uri, url, urn - uniform resource identifier (URI), includ
ing 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 pri
mary 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 destina
tions for tools such as web browsers. The string
"http://www.kernelnotes.org" is a URL (and thus it's 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 rela
tive 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 seg
ment of a relative URI path (e.g., "this:that"), because
it would be mistaken for a scheme name; precede such seg
ments with ./ (e.g., "./this:that"). Note that descen
dents of MS-DOS (e.g., Microsoft Windows) replace device
name colons with the vertical bar ("|") in URIs, so "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 user name, 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@xyz.com:8080/> logs
into a web server on host xyz.com as fred (using fredpass
word) using port 8080. Avoid including a password in a
URI if possible because of the many security risks of hav
ing a password written down. If the URL supplies a user
name 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 spe
cialized 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, con
sisting 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
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 user name "anonymous" is sup
plied, and in that case many clients provide as the pass
word 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 selec
tor%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@host
name. 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,
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, without the enclosing "<" and ">"; it takes the form
unique@full_domain_name. A message identifier may be dis
tinguished 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, host 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 con
tents 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, e.g.,
<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 parenthe
sis and section number; see man(7) for more information on
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 (gener
ated 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 sec
ond 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>. Exam
ples 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 descrip
tions 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 for
mat.
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
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). More information on the
LDAP URL scheme is available in RFC 2255. The components
of this URL are:
hostport the LDAP server to query, written as a host
name 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 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 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 pre
fixed with a '!' is critical (must be sup
ported to be valid), otherwise it's non-criti
cal (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=Univer
sity%20of%20Michigan,c=US
To just get its postal address attribute, request:
ldap://ldap.itd.umich.edu/o=Univer
sity%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://hostport/database
wais://hostport/database?search
wais://hostport/database/wtype/wpath
This scheme designates a WAIS database, search, or docu
ment (see IETF RFC 1625 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 loca
tions). 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. Unre
served characters include include upper and lower case
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 rep
resenting the octet code (you can use upper or lower case
letters for the hexadecimal digits). For example, a blank
space must be escaped as "%20", a tab character as "%09",
always has the reserved purpose of being the escape indi
cator, 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 rele
vant 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 doublequotes
(e.g., "http://www.kernelnotes.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 end
ing 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 hackers worldwide (see the Jargon File's sec
tion on Hacker Writing Style for more information). 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 (tele
vision, radio, newspapers, billboards, etc.) have increas
ingly 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 pri
marily intended for human interpretation rather than
machine, with the assumption that context-based heuristics
are sufficient to complete the URI (e.g., hostnames
"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 only be
used 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.
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 pro
gram 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 documentation 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 con
vergence 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
resource at some later point in time; such a guarantee can
only be obtained 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 damaging remote operation to
occur. The unsafe URL is typically constructed by speci
fying a port number other than that reserved for the net
work protocol in question. The client unwittingly con
tacts 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, espe
cially when it is a number within the reserved space.
Care should be taken when a URI contains escaped delim
iters for a given protocol (for example, CR and LF charac
ters 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 performed.
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 disrecommended except in those rare cases where
the 'password' parameter is intended to be public.
CONFORMING TO
IETF RFC 2396, HTML 4.0.
BUGS
Documentation may be placed in a variety of locations, so
there currently 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 requirements 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 filename 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
(e.g., "userdoc:") to permit programs to include cross-
references to more detailed documentation without having
to know the exact location of that documentation. Alter
natively, 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 for
mats; there should be a standard mechanism to load an
arbitrary URI that automatically detects the users' envi
ronment (e.g., text or graphics, desktop environment,
local user preferences, and currently-executing tools) and
invokes the right tool for any URI.
AUTHOR
David A. Wheeler (dwheeler@dwheeler.com) wrote this man
page.
SEE ALSO
lynx(1), mailaddr(7), utf-8(7), man2html(1), IETF RFC
2255.
Man(1) output converted with
man2html