A user agent is the client 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 via a network. These dumb terminals were clients of the time-sharing application used with a particular network protocol In computing, a protocol is a set of rules which is used by computers to communicate with each other across a network. A protocol is a convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax,; the phrase is most commonly used in reference to those which access the World Wide Web The World Wide Web is a system of interlinked hypertext documents accessed via the Internet. With a Web browser, one can view Web pages that may contain text, images, videos, and other multimedia and navigate between them using hyperlinks. Using concepts from earlier hypertext systems, the World Wide Web was invented in 1989 by the English. Other systems, such as Session Initiation Protocol The Session Initiation Protocol is a signalling protocol, widely used for setting up and tearing down multimedia communication sessions such as voice and video calls over Internet Protocol (IP). Other feasible application examples include video conferencing, streaming multimedia distribution, instant messaging, presence information and online (SIP), use the term user agent to refer to both end points of a phone call, server and client.[1]
Web user agents range from web browsers A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content. Hyperlinks present in resources enable users to easily navigate their browsers to and e-mail clients An e-mail client (also mail user agent or e-mail reader) is a frontend computer program used to manage e-mail to search engine A Web search engine is a tool designed to search for information on the World Wide Web. The search results are usually presented in a list and are commonly called hits. The information may consist of web pages, images, information and other types of files. Some search engines also mine data available in databases or open directories. Unlike Web crawlers A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner. Other terms for Web crawlers are ants, automatic indexers, bots, and worms or Web spider, Web robot, or—especially in the FOAF community—Web scutter ("spiders"), as well as mobile phones A mobile phone or mobile is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a mobile phone, telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging,, screen readers A screen reader is a software application that attempts to identify and interpret what is being displayed on the screen . This interpretation is then re-presented to the user with text-to-speech, sound icons, or a Braille output device. Screen readers are a form of assistive technology (AT) potentially useful to people who are blind, visually and braille The Braille system is a method that is widely used by blind people to read and write. Braille was devised in 1821 by Louis Braille, a Frenchman. Each Braille character or cell is made up of six dot positions, arranged in a rectangle containing two columns of three dots each. A dot may be raised at any of the six positions to form sixty-four browsers used by people with disabilities. When Internet users visit a web site, a text string In computer programming and some branches of mathematics, a string is an ordered sequence of symbols. These symbols are chosen from a predetermined set or alphabet is usually sent in the HTTP Hypertext Transfer Protocol is an application-level protocol for distributed, collaborative, hypermedia information systems. Its use for retrieving inter-linked resources led to the establishment of the World Wide Web header field User-Agent to identify the application and operating system An operating system is an interface between hardware and user; an OS is responsible for the management and coordination of activities and the sharing of the resources of the computer. The operating system acts as a host for computing applications that are run on the machine. As a host, one of the purposes of an operating system is to handle the names and versions to the server A web server has defined load limits, because it can handle only a limited number of concurrent client connections (usually between 2 and 60,000, by default between 500 and 1,000) per IP address (and TCP port) and it can serve only a certain maximum number of requests per second depending on:. Bots 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 possible for a human alone. The largest use of bots is in web spidering, in which an automated, such as web crawlers, often also include a URL In computing, a Uniform Resource Locator is a type of Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI. In popular language, a URL is also referred and/or e-mail address An e-mail address identifies a location to which e-mail messages can be delivered. An e-mail address on the modern Internet looks like, for example, jsmith@example.com and is usually read as "jsmith at example dot com". Many earlier e-mail systems had different formats for e-mail addresses and because modern e-mail systems are partially so that the webmaster A webmaster , also called a web architect, web developer, site author, website administrator, or (informally) webmeister, is a person responsible for maintaining a website(s). The duties of the webmaster may include ensuring that the web servers, hardware and software are operating accurately, designing the website, generating and revising web can contact the operator of the bot.
The user-agent string is one of the criteria by which web crawlers can be excluded from certain pages or parts of a website using the "Robots Exclusion Standard The robot 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. Robots are often used by search engines to categorize and archive web sites, or by webmasters to" (robots.txt file). This allows webmasters to force exclusion of individual web pages or directories from the data gathered by a particular crawler, or when a particular crawler is using up too much bandwidth In computer networking and computer science, digital bandwidth, network bandwidth or just bandwidth is a measure of available or consumed data communication resources expressed in bit/s or multiples of it, to request that crawler not to visit those pages.
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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 only with that particular browser, rather than according to standards from bodies such as the W3C The World Wide Web Consortium is the main international standards organization for the World Wide Web (abbreviated WWW or W3). It is arranged as a consortium where member organizations maintain full-time staff for the purpose of working together in the development of standards for the World Wide Web. As of June 2009, the W3C had 388 members and IETF The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standard bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite. It is an open standards organization, with no formal membership or membership requirements. All participants and leaders are. 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 Windows Internet Explorer , is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. It has been the most widely used web browser since 1999, attaining a peak of about 95% usage share during 2002 and 2003 with IE5 and IE6. That percentage share has since's use of a User-Agent string beginning "Mozilla/<version> (compatible; MSIE <version>...", in order to receive content intended for Netscape Navigator Netscape Navigator and Netscape are the names for the proprietary web browser popular in the 1990s, and the flagship product of the Netscape Communications Corporation, and the dominant web browser in terms of usage share. Yet by 2002 its users had almost disappeared. One of the reasons for this was due to the popularity of Microsoft's Internet, its main rival at the time of its development. This was not a reference to the open-source Mozilla Mozilla was the official, public, original name of Mozilla Application Suite by the Mozilla Foundation, currently known as SeaMonkey suite 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 the 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 Mozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. Firefox had 22.51% of the recorded usage share of web browsers as of May 2009[update], making it the second most popular browser in terms of current use worldwide, after Internet Explorer. See how Firefox performs, Safari Safari is a web browser developed by Apple Inc. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Apple has also made Safari the native browser for the iPhone OS. A version of Safari for the Microsoft Windows operating, and Opera Opera is a web browser and internet suite developed by the Opera Software company. Opera handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, IRC online chatting, downloading files via BitTorrent, and reading web feeds. Opera is offered free of charge for personal computers 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 an option of full masking as Internet Explorer or Firefox, which hides "Opera" completely.
Beside browsers, other programs using the HTTP protocol, like most download managers A download manager is a computer program dedicated to the task of downloading possibly unrelated stand-alone files from (and sometimes to) the Internet for storage. This is unlike a World Wide Web browser, which is mainly intended to browse web pages, composed of a multitude of smaller files, where error-free moving of files for permanent storage 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).
Some web developers have started a "Viewable With Any Browser" campaign[2] which encourages developers to design webpages that work regardless of the browser used.
One result of user agent spoofing is that the usage share The usage share of web browsers is the percentage of visitors to a group of websites that use a particular web browser. For example, when it is said that Internet Explorer has 66% usage share, it means that some version of Internet Explorer is used by 66% of visitors that visit a given set of sites 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 the practice of websites showing 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. An infamous example of this is Microsoft Exchange Server Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. It is part of the Microsoft Servers line of server products and is widely used by enterprises using Microsoft infrastructure solutions. Exchange's major features consist of electronic mail, calendaring, contacts and tasks; support for mobile and web- 2003's Outlook Web Access feature. When viewed with Internet Explorer, more functionality is displayed compared to the same page in any other browser.[citation needed] User agent sniffing is mostly considered poor practice, since it encourages browser-specific design and penalizes new browsers with unrecognized user agent identifications. Instead, Webmasters should create HTML markup that is as standard as possible, to allow correct rendering in as many browsers as possible, and test for specific browser features rather than particular browsers.[3]
Websites specifically targeted towards mobile phones, like NTT DoCoMo NTT docomo, Inc. is the predominant mobile phone operator in Japan. The name is officially an abbreviation of the phrase, "do communications over the mobile network", and is also from a phrase dokodemo, meaning "everywhere" in Japanese. docomo provides phone, video phone (FOMA and Some PHS), i-mode (internet), and mail (i-mode's I-Mode NTT DoCoMo's i-mode is a wireless internet service popular in Japan. Unlike Wireless Application Protocol or WAP, i-mode encompasses a wider variety of internet standards, including web access, e-mail and the packet-switched network that delivers the data. i-mode users have access to various services such as e-mail, sports results, weather or Vodafone Vodafone is a mobile network operator with its headquarters in Newbury, Berkshire, England, UK. It is the largest mobile telecommunications network company in the world by turnover and has a market value of about £75 billion . Vodafone currently has operations in 25 countries and partner networks in a further 42 countries's Vodafone Live! portals, often rely heavily on user agent sniffing, since mobile browsers A mobile browser, also called a microbrowser, minibrowser or wireless internet browser , is a web browser designed for use on a mobile device such as a mobile phone or PDA. Mobile browsers are optimized so as to display Web content most effectively for small screens on portable devices. Mobile browser software must be small and efficient to often differ greatly from each other. Many developments in mobile browsing have been made in the last few years,[when?] 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 The Extensible Hypertext Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax).
