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.
A typical user agent string looks like this: "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0".
Essentially, an user agent is a way for a browser to say “Hi, I’m Mozilla Firefox on Windows” or “Hi, I’m Safari on an iPhone” to a web server. The user agent string identifies the browser as IE 9 with the Trident 5 rendering engine.
The plot thickens: Chrome is pretending to be both Mozilla and Safari. To understand why, we’ll have to examine the history of user agents and browsers.
(Yes, this means that you can sometimes bypass registration screens by setting your user agent to Google bot.) He's written about technology for nearly a decade and was a World columnist for two years.
Chris has written for The New York Times, been interviewed as a technology expert on TV stations like Miami's NBC 6, and had his work covered by news outlets like the BBC. Since 2011, Chris has written over 2,000 articles that have been read more than 500 million times---and that's just here at How-To Geek.
There are ways to develop your website to progressively enhance itself based on the availability of features rather than by targeting specific browsers. Using the user agent to detect the browser looks simple, but doing it well is, in fact, a very hard problem.
Also, experts, or people with another point of view, can give you ideas for working around the bug. Your site needs to use a specific Web feature that some browsers don't yet support, and you want to send those users to an older Website with fewer features but that you know will work.
This is the worst reason to use user agent detection because odds are eventually all the other browsers will catch up. The difficulty of successfully using user agent detection is worth a few disruptions to the purity of your HTML.
Also, rethink your design: can you use progressive enhancement or fluid layouts to help remove the need to do this? In those rare cases where behavior differs between browsers, instead of checking the user agent string, you should instead implement a test to detect how the browser implements the API and determine how to use it from that.
Support could have been added to other browsers at any time, but this code would have continued choosing the inferior path. As the above code demonstrates, there is always a way to test browser support without user agent sniffing.
For example, in the above code snippets, using look behind in short-regexp notation (e.g. /reg/IGM) will cause a parser error in unsupported browsers. Progressive enhancement This design technique involves developing your Website in 'layers', using a bottom-up approach, starting with a simpler layer and improving the capabilities of the site in successive layers, each using more features.
Graceful degradation This is a top-down approach in which you build the best possible site using all the features you want, then tweak it to make it work on older browsers. People use user agent sniffing to detect if the users' device is touch-friendly and has a small screen, so they can optimize their website accordingly.
While user agent sniffing can sometimes detect these, not all devices are the same: some mobile devices have big screened sizes, some desktops have a small touchscreen, some people use smart TV's which are an entirely different ballgame altogether, and some people can dynamically change the width and height of their screen by flipping their tablet on its side! Rather, add in touch conveniences such as bigger, more easily clickable buttons (you can do this using CSS by increasing the font size).
Here is an example of code that increases the padding of #exampleButton to 1em on mobile devices. This effect can be easily achieved using CSS flex boxes, sometimes with floats as a partial fallback. Also try to move less relevant/important information down to the bottom and group the page's content together meaningfully.
Although it is off-topic, perhaps the following detailed example might give you insights and ideas that persuade you to forgo user agent sniffing. The boxes can be separated into multiple columns via two equally fair method.
The second method uses a Column layout and resents all the dogs to the left and all the cats to the right. Only in this particular scenario, it is appropriate to provide no fallback for the flex boxes/multi columns, resulting in a single column of very wide boxes on old browsers.
If more people visit the webpage to see the cats, then it might be a good idea to put all the cats higher in the source code than the dogs so that more people can find what they are looking for faster on smaller screens where the content collapses down to one column. Next, always make your code dynamic. The user can flip their mobile device on its side, changing the width and height of the page.
Or, there might be some weird flip-phone-like device thing in the future where flipping it out extends the screen. The simplest way to do this is to separate all the code that moves content around based on screen size to a single function that is called when the page is loaded and at each resize event thereafter.
One such case is using user agent sniffing as a fallback when detecting if the device has a touch screen. Internet Explorer (on Windows) and WebKit (on iOS) are two perfect examples.
WebKit is a bit worse because Apple forces all the browsers on IOS to use WebKit internally, thus the user has no way to get a better more updated browser on older devices. For example, WebKit 6 has a bug whereby when the device orientation changes, the browser might not fire MediaQueryList listeners when it should.
Most browsers set the name and version in the format BrowserName/VersionNumber, with the notable exception of Internet Explorer. So to detect Safari you have to check for the Safari string and the absence of the Chrome string, Chromium often reports itself as Chrome too or Sea monkey sometimes reports itself as Firefox.
The browser version is often, but not always, put in the value part of the BrowserName/VersionNumber token in the Sergeant String. There are five major rendering engines: Trident, Gecko, Presto, Blink, and WebKit.
It is therefore important to pay attention not to trigger false-positives when detecting the rendering engine. Must containGeckoGecko/xyzWebKitAppleWebKit/pay attention, WebKit browsers add a 'like Gecko' string that may trigger false positive for Gecko if the detection is not careful. PrestoOpera/XYZ Note: Presto is no longer used in Opera browser builds >= version 15 (see 'Blink')Trident Trident/Internet Explorer put this token in the comment part of the Sergeant StringEdgeHTMLEdge/the non-Chromium Edge puts its engine version after the Edge/ token, not the application version.
Note: Edge HTML is no longer used in Edge browser builds >= version 79 (see 'Blink'). BlinkChrome/XYZ Most rendering engines put the version number in the RenderingEngine/VersionNumber token, with the notable exception of Gecko. The Operating System is given in most Sergeant strings (although not web-focused platforms like Firefox OS), but the format varies a lot.
They indicate the OS, but also often its version and information on the relying on hardware (32 or 64 bits, or Intel/PPC for Mac). Like in all cases, these strings may change in the future, one should use them only in conjunction with the detection of already released browsers.
A technological survey must be in place to adapt the script when new browser versions are coming out. The most common reason to perform user agent sniffing is to determine which type of device the browser runs on.
Never assume that a browser or a rendering engine only runs on one type of device. Never use the OS token to define if a browser is on mobile, tablet or desktop.
Internet Explorer Immobile/XYZ token in the comment. Mozilla/5.0 (compatible; MSI 9.0; Windows Phone OS 7.5; Trident/5.0; Immobile/9.0)Edge on Windows 10 Mobile/XYZ & Edge/ tokens outside the comment. Mozilla/5.0 (Windows Phone 10.0; Android 6.0.1; Xbox; Xbox One) Apple WebKit/537.36 (HTML, like Gecko) Chrome/58.0.3029.110 Mobile Safari/537.36 Edge/16.16299In summary, we recommend looking for the string “Mob” anywhere in the Sergeant to detect a mobile device. If the device is large enough that it's not marked with “Mob”, you should serve your desktop site (which, as a best practice, should support touch input anyway, as more desktop machines are appearing with touchscreens).
Armed with this information, you can develop richer and more dynamic websites that deliver different experiences based on the user agent that's visiting. User agents are also critical in controlling search engine robots using the robots.txt file on your server.
But they don't function perfectly in every situation, and the information in an user agent string can be faked. In order to leverage this information, you need to understand the component parts of an user agent string and consider also the potential risks of using this method to deliver content.
It's easiest to understand user agents if we backtrack and look at the evolution of the web. When the internet was a text-based system, right back at the beginning of its use, users had to type commands to navigate and send messages.
We simply point and click, and the browser is acting as our agent,” turning our actions into commands. When your browser (or similar device) loads a website, it identifies itself as an agent when it retrieves the content you've requested.
Browsers : Including Microsoft Edge, Internet Explorer, Firefox, Safari, Chrome, BlackBerry, Opera, Opera Mini, iOS Safari, Chrome for Android, Samsung Internet, HQ browser, and others. Plus a whole range of feed readers, validators, cloud platforms, media players, email libraries, and scripts.
