uri(7) Miscellaneous Information 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" | "ftp" | "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 per-
sistent 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 syn-
onym 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 sys-
tems. A path segment which contains a colon character can't be used as
the first segment of a relative URI path (e.g., "this:that"), because it
would be mistaken for a scheme name; precede such segments with ./
(e.g., "./this:that"). Note that descendants of MS-DOS (e.g., Microsoft
Windows) replace devicename colons with the vertical bar ("|") in URIs,
so "C:" becomes "C|".
A fragment identifier, if included, refers to a particular named portion
(fragment) of a resource; text after a '#' identifies the fragment. A
URI beginning with '#' refers to that fragment in the current resource.
Usage
There are many different URI schemes, each with specific additional
rules and meanings, but they are intentionally made to be as similar as
possible. For example, many URL schemes permit the authority to be the
following format, called here an ip_server (square brackets show what's
optional):
ip_server = [user [ : password ] @ ] host [ : port]
This format allows you to optionally insert a username, a user plus
password, and/or a port number. The host is the name of the host com-
puter, either its name as determined by DNS or an IP address (numbers
separated by periods). Thus the URI <http://fred:fredpassword@exam-
ple.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 re-
quest 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' documen-
tation 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 di-
rectory (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 for-
mat; when such URIs with more than one key are embedded in SGML/XML doc-
uments (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 Gate-
way Interface specification at ]8;;http://www.w3.org/CGI\http://www.w3.org/CGI]8;;\ for more informa-
tion.
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 ad-
dress. 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 exam-
ple is <news:comp.lang.ada>.
A message-id corresponds to the Message-ID of ]8;;http://www.ietf.org/rfc/rfc1036.txt\IETF RFC 1036,]8;;\ 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, 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 re-
ferring 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 exam-
ple, <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 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 for-
bidden 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 Proto-
col (LDAP), a protocol for querying a set of servers for hierarchically
organized information (such as people and computing resources). See
]8;;http://www.ietf.org/rfc/rfc2255.txt\RFC 2255]8;;\ 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 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 ]8;;http://www.ietf.org/rfc/rfc2253.txt\RFC 2253]8;;\ 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 as-
sumed.
filter specifies the search filter (subset of entries to return). If
omitted, all entries should be returned. See ]8;;http://www.ietf.org/rfc/rfc2254.txt\RFC 2254]8;;\ 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 ]8;;http://www.ietf.org/rfc/rfc1625.txt\IETF
RFC 1625]8;;\ for more information on WAIS). Hostport is the hostname, op-
tionally 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 sup-
ported by the urn: scheme, with a hierarchical name space (e.g.,
urn:ietf:... would identify IETF documents); at this time URNs are not
widely implemented. Not all tools support all schemes.
Character encoding
URIs use a limited number of characters so that they can be typed in and
used in a variety of situations.
The following characters are reserved, that is, they may appear in a URI
but their use is limited to their reserved purpose (conflicting data
must be escaped before forming the URI):
; / ? : @ & = + $ ,
Unreserved characters may be included in a URI. Unreserved characters
include uppercase and lowercase Latin letters, decimal digits, and the
following limited set of punctuation marks and symbols:
- _ . ! ~ * ' ( )
All other characters must be escaped. An escaped octet is encoded as a
character triplet, consisting of the percent character "%" followed by
the two hexadecimal digits representing the octet code (you can use up-
percase 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 re-
served 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 ex-
ample, "%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 3986 (last
paragraph of section 2.5) recommend the following approach:
(1) translate the character sequences into UTF-8 (IETF RFC 3629)—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 in-
stead, or switch to a quoting system that never includes extraneous
characters inside quotation marks. This latter system, called the 'new'
or 'logical' quoting system by "Hart's Rules" and the "Oxford Dictionary
for Writers and Editors", is preferred practice in Great Britain and in
various European languages. Older documents suggested inserting the
prefix "URL:" just before the URI, but this form has never caught on.
The URI syntax was designed to be unambiguous. However, as URIs have
become commonplace, traditional media (television, radio, newspapers,
billboards, etc.) have increasingly used abbreviated URI references con-
sisting of only the authority and path portions of the identified re-
source (e.g., <www.w3.org/Addressing>). Such references are primarily
intended for human interpretation rather than machine, with the assump-
tion 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 ref-
erences. 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 de-
fined base (such as in dialog boxes). Don't use abbreviated URIs as hy-
pertext links inside a document; use the standard format as described
here.
STANDARDS
]8;;http://www.ietf.org/rfc/rfc2396.txt\(IETF RFC 2396)]8;;\, ]8;;http://www.w3.org/TR/REC-html40\(HTML 4.0)]8;;\.
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 (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 au-
thor of this man page prefers the KDE approach here, though a more regu-
lar 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 lo-
cate 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 per-
form a seemingly harmless operation, such as the retrieval of an entity
associated with the resource, will in fact cause a possibly damaging re-
mote operation to occur. The unsafe URL is typically constructed by
specifying a port number other than that reserved for the network proto-
col in question. The client unwittingly contacts a site that is in fact
running a different protocol. The content of the URL contains instruc-
tions 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 per-
formed.
It is clearly unwise to use a URI that contains a password which is in-
tended 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 pub-
lic.
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 arbi-
trary 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 Inter-
net (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 ver-
sion of the filesystem 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 im-
plement 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.
SEE ALSO
lynx(1), man2html(1), mailaddr(7), utf-8(7)
]8;;http://www.ietf.org/rfc/rfc2255.txt\IETF RFC 2255]8;;\
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