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".
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.
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”.
Mozilla/5.0 (Linux; U; Android 2.2; ends; HTC_DesireHD_A9191 Build/FRF91) Apple WebKit/533.1 (HTML, like Gecko) Version/4.0 Mobile Safari/533.1 ^ Emberton, Stephen. ^ “Chrome Phasing out Support for User Agent ".
In the early days of the web, developers would detect different browsers to determine features supported by them. The exception here is if there is a specific bug you're trying to workaround in a popular browser within your client base.
For example, if you had an intranet and many users had the same version of say Firefox installed and it had a known issue, you might consider adding a temporary workaround in your code for such things. They reveal a catalog of technical data about the device and software that the visitor is using.
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.
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.
This is a really set of data for web developers since it allows them to customize the experience depending on the user agent that's loaded the page. Browsers are a straightforward example of an user agent, but other tools can act as agents.
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. Note that a huge part of the user agent string is concerned with compatibility.
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.
More, we can feed that data back into a cycle of continuous improvement, analytics and other processes, like conversion optimization. As we mentioned in the introduction, search engine crawlers are a very specific type of user agent.
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. There's no “standard” way of writing an user agent string, so different web browsers use different formats (some are wildly different), and many web browsers cram loads of information into their user agents.
Providing regularly updated browscap.ini and related downloads since 1998! Browsers and search engine crawlers usually identify themselves through an HTTP user agent ID string field.
Some websites use the user agent string ID to detect if the visitor is a specific browser or search engine crawler. The HTTP user agent id string field allows websites to check and detect browser and versions; this information can be used to output different HTML and content.
You can also use the underneath HTTP user agent strings in our desktop crawler and site mapper software tools to test how your own website behaves when exposed to different browsers and robots. An user agent (UA) string is able to be used to detect what version of a specific browser is being used on a certain operating system.
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.
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.
Also note that there is a huge difference between the media queries (max-width: 25em), not all and (min-width: 25em), and (max-width: 24.99em) : (max-width: 25em) excludes (max-width: 25em), whereas not all and (min-width: 25em) includes (max-width: 25em). 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. However, Internet Explorer was such a special little wasp exception prior to version 9 that it was very easy to detect the browser based upon the browser-specific features available.
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).
If you ever wanted to make your web traffic seem like it was coming from a different browser–say, to trick a site that claims it’s incompatible with yours–you can. This allows you to request web pages intended for different browsers–or even different devices, like smartphones and tablets.
Click the menu button to the right of the “Console” tab at the bottom of the Developer Tools pane and select “Network Conditions” If you don’t see the console at the bottom, click the menu button in the top right corner of the Developer Tools pane–that’s the button just to the left of the “x”–and select “Show Console”.
On the Network conditions tab, uncheck “Select automatically” next to User agent. It only works while you have the Developer Tools pane open, and it only applies to the current tab.
To create the preference, right-click on the about:config page, point to New, and select String. This setting applies to every open tab and persists until you change it, even if you close and reopen Firefox.
To revert Firefox to the default user agent, right-click the “general.user agent.override” preference and select Reset. Microsoft Edge and Internet Explorer have user agent switchers in their developer tools, and they’re nearly identical.
To open them, click the settings menu and select “F12 Developer Tools” or just press F12 on your keyboard. The developer tools will open in a separate pane at the bottom of the window.
You can find extensive lists of user agents on various websites, such as this one. It only applies to the current tab, and only while the F12 Developer Tools pane is open.
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.