In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, the User-Agent header is an
HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide We ...
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
Client(s) or The Client may refer to:
* Client (business)
* Client (computing), hardware or software that accesses a remote service on another computer
* Customer or client, a recipient of goods or services in return for monetary or other valuabl ...
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
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
, device model, software vendor, or software revision, by submitting a characteristic identification
string to its operating peer. In HTTP,
SIP,
[RFC 3261, ''SIP: Session Initiation Protocol'', IETF, The Internet Society (2002)] 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
An email address identifies an email box to which messages are delivered. While early messaging systems used a variety of formats for addressing, today, email addresses follow a set of specific rules originally standardized by the Internet Engineer ...
so that the
Webmaster
A webmaster is a person responsible for maintaining one or more websites. The title may refer to web architects, web developers, site authors, website administrators, website owners, website coordinators, or website publishers.
The duties of ...
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 i
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
The robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visit.
Th ...
(''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 o
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
Browse, browser or browsing may refer to:
Programs
* Web browser, a program used to access the World Wide Web
*Code browser, a program for navigating source code
* File browser or file manager, a program used to manage files and related objects
* ...
, 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
Mozilla (stylized as moz://a) is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, spreads and supports Mozilla products, thereby promoting exclusively free software and open standards, w ...
only. Other browsers were considered to be older products such as
Mosaic
A mosaic is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and were particularly pop ...
,
Cello
The cello ( ; plural ''celli'' or ''cellos'') or violoncello ( ; ) is a bowed (sometimes plucked and occasionally hit) string instrument of the violin family. Its four strings are usually tuned in perfect fifths: from low to high, C2, G ...
, 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
Opera is a form of theatre in which music is a fundamental component and dramatic roles are taken by singers. Such a "work" (the literal translation of the Italian word "opera") is typically a collaboration between a composer and a libr ...
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
The robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visit.
Th ...
".
Encryption strength notations
Web browsers created in the United States, such as
Netscape Navigator
Netscape Navigator was a web browser, and the original browser of the Netscape line, from versions 1 to 4.08, and 9.x. It was the flagship product of the Netscape Communications Corp and was the dominant web browser in terms of usage share in ...
and
Internet Explorer
Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft which was used in the Windows line of operating systems ( ...
, previously used the letters U, I, and N to specify the
encryption
In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can de ...
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
Export restrictions, or a restriction on exportation, are limitations on the quantity of goods exported to a specific country or countries by a Government. Export restrictions could be aimed at achieving diverse policy objectives such as environ ...
, most vendors supported 256-bit encryption.
User agent spoofing
The popularity of various Web browser products has varied throughout the Web's history, and this has influenced the design of websites in such a way that websites are sometimes designed to work well only with particular browsers, rather than according to uniform standards by the
World Wide Web Consortium
The World Wide Web Consortium (W3C) is the main international standards organization for the World Wide Web. Founded in 1994 and led by Tim Berners-Lee, the consortium is made up of member organizations that maintain full-time staff working ...
(W3C) or the
Internet Engineering Task Force
The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...
(IETF). Websites 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. For example, the Android browser identifies itself as
Safari (among other things) in order to aid compatibility.
Other HTTP client programs, like
download manager
A download manager is a software tool that manages the downloading of files from the Internet, which may be built: into a Web browser
A web browser is application software for accessing websites. When a user requests a web page from a par ...
s and
offline browsers, often have the ability to change the user agent string.
A result of user agent spoofing may be that collected
statistics of Web browser usage are inaccurate.
User agent sniffing
''User agent sniffing'' is the practice of websites showing different or adjusted content when viewed with certain user agents. An example of this is
Microsoft Exchange Server
Microsoft Exchange Server is a mail server and calendaring server developed by Microsoft. It runs exclusively on Windows Server operating systems.
The first version was called Exchange Server 4.0, to position it as the successor to the related ...
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 browsers. User agent sniffing is considered poor practice, since it encourages browser-specific design and penalizes new browsers with unrecognized user agent identifications. Instead, the W3C recommends creating standard HTML markup,
allowing correct rendering in as many browsers as possible, and to test for specific browser features rather than particular browser versions or brands.
Websites intended for display by mobile phones often rely on user agent sniffing, since
mobile browser
A mobile browser is a web browser designed for use on a mobile device such as a mobile phone or PDA. Mobile browsers are optimized to display Web content most effectively on small screens on portable devices. Mobile browser software must be smal ...
s often differ greatly from each other.
Deprecation of User-Agent header
In 2020,
Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
announced that they would be freezing parts of the User-Agent header in their
Chrome browser as it's no longer required for determining browser capabilities and instead mainly used for
browser fingerprinting
A device fingerprint or machine fingerprint is information collected about the software and hardware of a remote computing device for the purpose of identification. The information is usually assimilated into a brief identifier using a fingerprinti ...
. They stated that other major web browser vendors were supportive of the move. Google stated that a new feature called
Client Hints HTTP Client Hints (or simply Client Hints) are a set of HTTP Header Fields for proactive content negotiation in the Hypertext Transfer Protocol (HTTP). The client can advertise information about itself through these fields so the server
Server may ...
would replace the functionality of the user agent string.
Starting with Chrome 113, released in April 2023, User-Agent header stays the same except for the major version part.
Browser misidentification
Starting with
Firefox
Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current ...
110 released in February 2023,
Mozilla
Mozilla (stylized as moz://a) is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, spreads and supports Mozilla products, thereby promoting exclusively free software and open standards, w ...
announced it would temporarily freeze portions of the browser's user agent string at version 109. This was done due to several websites incorrectly recognizing a development version of the browser (which identified itself by the string
Mozilla/5.0 (Windows NT 10.0; Win64; rv:110.0) Gecko/20100101 Firefox/110.0
) as the
deprecated
In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
Internet Explorer 11
Internet Explorer 11 (IE11) is the eleventh, final, and now deprecated version of the Internet Explorer web browser. It was initially included in the release of Windows 8.1, Windows RT 8.1 and Windows Server 2012 R2 on October 17, 2013, and was ...
(which reports
Mozilla/5.0 (Windows NT 10.0; Trident/7.0; rv:11.0) like Gecko
). The problem will self-correct after the release of Firefox 120, as only browsers identifying themselves as 110 through 119 were observed to be affected by it.
See also
*
Browser sniffing
*
List of HTTP header fields
*
Robots exclusion standard
The robots exclusion standard, also known as the robots exclusion protocol or simply robots.txt, is a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the site they are allowed to visit.
Th ...
*
User Agent Profile (UAProf)
*
*
Web browser engine
A browser engine (also known as a layout engine or rendering engine) is a core software component of every major web browser. The primary job of a browser engine is to transform HTML documents and other resources of a web page into an interactiv ...
*
Web crawler
A Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an Internet bot that systematically browses the World Wide Web and that is typically operated by search engines for the purpose of Web indexing (''web spid ...
*
Wireless Universal Resource File (WURFL)
References
{{DEFAULTSORT:User Agent
Clients (computing)
Hypertext Transfer Protocol headers