Once the user agent has identified itself to the web server, a process called content negotiation can begin. The user agent application is Mozilla version 5.0, or a piece of software compatible with it.
That's because Internet Explorer originally had to declare itself to be Mozilla compatible in order to receive content with frames. In practice, the majority of browsers now declare themselves to be Mozilla compatible to ensure that they can access all the content on the web.
Chrome Review (Lollipop and above) Mozilla/5.0 (Linux; Android 5.1.1; Nexus 5 Build/LMY48B; WV) Apple WebKit/537.36 (HTML, like Gecko) Version/4.0 Chrome/43.0.2357.65 Mobile Safari/537.36 An user agent is a particular string of characters in each browser that allows acts as an identification agent.
The user agent string contains the user application or software, the operating system (and their versions), the web client, the web client's version, and the engine responsible for the content display (such as Apple WebKit). It is a rendering engine that processes HTML and CSS to interpret and present web pages.
Platform identifiers change based on the operating system being used, and version numbers also increment as time passes. Mapping UA string tokens to a more human-readable browser name for use in code is a common pattern on the web today.
When mapping the new Edge token to a browser name, Microsoft recommends using a different name than the one developer used for the legacy version of Microsoft Edge to avoid accidentally applying any legacy workarounds that are not applicable to Chromium-based browsers. When Microsoft is notified about these types of issues, website owners are contacted and informed about the updated UA.
In these cases, Microsoft uses a list of UA overrides in our Beta and Stable channels to maximize compatibility for users who access these sites. When a 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.
For example, if a user's product were called WikiBrowser, their user agent string might be WikiBrowser/1.0 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.
Automated web crawling tools can use a simplified form, where an important field is contact information in case of problems. Automated agents are expected to follow rules in a special file called robots.txt “.
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 (W3C) or the Internet Engineering Task Force (IETF). Websites often include code to detect browser version to adjust the page design sent according to the user agent string received.
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.
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 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.
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 disallowed encryption with keys longer than 40 bits to be exported, vendors shipped various browser versions with different encryption strengths.
^ a b RFC 3261, SIP: Session Initiation Protocol, IETF, The Internet Society (2002) ^ RFC 7231, Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content, IETF, The Internet Society (June 2014) ^ Net news Article Format. Browser Versions Carry 10.5 Bits of Identifying Information on Average “, Electronic Frontier Foundation, 27 January 2010.
I've been rejected until I come back with Netscape” ^ “Android Browser Reports Itself as Apple Safari”. ^ User Agent String explained: Android WebKit Browser ".
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/
Newsreaders, screen readers and other web-related applications and systems act as agents as well. Some websites check the browser version in the user agent to ensure the user has a current browser and directs them to various vendors’ websites to download a newer version.
Many enterprise applications check browser type and versions due mostly to heavy customization of the UI and to ensure an optimal experience. The user is typically denied access without the application’s “certified” browser or is notified the experience will be less than optimal.
Even popular browsers such as Chrome, Firefox, and Safari can often display web content in inconsistent ways when compared with each other. But there are hundreds of user agents out there, ranging from browsers you’ve probably never heard of to email and RSS readers.
In this article, we’ll discuss the technical side of user agents and how you can take advantage of software and browser plugins to manipulate how content is shown on the web. This information is passed to web servers via HTTP headers, and it can be used to change the way that content is displayed.
While an user agent won’t identify you anywhere near as narrowly as an IP address, you’re still giving up the way you browse. Nowadays, most popular browsers include a developer console that will allow you to change all sorts of things, such as your viewing resolution and user agent.
The problem is that if you intend to change your user agent frequently, going through the steps to take it through a developer console can be tedious. This Chrome extension has nearly two million total users and was developed by Google, so its compatibility with the browser should be as good as you’ll ever find.
As previously mentioned, changing your user agent isn’t a guaranteed way to achieve the effects you might expect. 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. This post explains the pros and cons of using user agents when browsing the World Wide Web.