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 aq#aq identifies the fragment.
A URI beginning with aq#aq refers to that fragment in the current resource.
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
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
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,
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
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
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 prefixed with a aq!aq 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
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 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 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 hackers worldwide
(see the
Jargon File's section 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
(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.
CONFORMING TO
(IETF RFC 2396)
(HTML 4.0)
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 (viewers 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 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 guarantee 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 damaging remote operation to occur.
The unsafe URL
is typically constructed 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 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 recommended against except
in those rare cases where the "password" parameter is intended to be public.
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 local filesystem).
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 automatically
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
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Usage
-
- Character encoding
-
- Writing a URI
-
- CONFORMING TO
-
- NOTES
-
- Security
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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