# uri

## 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 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 '#' 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 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 &amp;.
           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_ %09 _search_
           gopher://_ip_server_ /_gophertype selector_ %09 _search_ %09 _gopher+_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 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 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩
           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 
                  ⟨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 portion 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 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 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 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 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 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 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 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.
    


## STANDARDS



           (IETF RFC 2396) ⟨http://www.ietf.org/rfc/rfc2396.txt⟩, (HTML 4.0)
           ⟨http://www.w3.org/TR/REC-html40⟩.
    


## 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.
    


* * *