Encryption strength "U" / "I" / "N"
Web browsers created in the United States, such as Netscape Navigator Netscape Navigator and Netscape are the names for the proprietary web browser popular in the 1990s, and the flagship product of the Netscape Communications Corporation, and the dominant web browser in terms of usage share. Yet by 2002 its users had almost disappeared. One of the reasons for this was due to the popularity of Microsoft's Internet, Internet Explorer Windows Internet Explorer , is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. It has been the most widely used web browser since 1999, attaining a peak of about 95% usage share during 2002 and 2003 with IE5 and IE6. That percentage share has since, and some others, use one of these three letters to specify the browser's encryption In cryptography, encryption is the process of transforming information using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext). In many contexts, the word encryption strength. Since the US government formerly would not allow encryption higher than 40-bit to be exported from the country, 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 of such browsers was only available for download to those in the USA. The US government has since loosened its policy and exporting high encryption is now permitted to most countries (see Export of cryptography for more information). Now Netscape Netscape Communications was a US computer services company, best known for its web browser. The browser was once dominant in terms of usage share, but lost most of that share to Internet Explorer during the first browser war. By the end of 2006, the usage share of Netscape browsers had fallen, from over 90% in the mid 1990s, to less than 1% and Mozilla The Mozilla Foundation is a non-profit organization that exists to support and provide leadership for the open source Mozilla project. The organization sets the policies that govern development, operate key infrastructure and control trademarks and other intellectual property. It owns two taxable for-profit subsidiaries: the Mozilla Corporation, distribute their browsers only in a "U" version, supporting up to 256-bit encryption, since an international version is no longer required.
This information can be seen in Mozilla Firefox browser by typing about:config and searching for the string general.useragent.security.
See also
- Robots Exclusion Standard The robot 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. Robots are often used by search engines to categorize and archive web sites, or by webmasters to
- Web crawler A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner. Other terms for Web crawlers are ants, automatic indexers, bots, and worms or Web spider, Web robot, or—especially in the FOAF community—Web scutter
- Wireless Universal Resource File (WURFL) WURFL stands for Wireless Universal Resource File. It is part of a FOSS community effort focused on the problem of presenting content on the wide variety of wireless devices. The WURFL itself is an XML configuration file which contains information about device capabilities and features for a variety of mobile devices. Device information is
- User Agent Profile (UAProf) The User Agent Profile specification is concerned with capturing capability and preference information for wireless devices. This information can be used by content providers to produce content in an appropriate format for the specific device
References
- ^ RFC 3261, SIP: Session Initiation Protocol, IETF, The Internet Society (2002)
- ^ "Viewable with Any Browser" campaign
- ^ "Browser Detection and Cross Browser Support". Mozilla Developer Center. 2007-09-07. https://developer.mozilla.org/en/Browser_Detection_and_Cross_Browser_Support. Retrieved on 2009-05-30.
External links
- User-agents.org, database of known user agent strings
- user-agent-string.info, User agent string detection system
Categories: Clients | HTTP headers
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