about URI scheme

Internal URI scheme

about is an internal URI scheme (also known as a "URL scheme" or, erroneously, "protocol") implemented in various Web browsers to reveal internal state and built-in functions. It is an IANA officially registered scheme, and is standardized.[1]

In early versions of Netscape, any URI beginning with about: that wasn't recognized 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 URI, 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).[2][3]

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. Some browsers use URIs beginning with the name of the browser for similar purposes, and many about URIs will be translated into the appropriate URI if entered. Examples are opera (Opera) or chrome (Google Chrome). An exception is about:blank, which is not translated.

Standardization

In 2010, and onwards, there are efforts to standardize the about URI scheme, and define the processing requirements for some specific URIs, in the IETF Applications Area Working Group (APPSAWG).[4] In August 2012, it was published as an official Request for Comments as RFC 6694. The about URIs that have since been defined and assigned by IANA are listed below.[5]

URIPurpose
about:blank[6]References 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-compatA reserved, though unresolvable, URI 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:srcdocA reserved, though unresolvable, URI defined within HTML5 intended to be the URI navigated to within iframes whose content comes from the srcdoc attribute.
about:invalidReferences a non-existent document with a generic error condition. Intended for the case where a URL is necessary, but it should not be resolveable.[7]
about:html-kindUsed as an identifier for kinds of media tracks.[8]

Common

Common "about:" URIs
URIPurpose
Firefox, SeaMonkey, MozillaGoogle Chrome, ChromiumKonquerorOperaEdgeInternet Explorer
about:Deprecated in Firefox 57 (Firefox Quantum)Redirects to "chrome://version"Redirects to "about:konqueror"Redirects to "opera:about"Redirects to "edge://version"
about:aboutLists all the about: URLs
(Firefox since version 4.0)
Redirects to "chrome://about"; Lists all the chrome: URIs (Works like "chrome://chrome-urls")Redirects to "about:konqueror"Redirects to "opera:about"Redirects to "edge://edge-urls"
about:blankShows a blank document
about:pluginsShows the filename, MIME type, description, suffix, and status of all installed plug-insDeprecated in Chrome 57[9]Shows the filename, MIME type, description, suffix, and status of all installed plug-insRedirects to "opera:plugins"
about:cacheShows information about the Cache Service.Deprecated in Chrome 66[10]Redirects to "about:konqueror"Redirects to "opera:cache"
about:mozillaShows 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.Redirects to "about:konqueror"Shows a blank HTML document with a blue background. Removed in Windows XP SP2. (The blue page can still be seen at "res://mshtml.dll/about.moz" instead.)

Chrome and Chromium derivatives

The following applies to all web browsers that derive from the Chromium project, including Google Chrome, Microsoft Edge, Opera, and Vivaldi.

Chrome-specific "about:" URIs
URI[11]Purpose
about:cacheDeprecated in version 66[10]
about:crashCrashes the current rendering process. (This causes the "Aw, snap!" crash notification page to be displayed.)
about:creditsDisplays the list of free and open source software used in the browser, and their associated licenses.
about:dinoDisplayed when the browser cannot connect to the target URL, this page contains an Easter Egg: Pressing the Space Bar key starts the _Dino_ minigame, which involves making a dinosaur jump over obstacles. Microsoft Edge doesn't feature this page, having replaced it with about:surf (see below.)[12]
about:dnsDisplays the DNS records
about:flagsDisplays a page where experimental features can be configured by setting feature flags.
about:gpuDisplays information about WebGL and hardware accelerated graphics.
about:histogramsDisplays histograms
about:inducebrowsercrashforrealzCrashes the Google Chrome browser. This is designed for developers to test what happens when the Google Chrome browser crashes. Additionally, crashes Chromebooks entirely due to them running on Google Chrome.
about:internetsOn particular versions of Windows, 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:killSame 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:labsMoved to about:flags in Chrome Dev channel 8.0.552.11
about:memoryDisplays the process manager
about:net-internalsProvides an interface for monitoring the network usage and performance statistics
about:pluginsShows installed plug-ins (Deprecated in Chrome 57)[9]
about:sandboxShows which sandbox protection mechanisms are currently enabled.
about:shorthangHangs the tab's process, making it become unresponsive. This is designed for testing the hanging protection.
about:statsDisplays statistics about processes. At the top of the page, it states "Shhh! This page is secret!"
about:versionDisplays version information, same as about:

The above list is not exhaustive; for a full list, see about:about URI.

Firefox

Many of these can also be used in Thunderbird, by setting them as the "Mail Start Page". Also, some extensions define additional about: URIs not listed here.[13]

Mozilla-specific "about:" URIs
URIsPurpose
FirefoxSeaMonkey, Mozilla Application Suite
about:addonsShows installed addons, plugins, themes, and provides an interface for installing new addons (Firefox 4+ and SeaMonkey 2.1+)
about:app-managerShows the Firefox OS Simulator
about:bloatDisplays BloatView output (disabled in release builds). Superseded by about:memory on Gecko 1.9.2.[14]
about:bloat?newTBA
about:bloat?clearTBA
about:blockedShows the malware protection page used when the browser identifies a page as not safe for viewing (Firefox 3+)
about:buildconfigShows the arguments and options used to compile the build in use
about:cacheShows information about the Cache Service. Number of entries, location of cache, size of cache, etc. for both memory and disk cache.
about:cache?device=memoryShows individual entries in memory
about:cache?device=diskShows individual entries on disk
about:cache?device=offlineShows individual entries for offline viewing (Gecko 1.9+)
about:cache-entryShows information about a cache entry. Used in about:cache links. Requires parameters.
about:certerrorShows the error page used when an SSL/TLS certificate is untrusted or otherwise invalid
about:configShows an interface for viewing and setting a wide variety of configuration variables, many of which are not otherwise accessible through the GUI (options panels).

The known earliest use of about:config dates back to Netscape 4.x, where it was not possible to edit the settings from the browser.

about:config settings first became modifiable in Mozilla Application Suite.

On first use, about:config displays a message which tells the user that the settings might void the warranty and they can be "Harmful to the stability, security, and performance of this application." The message about voiding one's warranty is a joke, as Mozilla Firefox ships without a warranty of any kind.[15] The message has a checkbox to turn it off.

about:crashesShows details of crashes submitted by Mozilla's crash reporter (Firefox 3+, SeaMonkey 2.0+)
about:creditsShows a list of all those who contributed to Mozilla
about:devtoolsShows the welcome page used by Firefox Developer Tools
about:downloadsShows a list of downloaded files
about:feedsShows the page used when clicking the rss feed icon in the address bar
about:healthreportShows the Firefox Health Report
about:homeShows the Firefox 4 default home page
about:jetpackShows an interface for viewing and setting the JetPack extension
about:licenceThe equivalent of about:license in the British English localizations of Firefox. (See American and British English spelling differences). Not available since Firefox 4.
about:licenseShows the Mozilla Public License (and the Netscape Public License) for the piece of software (Gecko 1.8+)
about:logoShows the logo used on the about: screen
about:memoryShows memory usage (Firefox 3.6+)
about:mozillaEaster egg, shows a verse from The Book of Mozilla
about:neterrorShows the error page used when the browser could not access the requested path
about:newaddonThis URI is not registered. However, when the user tries to sideload an add-on, a dialog box bearing this URI appears. asking for the user's explicit consent.
about:newtabShows a grid of favourite and most-visited websites (Firefox 13+)
about:permissionsShows permissions for all sites on Firefox 6.0a1
about:pluginsShows installed, currently enabled plug-ins
about:preferencesShows the new preferences page
about:privatebrowsingCan be used to switch to private browsing mode. It 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:reader?url=Opens the specified URL in Reader Mode. For example, about:reader?url=http://example.com.
about:rightsShows licensing information
about:robotsEaster egg, beginning with March 8, 2008 trunk builds. The title of a window cites the iconic phrase of The Day the Earth Stood Still.[16] The page's prose cites Isaac Asimov's first law of robotics.
about:sessionrestoreShows an interface for viewing about last session.
about:startpageIn Ubuntu, shows the Ubuntu start page with the current search engine (if the "Ubuntu Firefox Modifications – ubufox" add-on is installed)
about:supportShows troubleshooting information (Firefox 3.6+, SeaMonkey 2.1+)
about:sync-tabsShows list of tabs from other computers. Requires enabled synchronization.
about:tabcrashedContains the text displayed when a tab crashes. Added in Firefox 25.
about:webrtcShows an interface for WebRTC internals
about:welcomeShows the welcome page used by Firefox

Opera (v1–15)

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

None of these functions, except the opera:about combination, work in the Nintendo DS Browser, which is an Opera derivative.

Opera-specific "about:" URIs
URIsPurpose
opera:blankMaps to about:blank
opera:aboutProvides info about the browser and the configured paths
about:operaMaps to opera:about
opera:cacheShows the content of the cache
opera:configShows a page that allows changing of numerous browser preferences, many of which cannot be accessed through the normal Preferences window (Opera 9.0 and newer)
opera:drivesShows the local drives of the host system
opera:historyShows the content of the browser history
opera:historysearchShows the start page of internal search engine for the browser history (Opera 9.5 and newer)
opera:pluginsShows installed plug-ins. Opera 11 and newer allow individual plug-ins to be disabled from this page.
opera:buttonDefines a custom button as a set of Opera actions. (Note: This does not work directly, all button parameters as on CustomButtons need be specified.)
opera:helpShortcut to the help directory. This can be either local or external.
opera:debugShows a page that allows changing of settings for remote debug of Opera via Opera Dragonfly (Opera 9.5 and newer)
opera:memdebugShows debug information about memory usage (Opera 9.5 and newer)
opera:webstorageShows the browser's Web Storage (Opera 10.5 and newer)
opera:webdatabasesShows the browser's Web Storage databases (Opera 10.5 and newer)
opera:gpuShows information about hardware accelerated graphics and GPU support
opera:cpuDisplays CPU usage (Opera 12 beta and newer)

Internet Explorer (6–11)

Internet Explorer about URIs are configurable in Windows. It is therefore possible that some of the listed URIs 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 URIs are sometimes used for spyware and adware, most notably in CoolWebSearch, which made about:blank display advertisements.

Any about URI that is not recognized by Internet Explorer redirects to a page saying "Navigation to the webpage was canceled."

Internet Explorer-specific "about:" URIs
URIPurposeSupported versions
about:homeDisplays the user's home page.6, 7, 9, 10, 11 (not 8)
about:desktopitemnavigationfailureDisplays the "navigation cancelled" page.6–8
about:navigationcanceled
about:navigationfailure
about:noadd-onsAppears when add-ons are disabled to notify the users of a change in their web browsing experience.7–11
about:noadd-onsinfoContains information about add-ons and what they do.7–11
about:offlineinformationInforms the user that the current page cannot be viewed offline. Internet Explorer 9 and later do not support offline browsing.6–8
about:postnotcachedInforms the user that the current page needs to refresh and any information entered in a form will have to be re-posted.6–11
about:securityriskInforms the user not to browse with the current security settings because they may be harmful to the computer.6–11
about:tabsInforms the user about tabbed browsing. Internet Explorer shows this page upon creating a new tab if the "Open home page for new tabs instead of a blank page" setting is enabled.7–11
about:inprivateAppears when the use initiates InPrivate Browsing; contains information about this feature.9–11
about:compatDisplays a table of sites and the document rendering emulation mode configured for maximum backward compatibility.[17]11
about:newsfeedDisplays Internet Explorer 11's signature start page. It contains a Microsoft Bing search bar at the top, followed by a customizable row of tiles pertaining bookmarked website, a row of news topic category selection links, and an infinitely scrolling news aggregation feed.11

Microsoft Edge (v20–44)

The following applies to Microsoft Edge version 20 through 44, which Microsoft dubs "the legacy Edge." For newer versions, see the Chromium section.

Microsoft Edge-specific "about:" URIs
URIPurpose
about:homeShows the content that would be shown if the home button were pressed
about:flagsDisplays a list of Developer Setting and Experimental features
about:startShows the customizable (default) start page.
about:configRedirects to about:flags
about:tabsShows the (default) new tab page.
about:compatShows the Enterprise Mode list (if configured)
about:edgeShows the Edge logo.
about:inprivateShows the inprivate information tab.
about:surfA surfing game easter egg.

