Uniform Resource Locators T. Berners-Lee draft-ietf-uri-url-07.txt L. Masinter Expires March 13, 1995 M. McCahill Editors September 7, 1994 Uniform Resource Locators (URL) Status of this memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet-Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or munnari.oz.au. This Internet Draft expires April 7, 1995. 0. Abstract 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 The specification of a Uniform Resource Locator, defined in this document, 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 is based on the "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 editor, Tim Berners-Lee , or to the URI-WG . Discussions of the group are archived at 2. Definitions This document describes the syntax for "Uniform Resource Locators" (URLs): a compact representation of the location and access method for a resource available via the Internet. Just as there are many different methods of access to resources, there are several _schemes_ for describing the location of such resources. The generic syntax provides a framework for new URL schemes to be established using as yet undefined protocols. 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. URL SYNTAX URLs are written as follows: : A URL contains the name of the scheme being used () followed by a colon and then a string (the ) whose interpretation depends on the scheme. Scheme names consist of lower case letters "a"--"z", digits, and the characters plus ("+"), period ("."), and hyphen ("-"). 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"). A BNF description of the URL syntax is given in Section 5. 2.2. Encoding of reserved and unsafe characters URLs are represented as a sequence of characters taken from the NVT ASCII character set. Characters and other 8-bit bytes are _encoded_ by representing them as a percent sign "%" followed by two hexadecimal digits (0-9, A-F). In any circumstance, only printable ASCII characters are allowed in URLs: URLs may not contain space or other non-printable characters. If it is necessary to designate a byte in a component of a URL that would otherwise be represented by a space or a non-printable character, it is necessary to represent that byte encoded. There are a number of printable ASCII characters whose use in URLs is _unsafe_; characters can be unsafe for a number of reasons. 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 sometimes known to modify such characters. All unsafe characters should always be encoded within a URL. For example, the character "#" should 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. In general, only alphanumerics, reserved characters used for their reserved purposes, "$", "-", "_", ".", and "+" may be used unencoded. On the other hand, even safe characters such as alphanumerics _may_ be encoded, as long as they are not being used for a reserved purpose. 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 it is necessary to designate a byte in a component of a URL that would otherwise be represented by a reserved character, it is necessary to represent that byte 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 a byte is represented by a character and when it is represented by its hex encoding. However, this is not true for reserved characters: encoding a reserved character for a particular scheme may change the semantics of a URL. 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 "/". 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 initial part of the scheme-specific data: //:@: or //:@:/ This initial part starts with a double slash "//" to indicate its presence, and continues until the following slash "/", if any. Within this section are: user An optional user name. Some schemes (e.g., ftp) allow the specification of a user name. password 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. 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., has an empty user name and no password, has no user name, while has a user name of "foo" and an empty password. host The fully qualified domain name of a network host, or its IP address as a set of four decimal digits separated by periods. Fully qualified domain names take the form as described in Section 3.5 of RFC 1034: a sequence of parts separated by period. port 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. url-path 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 use, 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 : is omitted, the port defaults to 21. 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: //...//;type= Where through and are (possibly encoded) strings and is one of the characters "a", "i", or "d". The url-path is interpreted as a series of FTP commands as follows: Each of the 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 as the argument, and interpret the results as a file directory listing. Otherwise, perform a TYPE command with as the argument, and then access the file whose name is (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 necessary to encode each "/" as %2F. For example, the URL 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 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, , 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= 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. 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: http://:/? where and are as described in Section 3.1. If : is omitted, the port defaults to 80. No user name or password is allowed. is an HTTP selector, and is a query string. The is optional, as is the and its preceding "?". If neither nor is present, the "/" may also be omitted. Within the and components, "/", ";", "?" are reserved. The "/" character may be used within HTTP to designate a hierarchical structure. 3.4. GOPHER 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 attributes. 3.4.1. Gopher URL syntax A Gopher URL takes the form: gopher://:/ where is one of %09 %09 %09%09 If : is omitted, the port defaults to 70. is single-character field to denote the Gopher type of the resource to which the URL refers. The entire may also be empty, in which case the delimiting "/" is also optional and the defaults to "1". is the Gopher selector string. In the Gopher protocol, Gopher selector strings are a sequence of 8-bit bytes which may contain any characters other than tab, return, or linefeed. Gopher clients specify which item to retrieve by sending the Gopher selector string to a Gopher server. Within the , no additional characters have a reserved interpretation. Note that some Gopher strings begin with a copy of the 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 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 string must be supplied, although the element may be the empty string. The 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 optionally by a tab and the search string (if the element is not empty), followed by a tab and the Gopher+ commands. 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: +view_name%20language_name For example, a Gopher+ string of "+application/postscript%20Es_ES" refers to the Spanish language postscript alternate view of a Gopher+ item. 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: +%091%0D%0A+-1%0D%0Aask_item1_value%0D%0Aask_item2_value%0D%0A.