1URI(7)                     Linux Programmer's Manual                    URI(7)


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


10       URI = [ absoluteURI | relativeURI ] [ "#" fragment ]
12       absoluteURI = scheme ":" ( hierarchical_part | opaque_part )
14       relativeURI = ( net_path | absolute_path | relative_path ) [ "?" query ]
16       scheme = "http" | "ftp" | "gopher" | "mailto" | "news" | "telnet" |
17                  "file" | "man" | "info" | "whatis" | "ldap" | "wais" | ...
19       hierarchical_part = ( net_path | absolute_path ) [ "?" query ]
21       net_path = "//" authority [ absolute_path ]
23       absolute_path = "/"  path_segments
25       relative_path = relative_segment [ absolute_path ]


28       A Uniform Resource Identifier (URI) is a  short  string  of  characters
29       identifying an abstract or physical resource (for example, a web page).
30       A Uniform Resource Locator (URL) is a URI that  identifies  a  resource
31       through  its  primary  access mechanism (e.g., its network "location"),
32       rather than by name or some other attribute of that resource.   A  Uni‐
33       form  Resource Name (URN) is a URI that must remain globally unique and
34       persistent even when the resource ceases to exist or  becomes  unavail‐
35       able.
37       URIs are the standard way to name hypertext link destinations for tools
38       such as web browsers.  The string "http://www.kernel.org" is a URL (and
39       thus it is also a URI).  Many people use the term URL loosely as a syn‐
40       onym for URI (though technically URLs are a subset of URIs).
42       URIs can be absolute or relative.  An absolute identifier refers  to  a
43       resource  independent of context, while a relative identifier refers to
44       a resource by describing  the  difference  from  the  current  context.
45       Within  a  relative  path reference, the complete path segments "." and
46       ".." have special meanings: "the  current  hierarchy  level"  and  "the
47       level  above  this hierarchy level", respectively, just like they do in
48       UNIX-like systems.  A path segment which  contains  a  colon  character
49       can't  be  used  as  the  first  segment  of a relative URI path (e.g.,
50       "this:that"), because it would be mistaken for a scheme  name;  precede
51       such  segments with ./ (e.g., "./this:that").  Note that descendants of
52       MS-DOS (e.g., Microsoft Windows) replace  devicename  colons  with  the
53       vertical bar ("|") in URIs, so "C:" becomes "C|".
55       A  fragment  identifier, if included, refers to a particular named por‐
56       tion (fragment) of a resource; text after a '#'  identifies  the  frag‐
57       ment.   A URI beginning with '#' refers to that fragment in the current
58       resource.
60   Usage
61       There are many different URI schemes,  each  with  specific  additional
62       rules and meanings, but they are intentionally made to be as similar as
63       possible.  For example, many URL schemes permit the authority to be the
64       following format, called here an ip_server (square brackets show what's
65       optional):
67       ip_server = [user [ : password ] @ ] host [ : port]
69       This format allows you to optionally insert a  username,  a  user  plus
70       password,  and/or a port number.  The host is the name of the host com‐
71       puter, either its name as determined by DNS or an IP  address  (numbers
72       separated  by  periods).   Thus the URI <http://fred:fredpassword@exam‐
73       ple.com:8080/> logs into a web  server  on  host  example.com  as  fred
74       (using  fredpassword) using port 8080.  Avoid including a password in a
75       URI if possible because of the many security risks of having a password
76       written  down.  If the URL supplies a username but no password, and the
77       remote server requests a password, the  program  interpreting  the  URL
78       should request one from the user.
80       Here  are  some  of the most common schemes in use on UNIX-like systems
81       that are understood by many tools.  Note that  many  tools  using  URIs
82       also  have  internal  schemes  or specialized schemes; see those tools'
83       documentation for information on those schemes.
85       http - Web (HTTP) server
87       http://ip_server/path
88       http://ip_server/path?query
90       This is a URL accessing a web (HTTP) server.  The default port  is  80.
91       If  the  path refers to a directory, the web server will choose what to
92       return; usually if there is a file named  "index.html"  or  "index.htm"
93       its  content is returned, otherwise, a list of the files in the current
94       directory (with appropriate links) is generated and returned.  An exam‐
95       ple is <http://lwn.net>.
