win32-Namespaces] Microsoft Developer Network, "Naming Files, Paths, and Namespaces", June 2013.

Appendix A. Example URIs

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

Local files:

Non-local files:

Appendix B. System-specific Operations

This section provides examples of some system-specific thingies.

This section is not normative. [FIXME: it’s also incomplete]

B.1. POSIX Systems

B.2. DOS- and Windows-Like Systems

When mapping a DOS- or Windows-like file path to a URI, use the drive letter (e.g. “c:”) as the first path segment.

Some implementations leave the leading slash off before the drive letter. See Appendix C.1)

Some implementations replace “:” with “|”, while others leave it off completed. See Appendix C.1.1

B.3. Mac OS X Systems

B.4. OpenVMS Files-11 Systems

When mapping a VMS file path to a file URI, use the device name as the first path segment. Note that the dollars sign “$” is a reserved character ([RFC3986], Section 2.2), so should be percent-encoded.

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 5) 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 section 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 ":"

This is intended to support URIs of the form:

C.1.1. 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:

   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 URIs of the form:

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

C.2. UNC Paths

It is common to encounter file URIs that encode entire UNC strings in the path, 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"/" authority

For example:

       \_____/\__________________________ /
       Scheme     Transformed UNC string

       \_____/|\__________________________ /
       Scheme |    Transformed UNC string
              Extra slash

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

Non-local files:

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

C.3. 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

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
| (unknown) encoding of the file system:
|  o UTF-8:
|       <HIRAGANA LETTER TI (U+3061)>
|  o Codepage 437:
|       <GREEK SMALL LETTER PI (U+03C0)> +
|  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>

The precise format of sharename depends on the protocol; see: SMB [MS-SMB], NFS [RFC3530], NCP [NOVELL].

Author's Address

Matthew Kerwin QUT EMail: