What Does Your User Agent Say About You?


A user agent is a computer program representing a person, for example, a browser in a Web context.

Besides a browser, a user agent could be a bot scraping webpages, a download manager, or another app accessing the Web. Along with each request they make to the server, browsers include a self-identifying User-Agent HTTP header called a user agent (UA) string. This string often identifies the browser, its version number, and its host operating system.

Spam bots, download managers, and some browsers often send a fake UA string to announce themselves as a different client. This is known as user agent spoofing.

The user agent string can be accessed with JavaScript on the client side using the navigator.userAgent property.

A typical user agent string looks like this: "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0".


User Agent String

Browser Data

User Agent Example

Ellen Grant
• Tuesday, 24 November, 2020
• 16 min read

Rv: gecko version indicates the release version of Gecko (such as 17.0 “). The Chrome (or Chromium/Blink-based engines) user agent string is similar to Firefox’s.



For compatibility, it adds strings like HTML, like Gecko and Safari. The Opera browser is also based on the Blink engine, which is why it almost looks the same, but adds “Or/”.

The user agent application is Mozilla version 5.0. The operating system is NT version 10.0 (and is running on a Windows(64-bit) Machine).

Google Chrome Internet Explorer Firefox Safari Opera See your article appearing on the GeeksforGeeks main page and help other Geeks.

Basically, this works as the software that bridges the gap between the internet, the user and you. You might have an easier time understanding how user agents backtrack to look at the evolution of it.

Users were required to type in the commands and navigate and send messages. Basically, the browser acts as the agent, turning the actions into the commands.

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Whenever the browser loads the website, it will identify the agent for retrieving the content that has been requested. This puts the user agent identification and the browser in with a host of information.

User agents have importance because of how it controls the search engine robots for the robots.txt to crawl and file on the server. Before you can leverage such information usefully, you will need to have a deep understanding of the components and the user agent strings.

You will also have to look at the risks that come with depending on a method like this for delivering solid content to people. Ultimately, you could compare this set of information as data made for web developers to use.

This lets them customize the experience based on the user agent that loads the page. Not every user agent will be instructed or controlled by the human in real time.

When the web first began, user agents were intended as a programming design that could help the computer to distinguish between Mozilla from Mosaic. When it comes to the engine that displays the content, the device is referred to as Apple WebKit version 537.36.

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Unfortunately, dissecting the user agent strings can be somewhat complicated because of how you don’t have a standard format with this system. Before we ever start, we have to realize that the user agent will never be the fool-proof method for identifying devices, users and browsers.

It means that the Web browser you're using is accessing a program (such as Gmail) or a service (such as that operates on some other faraway computer. Your computer (the client) is connecting to the desired Web page (the server) through one of the TCP/IP protocols.

When you key in a “query” on your Web browser (user agent) and hit “enter,” a text string (a programming sequence of symbols) is sent to the server of that website. While the text string identifies itself to the server as an user agent, it simultaneously requests access to the website.

When the Internet was young, the World Wide Web was dominated by a few of the first generation of browsers. As a result, many Web servers were designed to interact and connect with only those leading browsers.

This was possible because the website could identify that Web browser as the user agent by its text string when it requested access to the site. To overcome this roadblock, competing browsers were modified to replicate or impersonate text strings that would be accepted by the website.


An early example of this is when the browser Internet Explorer spoofed its primary rival at the time, Netscape Navigator. They did it so people using Internet Explorer could gain access to websites that were being limited to Netscape browser users.

The data in the user agent string help the website to deliver the content in a format that suits your browser. Even though depending on user agents alone is no longer enough to optimize a website, they are still an important source of information.

Along with the user agent identification, the browser sends information about the device and the network that the user is on, like the IP address. The same is true for email clients and other programs that display website content.

For the breakdown we will concentrate on user agents that can be expected to be web browsers operated by humans. For these browsers the format of the user agent string is: Mozilla/ () () Since Opera, who were the last to adapt to this standard, also started using the Mozilla user agent string, every popular browser uses this and will start the user agent string with Mozilla and the version number.

Some browser extensions are noted in the user agent string if they need certain content to be rendered in a specific way. Of course sites with malicious intentions can use this information to deliver specific exploits that have a bigger chance of working on your system.