97       A  query  can be given in the archaic "isindex" format, consisting of a
98       word or phrase and not including an equal sign (=).  A query  can  also
99       be  in  the longer "GET" format, which has one or more query entries of
100       the form key=value separated by the ampersand character (&).  Note that
101       key  can  be  repeated more than once, though it's up to the web server
102       and its application programs to determine if  there's  any  meaning  to
103       that.   There  is an unfortunate interaction with HTML/XML/SGML and the
104       GET query format; when such URIs with more than one key are embedded in
105       SGML/XML  documents  (including  HTML),  the  ampersand  (&)  has to be
106       rewritten as &amp;.  Note that not all queries use this format;  larger
107       forms may be too long to store as a URI, so they use a different inter‐
108       action mechanism (called POST) which does not include the data  in  the
109       URI.     See    the   Common   Gateway   Interface   specification   at
110http://www.w3.org/CGI⟩ for more information.
112       ftp - File Transfer Protocol (FTP)
114       ftp://ip_server/path
116       This is a URL accessing a  file  through  the  file  transfer  protocol
117       (FTP).   The  default  port  (for  control)  is  21.  If no username is
118       included, the username "anonymous" is supplied, and in that  case  many
119       clients provide as the password the requestor's Internet email address.
120       An example is <ftp://ftp.is.co.za/rfc/rfc1808.txt>.
122       gopher - Gopher server
124       gopher://ip_server/gophertype selector
125       gopher://ip_server/gophertype selector%09search
126       gopher://ip_server/gophertype selector%09search%09gopher+_string
128       The default gopher port is 70.  gophertype is a single-character  field
129       to denote the Gopher type of the resource to which the URL refers.  The
130       entire path may also be empty, in which case the delimiting "/" is also
131       optional and the gophertype defaults to "1".
133       selector is the Gopher selector string.  In the Gopher protocol, Gopher
134       selector strings are a sequence of octets which may contain any  octets
135       except  09  hexadecimal  (US-ASCII HT or tab), 0A hexadecimal (US-ASCII
136       character LF), and 0D (US-ASCII character CR).
138       mailto - Email address
140       mailto:email-address
142       This is an email address,  usually  of  the  form  name@hostname.   See
143       mailaddr(7)  for  more  information  on  the correct format of an email
144       address.  Note that any % character must be rewritten as %25.  An exam‐
145       ple is <mailto:dwheeler@dwheeler.com>.
147       news - Newsgroup or News message
149       news:newsgroup-name
150       news:message-id
152       A  newsgroup-name  is  a  period-delimited  hierarchical  name, such as
153       "comp.infosystems.www.misc".   If  <newsgroup-name>  is  "*"   (as   in
154       <news:*>),  it  is  used  to  refer to "all available news groups".  An
155       example is <news:comp.lang.ada>.
157       A  message-id  corresponds  to  the  Message-ID   of   IETF   RFC 1036,
158http://www.ietf.org/rfc/rfc1036.txt⟩  without  the  enclosing  "<" and
159       ">"; it takes the form unique@full_domain_name.  A  message  identifier
160       may  be distinguished from a news group name by the presence of the "@"
161       character.
163       telnet - Telnet login
165       telnet://ip_server/
167       The Telnet URL scheme is used to designate  interactive  text  services
168       that  may  be accessed by the Telnet protocol.  The final "/" character
169       may be  omitted.   The  default  port  is  23.   An  example  is  <tel‐
170       net://melvyl.ucop.edu/>.
172       file - Normal file
174       file://ip_server/path_segments
175       file:path_segments
177       This  represents  a file or directory accessible locally.  As a special
178       case, ip_server can be the string "localhost" or the empty string; this
179       is  interpreted  as  "the  machine  from  which the URL is being inter‐
180       preted".  If the path is to a directory, the viewer should display  the
181       directory's contents with links to each containee; not all viewers cur‐
182       rently  do  this.   KDE  supports  generated  files  through  the   URL
183       <file:/cgi-bin>.   If  the  given file isn't found, browser writers may
184       want to try to expand the filename via filename globbing  (see  glob(7)
185       and glob(3)).
