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Network Working Group T. Berners-Lee Request for Comments: 1738 CERN Category: Standards Track L. Masinter

                                                     Xerox Corporation
                                                           M. McCahill
                                               University of Minnesota
                                                         December 1994
                  Uniform Resource Locators (URL)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.


 This document specifies a Uniform Resource Locator (URL), the syntax
 and semantics of formalized information for location and access of
 resources via the Internet.

1. Introduction

 This document describes the syntax and semantics for a compact string
 representation for a resource available via the Internet.  These
 strings are called "Uniform Resource Locators" (URLs).
 The specification is derived from concepts introduced by the World-
 Wide Web global information initiative, whose use of such objects
 dates from 1990 and is described in "Universal Resource Identifiers
 in WWW", RFC 1630. The specification of URLs is designed to meet the
 requirements laid out in "Functional Requirements for Internet
 Resource Locators" [12].
 This document was written by the URI working group of the Internet
 Engineering Task Force.  Comments may be addressed to the editors, or
 to the URI-WG <>. Discussions of the group are archived
 at <URL:>

Berners-Lee, Masinter & McCahill [Page 1] RFC 1738 Uniform Resource Locators (URL) December 1994

2. General URL Syntax

 Just as there are many different methods of access to resources,
 there are several schemes for describing the location of such
 The generic syntax for URLs provides a framework for new schemes to
 be established using protocols other than those defined in this
 URLs are used to `locate' resources, by providing an abstract
 identification of the resource location.  Having located a resource,
 a system may perform a variety of operations on the resource, as
 might be characterized by such words as `access', `update',
 `replace', `find attributes'. In general, only the `access' method
 needs to be specified for any URL scheme.

2.1. The main parts of URLs

 A full BNF description of the URL syntax is given in Section 5.
 In general, URLs are written as follows:
 A URL contains the name of the scheme being used (<scheme>) followed
 by a colon and then a string (the <scheme-specific-part>) whose
 interpretation depends on the scheme.
 Scheme names consist of a sequence of characters. The lower case
 letters "a"--"z", digits, and the characters plus ("+"), period
 ("."), and hyphen ("-") are allowed. For resiliency, programs
 interpreting URLs should treat upper case letters as equivalent to
 lower case in scheme names (e.g., allow "HTTP" as well as "http").

2.2. URL Character Encoding Issues

 URLs are sequences of characters, i.e., letters, digits, and special
 characters. A URLs may be represented in a variety of ways: e.g., ink
 on paper, or a sequence of octets in a coded character set. The
 interpretation of a URL depends only on the identity of the
 characters used.
 In most URL schemes, the sequences of characters in different parts
 of a URL are used to represent sequences of octets used in Internet
 protocols. For example, in the ftp scheme, the host name, directory
 name and file names are such sequences of octets, represented by
 parts of the URL.  Within those parts, an octet may be represented by

Berners-Lee, Masinter & McCahill [Page 2] RFC 1738 Uniform Resource Locators (URL) December 1994

 the chararacter which has that octet as its code within the US-ASCII
 [20] coded character set.
 In addition, octets may be encoded by a character triplet consisting
 of the character "%" followed by the two hexadecimal digits (from
 "0123456789ABCDEF") which forming the hexadecimal value of the octet.
 (The characters "abcdef" may also be used in hexadecimal encodings.)
 Octets must be encoded if they have no corresponding graphic
 character within the US-ASCII coded character set, if the use of the
 corresponding character is unsafe, or if the corresponding character
 is reserved for some other interpretation within the particular URL
 No corresponding graphic US-ASCII:
 URLs are written only with the graphic printable characters of the
 US-ASCII coded character set. The octets 80-FF hexadecimal are not
 used in US-ASCII, and the octets 00-1F and 7F hexadecimal represent
 control characters; these must be encoded.
 Characters can be unsafe for a number of reasons.  The space
 character is unsafe because significant spaces may disappear and
 insignificant spaces may be introduced when URLs are transcribed or
 typeset or subjected to the treatment of word-processing programs.
 The characters "<" and ">" are unsafe because they are used as the
 delimiters around URLs in free text; the quote mark (""") is used to
 delimit URLs in some systems.  The character "#" is unsafe and should
 always be encoded because it is used in World Wide Web and in other
 systems to delimit a URL from a fragment/anchor identifier that might
 follow it.  The character "%" is unsafe because it is used for
 encodings of other characters.  Other characters are unsafe because
 gateways and other transport agents are known to sometimes modify
 such characters. These characters are "{", "}", "|", "\", "^", "~",
 "[", "]", and "`".
 All unsafe characters must always be encoded within a URL. For
 example, the character "#" must be encoded within URLs even in
 systems that do not normally deal with fragment or anchor
 identifiers, so that if the URL is copied into another system that
 does use them, it will not be necessary to change the URL encoding.

