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       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 pri­
       mary 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  destina­
       tions   for  tools  such  as  web  browsers.   The  string
       "http://www.kernelnotes.org" is a URL  (and  thus  it's  a
       URI).   Many  people use the term URL loosely as a synonym
       for URI (though technically URLs are a subset of URIs).

       URIs can be absolute or relative.  An absolute  identifier
       refers to a resource independent of context, while a rela­
       tive 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 seg­
       ment  of  a relative URI path (e.g., "this:that"), because
       it would be mistaken for a scheme name; precede such  seg­
       ments  with  ./  (e.g., "./this:that").  Note that descen­
       dents of MS-DOS (e.g., Microsoft Windows) replace  device­
       name  colons  with the vertical bar ("|") in URIs, so "C:"
       becomes "C|".
       ip_server = [user [ : password ] @ ] host [ : port]

       This format allows you to optionally insert a user name, a
       user plus password, and/or a port number.  The host is the
       name of the host 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 fredpass­
       word) using port 8080.  Avoid including a  password  in  a
       URI if possible because of the many security risks of hav­
       ing a password written down.  If the URL supplies  a  user
       name  but  no  password,  and the remote server requests a
       password, the program interpreting the URL should  request
       one from the user.

       Here  are  some of the most common schemes in use on Unix-
       like systems that are understood by many tools.  Note that
       many  tools  using URIs also have internal schemes or spe­
       cialized  schemes;  see  those  tools'  documentation  for
       information on those schemes.

   http - Web (HTTP) server

       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, con­
       sisting 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)

       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

       This  is  an email address, usually of the form name@host­
       name.  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

   news - Newsgroup or News message

       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

       A  message-id  corresponds  to  the Message-ID of IETF RFC
       1036, without the enclosing "<" and ">"; it takes the form
       unique@full_domain_name.  A message identifier may be dis­
       tinguished from a news group name by the presence  of  the
       "@" character.

   telnet - Telnet login

       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

       This represents a file or  directory  accessible  locally.
       As  a  special case, host can be the string "localhost" or
       <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

       This refers to local online manual (man) reference  pages.
       The command name can optionally be followed by a parenthe­
       sis 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

   info - Info page documentation

       This scheme refers to online info reference pages  (gener­
       ated  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 sec­
       ond 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>.  Exam­
       ples of the KDE format are <info:(gcc)> and <info:(gcc)G++
       and GCC>.

   whatis - Documentation search

       This  scheme  searches  the  database  of short (one-line)
       descriptions of commands and returns a  list  of  descrip­
       tions  containing that string.  Only complete word matches
       are returned.  See whatis(1).  This URI scheme  is  unique

       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.  The  components
       of this URL are:

       hostport    the  LDAP  server to query, written as a host­
                   name optionally followed by a  colon  and  the
                   port  number.   The  default  LDAP port is TCP
                   port 389.  If  empty,  the  client  determines
                   which the LDAP server to use.

       dn          the  LDAP Distinguished Name, which identifies
                   the base object of the LDAP  search  (see  RFC
                   2253 section 3).

       attributes  a  comma-separated  list  of  attributes to be
                   returned; see  RFC  2251  section  4.1.5.   If
                   omitted, all attributes should be returned.

       scope       specifies  the  scope of the search, which can
                   be one of "base" (for a base  object  search),
                   "one"  (for a one-level search), or "sub" (for
                   a  subtree  search).   If  scope  is  omitted,
                   "base" is assumed.

       filter      specifies the search filter (subset of entries
                   to return). If omitted, all entries should  be
                   returned.  See RFC 2254 section 4.

       extensions  a  comma-separated  list  of type=value pairs,
                   where the =value portion may  be  omitted  for
                   options  not  requiring it.  An extension pre­
                   fixed with a '!' is  critical  (must  be  sup­
                   ported to be valid), otherwise it's non-criti­
                   cal (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.:

       To just get its postal address attribute, request:

       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  loca­
       tions).  There are many schemes that have been defined but
       are not as widely used at the current  time  (e.g.,  pros­
       pero).   The  nntp:  scheme  is deprecated in favor of the
       news: scheme.  URNs  are  to  be  supported  by  the  urn:
       scheme, with a hierarchical name space (e.g., urn:ietf:...
       would identify IETF documents); at this time URNs are  not
       widely implemented.  Not all tools support all schemes.


       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.  Unre­
       served characters include include  upper  and  lower  case
       English letters, decimal digits, and the following limited
       set of punctuation marks and symbols:

               - _ . ! ~ * ' ( )

       All other characters must be escaped.  An escaped octet is
       encoded  as a character triplet, consisting of the percent
       character "%" followed by the two hexadecimal digits  rep­
       resenting  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 indi­
       cator, 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  rele­

       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 doublequotes
       (e.g., "http://www.kernelnotes.org"),  enclosed  in  angle
       brackets  (e.g., <http://lwn.net>), or placed on a line by
       themselves.  A warning for those  who  use  double-quotes:
       never move extraneous punctuation (such as the period 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  preferred  practice  in  Great
       Britain  and hackers worldwide (see the Jargon File's sec­
       tion on Hacker Writing Style for more information).  Older
       documents  suggested  inserting  the  prefix  "URL:"  just
       before the URI, but this form has never caught on.

       The URI syntax was designed to be  unambiguous.   However,
       as  URIs have become commonplace, traditional media (tele­
       vision, radio, newspapers, billboards, etc.) have increas­
       ingly  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 pri­
       marily  intended  for  human  interpretation  rather  than
       machine, with the assumption that context-based heuristics
       are sufficient to complete the URI (e.g., hostnames begin­
       ning  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  hypertext  links
       inside  a  document;  use the standard format as described


       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
       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 con­
       vergence are encouraged.


       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
       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  speci­
       fying  a port number other than that reserved for the net­
       work protocol in question.  The  client  unwittingly  con­
       tacts 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, espe­
       cially when it is a number within the reserved space.

       Care should be taken when a URI  contains  escaped  delim­
       iters for a given protocol (for example, CR and LF charac­
       ters 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,
       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.   Alter­
       natively, 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 for­
       mats;  there  should  be  a  standard mechanism to load an
       arbitrary URI that automatically detects the users'  envi­
       ronment  (e.g.,  text  or  graphics,  desktop environment,
       local user preferences, and currently-executing tools) and
       invokes the right tool for any URI.


       David  A.  Wheeler  (dwheeler@dwheeler.com) wrote this man


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

Linux                       2000-03-14                     URI(7)
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