| 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
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.
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 (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
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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.
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.
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).
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.
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.
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.
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.
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.
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).
This document defines the following permanent URI scheme. The "Uniform Resource Identifier (URI) Schemes" registry has been updated accordingly. This registration complies with [BCP35].
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [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. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008. |
| [RFC6454] | Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011. |
| [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. |
| [STD63] | Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. |
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.
The syntax in Section 2 is intended to support file URIs that take the following forms:
Local files:
Non-local files:
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.
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.
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.
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).
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.
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.
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 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.
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:
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.
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
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.
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:
To update such an old URI, replace the vertical line "|" with a colon ":".
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.
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:
For example:
UNC String: \\host.example.com\Share\path\to\file.txt URI: file://host.example.com/Share/path/to/file.txt
The inverse algorithm for translating a file URI to a UNC filespace selector string is left as an exercise for the reader.
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].
This extended syntax is intended to support URIs that take the following forms, in addition to those in Appendix B:
Non-local files:
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.
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.
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 "|"
This collected syntax is intended to support file URIs that take the following forms:
Local files:
Non-local files:
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.