URI(7) Linux Programmer's Manual URI(7)
NAME
uri, url, urn - uniform resource identifier (URI), including a URL or
URN
SYNOPSIS
URI = [ absoluteURI | relativeURI ] [ "#" fragment ]
absoluteURI = scheme ":" ( hierarchical_part | opaque_part )
relativeURI = ( net_path | absolute_path | relative_path ) [ "?" query ]
scheme = "http" | "ftp" | "gopher" | "mailto" | "news" | "telnet" |
"file" | "man" | "info" | "whatis" | "ldap" | "wais" | ...
hierarchical_part = ( net_path | absolute_path ) [ "?" query ]
net_path = "//" authority [ absolute_path ]
absolute_path = "/" path_segments
relative_path = relative_segment [ absolute_path ]
DESCRIPTION
A Uniform Resource Identifier (URI) is a short string of characters
identifying an abstract or physical resource (for example, a web page).
A Uniform Resource Locator (URL) is a URI that identifies a resource
through its primary access mechanism (e.g., its network "location"),
rather than by name or some other attribute of that resource. A Uni-
form Resource Name (URN) is a URI that must remain globally unique and
persistent even when the resource ceases to exist or becomes unavail-
able.
URIs are the standard way to name hypertext link destinations for tools
such as web browsers. The string "http://www.kernel.org" is a URL (and
thus it is also a URI). Many people use the term URL loosely as a syn-
onym for URI (though technically URLs are a subset of URIs).
URIs can be absolute or relative. An absolute identifier refers to a
resource independent of context, while a relative identifier refers to
a resource by describing the difference from the current context.
Within a relative path reference, the complete path segments "." and
".." have special meanings: "the current hierarchy level" and "the
level above this hierarchy level", respectively, just like they do in
UNIX-like systems. A path segment which contains a colon character
can't be used as the first segment of a relative URI path (e.g.,
"this:that"), because it would be mistaken for a scheme name; precede
such segments with ./ (e.g., "./this:that"). Note that descendants of
MS-DOS (e.g., Microsoft Windows) replace devicename colons with the
vertical bar ("|") in URIs, so "C:" becomes "C|".
A fragment identifier, if included, refers to a particular named por-
tion (fragment) of a resource; text after a '#' identifies the frag-
ment. A URI beginning with '#' refers to that fragment in the current
resource.
Usage
There are many different URI schemes, each with specific additional
rules and meanings, but they are intentionally made to be as similar as
possible. For example, many URL schemes permit the authority to be the
following format, called here an ip_server (square brackets show what's
optional):
ip_server = [user [ : password ] @ ] host [ : port]
This format allows you to optionally insert a username, a user plus
password, and/or a port number. The host is the name of the host com-
puter, either its name as determined by DNS or an IP address (numbers
separated by periods). Thus the URI <http://fred:fredpassword@exam-
ple.com:8080/> logs into a web server on host example.com as fred
(using fredpassword) using port 8080. Avoid including a password in a
URI if possible because of the many security risks of having a password
written down. If the URL supplies a username but no password, and the
remote server requests a password, the program interpreting the URL
should request one from the user.
Here are some of the most common schemes in use on UNIX-like systems
that are understood by many tools. Note that many tools using URIs
also have internal schemes or specialized schemes; see those tools'
documentation for information on those schemes.
http - Web (HTTP) server
http://ip_server/path
http://ip_server/path?query
This is a URL accessing a web (HTTP) server. The default port is 80.
If the path refers to a directory, the web server will choose what to
return; usually if there is a file named "index.html" or "index.htm"
its content is returned, otherwise, a list of the files in the current
directory (with appropriate links) is generated and returned. An exam-
ple is <http://lwn.net>;.