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This post explains the pros and cons of using user agents when browsing the World Wide Web. Every six months, Mozilla gets all of its employees from around the world together in one place, for a week.

This morning was the plenary session, with Mozilla leadership talking about our successes this year, as well as plans for the future. I noticed a theme that, while not super overt, I find very interesting, and bodes well for Mozilla’s future.

In my mind, this user agent framing is a very useful and interesting way to think about Mozilla’s mission, and the goals we pursue in the future. Ideally, this header would give servers the ability to perform content negotiation, sending down exactly those bits that best represent the requested resource in a given user agent, optimizing both bandwidth and user experience.

Brand and version information (e.g. “Chrome 69”) allows websites to work around known bugs in specific releases that aren’t otherwise detectable. For example, implementations of Content Security Policy have varied wildly between vendors, and it’s difficult to know what policy to send in an HTTP response without knowing what browser is responsible for its parsing and execution.

Developers will often negotiate what content to send based on the user agent and platform. Similarly to #1, OS revisions and architecture can be responsible for specific bugs which can be worked around in website’s code, and narrowly useful for things like selecting appropriate executables for download (32 vs 64 bit, ARM vs Intel, etc).

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It introduces four new Client Hints () that can provide the client’s branding and version information, the underlying operating system’s branding and major version, as well as details about the underlying device. Rather than broadcasting this data to everyone, all the time, user agents can make reasonable decisions about how to respond to given sites' requests for more granular data, reducing the passive fingerprinting surface area exposed to the network.

The request header field gives a server information about the architecture of the platform on which a given user agent is executing. The request header field gives a server information about the device on which a given user agent is executing.

The request header field gives a server information about the platform on which a given user agent is executing. The request header field gives a server information about the platform version on which a given user agent is executing.

The request header field gives a server information about an useragent's branding and version. It is considered low entropy because it includes only the useragent's branding information, and the significant version number (both of which are fairly clearly sniffable by “examining the structure of other headers and by testing for the availability and semantics of the features introduced or modified between releases of a particular browser” ).

The request header field gives a server information about whether an user agent prefers a “mobile” user experience. It is considered low entropy because it is a single bit of information directly controllable by the user.

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Collect pairs of brands and which represent the user agent, its equivalence class and/or its rendering engine. Append additional items to list containing objects, initialized with arbitrary and combinations.

Note: One approach to minimize caching variance when generating these random components could be to determine them at build time, and keep them identical throughout the lifetime of the user agent's significant version. Client Hints will not be delivered to non-secure endpoints (see the secure transport requirements in Section 2.2.1 of).

This means that user agent information will not be leaked over plaintext channels, reducing the opportunity for network attackers to build a profile of a given agent ’s behavior over time. The primary goal of Sergeant Client Hints is to reduce the default entropy available to the network for passive fingerprinting.

User agents ought to exercise judgement before granting access to this information, and MAY impose restrictions above and beyond the secure transport and delegation requirements noted above. For instance, user agents could choose to reveal only on requests it intends to download, giving the server the opportunity to serve the right binary.

Resetting expectations may help to prevent abuse of the UA string’s brand in the short term, but probably won’t help in the long run. User agents brands containing more than a single entry could encourage standardized processing of the UA string.

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By randomly including additional, intentionally incorrect, comma-separated entries with arbitrary ordering, they would reduce the chance that we ossify on a few required strings. In order to encourage sites to rely on equivalence classes based on Chromium versions rather than exact UA sniffing, Chrome might remove itself from the set entirely.

Browsers based on Chromium may use a similar UA string, but use their own brand as part of the set, enabling sites to count them. When adding arbitrary values to brands, user agents MUST make sure that receivers of the header adhere to Structured Header parsing, by adding escaped double-quotes, commas and semi-colons to those values.

The purpose of this is to make non-compliant server implementations immediately aware that their parsing code is inadequate. Note: One approach to minimize caching variance could be to determine the GREASE parts of the UA set at build time, and keep them identical throughout the lifetime of the useragent's significant version.

Restricting user -land JavaScript code from influencing and modifying UA-CH headers has various security related advantages. As such and based on discussions with the TAG, it seems reasonable to forbid write access to these headers from JavaScript (e.g. through fetch or Service Workers), and demarcate them as browser-controlled client hints, so they can be documented and included in requests without triggering CORS preflights.

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