User-Agent header

A user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system. The term most notably refers to applications that access the World Wide Web, but other systems, such as the Session Initiation Protocol (SIP), use the term user agent to refer to both end points of a communications session.

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 a user agent operates, it typically identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer. In the HTTP and SIP protocols, this is transmitted in a header field User-Agent. 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.

User agent identification

Some user agents identify their software as part of the client–server conversation. In HTTP and SIP, the identity is transmitted via the User-Agent request header, as described by RFC 1945. This string is then used by the communications partner to characterize the client and optionally select suitable content or operating parameters for session. For example, this may be used to provide properly formatted content for desktop computers and for smartphones.

The user agent string is one of the criteria by which Web crawlers may be excluded from accessing certain parts of a Web site using the Robots Exclusion Standard (robots.txt file).

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 Internet Engineering Task Force (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. 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.

Spam bots and Web scrapers often use fake user agents.

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

A result of user agent spoofing may be that collected statistics of Web browser usage is inaccurate.

User agent sniffing

The term user agent sniffing refers to the practice of Web sites 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. A useful example of this is Microsoft Exchange Server 2003's Outlook Web Access feature. When viewed with Internet Explorer 6 (or newer), more functionality is displayed compared to the same page in any other browser, because other browsers could not render the same content. User agent sniffing is mostly considered poor practice, since it encourages browser-specific design and penalizes new browsers with unrecognized user agent identifications. Instead, developers recommend creating HTML markup that is standard, allowing correct rendering in as many browsers as possible, and to test for specific browser features rather than particular browser versions or brands.

Web sites 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, while many older phones that do not possess these new technologies are still heavily used. Therefore, mobile Web portals 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.

Encryption strength notations

Web browsers created in the United States, such as Netscape Navigator and Internet Explorer, use the letters U, I, and N, to specify the encryption strength in the user agent string. Until the United States government allowed encryption with keys longer than 40 bits to be exported, in 1996, vendors shipped various browser versions 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 — and "N" stands (de facto) for "None" (no encryption). Following the lifting of export restrictions, most vendors supported 256-bit encryption.

See also

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

External links

• User-agents.org, database of known user agent strings
• Analyze any user agent string
• Detailed analysis of user agent and browser capabilities