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URI(7)                     Linux Programmer's Manual                    URI(7)

       uri,  url,  urn - uniform resource identifier (URI), including a URL or

       URI = [ absoluteURI | relativeURI ] [ "#" fragment ]

       absoluteURI = scheme ":" ( hierarchical_part | opaque_part )

       relativeURI = ( net_path | absolute_path | relative_path ) [ "?" query ]

       scheme = "http" | "ftp" | "gopher" | "mailto" | "news" | "telnet" |
                  "file" | "man" | "info" | "whatis" | "ldap" | "wais" | ...

       hierarchical_part = ( net_path | absolute_path ) [ "?" query ]

       net_path = "//" authority [ absolute_path ]

       absolute_path = "/"  path_segments

       relative_path = relative_segment [ absolute_path ]

       A Uniform Resource Identifier (URI) is a  short  string  of  characters
       identifying an abstract or physical resource (for example, a web page).
       A Uniform Resource Locator (URL) is a URI that  identifies  a  resource
       through  its  primary  access mechanism (e.g., its network "location"),
       rather than by name or some other attribute of that resource.   A  Uni-
       form  Resource Name (URN) is a URI that must remain globally unique and
       persistent even when the resource ceases to exist or  becomes  unavail-

       URIs are the standard way to name hypertext link destinations for tools
       such as web browsers.  The string "http://www.kernelnotes.org" is a URL
       (and thus it is also a URI).  Many people use the term URL loosely as a
       synonym for URI (though technically URLs are a subset of URIs).

       URIs can be absolute or relative.  An absolute identifier refers  to  a
       resource  independent of context, while a relative identifier refers to
       a resource by describing  the  difference  from  the  current  context.
       Within  a  relative  path reference, the complete path segments "." and
       ".." have special meanings: "the  current  hierarchy  level"  and  "the
       level  above  this hierarchy level", respectively, just like they do in
       Unix-like systems.  A path segment which  contains  a  colon  character
       can't  be  used  as  the  first  segment  of a relative URI path (e.g.,
       "this:that"), because it would be mistaken for a scheme  name;  precede
       such  segments with ./ (e.g., "./this:that").  Note that descendants of
       MS-DOS (e.g., Microsoft Windows) replace  devicename  colons  with  the
       vertical bar ("|") in URIs, so "C:" becomes "C|".

       A  fragment  identifier, if included, refers to a particular named por-
       tion (fragment) of a resource; text after a '#'  identifies  the  frag-
       ment.   A URI beginning with '#' refers to that fragment in the current

       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

       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:fredpass-
       wordATxyz.com:8080/> logs into a web server  on  host  xyz.com  as  fred
       (using  fredpassword) using port 8080.  Avoid including a password in a
       URI if possible because of the many security risks of having a password
       written  down.  If the URL supplies a username but no password, and the
       remote server requests a password, the  program  interpreting  the  URL
       should request one from the user.

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

       http - Web (HTTP) server


       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 exam-
       ple 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 inter-
       action 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)


       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


       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 exam-
       ple is <mailto:dwheelerATdwheeler.com>.

       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 <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 "@"

       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  <tel-

       file - Normal file


       This  represents  a file or directory accessible locally.  As a special
       case, host can be the string "localhost" or the empty string;  this  is
       interpreted  as  "the machine from which the URL is being interpreted".
       If the path is to a directory, the viewer  should  display  the  direc-
       tory'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 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
       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 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


       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 syn-
       tax.  The first two formats are the GNOME format; in nodenames all spa-
       ces  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

       whatis - Documentation search


       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


       This  loads  GNOME  help for the given application.  Note that not much
       documentation currently exists in this format.

       ldap - Lightweight Directory Access Protocol


       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).   More
       information   on   the  LDAP  URL  scheme  is  available  in  RFC 2255.
       <http://www.ietf.org/rfc/rfc2255.txt>; The components of this URL are:

       hostport    the LDAP server to query, written as a hostname  optionally
                   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  sup-
                   ported   to   be   valid),  otherwise  it  is  non-critical

       LDAP queries are easiest to explain by example.  Here's  a  query  that
       asks  ldap.itd.umich.edu for information about the University of Michi-
       gan in the U.S.:


       To just get its postal address attribute, request:


       To ask a host.com at port 6666 for information about  the  person  with
       common name (cn) "Babs Jensen" at University of Michigan, request:


       wais - Wide Area Information Servers


       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 inter-
       nal 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.,  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.

   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 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  representing the octet code (you can use
       upper or lower case letters for the hexadecimal digits).  For  example,
       a  blank  space must be escaped as "%20", a tab character as "%09", and
       the "&" as "%26".  Because the percent "%"  character  always  has  the
       reserved  purpose  of being the escape indicator, it must be escaped as
       "%25".  It is common practice to escape space characters  as  the  plus
       symbol  (+) in query text; this practice isn't uniformly defined in the
       relevant RFCs (which recommend %20 instead) but any tool accepting URIs
       with  query text should be prepared for them.  A URI is always shown in
       its "escaped" form.

       Unreserved characters can be escaped without changing the semantics  of
       the  URI, but this should not be done unless the URI is being used in a
       context that does not allow the unescaped  character  to  appear.   For
       example,  "%7e"  is  sometimes used instead of "~" in an HTTP URL path,
       but the two are equivalent for an HTTP URL.

       For URIs which must handle characters outside the  US  ASCII  character
       set,  the HTML 4.01 specification (section B.2) and IETF RFC 2718 (sec-
       tion 2.2.5) recommend the following approach:

       1.  translate the character sequences into UTF-8 (IETF RFC 2279) -- see
           utf-8(7) -- and then

       2.  use  the  URI escaping mechanism, that is, use the %HH encoding for
           unsafe octets.

   Writing a URI
       When written,  URIs  should  be  placed  inside  double  quotes  (e.g.,
       "http://www.kernelnotes.org"),   enclosed   in  angle  brackets  (e.g.,
       <http://lwn.net>;), or placed on a line by themselves.   A  warning  for
       those who use double-quotes: never move extraneous punctuation (such as
       the period ending a sentence or the comma in  a  list)  inside  a  URI,
       since this will change the value of the URI.  Instead, use angle brack-
       ets instead, or switch to a quoting system that never includes extrane-
       ous  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 section on  Hacker
       Writing      Style,     http://www.fwi.uva.nl/~mes/jargon/h/HackerWrit-
       ingStyle.html,  for  more  information).   Older  documents   suggested
       inserting  the  prefix  "URL:"  just  before the URI, but this form has
       never caught on.

       The URI syntax was designed to be unambiguous.  However, as  URIs  have
       become  commonplace,  traditional media (television, radio, newspapers,
       billboards, etc.) have increasingly  used  abbreviated  URI  references
       consisting  of  only  the authority and path portions of the identified
       resource (e.g., <www.w3.org/Addressing>).  Such references are  primar-
       ily  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 pre-
       fix 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 here.

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

       Any  tool accepting URIs (e.g., a web browser) on a Linux system should
       be able to handle (directly or indirectly) all of the schemes described
       here,  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
       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  documenta-
       tion  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.

       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 guaran-
       tee  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 dam-
       aging  remote  operation  to  occur.   The unsafe URL is typically con-
       structed 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 per-

       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 pub-

       Documentation may be placed in a variety of locations,  so  there  cur-
       rently  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 require-
       ments 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  file-
       name 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 file sys-
       tem).  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 file-system 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 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.

       lynx(1),   man2html(1),   mailaddr(7),   utf-8(7),    IETF    RFC 2255.

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2000-03-14                            URI(7)