Berners-Lee, Masinter & McCahill [Page 3] RFC 1738 Uniform Resource Locators (URL) December 1994

 Many URL schemes reserve certain characters for a special meaning:
 their appearance in the scheme-specific part of the URL has a
 designated semantics. If the character corresponding to an octet is
 reserved in a scheme, the octet must be encoded.  The characters ";",
 "/", "?", ":", "@", "=" and "&" are the characters which may be
 reserved for special meaning within a scheme. No other characters may
 be reserved within a scheme.
 Usually a URL has the same interpretation when an octet is
 represented by a character and when it encoded. However, this is not
 true for reserved characters: encoding a character reserved for a
 particular scheme may change the semantics of a URL.
 Thus, only alphanumerics, the special characters "$-_.+!*'(),", and
 reserved characters used for their reserved purposes may be used
 unencoded within a URL.
 On the other hand, characters that are not required to be encoded
 (including alphanumerics) may be encoded within the scheme-specific
 part of a URL, as long as they are not being used for a reserved

2.3 Hierarchical schemes and relative links

 In some cases, URLs are used to locate resources that contain
 pointers to other resources. In some cases, those pointers are
 represented as relative links where the expression of the location of
 the second resource is in terms of "in the same place as this one
 except with the following relative path". Relative links are not
 described in this document. However, the use of relative links
 depends on the original URL containing a hierarchical structure
 against which the relative link is based.
 Some URL schemes (such as the ftp, http, and file schemes) contain
 names that can be considered hierarchical; the components of the
 hierarchy are separated by "/".

Berners-Lee, Masinter & McCahill [Page 4] RFC 1738 Uniform Resource Locators (URL) December 1994

3. Specific Schemes

 The mapping for some existing standard and experimental protocols is
 outlined in the BNF syntax definition.  Notes on particular protocols
 follow. The schemes covered are:
 ftp                     File Transfer protocol
 http                    Hypertext Transfer Protocol
 gopher                  The Gopher protocol
 mailto                  Electronic mail address
 news                    USENET news
 nntp                    USENET news using NNTP access
 telnet                  Reference to interactive sessions
 wais                    Wide Area Information Servers
 file                    Host-specific file names
 prospero                Prospero Directory Service
 Other schemes may be specified by future specifications. Section 4 of
 this document describes how new schemes may be registered, and lists
 some scheme names that are under development.

3.1. Common Internet Scheme Syntax

 While the syntax for the rest of the URL may vary depending on the
 particular scheme selected, URL schemes that involve the direct use
 of an IP-based protocol to a specified host on the Internet use a
 common syntax for the scheme-specific data:
 Some or all of the parts "<user>:<password>@", ":<password>",
 ":<port>", and "/<url-path>" may be excluded.  The scheme specific
 data start with a double slash "//" to indicate that it complies with
 the common Internet scheme syntax. The different components obey the
 following rules:
      An optional user name. Some schemes (e.g., ftp) allow the
      specification of a user name.
      An optional password. If present, it follows the user
      name separated from it by a colon.
 The user name (and password), if present, are followed by a
 commercial at-sign "@". Within the user and password field, any ":",
 "@", or "/" must be encoded.

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 Note that an empty user name or password is different than no user
 name or password; there is no way to specify a password without
 specifying a user name. E.g., <URL:> has an empty
 user name and no password, <URL:> has no user name,
 while <URL:> has a user name of "foo" and an
 empty password.
      The fully qualified domain name of a network host, or its IP
      address as a set of four decimal digit groups separated by
      ".". Fully qualified domain names take the form as described
      in Section 3.5 of RFC 1034 [13] and Section 2.1 of RFC 1123
      [5]: a sequence of domain labels separated by ".", each domain
      label starting and ending with an alphanumerical character and
      possibly also containing "-" characters. The rightmost domain
      label will never start with a digit, though, which
      syntactically distinguishes all domain names from the IP
      The port number to connect to. Most schemes designate
      protocols that have a default port number. Another port number
      may optionally be supplied, in decimal, separated from the
      host by a colon. If the port is omitted, the colon is as well.
      The rest of the locator consists of data specific to the
      scheme, and is known as the "url-path". It supplies the
      details of how the specified resource can be accessed. Note
      that the "/" between the host (or port) and the url-path is
      NOT part of the url-path.
 The url-path syntax depends on the scheme being used, as does the
 manner in which it is interpreted.

3.2. FTP

 The FTP URL scheme is used to designate files and directories on
 Internet hosts accessible using the FTP protocol (RFC959).
 A FTP URL follow the syntax described in Section 3.1.  If :<port> is
 omitted, the port defaults to 21.

Berners-Lee, Masinter & McCahill [Page 6] RFC 1738 Uniform Resource Locators (URL) December 1994

3.2.1. FTP Name and Password

 A user name and password may be supplied; they are used in the ftp
 "USER" and "PASS" commands after first making the connection to the
 FTP server.  If no user name or password is supplied and one is
 requested by the FTP server, the conventions for "anonymous" FTP are
 to be used, as follows:
      The user name "anonymous" is supplied.
      The password is supplied as the Internet e-mail address
      of the end user accessing the resource.
 If the URL supplies a user name but no password, and the remote
 server requests a password, the program interpreting the FTP URL
 should request one from the user.

3.2.2. FTP url-path

 The url-path of a FTP URL has the following syntax:
 Where <cwd1> through <cwdN> and <name> are (possibly encoded) strings
 and <typecode> is one of the characters "a", "i", or "d".  The part
 ";type=<typecode>" may be omitted. The <cwdx> and <name> parts may be
 empty. The whole url-path may be omitted, including the "/"
 delimiting it from the prefix containing user, password, host, and
 The url-path is interpreted as a series of FTP commands as follows:
    Each of the <cwd> elements is to be supplied, sequentially, as the
    argument to a CWD (change working directory) command.
    If the typecode is "d", perform a NLST (name list) command with
    <name> as the argument, and interpret the results as a file
    directory listing.
    Otherwise, perform a TYPE command with <typecode> as the argument,
    and then access the file whose name is <name> (for example, using
    the RETR command.)
 Within a name or CWD component, the characters "/" and ";" are
 reserved and must be encoded. The components are decoded prior to
 their use in the FTP protocol.  In particular, if the appropriate FTP
 sequence to access a particular file requires supplying a string
 containing a "/" as an argument to a CWD or RETR command, it is

Berners-Lee, Masinter & McCahill [Page 7] RFC 1738 Uniform Resource Locators (URL) December 1994

 necessary to encode each "/".
 For example, the URL <URL:ftp://myname@host.dom/%2Fetc/motd> is
 interpreted by FTP-ing to "host.dom", logging in as "myname"
 (prompting for a password if it is asked for), and then executing
 "CWD /etc" and then "RETR motd". This has a different meaning from
 <URL:ftp://myname@host.dom/etc/motd> which would "CWD etc" and then
 "RETR motd"; the initial "CWD" might be executed relative to the
 default directory for "myname". On the other hand,
 <URL:ftp://myname@host.dom//etc/motd>, would "CWD " with a null
 argument, then "CWD etc", and then "RETR motd".
 FTP URLs may also be used for other operations; for example, it is
 possible to update a file on a remote file server, or infer
 information about it from the directory listings. The mechanism for
 doing so is not spelled out here.

3.2.3. FTP Typecode is Optional

 The entire ;type=<typecode> part of a FTP URL is optional. If it is
 omitted, the client program interpreting the URL must guess the
 appropriate mode to use. In general, the data content type of a file
 can only be guessed from the name, e.g., from the suffix of the name;
 the appropriate type code to be used for transfer of the file can
 then be deduced from the data content of the file.

3.2.4 Hierarchy

 For some file systems, the "/" used to denote the hierarchical
 structure of the URL corresponds to the delimiter used to construct a
 file name hierarchy, and thus, the filename will look similar to the
 URL path. This does NOT mean that the URL is a Unix filename.

3.2.5. Optimization

 Clients accessing resources via FTP may employ additional heuristics
 to optimize the interaction. For some FTP servers, for example, it
 may be reasonable to keep the control connection open while accessing
 multiple URLs from the same server. However, there is no common
 hierarchical model to the FTP protocol, so if a directory change
 command has been given, it is impossible in general to deduce what
 sequence should be given to navigate to another directory for a
 second retrieval, if the paths are different.  The only reliable
 algorithm is to disconnect and reestablish the control connection.

Berners-Lee, Masinter & McCahill [Page 8] RFC 1738 Uniform Resource Locators (URL) December 1994

3.3. HTTP

 The HTTP URL scheme is used to designate Internet resources
 accessible using HTTP (HyperText Transfer Protocol).
 The HTTP protocol is specified elsewhere. This specification only
 describes the syntax of HTTP URLs.
 An HTTP URL takes the form:
 where <host> and <port> are as described in Section 3.1. If :<port>
 is omitted, the port defaults to 80.  No user name or password is
 allowed.  <path> is an HTTP selector, and <searchpart> is a query
 string. The <path> is optional, as is the <searchpart> and its
 preceding "?". If neither <path> nor <searchpart> is present, the "/"
 may also be omitted.
 Within the <path> and <searchpart> components, "/", ";", "?" are
 reserved.  The "/" character may be used within HTTP to designate a
 hierarchical structure.