GNOME Web

When GNOME Web (formerly Epiphany) used Gecko as its layout engine, all Firefox-specific about: URIs worked in it. After the adoption of WebKit as its layout engine, only the following URIs are supported.[citation needed]

GNOME Web-specific "about:" URIs
URIPurpose
about:pluginsShows installed plug-ins.
about:memoryShows details about memory usage.
about:epiphanyDisplays the following message:[18]

It is a quote from Chapter 3 of Saint-Exupéry's memoir Terre des Hommes (titled Wind, Sand and Stars in its English translation by Lewis Galantiere).

about:applicationsInterface for managing the Web Application Mode, supported since release 3.2.[19]
about:overviewDisplays a grid of thumbnails for the more visited websites.
about:gpuDisplays information about WebGL and hardware accelerated graphics.
about:webDisplays the browser and its engine information, same as about:
about:incognitoDisplays information about incognito mode. Is a blank page in non-incognito mode.

Netscape

The about: URL originated in and has existed in all versions of Netscape browsers. It was originally added as an Easter egg to display information about the development team.[20]

Older versions of the Netscape browser have an about:people URI 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).[21] 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.[22]

Some versions of Netscape would display the browser history for the about:global URI.[22]

Others

  • In Konqueror, any about URI except about:blank and about:plugins redirects to about:konqueror, which shows a friendly ‘start’ and navigation page.
  • In 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.[23]
  • Safari only recognizes about:blank.
  • Microsoft Outlook supports an additional outlook:today URI, which shows the Outlook Today Screen. This screen shows messages, tasks and appointments; it is also accessible from Internet Explorer. Outlook recognizes about:blank too, but no other "about:" URIs.

References

  1. ^ "Uniform Resource Identifier (URI) Schemes". www.iana.org. Retrieved 2016-09-28.
  2. ^ Dominik Hoffmann (1996-06-14). "Special Netscape URLs for cache diagnostics?". Newsgroup: comp.infosystems.www.browsers.mac. Retrieved 2012-09-29.
  3. ^ Matthew Paul Thomas (2000-06-23). "Weird easter egg in NN 3.0 or 3.01". Retrieved 2012-09-29.
  4. ^ "Appsawg Status Pages". IETF.
  5. ^ ""about" URI Tokens". www.iana.org. Retrieved 2017-06-28.
  6. ^ RFC 6694
  7. ^ "CSS Values and Units Module Level 3". www.w3.org. Retrieved 2017-06-28.
  8. ^ "HTML Standard".
  9. ^ a b "Issue 615738: Deprecate chrome://plugins". bugs.chromium.org. Retrieved 2022-03-28.
  10. ^ a b "Issue 811956: Remove chrome://view-http-cache". bugs.chromium.org. Retrieved 2022-03-28.
  11. ^ Gunnell, Marshall (26 September 2020). "36 Hidden Google Search Games and Easter Eggs". How-To Geek. LifeSavvy Media.
  12. ^ "Debugging memory leaks". Mozilla Developer Network. Archived from the original on 2010-12-23. Retrieved 2010-08-10.
  13. ^ "446704 - "This might void your warranty!" is not appreciated by corporate customers". Bugzilla. Mozilla. 2008-07-22. Retrieved 2014-07-07.
  14. ^ "Bug 417302 — about:robots". Mozilla Project Bugzilla.
  15. ^ "Enterprise Mode and the Enterprise Mode Site List (Internet Explorer 11 for IT Pros)". Docs.microsoft.com. Retrieved 2018-06-19.
  16. ^ "Screenshot of Epiphany 3.4".
  17. ^ "NEWS file". The GNOME Project's git repository. Retrieved 2011-10-17.
  18. ^ "A short history of the about: URL".
  19. ^ "About: Netscape People". Yikes!. Archived from the original on 2006-05-20. Retrieved 2006-04-21.
  20. ^ a b "This page is dedicated to Netscape Navigator 3.0b4 and followers, oops Mozilla 3.0b4 and friends!". Archived from the original on 2020-10-24. Retrieved 2010-11-16.
  21. ^ "Internet Explorer Easter Egg — IE5 Mac Team". The Easter Egg Archive. 2000-06-19.
  • RFC 6694 - The "about" URI Scheme
  • About protocol links in Mozilla and Mozilla Firefox.
  • about redirectors' source code[permanent dead link] in Firefox
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