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 Uniform Resource Name (URN) is a
URI that must remain globally unique and persistent even when the
resource ceases to exist or becomes unavailable.
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 synonym 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
portion (fragment) of a resource; text after a '#' identifies the
fragment. 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 computer, either its name as determined by DNS or an IP
address (numbers separated by periods). Thus the URI
<http://fred:fredpassword@example.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 example 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 interaction 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 example is <mailto:dwheeler@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 <telnet://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 interpreted". If the path is to a directory, the
viewer should display the directory's contents with links to each
containee; not all viewers currently 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 example, <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
texinfo 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 syntax. The first two formats are the
GNOME format; in nodenames all spaces 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 currently 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
Protocol (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 supported 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 Michigan 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 internal 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., prospero). 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 Latin 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 example, 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 (section 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 brackets instead, or switch to a quoting
system that never includes extraneous 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 in
various European languages. Older documents suggested 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 primarily 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 prefix 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.