 The Gopher URL scheme is used to designate Internet resources
 accessible using the Gopher protocol.
 The base Gopher protocol is described in RFC 1436 and supports items
 and collections of items (directories). The Gopher+ protocol is a set
 of upward compatible extensions to the base Gopher protocol and is
 described in [2]. Gopher+ supports associating arbitrary sets of
 attributes and alternate data representations with Gopher items.
 Gopher URLs accommodate both Gopher and Gopher+ items and item

3.4.1. Gopher URL syntax

 A Gopher URL takes the form:
 where <gopher-path> is one of

Berners-Lee, Masinter & McCahill [Page 9] RFC 1738 Uniform Resource Locators (URL) December 1994

 If :<port> is omitted, the port defaults to 70.  <gophertype> is a
 single-character field to denote the Gopher type of the resource to
 which the URL refers. The entire <gopher-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).
 Gopher clients specify which item to retrieve by sending the Gopher
 selector string to a Gopher server.
 Within the <gopher-path>, no characters are reserved.
 Note that some Gopher <selector> strings begin with a copy of the
 <gophertype> character, in which case that character will occur twice
 consecutively. The Gopher selector string may be an empty string;
 this is how Gopher clients refer to the top-level directory on a
 Gopher server.

3.4.2 Specifying URLs for Gopher Search Engines

 If the URL refers to a search to be submitted to a Gopher search
 engine, the selector is followed by an encoded tab (%09) and the
 search string. To submit a search to a Gopher search engine, the
 Gopher client sends the <selector> string (after decoding), a tab,
 and the search string to the Gopher server.

3.4.3 URL syntax for Gopher+ items

 URLs for Gopher+ items have a second encoded tab (%09) and a Gopher+
 string. Note that in this case, the %09<search> string must be
 supplied, although the <search> element may be the empty string.
 The <gopher+_string> is used to represent information required for
 retrieval of the Gopher+ item. Gopher+ items may have alternate
 views, arbitrary sets of attributes, and may have electronic forms
 associated with them.
 To retrieve the data associated with a Gopher+ URL, a client will
 connect to the server and send the Gopher selector, followed by a tab
 and the search string (which may be empty), followed by a tab and the
 Gopher+ commands.

Berners-Lee, Masinter & McCahill [Page 10] RFC 1738 Uniform Resource Locators (URL) December 1994

3.4.4 Default Gopher+ data representation

 When a Gopher server returns a directory listing to a client, the
 Gopher+ items are tagged with either a "+" (denoting Gopher+ items)
 or a "?" (denoting Gopher+ items which have a +ASK form associated
 with them). A Gopher URL with a Gopher+ string consisting of only a
 "+" refers to the default view (data representation) of the item
 while a Gopher+ string containing only a "?" refer to an item with a
 Gopher electronic form associated with it.

3.4.5 Gopher+ items with electronic forms

 Gopher+ items which have a +ASK associated with them (i.e. Gopher+
 items tagged with a "?") require the client to fetch the item's +ASK
 attribute to get the form definition, and then ask the user to fill
 out the form and return the user's responses along with the selector
 string to retrieve the item.  Gopher+ clients know how to do this but
 depend on the "?" tag in the Gopher+ item description to know when to
 handle this case. The "?" is used in the Gopher+ string to be
 consistent with Gopher+ protocol's use of this symbol.

3.4.6 Gopher+ item attribute collections

 To refer to the Gopher+ attributes of an item, the Gopher URL's
 Gopher+ string consists of "!" or "$". "!" refers to the all of a
 Gopher+ item's attributes. "$" refers to all the item attributes for
 all items in a Gopher directory.

3.4.7 Referring to specific Gopher+ attributes

 To refer to specific attributes, the URL's gopher+_string is
 "!<attribute_name>" or "$<attribute_name>". For example, to refer to
 the attribute containing the abstract of an item, the gopher+_string
 would be "!+ABSTRACT".
 To refer to several attributes, the gopher+_string consists of the
 attribute names separated by coded spaces. For example,
 "!+ABSTRACT%20+SMELL" refers to the +ABSTRACT and +SMELL attributes
 of an item.

3.4.8 URL syntax for Gopher+ alternate views

 Gopher+ allows for optional alternate data representations (alternate
 views) of items. To retrieve a Gopher+ alternate view, a Gopher+
 client sends the appropriate view and language identifier (found in
 the item's +VIEW attribute). To refer to a specific Gopher+ alternate
 view, the URL's Gopher+ string would be in the form:

Berners-Lee, Masinter & McCahill [Page 11] RFC 1738 Uniform Resource Locators (URL) December 1994

 For example, a Gopher+ string of "+application/postscript%20Es_ES"
 refers to the Spanish language postscript alternate view of a Gopher+

3.4.9 URL syntax for Gopher+ electronic forms

 The gopher+_string for a URL that refers to an item referenced by a
 Gopher+ electronic form (an ASK block) filled out with specific
 values is a coded version of what the client sends to the server.
 The gopher+_string is of the form:


 To retrieve this item, the Gopher client sends:
 to the Gopher server.