187       The  second  format  (e.g., <file:/etc/passwd>) is a correct format for
188       referring to a local file.  However, older  standards  did  not  permit
189       this  format,  and some programs don't recognize this as a URI.  A more
190       portable syntax is to use an empty string as the server name, for exam‐
191       ple,  <file:///etc/passwd>; this form does the same thing and is easily
192       recognized by pattern matchers and older programs as a URI.  Note  that
193       if  you  really  mean  to  say "start from the current location," don't
194       specify the scheme at all; use a relative address  like  <../test.txt>,
195       which  has  the side-effect of being scheme-independent.  An example of
196       this scheme is <file:///etc/passwd>.
198       man - Man page documentation
200       man:command-name
201       man:command-name(section)
203       This refers to local online manual (man) reference pages.  The  command
204       name  can  optionally  be followed by a parenthesis and section number;
205       see man(7) for more information on the meaning of the section  numbers.
206       This  URI  scheme is unique to UNIX-like systems (such as Linux) and is
207       not currently registered by the IETF.  An example is <man:ls(1)>.
209       info - Info page documentation
211       info:virtual-filename
212       info:virtual-filename#nodename
213       info:(virtual-filename)
214       info:(virtual-filename)nodename
216       This scheme refers to online info reference pages (generated from  tex‐
217       info  files),  a  documentation format used by programs such as the GNU
218       tools.  This URI scheme is unique to UNIX-like systems (such as  Linux)
219       and is not currently registered by the IETF.  As of this writing, GNOME
220       and KDE differ in their URI syntax and do not accept the  other's  syn‐
221       tax.  The first two formats are the GNOME format; in nodenames all spa‐
222       ces are written as underscores.  The second two  formats  are  the  KDE
223       format; spaces in nodenames must be written as spaces, even though this
224       is forbidden by the URI standards.  It's hoped that in the future  most
225       tools  will  understand  all  of  these  formats and will always accept
226       underscores for spaces in nodenames.  In both GNOME  and  KDE,  if  the
227       form  without the nodename is used the nodename is assumed to be "Top".
228       Examples of the GNOME format are <info:gcc> and <info:gcc#G++_and_GCC>.
229       Examples  of  the  KDE  format  are <info:(gcc)> and <info:(gcc)G++ and
230       GCC>.
232       whatis - Documentation search
234       whatis:string
236       This scheme searches the database of short (one-line)  descriptions  of
237       commands  and  returns  a  list of descriptions containing that string.
238       Only complete word matches are  returned.   See  whatis(1).   This  URI
239       scheme  is  unique to UNIX-like systems (such as Linux) and is not cur‐
240       rently registered by the IETF.
242       ghelp - GNOME help documentation
244       ghelp:name-of-application
246       This loads GNOME help for the given application.  Note  that  not  much
247       documentation currently exists in this format.
249       ldap - Lightweight Directory Access Protocol
251       ldap://hostport
252       ldap://hostport/
253       ldap://hostport/dn
254       ldap://hostport/dn?attributes
255       ldap://hostport/dn?attributes?scope
256       ldap://hostport/dn?attributes?scope?filter
257       ldap://hostport/dn?attributes?scope?filter?extensions
259       This scheme supports queries to the Lightweight Directory Access Proto‐
260       col (LDAP), a protocol for querying a set of servers for hierarchically
261       organized  information  (such  as people and computing resources).  See
262       RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt⟩ for more information  on
263       the LDAP URL scheme.  The components of this URL are:
265       hostport    the  LDAP server to query, written as a hostname optionally
266                   followed by a colon and the port number.  The default  LDAP
267                   port  is  TCP  port  389.   If empty, the client determines
268                   which the LDAP server to use.
270       dn          the LDAP Distinguished  Name,  which  identifies  the  base
271                   object     of     the    LDAP    search    (see    RFC 2253
272http://www.ietf.org/rfc/rfc2253.txt⟩ section 3).
274       attributes  a comma-separated list of attributes to  be  returned;  see
275                   RFC 2251  section 4.1.5.  If omitted, all attributes should
276                   be returned.
278       scope       specifies the scope of the search,  which  can  be  one  of
279                   "base"  (for  a base object search), "one" (for a one-level
280                   search), or "sub" (for a  subtree  search).   If  scope  is
281                   omitted, "base" is assumed.
