man > urn(7)

uri(7)                            Miscellaneous Information Manual                            uri(7)

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

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

       absoluteURI = scheme ":" ( hierarchical_part | opaque_part )

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

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

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

       net_path = "//" authority [ absolute_path ]

       absolute_path = "/" path_segments

       relative_path = relative_segment [ absolute_path ]

DESCRIPTION
       A  Uniform  Resource Identifier (URI) is a short string of characters identifying an abstract
       or physical resource (for example, a web page).  A Uniform Resource Locator (URL)  is  a  URI
       that  identifies  a  resource  through its primary access mechanism (e.g., its network "loca‐
       tion"), 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 de‐
       vicename 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 re‐
       source; 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 per‐
       mit 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@exam‐
       ple.com:8080/> logs into a web server on host example.com as fred (using fredpassword)  using
       port  8080.   Avoid  including  a  password in a URI if possible because of the many security
       risks of having a password written down.  If the URL supplies a username but no password, and
       the remote server requests a password, the program interpreting the URL  should  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 "in‐
       dex.html" or "index.htm" its content is returned, otherwise, a list of the files in the  cur‐
       rent   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 embed‐
       ded 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 infor‐
       mation on the correct format of an email address.  Note that any % character must be  rewrit‐
       ten as %25.  An example is <mailto:dwheeler AT 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.infosys‐
       tems.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, 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 ex‐
       ample, <file:///etc/passwd>; this form does the same thing and is easily recognized  by  pat‐
       tern  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  documen‐
       tation 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 under‐
       stand  all  of  these formats and will always accept underscores for spaces in nodenames.  In
       both GNOME and KDE, if the form without the nodename is used the nodename is  assumed  to  be
       "Top".   Examples of the GNOME format are <info:gcc> and <info:gcc#G++_and_GCC>.  Examples of
       the KDE format are <info:(gcc)> and <info:(gcc)G++ and GCC>.

       whatis - Documentation search

       whatis:string

       This scheme searches the database of short (one-line) descriptions of commands and returns  a
       list  of  descriptions containing that string.  Only complete word matches are returned.  See
       whatis(1).  This URI scheme is unique to UNIX-like systems (such as Linux) and  is  not  cur‐
       rently registered by the IETF.

       ghelp - GNOME help documentation

       ghelp:name-of-application

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

       ldap - Lightweight Directory Access Protocol

       ldap://hostport
       ldap://hostport/
       ldap://hostport/dn
       ldap://hostport/dn?attributes
       ldap://hostport/dn?attributes?scope
       ldap://hostport/dn?attributes?scope?filter
       ldap://hostport/dn?attributes?scope?filter?extensions

       This scheme supports queries to the Lightweight Directory Access Protocol (LDAP), a  protocol
       for  querying  a  set of servers for hierarchically organized information (such as people and
       computing resources).  See RFC 2255 for more information on the LDAP URL scheme.  The  compo‐
       nents 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 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 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 for  more  in‐
       formation 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 par‐
       ticular search of the WAIS database database.  The third form designates a  particular  docu‐
       ment  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  de‐
       fined  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  un‐
       escaped  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 spec‐
       ification (section B.2) and IETF RFC 3986 (last paragraph of section 2.5) recommend the  fol‐
       lowing 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"), en‐
       closed  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  pe‐
       riod 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 Edi‐
       tors", is preferred practice in Great Britain and in various European languages.  Older docu‐
       ments  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  identi‐
       fied  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 heuris‐
       tics may change over time, particularly when new schemes are introduced.  Since  an  abbrevi‐
       ated  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), (HTML 4.0).

NOTES
       Any tool accepting URIs (e.g., a web browser) on a Linux system should be able to handle (di‐
       rectly 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  direc‐
       tory  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 con‐
       verged 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 guaran‐
       tee that a URL will not locate a different resource at some later point in time; such a guar‐
       antee  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 harm‐
       less 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  pro‐
       tocol,  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  exam‐
       ple, CR and LF characters for telnet protocols) that these are not unescaped before transmis‐
       sion.   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 un‐
       expected 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 re‐
       quirements may place the files in different directories (it may be in /usr/doc,  or  /usr/lo‐
       cal/doc,  or  /usr/share, or somewhere else).  Also, the directory ZZZ usually changes when a
       version changes (though filename globbing could partially overcome this).  Finally, using the
       file: scheme doesn't easily support people who dynamically load documentation from the Inter‐
       net (instead of loading the files onto a local filesystem).  A future URI scheme may be added
       (e.g., "userdoc:") to permit programs to include cross-references to more detailed documenta‐
       tion without having to know the exact location of that documentation.  Alternatively,  a  fu‐
       ture  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

Linux man-pages 6.7                          2023-10-31                                       uri(7)