url
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 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.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 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 descendents 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 user name, 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:fredpass-
word@xyz.com:8080/> logs into a web server on host xyz.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 pass-
word 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 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 mean-
ing 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 differ-
ent 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 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 <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, 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 direc-
tory’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, e.g.,
<file:///etc/passwd>; this form does the same thing and is easily rec-
ognized by pattern matchers and older programs as a URI. Note that if
you really mean to say "start from the current location," don’t spec-
ify 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
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 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 Pro-
tocol (LDAP), a protocol for querying a set of servers for hierarchi-
cally-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 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 ’!’ is critical (must be
supported to be valid), otherwise it’s non-critical
(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%20Michi-
gan,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%20Michi-
gan,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 num-
ber 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 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 representing 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", 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 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 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 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
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, par-
ticularly 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 stan-
dard 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 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 for-
mat, 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. Nei-
ther 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 guar-
antee can only be obtained from the person(s) controlling that names-
pace 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 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 num-
ber 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 proto-
cols) 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 opera-
tion 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 pub-
lic.
CONFORMING TO
IETF RFC 2396, HTML 4.0.
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 docu-
mentation 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 documen-
tation without having to know the exact location of that documenta-
tion. 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.
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.
Linux 2000-03-14 URI(7)
