about URI scheme

about is an internal URI scheme (also known as a "URL scheme" or, erroneously, "protocol") in various web browsers to display certain built-in functions. It is not an officially registered scheme, and has no standard syntax.

In early versions of Netscape, any address beginning about: which wasn't recognised as a built-in command would simply result in the text after the colon being displayed. Similarly, in early versions of Internet Explorer, about: followed by a string of HTML (e.g. about:<em>hello world</em>) would render that string as though it were the source of the page - thus providing a similar (though more limited) facility to the data: URI scheme defined by RFC 2397. Still other versions of Netscape would return various phrases in response to an unknown about address, including "Whatchew talkin' 'bout, Willis?" (a catch phrase from the TV show Diff'rent Strokes) or "Homey don't play dat!" (from a recurring skit on the TV show In Living Color).

The most commonly implemented about URIs are about:blank, which displays a blank HTML document, and simply about:, which may display information about the browser. Opera uses URIs beginning with opera for similar purposes, and many about URIs will be translated into an appropriate opera URI if entered; an exception is about:blank, which is not translated.

Note: In more recent versions of Internet Explorer, many of the following do not work or have been removed.

Contents

Standardisation

In 2010, and onwards, there were efforts to standardise the about URI scheme, and define the processing requirements for some specific URIs[1]. Note that Opera ASA, the proponents, had long used and advocated alternative browser specific URIs[2]. The about URIs that are known to be defined for specific purposes are listed below.

Address Purpose
about:blank[3] Returns a blank HTML document with the media type text/html and character encoding UTF-8. This is widely used to load blank pages into browsing contexts, such as iframes within HTML, which may then be modified by scripts.
about:legacy-compat A reserved, though unresolvable address defined within HTML5 intended for use in the DOCTYPE designed for compatibility with some legacy authoring tools, such as XSLT, which may not be capable of outputting the more common, shorter alternative that lacks both the PUBLIC and SYSTEM identifiers. <!DOCTYPE html SYSTEM "about:legacy-compat">.
about:srcdoc A reserved, though unresolvable address defined within HTML5 intended to be the address navigated to within iframes whose content comes from the srcdoc attribute

Common about: addresses

Common about: addresses
Addresses Mozilla / SeaMonkey Mozilla Firefox Google Chrome Konqueror Opera Safari Internet Explorer MSN Explorer
about: Shows version information and copyright information Translated to "about:konqueror" Translated to "opera:about" Shows a blank page
about:about Lists all the about: addresses
(Firefox since version 4.0)
Translated to "about:konqueror" Translated to "opera:about" Shows nothing but the text:"about"
about:blank[4] Shows a blank HTML document
about:plugins Shows the filename, MIME type, description, suffix, and status of all installed plug-ins Translated to "opera:plugins"
about:cache Shows information about the Cache Service. Lists the URIs of pages stored within the cache, with each linking to view-cache:uri that provides more information about its content. Translated to "about:konqueror" Translated to "opera:cache"
about:mozilla Shows a page from the fictional Book of Mozilla. In some versions of Netscape, also replaced the meteors in the standard Netscape throbber with a large fire-breathing lizard. Translated to "about:konqueror" The Windows version of IE showed a blank HTML document with blue background color. Possibly a joke reference to the "Blue Screen of Death." Removed in Windows XP SP2 but it can still be shown by typing "res://mshtml.dll/about.moz" instead.

Mozilla-specific about: addresses

Many of these can also be used in Thunderbird, by setting them as the "Mail Start Page". Also, some extensions define additional about: addresses not listed here.

