About URI scheme

Short description: Internal URI scheme

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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.

## Contents

* 1 Standardization
* 2 Common
* 3 Chrome and Chromium derivatives
* 4 Firefox
* 5 Opera (v1–15)
* 6 Internet Explorer (6–11)
* 7 Microsoft Edge (v20–44)
* 8 GNOME Web
* 9 Netscape
* 10 Others
* 11 References
* 12 External links

## 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]

URI  | Purpose

|

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-compat | A 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:srcdoc | A reserved, though unresolvable, URI defined within HTML5 intended to be the URI navigated to within iframes whose content comes from the srcdoc attribute.
about:invalid | References 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-kind | Used as an identifier for kinds of media tracks.[8]

## Common

Common "about:" URIs  URI  | Purpose

|

Firefox, SeaMonkey, Mozilla | Google Chrome, Chromium | Konqueror | Opera | Edge | Internet Explorer
about: | Deprecated in Firefox 57 (Firefox Quantum)  | Redirects to "chrome://version"  | Redirects to "about:konqueror"  | Redirects to "opera:about"  | Redirects to "edge://version"  | N/A
about:about | Lists 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"  | N/A
about:blank | Shows a blank document
about:plugins | Shows the filename, MIME type, description, suffix, and status of all installed plug-ins  | Deprecated in Chrome 57[9] | Shows the filename, MIME type, description, suffix, and status of all installed plug-ins  | Redirects to "opera:plugins"  | N/A  | N/A
about:cache | Shows information about the Cache Service.  | Deprecated in Chrome 66[10] | Redirects to "about:konqueror"  | Redirects to "opera:cache"  | N/A  | N/A
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.  | N/A  | Redirects to "about:konqueror"  | N/A  | N/A  | 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:cache | Deprecated in version 66[10]
about:crash | Crashes the current rendering process. (This causes the "Aw, snap!" crash notification page to be displayed.)
about:credits | Displays the list of free and open source software used in the browser, and their associated licenses.
about:dino | Displayed 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:dns | Displays the DNS records
about:flags | Displays a page where experimental features can be configured by setting feature flags.
about:gpu | Displays information about WebGL and hardware accelerated graphics.
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. Additionally, crashes Chromebooks entirely due to them running on Google Chrome.
about:internets | On 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: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 | 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 (Deprecated in Chrome 57)[9]
about:sandbox | Shows which sandbox protection mechanisms are currently enabled.
about:shorthang | Hangs the tab's process, making it become unresponsive. This is designed for testing the hanging protection.
about:stats | Displays statistics about processes. At the top of the page, it states "Shhh! This page is secret!"
about:version | Displays 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  URIs  | Purpose

|

Firefox | SeaMonkey, Mozilla Application Suite
about:addons | Shows installed addons, plugins, themes, and provides an interface for installing new addons (Firefox 4+ and SeaMonkey 2.1+)
about:app-manager | Shows the Firefox OS Simulator  | N/A
about:bloat | Displays BloatView output (disabled in release builds). Superseded by about:memory on Gecko 1.9.2.[14] | N/A
about:bloat?new | TBA  | N/A
about:bloat?clear | TBA  | N/A
about:blocked | Shows the malware protection page used when the browser identifies a page as not safe for viewing (Firefox 3+)  | N/A
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).

