INFO: The "About:" Protocol in Internet Explorer

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Article ID: 183978
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Summary

Internet Explorer 4.0 supports a number of special protocol schemes. One of these is the "about:" protocol, which is used to display descriptive HTML pages for various predefined "about:" strings.

For example, "about:NavigationFailure" directs Internet Explorer to display a brief page with more information about why a navigation might have failed and helpful hints for avoiding this error in the future.

Microsoft now supports an about: protocol that works as follows. If you enter a URL "about:example", MSHTML (the HTML parsing and rendering engine for Internet Explorer 4) looks in the registry under "HKML\SOFTWARE\Microsoft\Internet Explorer\AboutURLs" for an string value named "example" and then loads the URL associated with it. If it cannot find the value "example," it generates and displays a page "<HTML>example</HTML>."

More information

"about:" URLs have a simple form:
about:<string>
where <string> should be a simple combination of alphanumeric characters.

When resolving "about:<string>" URLs, Internet Explorer will look for a string value with a name that matches <string> under the registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\AboutURLs
If found, Internet Explorer will load the content from the URL specified in the value and display this in the browser window. It is acceptable to use different protocols in referring to the content location; in fact, all predefined "about:" protocols refer to "res:" URLs, which load HTML content from DLL resources.

For example, "NavigationFailure" is listed as a value name under the aforementioned registry entry that has a string data value of "res://shdoclc.dll/navcancl.htm" on Internet Explorer 5.

If Internet Explorer does not find a match between <string> and a string value in this location, Internet Explorer will enclose <string> in <HTML> start and end tags and render it as HTML content.

NOTE: Other than the about:URLs included under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\InternetExplorer\AboutURLS, about:URLs serve no purpose. For security reasons, as of the release of Microsoft Internet Explorer 6.0 Service Pack 1, when you resolve "about:<string>" URLs, Internet Explorer causes navigation to fail.

References

Internet Client SDK: Internet Tools & Technologies; Predefined Protocols

Properties

Article ID: 183978 - Last Review: June 22, 2014 - Revision: 4.0
Keywords: 
kbinfo KB183978
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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