A query can be given in the archaic "isindex" format, consisting of a
word or phrase and not including an equal sign (=). A query can also
be in the longer "GET" format, which has one or more query entries of
the form key=value separated by the ampersand character (&). Note that
key can be repeated more than once, though it's up to the web server
and its application programs to determine if there's any meaning to
that. There is an unfortunate interaction with HTML/XML/SGML and the
GET query format; when such URIs with more than one key are embedded in
SGML/XML documents (including HTML), the ampersand (&) has to be
rewritten as &. Note that not all queries use this format; larger
forms may be too long to store as a URI, so they use a different inter-
action mechanism (called POST) which does not include the data in the
URI. See the Common Gateway Interface specification at
<http://www.w3.org/CGI>; for more information.
ftp - File Transfer Protocol (FTP)
ftp://ip_server/path
This is a URL accessing a file through the file transfer protocol
(FTP). The default port (for control) is 21. If no username is
included, the username "anonymous" is supplied, and in that case many
clients provide as the password the requestor's Internet email address.
An example is <ftp://ftp.is.co.za/rfc/rfc1808.txt>;.
gopher - Gopher server
gopher://ip_server/gophertype selector
gopher://ip_server/gophertype selector%09search
gopher://ip_server/gophertype selector%09search%09gopher+_string
The default gopher port is 70. gophertype is a single-character field
to denote the Gopher type of the resource to which the URL refers. The
entire path may also be empty, in which case the delimiting "/" is also
optional and the gophertype defaults to "1".
selector is the Gopher selector string. In the Gopher protocol, Gopher
selector strings are a sequence of octets which may contain any octets
except 09 hexadecimal (US-ASCII HT or tab), 0A hexadecimal (US-ASCII
character LF), and 0D (US-ASCII character CR).
mailto - Email address
mailto:email-address
This is an email address, usually of the form name@hostname. See
mailaddr(7) for more information on the correct format of an email
address. Note that any % character must be rewritten as %25. An exam-
ple is <mailto:dwheeler AT dwheeler.com>.
news - Newsgroup or News message
news:newsgroup-name
news:message-id
A newsgroup-name is a period-delimited hierarchical name, such as
"comp.infosystems.www.misc". If <newsgroup-name> is "*" (as in
<news:*>), it is used to refer to "all available news groups". An
example is <news:comp.lang.ada>.
A message-id corresponds to the Message-ID of IETF RFC 1036,
<http://www.ietf.org/rfc/rfc1036.txt>; without the enclosing "<" and
">"; it takes the form unique@full_domain_name. A message identifier
may be distinguished from a news group name by the presence of the "@"
character.
telnet - Telnet login
telnet://ip_server/
The Telnet URL scheme is used to designate interactive text services
that may be accessed by the Telnet protocol. The final "/" character
may be omitted. The default port is 23. An example is <tel-
net://melvyl.ucop.edu/>;.
file - Normal file
file://ip_server/path_segments
file:path_segments
This represents a file or directory accessible locally. As a special
case, ip_server can be the string "localhost" or the empty string; this
is interpreted as "the machine from which the URL is being inter-
preted". If the path is to a directory, the viewer should display the
directory's contents with links to each containee; not all viewers cur-
rently do this. KDE supports generated files through the URL
<file:/cgi-bin>. If the given file isn't found, browser writers may
want to try to expand the filename via filename globbing (see glob(7)
and glob(3)).
The second format (e.g., <file:/etc/passwd>) is a correct format for
referring to a local file. However, older standards did not permit
this format, and some programs don't recognize this as a URI. A more
portable syntax is to use an empty string as the server name, for exam-
ple, <file:///etc/passwd>;; this form does the same thing and is easily
recognized by pattern matchers and older programs as a URI. Note that
if you really mean to say "start from the current location," don't
specify the scheme at all; use a relative address like <../test.txt>,
which has the side-effect of being scheme-independent. An example of
this scheme is <file:///etc/passwd>;.
man - Man page documentation
man:command-name
man:command-name(section)
This refers to local online manual (man) reference pages. The command
name can optionally be followed by a parenthesis and section number;
see man(7) for more information on the meaning of the section numbers.
This URI scheme is unique to UNIX-like systems (such as Linux) and is
not currently registered by the IETF. An example is <man:ls(1)>.
info - Info page documentation
info:virtual-filename
info:virtual-filename#nodename
info:(virtual-filename)
info:(virtual-filename)nodename
This scheme refers to online info reference pages (generated from tex-
info files), a documentation format used by programs such as the GNU
tools. This URI scheme is unique to UNIX-like systems (such as Linux)
and is not currently registered by the IETF. As of this writing, GNOME
and KDE differ in their URI syntax and do not accept the other's syn-
tax. The first two formats are the GNOME format; in nodenames all spa-
ces are written as underscores. The second two formats are the KDE
format; spaces in nodenames must be written as spaces, even though this
is forbidden by the URI standards. It's hoped that in the future most
tools will understand all of these formats and will always accept
underscores for spaces in nodenames. In both GNOME and KDE, if the
form without the nodename is used the nodename is assumed to be "Top".
