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 Viewer

Ellen Grant
• Tuesday, 06 October, 2020
• 17 min read

For instance, if a web server identifies a browser as Firefox running on Android, it will serve web pages specialized for mobile device instead of desktop. But, in case you are curious about the user agent of the web browser you are using, there are several resources you can take advantage of.

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Simply visit this page to view the user agent of your web browser. Following is the example of the user agent of Google Chrome version 69 running on Windows 10.

You might be wondering, why is the user agent of Google Chrome on Windows 10 so complicated? For instance, if you are using Google Chrome running on Windows 10, you can change the user agent to Firefox on iPhone so the web servers will serve the web pages specialized for mobile device, iPhone in this case.

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. 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.

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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.

Check the capabilities of the browser or device, and load different CSS based on the outcome; Deliver custom JavaScript to one device compared with another; Send an entirely different page layout to a phone, compared to a desktop computer; Automatically send the correct translation of a document, based on the user agent language preference; Push special offers to particular people, based on their device type or other factors; Gather statistics about visitors to inform our web design and content production process, or simply measure who's hitting our site, and from which referral sources. Overall, we can empower our scripts to make the best choice for our visitors, based on their user agent.

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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. WebKit is also responsible for allowing you to navigate through links on a web page.

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.

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For DELETE, GET, HEAD, PATCH, POST, and PUT requests, if you configure CloudFront to forward the Authorization header to your origin, you can configure your origin server to request client authentication. Configure CloudFront to not cache the response to OPTIONS requests.

Configure your origin to add a Cache-Control or an Expires header field to each object. Specify a value for Minimum TTL in CloudFront cache behaviors.

CloudFront does not support client authentication with client-side SSL certificates. When CloudFront receives a request for an object that has expired from an edge cache, it forwards the request to the origin either to get the latest version of the object or to get confirmation from the origin that the CloudFront edge cache already has the latest version.

An If-Match or If-None-Match header that contains the Tag value for the expired version of the object. For more information, see Caching content based on request headers.

You can require viewers to use HTTPS to send requests to CloudFront and require CloudFront to forward requests to your custom origin by using the protocol that is used by the viewer. CloudFront forwards HTTPS requests to the origin server using the SSLv3, TLSv1.0, TLSv1.1, and TLSv1.2 protocols.

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For more information, see OriginSslProtocols and DistributionConfig in the Amazon CloudFront API Reference. If you configure CloudFront to accept and forward to your origin all the HTTP methods that CloudFront supports, configure your origin server to handle all methods.

For example, if you configure CloudFront to accept and forward these methods because you want to use POST, you must configure your origin server to handle DELETE requests appropriately so viewers can't delete resources that you don't want them to. The following table lists HTTP request headers that you can forward to both custom and Amazon S3 origins (with the exceptions that are noted).

CloudFront does not add the header before forwarding the request to your origin. CloudFront does not add the header before forwarding the request to your origin.

CloudFront does not add the header before forwarding the request to your origin. CloudFront does not add the header before forwarding the request to your origin.

CloudFront does not add the header before forwarding the request to your origin. CloudFront replaces this header with Connection: Keep-Alive before forwarding the request to your origin.

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CloudFront sets the value to the domain name of the origin that is associated with the requested object. You can't cache based on the Host header for Amazon S3 or MediaS tore origins.

CloudFront removes the header, unless you've established a Socket connection. The header value contains an encrypted string that uniquely identifies the request.

The maximum length of a request, including the path, the query string (if any), and headers, is 20,480 bytes. If a request or a URL exceeds these maximums, CloudFront returns HTTP status code 413, Request Entity Too Large, to the viewer, and then terminates the TCP connection to the viewer.

When a viewer submits an HTTPS request for an object, either CloudFront or the viewer must confirm with the certificate authority (CA) that the SSL certificate for the domain has not been revoked. The performance improvement of CSP stapling is more pronounced when CloudFront receives numerous HTTPS requests for objects in the same domain.

Each server in a CloudFront edge location must submit a separate validation request. When CloudFront receives a lot of HTTPS requests for the same domain, every server in the edge location soon has a response from the CA that it can “staple” to a packet in the SSL handshake; when the viewer is satisfied that the certificate is valid, CloudFront can serve the requested object.

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If your distribution doesn't get much traffic in a CloudFront edge location, new requests are more likely to be directed to a server that hasn't validated the certificate with the CA yet. In that case, the viewer separately performs the validation step and the CloudFront server serves the object.

That CloudFront server also submits a validation request to the CA, so the next time it receives a request that includes the same domain name, it has a validation response from the CA. When CloudFront gets a response from your origin, it tries to maintain the connection for several seconds in case another request arrives during that period.

CloudFront forwards HTTP or HTTPS requests to the origin server based on the following: The protocol of the request that the viewer sends to CloudFront, either HTTP or HTTPS.

In the CloudFront console, the options are HTTP Only, HTTPS Only, and Match Viewer. Note that CloudFront caches the object only once even if viewers make requests using both HTTP and HTTPS protocols.

If CloudFront forwards a request to the origin using the HTTPS protocol, and if the origin server returns an invalid certificate or a self-signed certificate, CloudFront drops the TCP connection. You can reduce this time by specifying a shorter connection timeout, fewer attempts, or both.

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If your origin sends another 100-Continue response after the first one, CloudFront will return an error. Ensure that the origin server sets valid and accurate values for the Date and Last-Modified header fields.

If you do not specify an Tag value, CloudFront ignores subsequent If-Match or If-None-Match headers. CloudFront normally respects a Cache-Control: no-cache header in the response from the origin.

As a result, your origin returns the object to CloudFront in response to every request. Without a Content-Length header, CloudFront cannot determine whether the TCP connection was dropped accidentally or on purpose.

The maximum size of a response body that CloudFront will return to the viewer is 20 GB. This includes chunked transfer responses that don't specify the Content-Length header value.

If your origin server is unavailable and CloudFront gets a request for an object that is in the edge cache but that has expired (for example, because the period of time specified in the Cache-Control max-age directive has passed), CloudFront either serves the expired version of the object or serves a custom error page. For more information about CloudFront behavior when you've configured custom error pages, see.

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In some cases, an object that is seldom requested is evicted and is no longer available in the edge cache. After you configure the redirect, the first time a viewer submits a request for the object, CloudFront Front sends the request to the origin, and the origin responds with a redirect (for example, 302 Moved Temporarily).

You can configure your web server to redirect requests to one of the following locations: As a result, we recommend that you not redirect requests to the new URL of the object on the origin.

When the viewer submits the request that contains the new CloudFront URL, CloudFront gets the object from the new location on your origin, caches it at the edge location, and returns the object to the viewer. Subsequent requests for the object will be served by the edge location.

This avoids the latency and load associated with viewers requesting the object from the origin. If the viewer makes a Range GET request and the origin returns Transfer-Encoding: chunked, CloudFront returns the entire object to the viewer instead of the requested range.

We recommend that you use chunked encoding if the content length of your response cannot be predetermined. There are ways to develop your website to progressively enhance itself based on the availability of features rather than by targeting specific browsers.

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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.

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.

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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.

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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. Also, pay attention not to use a simple regular expression on the BrowserName, user agents also contain strings outside the Keyword/Value syntax.


Must containment not containFirefoxFirefox/xyzSeamonkey/xyzSeamonkeySeamonkey/xyzChromeChrome/chromium/xyzChromiumChromium/xyzSafariSafari/chrome/XYZ or Chromium/safari gives two version numbers: one technical in the Safari/XYZ token, and one user -friendly in a Version/XYZ tokenizer Opera 15+ (Blink-based engine) 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).

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