Mozilla-specific about: addresses
Addresses Mozilla Application Suite / SeaMonkey Mozilla Firefox
about:bloat Displays BloatView output (disabled in release builds). Superseded by about:memory on Gecko 1.9.2[1]
about:blocked Shows the malware protection page used when the browser identifies a page as not safe for viewing. Firefox 3+
about:addons Shows installed addons, plugins, themes, and provides an interface for installing new addons. Firefox 4+, SeaMonkey 2.1+
about:buildconfig Shows the arguments and options used to compile the build in use
about:cache Shows information about the Cache Service. Number of entries, location of cache, size of cache, etc. for both memory and disk cache.
about:cache?device=memory Shows individual entries in memory
about:cache?device=disk Shows individual entries on disk
about:cache?device=offline Shows individual entries for offline viewing. Gecko 1.9+
about:cache-entry Shows information about a cache entry. Used in about:cache links. Requires parameters.
about:certerror Shows the error page used when an SSL/TLS certificate is untrusted or otherwise invalid.
about:config Shows an interface for viewing and setting a wide variety of configuration variables, many of which are not otherwise accessible through the GUI (options panels)
about:crashes Shows details of crashes submitted by Mozilla's crash reporter. Firefox 3+, SeaMonkey 2.0+
about:credits Shows a list of all those who contributed to Mozilla
about:feeds Shows the page used when clicking the rss feed icon in the address bar.
about:home Shows the Firefox 4 default home page.
about:jetpack Shows an interface for viewing and setting the JetPack extension [5]
about:licence In the British English localised version, shows the content of about:license but with all occurrences of license used as a noun replaced with licence (see American and British English spelling differences). Not available in Firefox 4
about:license Shows the Mozilla Public License (and the Netscape Public License) for the piece of software. Gecko 1.8+
about:logo Shows the logo used on the about: screen
about:memory Shows memory usage. Firefox 3.6+
about:mozilla Easter egg: Shows a verse from The Book of Mozilla
about:neterror Shows the error page used when the browser could not access the requested path.
about:permissions Shows permissions for sites on Firefox 6.0a1.
about:plugins Shows installed, currently enabled plug-ins.
about:privatebrowsing Can be used to switch to private browsing mode, shows a message indicating that Firefox will not remember any history for the current session. Note: It does not show URI on the address bar.
about:rights Shows licensing information.
about:robots Easter egg, beginning with March 8, 2008 trunk builds. The title of a window contains a citation from The Day the Earth Stood Still. [6]
about:sessionrestore Shows an interface for viewing about last session
about:support Shows troubleshooting information. Firefox 3.6+, SeaMonkey 2.1+

Flock-specific about: addresses

Beginning in Flock 0.9, Flock introduced a feature called My World, a place where the user's favorites, RSS feeds and favorite media can be found.

Address Flock
about:myworld Shows the My World page.
about:tutorial Shows the Flock tutorial page.
about:mozilla Shows a different version of The Book of Mozilla against a blue screen.

Opera-specific about: addresses

about is an alias for opera, therefore all these addresses also work with about prefixed. User JavaScript is disabled for all addresses in the about or opera URI schemes as a security feature. These pages can, however, be styled using local stylesheets.

None of these functions, except the about:blank combination, work in the Nintendo DS version of the Opera browser, the Nintendo DS Browser.

Opera-specific about: addresses
Addresses Opera
opera:blank Maps to about:blank
opera:about Provides info about the browser and the configured paths.
about:opera Maps to opera:about
opera:cache Shows the content of the cache.
opera:config (Opera 9.0 and newer) Shows a page that allows changing of numerous browser preferences, many of which cannot be accessed through the normal Preferences window
opera:drives (Operating system specific) Shows the local drives of the host system.
opera:history Shows the content of the browser history.
opera:historysearch (Opera 9.5 and newer) Shows the start page of internal search engine for the browser history.
opera:plugins Shows installed plug-ins. Opera 11 and newer allow you to disable individual plug-ins from this page.
opera:button Defines a custom button as a set of Opera actions. (Note: This does not work directly, you need to specify all button parametres as on CustomButtons.)
opera:help Shortcut to the help directory. This can be either local or external.
opera:debug (Opera 9.5 and newer) Shows a page that allows changing of settings for remote debug of Opera via Opera Dragonfly
opera:memdebug (Opera 9.5 and newer) Shows debug information about memory usage.
opera:webstorage (Opera 10.5 and newer) Shows information Web Storage (see HTML5).
opera:webdatabases (Opera 10.5 and newer) Shows information Web Storage databases (see HTML5).

Internet Explorer-specific about: addresses

Internet Explorer about addresses are configurable in Windows. It is therefore possible that some of the listed addresses will not work on a particular computer. For example "about:mozilla" was removed in SP2 (although the page can still be found at "res://mshtml.dll/about.moz"). These about addresses are sometimes used for spyware and adware, most notably in CoolWebSearch, which made about:blank display advertisements.

Address Internet Explorer 6 (SP2) Internet Explorer 7 Internet Explorer 8
about:home Displays the user's home page.
about:blank Displays the blank page.
about:desktopitemnavigationfailure Displays "navigation cancelled" page.
about:navigationcanceled Displays "navigation cancelled" page.
about:navigationfailure Displays "navigation cancelled" page.
about:noadd-ons Displays information page when add-ons are disabled.
about:noadd-onsinfo Displays information page about the effects when you use add-ons.
about:offlineinformation Informs the user that the current page can not be viewed off line.
about:postnotcached Informs the user that to refresh the current page, information entered in a form will have to be re-posted.
about:securityrisk Informs the user not to browse with the current security settings because they may be harmful to the computer.
about:tabs Informs the user about tabbed browsing. It is also loaded when a new tab is created if the "Open home page for new tabs instead of a blank page" setting is enabled
about:inprivate Informs the user about InPrivate Browsing. It is loaded when InPrivate Browsing is initiated.