283       filter      specifies  the search filter (subset of entries to return).
284                   If omitted, all entries should be returned.   See  RFC 2254
285http://www.ietf.org/rfc/rfc2254.txt⟩ section 4.
287       extensions  a  comma-separated  list  of  type=value  pairs,  where the
288                   =value portion may be omitted for options not requiring it.
289                   An  extension prefixed with a '!' is critical (must be sup‐
290                   ported  to  be  valid),   otherwise   it   is   noncritical
291                   (optional).
293       LDAP  queries  are  easiest to explain by example.  Here's a query that
294       asks ldap.itd.umich.edu for information about the University of  Michi‐
295       gan in the U.S.:
297       ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US
299       To just get its postal address attribute, request:
301       ldap://ldap.itd.umich.edu/o=University%20of%20Michigan,c=US?postalAddress
303       To  ask  a  host.com at port 6666 for information about the person with
304       common name (cn) "Babs Jensen" at University of Michigan, request:
306       ldap://host.com:6666/o=University%20of%20Michigan,c=US??sub?(cn=Babs%20Jensen)
308       wais - Wide Area Information Servers
310       wais://hostport/database
311       wais://hostport/database?search
312       wais://hostport/database/wtype/wpath
314       This scheme designates a WAIS database, search, or document  (see  IETF
315       RFC 1625  ⟨http://www.ietf.org/rfc/rfc1625.txt⟩ for more information on
316       WAIS).  Hostport is the hostname, optionally followed by  a  colon  and
317       port number (the default port number is 210).
319       The  first  form  designates a WAIS database for searching.  The second
320       form designates a particular search of the WAIS database database.  The
321       third  form  designates a particular document within a WAIS database to
322       be retrieved.  wtype is the WAIS designation of the type of the  object
323       and wpath is the WAIS document-id.
325       other schemes
327       There  are many other URI schemes.  Most tools that accept URIs support
328       a set of internal URIs (e.g., Mozilla has the about: scheme for  inter‐
329       nal  information,  and  the  GNOME help browser has the toc: scheme for
330       various starting locations).  There are many  schemes  that  have  been
331       defined  but  are  not  as widely used at the current time (e.g., pros‐
332       pero).  The nntp: scheme is deprecated in favor of  the  news:  scheme.
333       URNs  are  to be supported by the urn: scheme, with a hierarchical name
334       space (e.g., urn:ietf:... would identify IETF documents); at this  time
335       URNs are not widely implemented.  Not all tools support all schemes.
337   Character encoding
338       URIs  use  a  limited number of characters so that they can be typed in
339       and used in a variety of situations.
341       The following characters are reserved, that is, they may  appear  in  a
342       URI  but  their  use  is limited to their reserved purpose (conflicting
343       data must be escaped before forming the URI):
345                 ; / ? : @ & = + $ ,
347       Unreserved characters may be included in a URI.  Unreserved  characters
348       include  uppercase  and  lowercase English letters, decimal digits, and
349       the following limited set of punctuation marks and symbols:
351               - _ . ! ~ * ' ( )
353       All other characters must be escaped.  An escaped octet is encoded as a
354       character  triplet, consisting of the percent character "%" followed by
355       the two hexadecimal digits representing the octet  code  (you  can  use
356       uppercase  or lowercase letters for the hexadecimal digits).  For exam‐
357       ple, a blank space must be escaped as "%20", a tab character as  "%09",
358       and the "&" as "%26".  Because the percent "%" character always has the
359       reserved purpose of being the escape indicator, it must be  escaped  as
360       "%25".   It  is  common practice to escape space characters as the plus
361       symbol (+) in query text; this practice isn't uniformly defined in  the
362       relevant RFCs (which recommend %20 instead) but any tool accepting URIs
363       with query text should be prepared for them.  A URI is always shown  in
364       its "escaped" form.
366       Unreserved  characters can be escaped without changing the semantics of
367       the URI, but this should not be done unless the URI is being used in  a
368       context  that  does  not  allow the unescaped character to appear.  For
369       example, "%7e" is sometimes used instead of "~" in an  HTTP  URL  path,
370       but the two are equivalent for an HTTP URL.