Examples of the GNOME format are <info:gcc> and <info:gcc#G++_and_GCC>.
Examples of the KDE format are <info:(gcc)> and <info:(gcc)G++ and
GCC>.
whatis - Documentation search
whatis:string
This scheme searches the database of short (one-line) descriptions of
commands and returns a list of descriptions containing that string.
Only complete word matches are returned. See whatis(1). This URI
scheme is unique to UNIX-like systems (such as Linux) and is not cur-
rently registered by the IETF.
ghelp - GNOME help documentation
ghelp:name-of-application
This loads GNOME help for the given application. Note that not much
documentation currently exists in this format.
ldap - Lightweight Directory Access Protocol
ldap://hostport
ldap://hostport/
ldap://hostport/dn
ldap://hostport/dn?attributes
ldap://hostport/dn?attributes?scope
ldap://hostport/dn?attributes?scope?filter
ldap://hostport/dn?attributes?scope?filter?extensions
This scheme supports queries to the Lightweight Directory Access Proto-
col (LDAP), a protocol for querying a set of servers for hierarchically
organized information (such as people and computing resources). See
RFC 2255 <http://www.ietf.org/rfc/rfc2255.txt>; for more information on
the LDAP URL scheme. The components of this URL are:
hostport the LDAP server to query, written as a hostname optionally
followed by a colon and the port number. The default LDAP
port is TCP port 389. If empty, the client determines
which the LDAP server to use.
dn the LDAP Distinguished Name, which identifies the base
object of the LDAP search (see RFC 2253
<http://www.ietf.org/rfc/rfc2253.txt>; section 3).
attributes a comma-separated list of attributes to be returned; see
RFC 2251 section 4.1.5. If omitted, all attributes should
be returned.
scope specifies the scope of the search, which can be one of
"base" (for a base object search), "one" (for a one-level
search), or "sub" (for a subtree search). If scope is
omitted, "base" is assumed.
filter specifies the search filter (subset of entries to return).
If omitted, all entries should be returned. See RFC 2254
<http://www.ietf.org/rfc/rfc2254.txt>; section 4.
extensions a comma-separated list of type=value pairs, where the
=value portion may be omitted for options not requiring it.
An extension prefixed with a '!' is critical (must be sup-
ported to be valid), otherwise it is noncritical
(optional).
LDAP queries are easiest to explain by example. Here's a query that
asks ldap.itd.umich.edu for information about the University of Michi-
gan in the U.S.:
ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US
To just get its postal address attribute, request:
ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US?postalAddress
To ask a host.com at port 6666 for information about the person with
common name (cn) "Babs Jensen" at University of Michigan, request:
ldap://host.com:6666/o=University%20of%20Michigan,c=US??sub?(cn=Babs%20Jensen)
wais - Wide Area Information Servers
wais://hostport/database
wais://hostport/database?search
wais://hostport/database/wtype/wpath
This scheme designates a WAIS database, search, or document (see IETF
RFC 1625 <http://www.ietf.org/rfc/rfc1625.txt>; for more information on
WAIS). Hostport is the hostname, optionally followed by a colon and
port number (the default port number is 210).
The first form designates a WAIS database for searching. The second
form designates a particular search of the WAIS database database. The
third form designates a particular document within a WAIS database to
be retrieved. wtype is the WAIS designation of the type of the object
and wpath is the WAIS document-id.
other schemes
There are many other URI schemes. Most tools that accept URIs support
a set of internal URIs (e.g., Mozilla has the about: scheme for inter-
nal information, and the GNOME help browser has the toc: scheme for
various starting locations). There are many schemes that have been
defined but are not as widely used at the current time (e.g., pros-
pero). The nntp: scheme is deprecated in favor of the news: scheme.