 The mailto URL scheme is used to designate the Internet mailing
 address of an individual or service. No additional information other
 than an Internet mailing address is present or implied.
 A mailto URL takes the form:
 where <rfc822-addr-spec> is (the encoding of an) addr-spec, as
 specified in RFC 822 [6]. Within mailto URLs, there are no reserved
 Note that the percent sign ("%") is commonly used within RFC 822
 addresses and must be encoded.
 Unlike many URLs, the mailto scheme does not represent a data object
 to be accessed directly; there is no sense in which it designates an
 object. It has a different use than the message/external-body type in

Berners-Lee, Masinter & McCahill [Page 12] RFC 1738 Uniform Resource Locators (URL) December 1994

3.6. NEWS

 The news URL scheme is used to refer to either news groups or
 individual articles of USENET news, as specified in RFC 1036.
 A news URL takes one of two forms:
 A <newsgroup-name> is a period-delimited hierarchical name, such as
 "comp.infosystems.www.misc". A <message-id> corresponds to the
 Message-ID of section 2.1.5 of 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 commercial at "@" character. No additional characters
 are reserved within the components of a news URL.
 If <newsgroup-name> is "*" (as in <URL:news:*>), it is used to refer
 to "all available news groups".
 The news URLs are unusual in that by themselves, they do not contain
 sufficient information to locate a single resource, but, rather, are

3.7. NNTP

 The nntp URL scheme is an alternative method of referencing news
 articles, useful for specifying news articles from NNTP servers (RFC
 A nntp URL take the form:
 where <host> and <port> are as described in Section 3.1. If :<port>
 is omitted, the port defaults to 119.
 The <newsgroup-name> is the name of the group, while the <article-
 number> is the numeric id of the article within that newsgroup.
 Note that while nntp: URLs specify a unique location for the article
 resource, most NNTP servers currently on the Internet today are
 configured only to allow access from local clients, and thus nntp
 URLs do not designate globally accessible resources. Thus, the news:
 form of URL is preferred as a way of identifying news articles.

Berners-Lee, Masinter & McCahill [Page 13] RFC 1738 Uniform Resource Locators (URL) December 1994


 The Telnet URL scheme is used to designate interactive services that
 may be accessed by the Telnet protocol.
 A telnet URL takes the form:
 as specified in Section 3.1. The final "/" character may be omitted.
 If :<port> is omitted, the port defaults to 23.  The :<password> can
 be omitted, as well as the whole <user>:<password> part.
 This URL does not designate a data object, but rather an interactive
 service. Remote interactive services vary widely in the means by
 which they allow remote logins; in practice, the <user> and
 <password> supplied are advisory only: clients accessing a telnet URL
 merely advise the user of the suggested username and password.

3.9. WAIS

 The WAIS URL scheme is used to designate WAIS databases, searches, or
 individual documents available from a WAIS database. WAIS is
 described in [7]. The WAIS protocol is described in RFC 1625 [17];
 Although the WAIS protocol is based on Z39.50-1988, the WAIS URL
 scheme is not intended for use with arbitrary Z39.50 services.
 A WAIS URL takes one of the following forms:
 where <host> and <port> are as described in Section 3.1. If :<port>
 is omitted, the port defaults to 210.  The first form designates a
 WAIS database that is available for searching. The second form
 designates a particular search.  <database> is the name of the WAIS
 database being queried.
 The third form designates a particular document within a WAIS
 database to be retrieved. In this form <wtype> is the WAIS
 designation of the type of the object. Many WAIS implementations
 require that a client know the "type" of an object prior to
 retrieval, the type being returned along with the internal object
 identifier in the search response.  The <wtype> is included in the
 URL in order to allow the client interpreting the URL adequate
 information to actually retrieve the document.

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 The <wpath> of a WAIS URL consists of the WAIS document-id, encoded
 as necessary using the method described in Section 2.2. The WAIS
 document-id should be treated opaquely; it may only be decomposed by
 the server that issued it.

3.10 FILES

 The file URL scheme is used to designate files accessible on a
 particular host computer. This scheme, unlike most other URL schemes,
 does not designate a resource that is universally accessible over the
 A file URL takes the form:
 where <host> is the fully qualified domain name of the system on
 which the <path> is accessible, and <path> is a hierarchical
 directory path of the form <directory>/<directory>/.../<name>.
 For example, a VMS file
 might become
 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'.
 The file URL scheme is unusual in that it does not specify an
 Internet protocol or access method for such files; as such, its
 utility in network protocols between hosts is limited.