372       For  URIs  which  must handle characters outside the US ASCII character
373       set, the HTML 4.01 specification (section B.2) and IETF RFC 2718  (sec‐
374       tion 2.2.5) recommend the following approach:
376       1.  translate  the  character  sequences into UTF-8 (IETF RFC 2279)—see
377           utf-8(7)—and then
379       2.  use the URI escaping mechanism, that is, use the %HH  encoding  for
380           unsafe octets.
382   Writing a URI
383       When  written,  URIs  should  be  placed  inside  double  quotes (e.g.,
384       "http://www.kernel.org"),   enclosed   in   angle    brackets    (e.g.,
385       <http://lwn.net>),  or  placed  on a line by themselves.  A warning for
386       those who use double-quotes: never move extraneous punctuation (such as
387       the  period  ending  a  sentence  or the comma in a list) inside a URI,
388       since this will change the value of the URI.  Instead, use angle brack‐
389       ets instead, or switch to a quoting system that never includes extrane‐
390       ous characters inside quotation marks.  This latter system, called  the
391       'new'  or  'logical'  quoting  system by "Hart's Rules" and the "Oxford
392       Dictionary for Writers and Editors", is  preferred  practice  in  Great
393       Britain  and hackers worldwide (see the Jargon File's section on Hacker
394       Writing           Style,           ⟨http://www.fwi.uva.nl/~mes/jargon/h
395       /HackerWritingStyle.html⟩, for more information).  Older documents sug‐
396       gested inserting the prefix "URL:" just before the URI, but  this  form
397       has never caught on.
399       The  URI  syntax was designed to be unambiguous.  However, as URIs have
400       become commonplace, traditional media (television,  radio,  newspapers,
401       billboards,  etc.)  have  increasingly  used abbreviated URI references
402       consisting of only the authority and path portions  of  the  identified
403       resource  (e.g., <www.w3.org/Addressing>).  Such references are primar‐
404       ily intended for human interpretation rather  than  machine,  with  the
405       assumption that context-based heuristics are sufficient to complete the
406       URI (e.g., hostnames beginning with "www" are likely to have a URI pre‐
407       fix  of  "http://"  and hostnames beginning with "ftp" likely to have a
408       prefix of "ftp://").  Many client implementations heuristically resolve
409       these  references.   Such heuristics may change over time, particularly
410       when new schemes are introduced.  Since an abbreviated URI has the same
411       syntax  as  a  relative  URL path, abbreviated URI references cannot be
412       used where relative URIs are permitted, and can be used only when there
413       is  no  defined  base (such as in dialog boxes).  Don't use abbreviated
414       URIs as hypertext links inside a document; use the standard  format  as
415       described here.


418       (IETF   RFC 2396)   ⟨http://www.ietf.org/rfc/rfc2396.txt⟩,  (HTML  4.0)


422       Any tool accepting URIs (e.g., a web browser) on a Linux system  should
423       be able to handle (directly or indirectly) all of the schemes described
424       here, including the man: and info: schemes.  Handling them by  invoking
425       some other program is fine and in fact encouraged.
427       Technically the fragment isn't part of the URI.
429       For information on how to embed URIs (including URLs) in a data format,
430       see documentation on that format.  HTML uses the format <A  HREF="uri">
431       text </A>.  Texinfo files use the format @uref{uri}.  Man and mdoc have
432       the recently added UR macro, or just include the URI in the text (view‐
433       ers should be able to detect :// as part of a URI).
435       The  GNOME and KDE desktop environments currently vary in the URIs they
436       accept, in particular in their respective help browsers.  To  list  man
437       pages,  GNOME  uses <toc:man> while KDE uses <man:(index)>, and to list
438       info pages, GNOME uses <toc:info>  while  KDE  uses  <info:(dir)>  (the
439       author  of  this  man page prefers the KDE approach here, though a more
440       regular format would be even better).  In general, KDE uses <file:/cgi-
441       bin/>  as a prefix to a set of generated files.  KDE prefers documenta‐
442       tion  in  HTML,  accessed  via  the  <file:/cgi-bin/helpindex>.   GNOME
443       prefers  the  ghelp  scheme  to  store and find documentation.  Neither
444       browser handles file: references to directories at  the  time  of  this
445       writing,  making  it  difficult  to refer to an entire directory with a
446       browsable URI.  As noted above, these environments differ in  how  they
447       handle  the info: scheme, probably the most important variation.  It is
448       expected that GNOME and KDE will converge to common URI formats, and  a
449       future  version  of  this  man page will describe the converged result.
