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Information about User Agent

A user agent is the client application used with a particular network protocol; the phrase is most commonly used in reference to those which access the World Wide Web, but other systems such as SIP uses the term user agent to refer to the user's phone. Web user agents range from web browsers to search engine crawlers ("spiders"), as well as mobile phones, screen readers and braille browsers used by people with disabilities. When Internet users visit a web site, a text string is generally sent to identify the user agent to the server. This forms part of the HTTP request, prefixed with User-agent: or User-Agent: and typically includes information such as the application name, version, host operating system, and language. Bots, such as web crawlers, often also include a URL and/or e-mail address so that the webmaster can contact the operator of the bot.

The user-agent string is one of the criteria by which crawlers can be excluded from certain pages or parts of a website using the "Robots Exclusion Standard" (robots.txt). This allows webmasters who feel that certain parts of their website should not be included in the data gathered by a particular crawler, or that a particular crawler is using up too much bandwidth, to request that crawler not to visit those pages.

User agent spoofing

At various points in its history, use of the Web has been dominated by one browser to the extent that many websites are designed to work with that particular browser, rather than according to standards from bodies such as the W3C and IETF. Such sites often include "browser sniffing" code, which alters the information sent out depending on the User-Agent string received. This can mean that less popular browsers are not sent complex content, even though they might be able to deal with it correctly, or in extreme cases refused all content. Thus various browsers "cloak" or "spoof" this string, in order to identify themselves as something else to such detection code; often, the browser's real identity is then included later in the string.

The earliest example of this is Internet Explorer's use of a User-Agent string beginning "Mozilla/<version> (compatible; MSIE <version>...", in order to receive content intended for Netscape Navigator, its main rival at the time of its development. This was not a reference to the open-source Mozilla browser, which was developed much later, but to the original codename for Navigator, which was also the name of the Netscape company mascot. This format of User-Agent string has since been copied by other user agents, partly because Explorer, in turn, came to dominate.

When Internet Explorer became the dominant web browser, rivals such as Firefox, Safari, and Opera implemented systems whereby the user could select a false User-Agent string to send, such as that of a recent version of Explorer. Some – e.g. Firefox and Safari – duplicate the User-Agent string they are trying to spoof exactly; others – e.g. Opera – duplicate the User-Agent string but add the genuine browser name to the end. This latter approach, of course, leads to a string containing three names and versions: first, the user agent claims to be "Mozilla" (i.e. Netscape Navigator); then, "MSIE" (Internet Explorer); and finally, the actual browser, such as "Opera". Opera also offers a full masking as Internet Explorer or Firefox, which hides "Opera" completely.

Beside browsers, other programs utilizing HTTP protocol, like most download managers and offline browsers, also had the ability to change the user agent string sent to servers to user's liking. This is presumably done in an effort to maintain compatibility with certain servers (some servers refused to serve those programs right away because they are mostly used carelessly, thus burdening the server).

This vicious circle is expected to continue in the area of web browsers. Some standards-based web developers have started the "Viewable With Any Browser" campaign which encourages developers to design webpages according to official standards, not for any particular browser(s).

One result of user agent spoofing is that the usage share of Internet Explorer, the user agent browsers typically spoof, is probably overestimated, and the usage share of other browsers may be underestimated. User agent spoofing can also provide a security issue by spoofing search engine bots and bypassing key parts in a website.

User agent sniffing

The term user agent sniffing refers to websites that show different content when viewed with a certain user agent. On the Internet, this will result in a different site being shown when browsing the page with a specific browser (e.g. Microsoft Internet Explorer). An infamous example of this is Microsoft's Outlook 2003 Web Access. When viewed with IE, much more functionality is displayed compared to the same page in any other browser. User agent sniffing is mostly considered poor practice for Web 2.0 web sites, since it encourages browser specific design. Many webmasters are recommended to create an HTML markup that is as standardised as possible, to allow correct rendering in as many browsers as possible.

Websites specifically targeted towards mobile phones, like NTT DoCoMo's I-Mode or Vodafone's Vodafone Live! portals, often rely heavily on user agent sniffing, since browsers on mobile phones often differ a lot from each other. Many developments in mobile browsing have been made in the last few years, while many older phones that do not possess these new technologies are still heavily used. Therefore, mobile webportals will often generate completely different markup code depending on the mobile phone used to browse them. These differences can be small (e.g. resizing of certain images to fit smaller screens), or quite extensive (e.g. rendering of the page in WML instead of XHTML).

There are a number of ways to perform user agent sniffing within web applications, including using public domain scripts and even commercial products such as BrowserHawk. See the External Links section for details.

Encryption strength "U" / "I" / "N"

Netscape, Mozilla, Opera and some others use one of these three letters to specify the browser's encryption strength. Since the US government formerly would not allow encryption higher than 40-bit to be exported from the USA, different versions were released with different encryption strengths. "U" stands for "USA" (for the version with 128-bit encryption), "I" stands for "International" (the browser has 40-bit encryption and can be used anywhere in the world), "N" stands for "None" (no encryption). Originally the "U" version was allowed for download only from the USA, but since then the US government has loosened its policy and exporting high encryption is now permitted to most countries (see Export of cryptography for more information). Now Netscape and Mozilla distribute their browsers only in a "U" version, supporting up to 256-bit encryption, since an international version is no longer required.

See also

External links

Lists of user-agent strings in common use

Tools for examining user-agent strings

Other links

A client is an application or system that accesses a (remote) service on another computer system known as a server by way of a network. The term was first applied to devices that were not capable of running their own stand-alone programs, but could interact with remote computers
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protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, semantics, and synchronization of communication.
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World Wide Web (commonly shortened to the Web) is a system of interlinked, hypertext documents accessed via the Internet. With a web browser, a user views web pages that may contain text, images, videos, and other multimedia and navigates between them using hyperlinks.
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The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants.
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A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network.
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search engine is an information retrieval system designed to help find information stored on a computer system. Search engines help to minimize the time required to find information and the amount of information which must be consulted, akin to other techniques for managing
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For the search engine of the same name, see .


A web crawler (also known as a web spider or web robot) is a program or automated script which browses the World Wide Web in a methodical, automated manner.
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mobile phone or cell phone is a long-range, portable electronic device used for mobile communication. In addition to the standard voice function of a telephone, current mobile phones can support many additional services such as SMS for text messaging, email, packet switching
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A screen reader is a software application that attempts to identify and interpret what is being displayed on the screen. This interpretation is then represented to the user with text-to-speech, sound icons, or a braille output.
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Braille

Unicode range U+2800 to U+28FF
ISO 15924 Brai

Note: This page may contain IPA phonetic symbols in Unicode.
The braille
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string is an ordered sequence of symbols. These symbols are chosen from a predetermined set.

In programming, when stored in memory each symbol is represented using a numeric value.
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web server can mean one of two things:
  1. A computer program that is responsible for accepting HTTP requests from clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and

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Hypertext Transfer Protocol (HTTP) is a communications protocol used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML hypertext pages.
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An operating system (OS) is the software that manages the sharing of the resources of a computer. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the
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Internet bots, also known as web robots, WWW robots or simply bots, are software applications that run automated tasks over the internet. Typically, bots perform tasks that are both simple and structurally repetitive, at a much higher rate than would be
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Uniform Resource Locator (URL) formerly known as Universal Resource Locator, is a technical, Web-related term used in two distinct meanings:
  • In popular usage, many technical documents, it is a synonym for Uniform Resource Identifier (URI);

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e-mail address identifies a location to which e-mail messages can be delivered. The term "e-mail address" is also used as the formal pre-registered authoritative electronic mailing delivery site for an individual (example: an attorney's e-mail address registered for delivery of
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The webmaster (feminine: webmistress), also called the system administrator, the author, or the website administrator, is the person responsible for designing, developing, marketing, or maintaining a website.
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robots exclusion standard, also known as the Robots Exclusion Protocol or robots.txt protocol is a convention to prevent cooperating web spiders and other web robots from accessing all or part of a website which is, otherwise, publicly viewable.
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Bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a filter, a communication channel, or a signal spectrum, and is typically measured in hertz.
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World Wide Web Consortium

Consortium
Founded October 1994
Founder Tim Berners-Lee
Headquarters MIT/CSAIL in USA
ERCIM in France
Keio University in Japan
and many other offices around the world

Website www.w3.
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Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE), commonly abbreviated to IE, is a series of proprietary graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems
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Maintainer: Netscape Communications Corporation

OS: Cross-platform

Use: Web browser

Website: [1] Netscape Navigator, also known as Netscape
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Mozilla was the official, public, original name of Mozilla Application Suite by the Mozilla Foundation, currently known as SeaMonkey suite.

In informal use it has been used in a number of ways and in combination with other phrases, though all of them have been
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Mozilla (or Mozzie or Moz) was the mascot of the now disbanded Netscape Communications Corporation.

Mozilla takes the form of a cute green and purple cartoon lizard.
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Maintainer: Mozilla Corporation / Mozilla Foundation

OS: Cross-platform
Available language(s): Multilingual,[1] EULA in English only[2]
Use: Web browser
License: Mozilla EULA for binary redistribution

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Maintainer: Apple Inc.

OS: Mac OS X, Microsoft Windows

Use: Web browser
License: Proprietary EULA, LGPL
Website: Apple: Safari

Safari is a web browser developed by Apple Inc. and included in Mac OS X.
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Maintainer: Opera Software ASA

OS: Cross-platform

Use: Internet suite
License: Proprietary
Website: [1] Opera is a cross-platform web browser and Internet suite developed by the Opera Software corporation.
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This article or section may contain original research or unverified claims.
Please help Wikipedia by adding references. See the for details.
This article has been tagged since September 2007.

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on-line and off-line have specific meanings with respect to computer technology and telecommunication. The concepts have however been extended from their computing and telecommunication meanings into the area of human interaction and conversation.
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