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: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:devtools | Shows the welcome page used by Firefox Developer Tools
about:downloads | Shows a list of downloaded files
about:feeds | Shows the page used when clicking the rss feed icon in the address bar
about:healthreport | Shows the Firefox Health Report
about:home | Shows the Firefox 4 default home page  | N/A
about:jetpack | Shows an interface for viewing and setting the JetPack extension  | N/A
about:licence | The equivalent of about:license in the British English localizations of Firefox. (See American and British English spelling differences). Not available since 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:newaddon | This 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.  | N/A
about:newtab | Shows a grid of favourite and most-visited websites (Firefox 13+)
about:permissions | Shows permissions for all sites on Firefox 6.0a1  | N/A
about:plugins | Shows installed, currently enabled plug-ins
about:preferences | Shows the new preferences page
about:privatebrowsing | Can 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  | N/A
about:reader?url= | Opens the specified URL in Reader Mode. For example, `about:reader?url=http://example.com`.  | N/A
about:rights | Shows licensing information
about:robots | Easter 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.  | N/A
about:sessionrestore | Shows an interface for viewing about last session.
about:startpage | In Ubuntu, shows the Ubuntu start page with the current search engine (if the "Ubuntu Firefox Modifications – ubufox" add-on is installed)
about:support | Shows troubleshooting information (Firefox 3.6+, SeaMonkey 2.1+)
about:sync-tabs | Shows list of tabs from other computers. Requires enabled synchronization.
about:tabcrashed | Contains the text displayed when a tab crashes. Added in Firefox 25.  | N/A
about:webrtc | Shows an interface for WebRTC internals
about:welcome | Shows 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  URIs  | Purpose

|

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 | Shows 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:drives | Shows the local drives of the host system
opera:history | Shows the content of the browser history
opera:historysearch | Shows the start page of internal search engine for the browser history (Opera 9.5 and newer)
opera:plugins | Shows installed plug-ins. Opera 11 and newer allow individual plug-ins to be disabled from this page.
opera:button | Defines 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:help | Shortcut to the help directory. This can be either local or external.
opera:debug | Shows a page that allows changing of settings for remote debug of Opera via Opera Dragonfly (Opera 9.5 and newer)
opera:memdebug | Shows debug information about memory usage (Opera 9.5 and newer)
opera:webstorage | Shows the browser's Web Storage (Opera 10.5 and newer)
opera:webdatabases | Shows the browser's Web Storage databases (Opera 10.5 and newer)
opera:gpu | Shows information about hardware accelerated graphics and GPU support
opera:cpu | Displays 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  URI  | Purpose  | Supported versions

|
|

about:home | Displays the user's home page.  | 6, 7, 9, 10, 11 (not 8)
about:desktopitemnavigationfailure | Displays the "navigation cancelled" page.  | 6–8
about:navigationcanceled
about:navigationfailure
about:noadd-ons | Appears when add-ons are disabled to notify the users of a change in their web browsing experience.  | 7–11
about:noadd-onsinfo | Contains information about add-ons and what they do.  | 7–11
about:offlineinformation | Informs the user that the current page cannot be viewed offline. Internet Explorer 9 and later do not support offline browsing.  | 6–8
about:postnotcached | Informs 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:securityrisk | Informs the user not to browse with the current security settings because they may be harmful to the computer.  | 6–11
about:tabs | Informs 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:inprivate | Appears when the use initiates InPrivate Browsing; contains information about this feature.  | 9–11
about:compat | Displays a table of sites and the document rendering emulation mode configured for maximum backward compatibility.[17] | 11
about:newsfeed | Displays 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  URI  | Purpose

|

about:home | Shows the content that would be shown if the home button were pressed
about:flags | Displays a list of Developer Setting and Experimental features
about:start | Shows the customizable (default) start page.
about:config | Redirects to about:flags
about:tabs | Shows the (default) new tab page.
about:compat | Shows the Enterprise Mode list (if configured)
about:edge | Shows the Edge logo.
about:inprivate | Shows the inprivate information tab.
about:surf | A 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  URI  | Purpose

|

about:plugins | Shows installed plug-ins.
about:memory | Shows details about memory usage.
about:epiphany | Displays the following message:[18]

> |

> 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.

> |

> It seems that perfection is attained not when there remains nothing to add, but when there remains nothing to remove.

> 
|

> |

> - Antoine de Saint-Exupéry

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:applications | Interface for managing the Web Application Mode, supported since release 3.2.[19]
about:overview | Displays a grid of thumbnails for the more visited websites.
about:gpu | Displays information about WebGL and hardware accelerated graphics.
about:web | Displays the browser and its engine information, same as about:
about:incognito | Displays 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". https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml.
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. http://tools.ietf.org/wg/appsawg/.
5. ↑ ""about" URI Tokens". https://www.iana.org/assignments/about-uri-tokens/about-uri-tokens.xhtml.
6. ↑ RFC 6694
7. ↑ "CSS Values and Units Module Level 3" (in en). https://www.w3.org/TR/css-values/#about-invalid.
8. ↑ "HTML Standard". https://html.spec.whatwg.org/multipage/urls-and-fetching.html#about:html-kind.
9. ↑ 9.0 9.1 "Issue 615738: Deprecate chrome://plugins". https://bugs.chromium.org/p/chromium/issues/detail?id=615738.
10. ↑ 10.0 10.1 "Issue 811956: Remove chrome://view-http-cache". https://bugs.chromium.org/p/chromium/issues/detail?id=811956.
11. ↑ "AboutLinks". code.google.com/. Google Code. 22 December 2010. https://code.google.com/p/chromium/wiki/AboutLinks.
12. ↑ Gunnell, Marshall (26 September 2020). "36 Hidden Google Search Games and Easter Eggs". LifeSavvy Media. https://www.howtogeek.com/690469/36-hidden-google-search-games-and-easter-eggs/.
13. ↑ "About protocol links". mozillaZine. http://kb.mozillazine.org/About_protocol_links.
14. ↑ "Debugging memory leaks". Mozilla Developer Network. https://developer.mozilla.org/en/Debugging_memory_leaks.
15. ↑ "446704 - "This might void your warranty!" is not appreciated by corporate customers.". Mozilla. 2008-07-22. https://bugzilla.mozilla.org/show_bug.cgi?id=446704.
16. ↑ "Bug 417302 — about:robots". Mozilla Project Bugzilla. https://bugzilla.mozilla.org/show_bug.cgi?id=417302.
17. ↑ "Enterprise Mode and the Enterprise Mode Site List (Internet Explorer 11 for IT Pros)". Docs.microsoft.com. https://docs.microsoft.com/en-us/internet-explorer/ie11-deploy-guide/what-is-enterprise-mode#enterprise-mode-and-the-enterprise-mode-site-list-xml-file.
18. ↑ "Screenshot of Epiphany 3.4". //blogs.gnome.org/xan/files/2012/03/about-epiphany.png.
19. ↑ "NEWS file". The GNOME Project's git repository. https://git.gnome.org/browse/epiphany/plain/NEWS.
20. ↑ "A short history of the about: URL". http://www.montulli.org/lou/about_urls.
21. ↑ "About: Netscape People". Yikes!. http://www.yikes.com/netscape/people.html.
22. ↑ 22.0 22.1 "This page is dedicated to Netscape Navigator 3.0b4 and followers, oops Mozilla 3.0b4 and friends!". http://www.rigaut.com/benoit/CERN/about/.
23. ↑ "Internet Explorer Easter Egg — IE5 Mac Team". The Easter Egg Archive. 2000-06-19. http://www.eeggs.com/items/14448.html.

## External links

* RFC 6694 \- The "about" URI Scheme
* About protocol links in Mozilla and Mozilla Firefox.
* about redirectors' source code[yes|permanent dead link|dead link}}] in Firefox

* v
* t
* e

Uniform Resource Identifier (URI) schemes

Official|

* about
* acct
* crid
* data
* file
* ftp
* geo
* gopher
* http
* https
* info
* ldap
* mailto
* nfs
* nntp
* sip / sips
* tag
* telnet
* urn
* view-source
* ws / wss
* xmpp

Unofficial|

* coffee
* ed2k
* feed
* finger
* irc / irc6 / ircs
* ldaps
* magnet
* rsync
* ymsgr

Protocol list

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