 The Prospero URL scheme is used to designate resources that are
 accessed via the Prospero Directory Service. The Prospero protocol is
 described elsewhere [14].
 A prospero URLs takes the form:
 where <host> and <port> are as described in Section 3.1. If :<port>
 is omitted, the port defaults to 1525. No username or password is

Berners-Lee, Masinter & McCahill [Page 15] RFC 1738 Uniform Resource Locators (URL) December 1994

 The <hsoname> is the host-specific object name in the Prospero
 protocol, suitably encoded.  This name is opaque and interpreted by
 the Prospero server.  The semicolon ";" is reserved and may not
 appear without quoting in the <hsoname>.
 Prospero URLs are interpreted by contacting a Prospero directory
 server on the specified host and port to determine appropriate access
 methods for a resource, which might themselves be represented as
 different URLs. External Prospero links are represented as URLs of
 the underlying access method and are not represented as Prospero
 Note that a slash "/" may appear in the <hsoname> without quoting and
 no significance may be assumed by the application.  Though slashes
 may indicate hierarchical structure on the server, such structure is
 not guaranteed. Note that many <hsoname>s begin with a slash, in
 which case the host or port will be followed by a double slash: the
 slash from the URL syntax, followed by the initial slash from the
 <hsoname>. (E.g., <URL:prospero://host.dom//pros/name> designates a
 <hsoname> of "/pros/name".)
 In addition, after the <hsoname>, optional fields and values
 associated with a Prospero link may be specified as part of the URL.
 When present, each field/value pair is separated from each other and
 from the rest of the URL by a ";" (semicolon).  The name of the field
 and its value are separated by a "=" (equal sign). If present, these
 fields serve to identify the target of the URL.  For example, the
 OBJECT-VERSION field can be specified to identify a specific version
 of an object.


 A new scheme may be introduced by defining a mapping onto a
 conforming URL syntax, using a new prefix. URLs for experimental
 schemes may be used by mutual agreement between parties. Scheme names
 starting with the characters "x-" are reserved for experimental
 The Internet Assigned Numbers Authority (IANA) will maintain a
 registry of URL schemes. Any submission of a new URL scheme must
 include a definition of an algorithm for accessing of resources
 within that scheme and the syntax for representing such a scheme.
 URL schemes must have demonstrable utility and operability.  One way
 to provide such a demonstration is via a gateway which provides
 objects in the new scheme for clients using an existing protocol.  If

Berners-Lee, Masinter & McCahill [Page 16] RFC 1738 Uniform Resource Locators (URL) December 1994

 the new scheme does not locate resources that are data objects, the
 properties of names in the new space must be clearly defined.
 New schemes should try to follow the same syntactic conventions of
 existing schemes, where appropriate.  It is likewise recommended
 that, where a protocol allows for retrieval by URL, that the client
 software have provision for being configured to use specific gateway
 locators for indirect access through new naming schemes.
 The following scheme have been proposed at various times, but this
 document does not define their syntax or use at this time. It is
 suggested that IANA reserve their scheme names for future definition:
 afs              Andrew File System global file names.
 mid              Message identifiers for electronic mail.
 cid              Content identifiers for MIME body parts.
 nfs              Network File System (NFS) file names.
 tn3270           Interactive 3270 emulation sessions.
 mailserver       Access to data available from mail servers.
 z39.50           Access to ANSI Z39.50 services.

5. BNF for specific URL schemes

 This is a BNF-like description of the Uniform Resource Locator
 syntax, using the conventions of RFC822, except that "|" is used to
 designate alternatives, and brackets [] are used around optional or
 repeated elements. Briefly, literals are quoted with "", optional
 elements are enclosed in [brackets], and elements may be preceded
 with <n>* to designate n or more repetitions of the following
 element; n defaults to 0.

; The generic form of a URL is:

genericurl = scheme ":" schemepart

; Specific predefined schemes are defined here; new schemes ; may be registered with IANA

url = httpurl | ftpurl | newsurl |

               nntpurl | telneturl | gopherurl |
               waisurl | mailtourl | fileurl |
               prosperourl | otherurl

; new schemes follow the general syntax otherurl = genericurl

; the scheme is in lower case; interpreters should use case-ignore scheme = 1*[ lowalpha | digit | "+" | "-" | "." ]

Berners-Lee, Masinter & McCahill [Page 17] RFC 1738 Uniform Resource Locators (URL) December 1994

schemepart = *xchar | ip-schemepart

; URL schemeparts for ip based protocols:

ip-schemepart = "" login [ "/" urlpath ] login = [ user [ ":" password ] "@" ] hostport hostport = host [ ":" port ] host = hostname | hostnumber hostname = *[ domainlabel "." ] toplabel domainlabel = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit toplabel = alpha | alpha *[ alphadigit | "-" ] alphadigit alphadigit = alpha | digit hostnumber = digits "." digits "." digits "." digits port = digits user = *[ uchar | ";" | "?" | "&" | "=" ] password = *[ uchar | ";" | "?" | "&" | "=" ] urlpath = *xchar ; depends on protocol see section 3.1 ; The predefined schemes: ; FTP (see also RFC959) ftpurl = "ftp:" login [ "/" fpath [ ";type=" ftptype ]] fpath = fsegment *[ "/" fsegment ] fsegment = *[ uchar | "?" | ":" | "@" | "&" | "=" ] ftptype = "A" | "I" | "D" | "a" | "i" | "d"


fileurl = "file:" [ host | "localhost" ] "/" fpath ; HTTP httpurl = "http:" hostport [ "/" hpath [ "?" search ]] hpath = hsegment *[ "/" hsegment ] hsegment = *[ uchar | ";" | ":" | "@" | "&" | "=" ] search = *[ uchar | ";" | ":" | "@" | "&" | "=" ]