450       Efforts to aid this convergence are encouraged.
452   Security
453       A URI does not in itself pose a security threat.  There is  no  general
454       guarantee  that a URL, which at one time located a given resource, will
455       continue to do so.  Nor is there any guarantee  that  a  URL  will  not
456       locate a different resource at some later point in time; such a guaran‐
457       tee can be obtained only from the person(s) controlling that  namespace
458       and the resource in question.
460       It  is  sometimes  possible  to construct a URL such that an attempt to
461       perform a seemingly harmless operation, such as  the  retrieval  of  an
462       entity associated with the resource, will in fact cause a possibly dam‐
463       aging remote operation to occur.  The  unsafe  URL  is  typically  con‐
464       structed  by  specifying a port number other than that reserved for the
465       network protocol in question.  The client unwittingly contacts  a  site
466       that  is  in fact running a different protocol.  The content of the URL
467       contains instructions that, when interpreted according  to  this  other
468       protocol,  cause  an unexpected operation.  An example has been the use
469       of a gopher URL to cause an unintended or impersonating message  to  be
470       sent via a SMTP server.
472       Caution  should be used when using any URL that specifies a port number
473       other than the default for the protocol, especially when it is a number
474       within the reserved space.
476       Care should be taken when a URI contains escaped delimiters for a given
477       protocol (for example, CR and LF characters for telnet protocols)  that
478       these  are  not  unescaped before transmission.  This might violate the
479       protocol, but avoids the potential for such characters to  be  used  to
480       simulate  an extra operation or parameter in that protocol, which might
481       lead to an unexpected and possibly harmful remote operation to be  per‐
482       formed.
484       It  is  clearly  unwise  to use a URI that contains a password which is
485       intended to be secret.  In particular, the use of a password within the
486       "userinfo" component of a URI is strongly recommended against except in
487       those rare cases where the "password" parameter is intended to be  pub‐
488       lic.


491       Documentation  may  be  placed in a variety of locations, so there cur‐
492       rently isn't a good URI scheme  for  general  online  documentation  in
493       arbitrary  formats.  References of the form <file:///usr/doc/ZZZ> don't
494       work because different distributions and  local  installation  require‐
495       ments  may  place  the  files  in  different  directories (it may be in
496       /usr/doc, or /usr/local/doc, or /usr/share, or somewhere else).   Also,
497       the  directory ZZZ usually changes when a version changes (though file‐
498       name globbing could partially overcome this).  Finally, using the file:
499       scheme doesn't easily support people who dynamically load documentation
500       from the Internet (instead of loading the files onto a  local  filesys‐
501       tem).   A  future  URI scheme may be added (e.g., "userdoc:") to permit
502       programs to include cross-references  to  more  detailed  documentation
503       without  having  to  know  the  exact  location  of that documentation.
504       Alternatively, a future version of  the  filesystem  specification  may
505       specify  file  locations  sufficiently so that the file: scheme will be
506       able to locate documentation.
508       Many programs and file formats don't include a way  to  incorporate  or
509       implement links using URIs.
511       Many  programs  can't  handle all of these different URI formats; there
512       should be a standard mechanism to load an arbitrary URI that  automati‐
513       cally  detects  the users' environment (e.g., text or graphics, desktop
514       environment, local user preferences, and currently executing tools) and
515       invokes the right tool for any URI.


518       lynx(1), man2html(1), mailaddr(7), utf-8(7)
520       IETF RFC 2255 ⟨http://www.ietf.org/rfc/rfc2255.txt


523       This  page  is  part of release 4.16 of the Linux man-pages project.  A
524       description of the project, information about reporting bugs,  and  the
525       latest     version     of     this    page,    can    be    found    at
526       https://www.kernel.org/doc/man-pages/.
530Linux                             2017-09-15                            URI(7)