URNs are to be supported by the urn: scheme, with a hierarchical name
space (e.g., urn:ietf:... would identify IETF documents); at this time
URNs are not widely implemented. Not all tools support all schemes.
Character encoding
URIs use a limited number of characters so that they can be typed in
and used in a variety of situations.
The following characters are reserved, that is, they may appear in a
URI but their use is limited to their reserved purpose (conflicting
data must be escaped before forming the URI):
; / ? : @ & = + $ ,
Unreserved characters may be included in a URI. Unreserved characters
include uppercase and lowercase English letters, decimal digits, and
the following limited set of punctuation marks and symbols:
- _ . ! ~ * ' ( )
All other characters must be escaped. An escaped octet is encoded as a
character triplet, consisting of the percent character "%" followed by
the two hexadecimal digits representing the octet code (you can use
uppercase or lowercase letters for the hexadecimal digits). For exam-
ple, a blank space must be escaped as "%20", a tab character as "%09",
and the "&" as "%26". Because the percent "%" character always has the
reserved purpose of being the escape indicator, it must be escaped as
"%25". It is common practice to escape space characters as the plus
symbol (+) in query text; this practice isn't uniformly defined in the
relevant RFCs (which recommend %20 instead) but any tool accepting URIs
with query text should be prepared for them. A URI is always shown in
its "escaped" form.
Unreserved characters can be escaped without changing the semantics of
the URI, but this should not be done unless the URI is being used in a
context that does not allow the unescaped character to appear. For
example, "%7e" is sometimes used instead of "~" in an HTTP URL path,
but the two are equivalent for an HTTP URL.
For URIs which must handle characters outside the US ASCII character
set, the HTML 4.01 specification (section B.2) and IETF RFC 2718 (sec-
tion 2.2.5) recommend the following approach:
1. translate the character sequences into UTF-8 (IETF RFC 2279)--see
utf-8(7)--and then
2. use the URI escaping mechanism, that is, use the %HH encoding for
unsafe octets.
Writing a URI
When written, URIs should be placed inside double quotes (e.g.,
"http://www.kernel.org"), enclosed in angle brackets (e.g.,
<http://lwn.net>;), or placed on a line by themselves. A warning for
those who use double-quotes: never move extraneous punctuation (such as
the period ending a sentence or the comma in a list) inside a URI,
since this will change the value of the URI. Instead, use angle brack-
ets instead, or switch to a quoting system that never includes extrane-
ous characters inside quotation marks. This latter system, called the
'new' or 'logical' quoting system by "Hart's Rules" and the "Oxford
Dictionary for Writers and Editors", is preferred practice in Great
Britain and hackers worldwide (see the Jargon File's section on Hacker
Writing Style, <http://www.fwi.uva.nl/~mes/jargon/h
/HackerWritingStyle.html>, for more information). Older documents sug-
gested inserting the prefix "URL:" just before the URI, but this form
has never caught on.
The URI syntax was designed to be unambiguous. However, as URIs have
become commonplace, traditional media (television, radio, newspapers,
billboards, etc.) have increasingly used abbreviated URI references
consisting of only the authority and path portions of the identified
resource (e.g., <www.w3.org/Addressing>). Such references are primar-
ily intended for human interpretation rather than machine, with the
assumption that context-based heuristics are sufficient to complete the
URI (e.g., hostnames beginning with "www" are likely to have a URI pre-
fix of "http://" and hostnames beginning with "ftp" likely to have a
prefix of "ftp://"). Many client implementations heuristically resolve
these references. Such heuristics may change over time, particularly
when new schemes are introduced. Since an abbreviated URI has the same
syntax as a relative URL path, abbreviated URI references cannot be
used where relative URIs are permitted, and can be used only when there
is no defined base (such as in dialog boxes). Don't use abbreviated
URIs as hypertext links inside a document; use the standard format as
described here.
CONFORMING TO
(IETF RFC 2396) <http://www.ietf.org/rfc/rfc2396.txt>;, (HTML 4.0)
<http://www.w3.org/TR/REC-html40>;.
NOTES
Any tool accepting URIs (e.g., a web browser) on a Linux system should
be able to handle (directly or indirectly) all of the schemes described
here, including the man: and info: schemes. Handling them by invoking
some other program is fine and in fact encouraged.
