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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%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 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.

SEE ALSO

       lynx(1), man2html(1), mailaddr(7), utf-8(7)

       IETF RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt