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. Other systems, such as Session Initiation Protocol (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 and e-mail clients 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 usually sent in the HTTP header field User-Agent to identify the application and operating system names and versions to the server. 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 web crawlers can be excluded from certain pages or parts of a website using the "Robots Exclusion Standard" (robots.txt file). This allows webmasters to request 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, by requesting that crawler not to visit certain pages.

Contents 1 User agent spoofing 2 User agent sniffing 3 Encryption strength notations 4 See also 5 References 6 External links

[edit] User agent spoofing

The popularity of various Web browser products has varied through out the Web's history, and this has influenced the design of Web sites in such a way that Web sites are sometimes designed to work well only with particular browsers, rather than according to uniform standards by the World Wide Web Consortium (W3C) or the IETF. Web sites often include code to detect browser version to adjust the page design sent according to the user agent string received. This may 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.^{[citation needed]} Thus various browsers have a feature to cloak or spoof their identification to force certain server-side content.

Other HTTP client programs, like download managers and offline browsers, also have the ability to change the user agent string.

At times it has been popular among Web developers to intiate Viewable With Any Browser campaigns^[2], encouraging developers to design Web pages that work equally well with any browser.

One result of user agent spoofing may be that the usage share of web browsers is falsified in Web statistics.

[edit] 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 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's I-Mode or Vodafone's Vodafone Live! portals, often rely heavily on user agent sniffing, since mobile browsers 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).

[edit] Encryption strength notations

Web browsers created in the United States, such as Netscape Navigator, Internet Explorer, and some others, use the letters U, I, and N, to specify the encryption strength in the user agent string. Since the US government formerly did not allow encryption with larger than 40-bit keys to be exported, different browser versions were released with other 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, however the export restrictions have been lifted and most vendors distribute their browsers only in a "U" version, supporting up to 256-bit encryption.

[edit] See also

• Robots Exclusion Standard
• Web crawler
• Wireless Universal Resource File (WURFL)
• User Agent Profile (UAProf)
• List of user agents for mobile phones

[edit] References

[edit] External links

• User-agents.org, database of known user agent strings
• user-agent-string.info, User agent string detection system