| NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON | The Linux Programming Interface |
URI(7) Linux Programmer's Manual URI(7)
uri, url, urn - uniform resource identifier (URI), including a URL or URN
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 ]
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 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.
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@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
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 distin-
guished 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 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 with-
out 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 documen-
tation 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). More information on the
LDAP URL scheme is available in RFC 2255.
<http://www.ietf.org/rfc/rfc2255.txt> The components of this URL are:
hostport the LDAP server to query, written as a hostname optionally fol-
lowed 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 por-
tion 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 des-
ignates 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 informa-
tion, 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 depre-
cated 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 sup-
port all schemes.
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
upper and lower case English letters, decimal digits, and the following lim-
ited set of punctuation marks and symbols:
- _ . ! ~ * ' ( )
All other characters must be escaped. An escaped octet is encoded as a char-
acter 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 (section 2.2.5) recom-
mend 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.
When written, URIs should be placed inside double quotes (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 pre-
ferred 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/Hacker-
WritingStyle.html, 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 only be used when there is no
defined base (such as in dialog boxes). Don't use abbreviated URIs as hyper-
text links inside a document; use the standard format as described here.
http://www.ietf.org/rfc/rfc2396.txt (IETF RFC 2396), http://www.w3.org/TR/REC-
html40 (HTML 4.0).
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, includ-
ing 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 bet-
ter). 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 documenta-
tion. 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 conver-
gence are encouraged.
A URI does not in itself pose a security threat. There is no general guaran-
tee 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 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 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 proto-
col (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 oper-
ation 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" com-
ponent of a URI is strongly recommended against except in those rare cases
where the "password" parameter is intended to be public.
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 dis-
tributions and local installation requirements may place the files in differ-
ent 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 doc-
umentation from the Internet (instead of loading the files onto a local file
system). A future URI scheme may be added (e.g., "userdoc:") to permit pro-
grams to include cross-references to more detailed documentation without hav-
ing to know the exact location of that documentation. Alternatively, a future
version of the file-system specification may specify file locations suffi-
ciently 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.
lynx(1), man2html(1), mailaddr(7), utf-8(7), IETF RFC 2255
<http://www.ietf.org/rfc/rfc2255.txt>
This page is part of release 3.32 of the Linux man-pages project. A descrip-
tion of the project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.
Linux 2000-03-14 URI(7)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface