In computing, a user agent is software (a software agent) that is acting on behalf of a user. For example, an email reader is a Mail User Agent, and in the Session Initiation Protocol (SIP), the term user agent refers to both end points of a communications session.[1]
In many cases, a user agent acts as a client in a network protocol used in communications within a client–server distributed computing system. In particular, the Hypertext Transfer Protocol identifies the client software originating the request, using a "User-Agent" header, even when the client is not operated by a user. The SIP protocol (based on HTTP) followed this usage.
HTTP is used in a wide variety of applications; not only the traditional web-related Web browsers, search engine crawlers (spiders), screen readers and braille browsers used by people with disabilities, applications on mobile phones, but also print drivers and home appliance control.
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When software agent operates in a network protocol, it often identifies itself, its application type, operating system, software vendor, or software revision, by submitting a characteristic identification string to its operating peer. In HTTP, SIP, and SMTP/NNTP[2] protocols, this identification 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.
In HTTP, the User-Agent string is often used for content negotiation, where the origin server selects suitable content or operating parameters for the response. For example, the User-Agent string might be used by a web server to choose variants based on the known capabilities of a particular version of client software.
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).
As with many other HTTP request headers, the information in the "User-Agent" string contributes to the information that the client sends to the server, since the string can vary considerably from user to user.[3]
The User-Agent string format is currently specified by Section 14.43 of RFC 2616 (HTTP/1.1) The format of the User-Agent string in HTTP is a list of product tokens (keywords) with optional comments. For example if your product were called WikiBrowser, your user agent string might be WikiBrowser/1.0 Gecko/1.0. The "most important" product component is listed first. The parts of this string are as follows:
Unfortunately, during the browser wars, many web servers were configured to only send web pages that required advanced features to clients that were identified as some version of Mozilla.
For this reason, most Web browsers use a User-Agent value as follows: Mozilla/[version] ([system and browser information]) [platform] ([platform details]) [extensions]. For example, Safari on the iPad has used the following:
Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (KHTML, like Gecko) Mobile/7B405
The components of this string are as follows:
The popularity of various Web browser products has varied throughout 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.[4] Thus, various browsers have a feature to cloak or spoof their identification to force certain server-side content. For example, the Android browser identifies itself as Safari in order to aid compatibility.[5]
Other HTTP client programs, like download managers and offline browsers, often 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,[6] 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 are inaccurate.
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 older browsers, because older 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, the W3C recommends creating HTML markup that is standard,[7] allowing correct rendering in as many browsers as possible, and to test for specific browser features rather than particular browser versions or brands.[8]
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.
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).[9] Following the lifting of export restrictions, most vendors supported 256-bit encryption.