; GOPHER (see also RFC1436)

gopherurl = "gopher:" hostport [ / [ gtype [ selector [ "%09" search [ "%09" gopher+_string ] ] ] ] ] gtype = xchar selector = *xchar gopher+_string = *xchar Berners-Lee, Masinter & McCahill [Page 18] RFC 1738 Uniform Resource Locators (URL) December 1994 ; MAILTO (see also RFC822) mailtourl = "mailto:" encoded822addr encoded822addr = 1*xchar ; further defined in RFC822 ; NEWS (see also RFC1036) newsurl = "news:" grouppart grouppart = "*" | group | article group = alpha *[ alpha | digit | "-" | "." | "+" | "_" ] article = 1*[ uchar | ";" | "/" | "?" | ":" | "&" | "=" ] "@" host ; NNTP (see also RFC977) nntpurl = "nntp:" hostport "/" group [ "/" digits ]


telneturl = "telnet:" login [ "/" ] ; WAIS (see also RFC1625) waisurl = waisdatabase | waisindex | waisdoc waisdatabase = "wais:" hostport "/" database waisindex = "wais:" hostport "/" database "?" search waisdoc = "wais:" hostport "/" database "/" wtype "/" wpath database = *uchar wtype = *uchar wpath = *uchar


prosperourl = "prospero:" hostport "/" ppath *[ fieldspec ] ppath = psegment *[ "/" psegment ] psegment = *[ uchar | "?" | ":" | "@" | "&" | "=" ] fieldspec = ";" fieldname "=" fieldvalue fieldname = *[ uchar | "?" | ":" | "@" | "&" ] fieldvalue = *[ uchar | "?" | ":" | "@" | "&" ] ; Miscellaneous definitions lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" hialpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" Berners-Lee, Masinter & McCahill [Page 19] RFC 1738 Uniform Resource Locators (URL) December 1994 alpha = lowalpha | hialpha digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" safe = "$" | "-" | "_" | "." | "+" extra = "!" | "*" | "'" | "(" | ")" | "," national = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`" punctuation = "<" | ">" | "#" | "%" | <"> reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" hex = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f" escape = "%" hex hex unreserved = alpha | digit | safe | extra uchar = unreserved | escape xchar = unreserved | reserved | escape digits = 1*digit 6. Security Considerations The URL scheme does not in itself pose a security threat. Users should beware that there is no general guarantee that a URL which at one time points to a given object continues to do so, and does not even at some later time point to a different object due to the movement of objects on servers. A URL-related security threat is that it is sometimes possible to construct a URL such that an attempt to perform a harmless idempotent operation such as the retrieval of the object 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 server which is in fact running a different protocol. The content of the URL contains instructions which when interpreted according to this other protocol cause an unexpected operation. An example has been the use of gopher URLs to cause a rude message to be sent via a SMTP server. Caution should be used when using any URL which 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 URLs contain embedded encoded delimiters for a given protocol (for example, CR and LF characters for telnet protocols) that these are not unencoded before transmission. This would violate the protocol but could be used to simulate an extra operation or parameter, again causing an unexpected and possible harmful remote operation to be performed. Berners-Lee, Masinter & McCahill [Page 20] RFC 1738 Uniform Resource Locators (URL) December 1994 The use of URLs containing passwords that should be secret is clearly unwise. 7. Acknowledgements This paper builds on the basic WWW design (RFC 1630) and much discussion of these issues by many people on the network. The discussion was particularly stimulated by articles by Clifford Lynch, Brewster Kahle [10] and Wengyik Yeong [18]. Contributions from John Curran, Clifford Neuman, Ed Vielmetti and later the IETF URL BOF and URI working group were incorporated. Most recently, careful readings and comments by Dan Connolly, Ned Freed, Roy Fielding, Guido van Rossum, Michael Dolan, Bert Bos, John Kunze, Olle Jarnefors, Peter Svanberg and many others have helped refine this RFC. Berners-Lee, Masinter & McCahill [Page 21] RFC 1738 Uniform Resource Locators (URL) December 1994 APPENDIX: Recommendations for URLs in Context URIs, including URLs, are intended to be transmitted through protocols which provide a context for their interpretation. In some cases, it will be necessary to distinguish URLs from other possible data structures in a syntactic structure. In this case, is recommended that URLs be preceeded with a prefix consisting of the characters "URL:". For example, this prefix may be used to distinguish URLs from other kinds of URIs. In addition, there are many occasions when URLs are included in other kinds of text; examples include electronic mail, USENET news messages, or printed on paper. In such cases, it is convenient to have a separate syntactic wrapper that delimits the URL and separates it from the rest of the text, and in particular from punctuation marks that might be mistaken for part of the URL. For this purpose, is recommended that angle brackets ("<" and ">"), along with the prefix "URL:", be used to delimit the boundaries of the URL. This wrapper does not form part of the URL and should not be used in contexts in which delimiters are already specified. In the case where a fragment/anchor identifier is associated with a URL (following a "#"), the identifier would be placed within the brackets as well. In some cases, extra whitespace (spaces, linebreaks, tabs, etc.) may need to be added to break long URLs across lines. The whitespace should be ignored when extracting the URL. No whitespace should be introduced after a hyphen ("-") character. Because some typesetters and printers may (erroneously) introduce a hyphen at the end of line when breaking a line, the interpreter of a URL containing a line break immediately after a hyphen should ignore all unencoded whitespace around the line break, and should be aware that the hyphen may or may not actually be part of the URL. Examples: Yes, Jim, I found it under <URL:; type=d> but you can probably pick it up from <URL:>. Note the warning in <URL:http://ds.internic. net/instructions/overview.html#WARNING>. Berners-Lee, Masinter & McCahill [Page 22] RFC 1738 Uniform Resource Locators (URL) December 1994 References [1] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey, D., and B. Alberti, "The Internet Gopher Protocol (a distributed document search and retrieval protocol)", RFC 1436, University of Minnesota, March 1993. <URL:;type=a> [2] Anklesaria, F., Lindner, P., McCahill, M., Torrey, D., Johnson, D., and B. Alberti, "Gopher+: Upward compatible enhancements to the Internet Gopher protocol", University of Minnesota, July 1993. <URL: /Gopher+/Gopher+.txt> [3] Berners-Lee, T., "Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web", RFC 1630, CERN, June 1994. <URL:> [4] Berners-Lee, T., "Hypertext Transfer Protocol (HTTP)", CERN, November 1993. <URL:> [5] Braden, R., Editor, "Requirements for Internet Hosts – Application and Support", STD 3, RFC 1123, IETF, October 1989. <URL:> [6] Crocker, D. "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, April 1982. <URL:> [7] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype Functional Specification", (v1.5), Thinking Machines Corporation, April 1990. <URL:> [8] Horton, M. and R. Adams, "Standard For Interchange of USENET Messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic Studies, December 1987. <URL:> [9] Huitema, C., "Naming: Strategies and Techniques", Computer Networks and ISDN Systems 23 (1991) 107-110. Berners-Lee, Masinter & McCahill [Page 23] RFC 1738 Uniform Resource Locators (URL) December 1994 [10] Kahle, B., "Document Identifiers, or International Standard Book Numbers for the Electronic Age", 1991. <URL:> [11] Kantor, B. and P. Lapsley, "Network News Transfer Protocol: A Proposed Standard for the Stream-Based Transmission of News", RFC 977, UC San Diego & UC Berkeley, February 1986. <URL:> [12] Kunze, J., "Functional Requirements for Internet Resource Locators", Work in Progress, December 1994. <URL: /draft-ietf-uri-irl-fun-req-02.txt> [13] Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC 1034, USC/Information Sciences Institute, November 1987. <URL:> [14] Neuman, B., and S. Augart, "The Prospero Protocol", USC/Information Sciences Institute, June 1993. <URL: /prospero-protocol.PS.Z> [15] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD 9, RFC 959, USC/Information Sciences Institute, October 1985. <URL:> [16] Sollins, K. and L. Masinter, "Functional Requirements for Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation, December 1994. <URL:> [17] St. Pierre, M, Fullton, J., Gamiel, K., Goldman, J., Kahle, B., Kunze, J., Morris, H., and F. Schiettecatte, "WAIS over Z39.50-1988", RFC 1625, WAIS, Inc., CNIDR, Thinking Machines Corp., UC Berkeley, FS Consulting, June 1994. <URL:> [18] Yeong, W. "Towards Networked Information Retrieval", Technical report 91-06-25-01, Performance Systems International, Inc. <URL:>, June 1991. [19] Yeong, W., "Representing Public Archives in the Directory", Work in Progress, November 1991. Berners-Lee, Masinter & McCahill [Page 24] RFC 1738 Uniform Resource Locators (URL) December 1994 [20] "Coded Character Set – 7-bit American Standard Code for Information Interchange", ANSI X3.4-1986. Editors' Addresses Tim Berners-Lee World-Wide Web project CERN, 1211 Geneva 23, Switzerland Phone: +41 (22)767 3755 Fax: +41 (22)767 7155 EMail: Larry Masinter Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94034 Phone: (415) 812-4365 Fax: (415) 812-4333 EMail: Mark McCahill Computer and Information Services, University of Minnesota Room 152 Shepherd Labs 100 Union Street SE Minneapolis, MN 55455 Phone: (612) 625 1300 EMail: Berners-Lee, Masinter & McCahill [Page 25]

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