In computing, the User-Agent header is an HTTP header intended to identify the user agent responsible for making a given HTTP request. Whereas the character sequence User-Agent comprises the name of the header itself, the header value that a given user agent uses to identify itself is colloquially known as its user agent string. The user agent for the operator of a computer used to access the Web has encoded within the rules that govern its behavior the knowledge of how to negotiate its half of a request-response transaction; the user agent thus plays the role of the client in a client–server system. Often considered useful in networks is the ability to identify and distinguish the software facilitating a network session. For this reason, the User-Agent HTTP header exists to identify the client software to the responding server.
Use in client requests When a software agent operates in a network protocol, it often identifies itself, its application type, operating system, device model, software vendor, or software revision, by submitting a characteristic identification string to its operating peer. In HTTP, SIP, and NNTP 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 concept of content tailoring is built into the HTTP standard in RFC 1945 "for the sake of tailoring responses to avoid particular user agent limitations". The user agent string is one of the criteria by which Web crawlers may be excluded from accessing certain parts of a website 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.
Format for human-operated web browsers The user agent string format is currently specified by section 10.1.5 of RFC 9110, HTTP Semantics. The format of the user agent string in HTTP is a list of product tokens (keywords) with optional comments. For example, if a user's product were called WikiBrowser, their 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:
product name and version (WikiBrowser/1.0) layout engine and version (Gecko/1.0) During the first browser war, many web servers were configured to send web pages that required advanced features, including frames, to clients that were identified as some version of Mozilla only. Other browsers were considered to be older products such as Mosaic, Cello, or Samba, and would be sent a bare bones HTML document. For this reason, most Web browsers use a user agent string value as follows:
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:
Mozilla/5.0: Previously used to indicate compatibility with the Mozilla rendering engine. (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us): Details of the system in which the browser is running. AppleWebKit/531.21.10: The platform the browser uses. (KHTML, like Gecko): Browser platform details. Mobile/7B405: This is used by the browser to indicate specific enhancements that are available directly in the browser or through third parties. An example of this is Microsoft Live Meeting which registers an extension so that the Live Meeting service knows if the software is already installed, which means it can provide a streamlined experience to joining meetings. Before migrating to the Chromium code base, Opera was the most widely used web browser that did not have the user agent string with "Mozilla" (instead beginning it with "Opera"). Since July 15, 2013, Opera's user agent string begins with "Mozilla/5.0" and, to avoid encountering legacy server rules, no longer includes the word "Opera" (instead using the string "OPR" to denote the Opera version).
Format for automated agents (bots) Automated web crawling tools can use a simplified form, where an important field is contact information in case of problems. By convention the word "bot" is included in the name of the agent. For example:
Googlebot/2.1 (+http://www.google.com/bot.html)
Automated agents are expected to follow rules in a special file called "robots.txt".
Encryption strength notations Web browsers created in the United States, such as Netscape Navigator and Internet Explorer, previously used the letters U, I, and N to specify the encryption strength in the user agent string. Until 1996, when the United States government allowed encryption with keys longer than 40 bits to be exported, 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.
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