%0D%0A To retrieve this item, the Gopher client sends: a_gopher_selector+1 +-1 ask_item1_value ask_item2_value . to the Gopher server. 3.5. MAILTO 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: mailto: where is (the encoding of an) addr-spec, as specified in RFC 822. Within mailto URLs, no additional characters are reserved within the component. Note that the percent sign ("%") is commonly used within RFC 822 addresses and must be URL-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 MIME. 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: news: news: A is a period-delimited hierarchical name, such as "comp.infosystems.www.misc". A corresponds to the Message-ID of section 2.1.5 of RFC 1036, without the enclosing "<" and ">"; it takes the form @. 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 is "*" (as in ), 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 location-independent. 3.7. NNTP The nntp URL scheme is an alternative method of referencing news articles, useful for specifying news articles from NNTP servers (RFC 977). A nntp URL take the form: nntp://:// where and are as described in Section 3.1. If : is omitted, the port defaults to 119. The is the name of the group, while the 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. 3.8. TELNET The Telnet URL scheme is used to designate interactive services that may be accessed by the Telnet protocol. A telnet URL takes the form: telnet://:@: [ / ] as specified in Section 3.1. The port defaults to 23; the and segments are completely optional (a requires a element.) 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 and 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 [6]; the WAIS protocol is described in RFC 1625 [17]. A WAIS URLs takes one the following forms: wais://:/ wais://:/? wais://:/// where and are as described in Section 3.1. If : 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. 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 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 is included in the URL in order to allow the client interpreting the URL adequate information to actually retrieve the document. The 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 Internet. A file URL takes the form: file:/// where is the fully qualified domain name of the system on which the is accessible, and is a hierarchical directory path of the form //.../. For example, a VMS file DISK$USER:[MY.NOTES]NOTE123456.TXT might become As a special case, 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. 3.11 PROSPERO 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: prospero://:/;= where and are as described in Section 3.1. If : is omitted, the port defaults to 1525. No username or password is allowed. The 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 . 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 URLs. Note that a slash "/" may appear in the 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 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 . (E.g., designates a of "/pros/name".) In addition, after the , 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. 4. REGISTRATION OF NEW SCHEMES A new scheme may be introduced by defining a mapping onto a conforming URL syntax, using a new prefix. Experimental prefixes may be used by mutual agreement between parties. Scheme names starting with the characters "x-" are reserved for experimental purposes. 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 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 * 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 | "+" | "-" | "." ] 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 = alpha *uchar 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" ; FILE fileurl = "file://" host [ "/" 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 ; 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 ] ; TELNET 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 ; PROSPERO 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" 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 | national 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. 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, and many others have helped refine the current draft. APPENDIX: Recommendations for URLs in Context URIs, including URLs, are intended to be transmitted though 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. 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 but you can probably pick it up from . Note the warning in . REFERENCES [1] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey, D., and Alberti, B., "The Internet Gopher Protocol: A distributed document search and retrieval protocol", RFC 1436, , March 1993. [2] Anklesaria, F., Lindner, P., McCahill, M., Torrey, D., Johnson, D., and Alberti, B., "Gopher+: Upward compatible enhancements to the Internet Gopher protocol", University of Minnesota, , July 1993. [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, , June 1994. [4] Berners-Lee, T ., "Hypertext Transfer Protocol (HTTP)" , CERN, , November 1993. [5] Crocker, D. H., "Standard for the Format of ARPA Internet Text Messages", RFC 822, , April 1982. [6] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., Sui, J., and Grinbaum, M., "WAIS Interface Protocol Prototype Functional Specification", (v1.5), Thinking Machines Corporation, , April 1990. [7] Deutsch, P., Emtage, A. & Marine, A. "How to Use Anonymous FTP", RFC1635, , May 1994. [8] Horton, M. and Adams, R., "Standard For Interchange of USENET messages", RFC 1036, , December 1987. [9] Huitema, C., "Naming: strategies and techniques", Computer Networks and ISDN Systems 23 (1991) 107-110. [10] Kahle, B., "Document Identifiers, or International Standard Book Numbers for the Electronic Age", , 1991. [11] Kantor, B. and Lapsley, P., "Network News Transfer Protocol: A Proposed Standard for the Stream-Based Transmission of News", RFC977, , February 1986. [12] Kunze, J., "Functional Requirements for Internet Resource Locators", Internet-Draft (work in progress), , July 1994. [13] Mockapetris, P., "Domain Names - Concepts and Facilities", RFC1034, USC-ISI, , November, 1987. [14] Neuman, B.C., and Augart, S. "The Prospero Protocol", USC Information Sciences Institute, , June 1993. [15] Postel, J. and Reynolds, J.K., "File Transfer Protocol (FTP)", RFC 959, , October 1985. [16] Sollins, K. and Masinter, L. "Requirements for Uniform Resource Names", Internet-Draft (work in progress), [17] St. Pierre, M, Fullton, J., Gamiel, K., Goldman, J., Kahle, B., Kunze, J., Morris, H., and Schiettecatte, F., "WAIS over Z39.50-1988", RFC 1625, , June 1994. [18] Yeong, W. "Towards Networked Information Retrieval", Technical report 91-06-25-01, Performance Systems International, Inc. , June 1991. [19] Yeong, W., "Representing Public Archives in the Directory", Internet Draft, November 1991, now expired. EDITORS' ADDRESSES Tim Berners-Lee World-Wide Web project CERN, 1211 Geneva 23, Switzerland Tel: +41 (22)767 3755 Fax: +41 (22)767 7155 Email: timbl@info.cern.ch Larry Masinter Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94034 Tel: (415) 812-4365 Fax: (415) 812-4333 Email: masinter@parc.xerox.com Mark McCahill Computer and Information Services, University of Minnesota Room 152 Shepherd Labs 100 Union Street SE Minneapolis, MN 55455 Tel: (612) 625 1300 EMail: mpm@boombox.micro.umn.edu