Technically the fragment isn't part of the URI.
For information on how to embed URIs (including URLs) in a data format,
see documentation on that format. HTML uses the format <A HREF="uri">
text </A>. Texinfo files use the format @uref{uri}. Man and mdoc have
the recently added UR macro, or just include the URI in the text (view-
ers should be able to detect :// as part of a URI).
The GNOME and KDE desktop environments currently vary in the URIs they
accept, in particular in their respective help browsers. To list man
pages, GNOME uses <toc:man> while KDE uses <man:(index)>, and to list
info pages, GNOME uses <toc:info> while KDE uses <info:(dir)> (the
author of this man page prefers the KDE approach here, though a more
regular format would be even better). In general, KDE uses <file:/cgi-
bin/> as a prefix to a set of generated files. KDE prefers documenta-
tion in HTML, accessed via the <file:/cgi-bin/helpindex>. GNOME
prefers the ghelp scheme to store and find documentation. Neither
browser handles file: references to directories at the time of this
writing, making it difficult to refer to an entire directory with a
browsable URI. As noted above, these environments differ in how they
handle the info: scheme, probably the most important variation. It is
expected that GNOME and KDE will converge to common URI formats, and a
future version of this man page will describe the converged result.
Efforts to aid this convergence are encouraged.
Security
A URI does not in itself pose a security threat. There is no general
guarantee that a URL, which at one time located a given resource, will
continue to do so. Nor is there any guarantee that a URL will not
locate a different resource at some later point in time; such a guaran-
tee can be obtained only from the person(s) controlling that namespace
and the resource in question.
It is sometimes possible to construct a URL such that an attempt to
perform a seemingly harmless operation, such as the retrieval of an
entity associated with the resource, will in fact cause a possibly dam-
aging remote operation to occur. The unsafe URL is typically con-
structed by specifying a port number other than that reserved for the
network protocol in question. The client unwittingly contacts a site
that is in fact running a different protocol. The content of the URL
contains instructions that, when interpreted according to this other
protocol, cause an unexpected operation. An example has been the use
of a gopher URL to cause an unintended or impersonating message to be
sent via a SMTP server.
Caution should be used when using any URL that specifies a port number
other than the default for the protocol, especially when it is a number
within the reserved space.
Care should be taken when a URI contains escaped delimiters for a given
protocol (for example, CR and LF characters for telnet protocols) that
these are not unescaped before transmission. This might violate the
protocol, but avoids the potential for such characters to be used to
simulate an extra operation or parameter in that protocol, which might
lead to an unexpected and possibly harmful remote operation to be per-
formed.
It is clearly unwise to use a URI that contains a password which is
intended to be secret. In particular, the use of a password within the
"userinfo" component of a URI is strongly recommended against except in
those rare cases where the "password" parameter is intended to be pub-
lic.
BUGS
Documentation may be placed in a variety of locations, so there cur-
rently isn't a good URI scheme for general online documentation in
arbitrary formats. References of the form <file:///usr/doc/ZZZ>; don't
work because different distributions and local installation require-
ments may place the files in different directories (it may be in
/usr/doc, or /usr/local/doc, or /usr/share, or somewhere else). Also,
the directory ZZZ usually changes when a version changes (though file-
name globbing could partially overcome this). Finally, using the file:
scheme doesn't easily support people who dynamically load documentation
from the Internet (instead of loading the files onto a local filesys-
tem). A future URI scheme may be added (e.g., "userdoc:") to permit
programs to include cross-references to more detailed documentation
without having to know the exact location of that documentation.
Alternatively, a future version of the filesystem specification may
specify file locations sufficiently so that the file: scheme will be
able to locate documentation.
Many programs and file formats don't include a way to incorporate or
implement links using URIs.
Many programs can't handle all of these different URI formats; there
should be a standard mechanism to load an arbitrary URI that automati-
cally detects the users' environment (e.g., text or graphics, desktop
environment, local user preferences, and currently executing tools) and
invokes the right tool for any URI.
SEE ALSO
lynx(1), man2html(1), mailaddr(7), utf-8(7)
IETF RFC 2255 <http://www.ietf.org/rfc/rfc2255.txt>;
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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