HTML 4.0 Sourcebook
(Publisher: John Wiley & Sons, Inc.)
Author(s): Ian S. Graham
ISBN: 0471257249
Publication Date: 04/01/98

Tn3270 and Rlogin URLs

Some browsers support a variant of a telnet URL, called a tn3270 URL, which references a connection requiring IBM 3270 terminal emulation on the part of the client. In general, this is only useful if the client computer supports a tn3270 client program (tn3270 clients are rarely built into Web browsers) and provided the browser knows about this program. If not, most browsers default to a regular Telnet connection.

A few browsers support rlogin connections via rlogin URLs. The specification is

rlogin:// username@int.domain.nam

where username is the account name for the rlogin. The user will be prompted for a password in the resulting window unless one is not required. Most browsers that support this emulate rlogin URLs using a Telnet connection. As these are not widely supported, it is best to stick to telnet URLs.

WAIS URLs

WAIS servers can be accessed via URLs in a manner similar to HTTP servers. The major difference is in the file specification: The URL must pass the correct search instructions to the WAIS server, in addition to information about what is to be searched.

The standard form for accessing a WAIS server is

wais:// int.domain.nam:port/database?search

where int.domain.nam is the Internet domain name of the host running a WAIS server at the indicated port (the default port is 210); database is the name of the WAIS database to be searched; and search is a list of search instructions to pass to the database. Another form is (leaving out the port number for brevity)

wais:// int.domain.nam/database

which designates a particular searchable database. A browser will understand that this URL references a searchable database and will prompt for query string input.

Finally, wais URLs can reference individual resources on a WAIS database. The form for this URL is:

wais:// int.domain.nam/database/wais_type/resource_path

where wais_type is a type indicator, which gives the type of the object being accessed, and resource_path is the document ID, used internally by the WAIS database. In general these strings are generated by the WAIS server itself, and it is rare that a user (or document author) will actually compose the wais_type and resource_path fields.

Most browsers do not support wais URLs directly, and instead must forward WAIS queries through a WAIS proxy, identified via a browser’s proxy configuration menu.

Special Class ID URLs

There are several special URL formats that currently are only used as CLASSID attribute values of OBJECT elements. These URLs define class ID values for special classes of executable objects, so that the syntax of the URL schemes varies widely depending on the object type. In addition, the URL syntax does not, in general, specify the location of the object—this must be obtained from an object catalog, maintained by the browser, or from the CODEBASE attribute.

Clsid URLs

Clsid URLs were introduced by Microsoft, and are used, within an OBJECT element, to reference an Active-X/COM class to be instantiated (loaded and run) when the object is created. The general form for a clsid URL is

clsid: class-id

where class-id is the identifier for the desired COM class. An example is:

clsid:EFE6733J-1321-43Cf-J43B-08113F412501

Clsid URLs are only valid within an OBJECT element, as the value of the CLASSID attribute. They are currently only supported by Internet Explorer 3 and 4.

Java URLs

Java URLs were introduced by Netscape and are used, within an OBJECT element, to reference a Java class to be instantiated and run. The general form is

java: packagelocation.packageobject

where packagelocation is the location of the package containing the desired class, and packageobject is the name of the class to be run. An example is:

java:myjava.applet.start.class

which references an applet from my personal applet collection. Java URLs are only valid within an OBJECT element, as the value of the CLASSID attribute.

Javabean URLs

Javabean URLs were introduced by Netscape to identify java objects built using the special JavaBean library. Thus, referencing a JavaBean tells the browser which other supporting classes will be required. The general form is:

javabean:packagelocation.packageobject

where packagelocation is the location of the package containing the desired class and packageobject is the name of the class to be run. An example is:

javabean:netscape.application.Button

which references a Netscape Internet Foundation Class (IFC) button object. Note that javabean URLs are only valid within an OBJECT element, as the value of the CLASSID attribute. They are only supported by Netscape Navigator 4.

“Pseudo”-URLs

Last, there are several, often proprietary, “pseudo”-URLs frequently seen in a browser’s Location window. The most common such schemes are about, javascript, mailbox, res, view-source, and wysiwyg. There are no formal specifications for these URLs: Indeed, the about URL was initially introduced—partly as a practical joke—by the Netscape development team, while the javascript URL was introduced to allow for easy testing of typed-in strings of JavaScript code. In general, these schemes are not of use to an author, although it is always nice to know what they mean! However, the special about:blank URL turned out to have several practical uses, such that this form is now supported by most browsers. Similarly, javascript URLs proved to be a simple (if not ideal) way of binding script programs to hypertext anchors, so that this URL form is also widely supported by browser vendors.