Google Chrome-specific about: addresses

Address Google Chrome
about:cache Shows the content of the cache.
about:crash Displays "Aw, snap!" crash notification page.
about:credits Displays the list of free and open source software used in the browser, and their associated licences.
about:dns Displays the DNS records
about:flags See about:labs
about:histograms Displays histograms
about:inducebrowsercrashforrealz Crashes the Google Chrome browser. This is designed for developers to test what happens when the Google Chrome browser crashes.
about:internets Displays a page entitled "Don't Clog the Tubes!" which renders a page with an animation of the Microsoft Windows "3D Pipes" screensaver. Does not work on Windows Vista due to the pipes screen saver no longer being supported for releases after the 2.0.169.1 release.
about:kill Same as about:crash, but with a violet background color and the message "He's Dead, Jim!" Occurs when the OS runs out of memory or the browser is killed by an external cause, such as the OS shutting down.
about:labs Displays a page for testing features. Moved to about:flags in Chrome Dev channel 8.0.552.11
about:memory Displays the process manager
about:net-internals Provides an interface for monitoring the network usage and performance statistics
about:plugins Shows installed plug-ins.
about:stats Displays statistics about processes. At the top of the page, it states "Shhh! This page is secret!"
about:shorthang Hangs the tab's process, making it become unresponsive. This is designed for testing the hanging protection.
about:version Displays version information, same as about:

Note: Any "about:" URI is accepted, but only a page with a blank source is returned, as with the "about:blank" of other browsers. Similar to Safari.

Konqueror

Note: Any about URI except about:blank and about:plugins redirects to about:konqueror.

Address Konqueror
about:konqueror Shows a friendly 'start' and navigation page

Epiphany

Note: When Epiphany used Gecko as its layout engine, the general and Mozilla-specific about: addresses worked as in Mozilla Firefox. Since WebKit became Epiphany's layout engine, only these addresses are supported:[citation needed]

Address Epiphany
about:blank Displays the blank page.
about:plugins Shows installed plug-ins.
about:epiphany Displays the following message:[citation needed]

   Il semble que la perfection soit atteinte non quand il n'y a plus rien à ajouter, mais quand il n'y a plus rien à retrancher.[2]   

Antoine de Saint-Exupéry   

about:applications interface for managing web applications, supported since release 3.2, to be removed in release 3.4.[3]

Netscape (historic)

  • Older versions of the Netscape browser have an about:people URL that was similar to about:credits above, but it would redirect to Netscape's active employee listing. In addition, about:username, where username is the username of a Netscape employee, would redirect to the Netscape homepage of the employee specified. For example, about:jwz would redirect to http://people.netscape.com/jwz/ (not an active link) [7] [8].
  • Not all employee pages were accessible through this scheme. Only developers who knew which file in the source tree and the encoding scheme used to obfuscate the directory of employee about entries could add their names.[citation needed]
  • Many other about: easter eggs existed, including the famous about:mozilla, and other less known ones that showed pictures of the Netscape mascot Mozilla in various foreign garb, such as about:deutsch showing Mozilla in Lederhosen.[4]
  • Some versions of Netscape would display the browser history for the about:global address[4]

Internet Explorer for Mac OS 5 (historic)

In the Internet Explorer for Mac 5 was an offline Easter egg, accessible by typing 'about:tasman', showing the Acid1 test with the text replaced by the names of the developers.[5]

Safari

Any about URI is accepted, but only a page with a blank source is returned, as with the about:blank of other browsers.

Microsoft Outlook

In Microsoft Outlook, an additional outlook:today URI was added, which showed the Outlook Today Screen (it shows messages, tasks and appointments and is also accessible from IE). Also, although about:blank is supported, anything else (including about:about) typed after about: would show that text as a web page in the browser.

External links

Notes and References

  1. ^ The 'about' URI scheme (IETF draft)
  2. ^ The term "about:blank" is also the generic name for a group of variants of the CoolWebSearch malware program group.
  1. ^ MDN Debugging memory leaks
  2. ^ It seems that perfection is attained not when there remains nothing to add, but when there remains nothing to remove.
  3. ^ NEWS file, The GNOME Project's git repository, http://git.gnome.org/browse/epiphany/plain/NEWS, retrieved 2011-10-17 
  4. ^ a b The hidden features of Netscape Navigator ouuups Mozilla !
  5. ^ Internet Explorer Easter Egg - IE5 Mac Team, The Easter Egg Archive, 2000-06-19, http://www.eeggs.com/items/14448.html 

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