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 Format

James Lee
• Tuesday, 08 December, 2020
• 17 min read

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. Other browsers were considered to be older products such as Mosaic, Cello, or Samba, and would be sent a bare-bones HTML document.

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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.

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^ 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 ".

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/”. In this example, the user agent string is mobile Safari’s version.

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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. Making statements based on opinion; back them up with references or personal experience.

Field Description Required $(DEVICE\_TYPE) SMART-TV is used for Samsung Smart TV. Optional Build/$(BUILD\_TAG)) Platform Build Tag is used on Android devices.

Optional $(APP\_NAME)/$(APP\_Very) Web Browsers on Samsung devices (Mobile and Smart TV) use SamsungBrowser/version ”. Mandatory (Chrome/$(CHROME\_Very)) This field is presented in a Chrome-based web browser only.

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This will also reflect on the Ti zen Samsung Browser if it will be based on Chrome in the future. For devices supporting Virtual Reality contents, use VR ”.

If PC UX is appropriate for the device, this field is empty. Optional Users can explicitly request PC content from Samsung Internet for Android via the “More > Desktop version”.

User agent string format for desktop version request is as follows. Please check below for existing Samsung Internet for Smart TV UA.

Current: Mozilla/5.0 (Linux; Android 5.0.2; SAMSUNG SM-G925K Build/LRX22G) Apple WebKit/537.36 (HTML, like Gecko) SamsungBrowser/4.0 Chrome/44.0.2403.133 Mobile VR Safari/537.36 consider the Resize Event On the Resize Event, focus should be maintained on the Input field so that the user of the TV web browser can input characters and symbols using Samsung IME.

The following table shows how the identifiers and proper contents are related. Open the test web page on the desktop Chrome Browser.

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(Previous versions of this document said they should be the output of name SRM, but the release field of the misname structure was considered to reveal too much information about the system, such as potential security holes.) By convention, used to indicate pre-release versions, such as beta quality software, or milestones.

GeckoProductToken Gecko/ Reconversion The Gecko product token allows products that embed the Gecko engine, including Mozilla, to identify this significant sub-product. For official Mozilla builds, this will correspond to the date portion of the Build ID.

For branded versions of Mozilla, the Reconversion should correspond to the date the code was pulled from, and may not necessarily correspond to the date portion of the generated Build ID. The form of their product token and comment is not specified here, but should adhere to the HTTP standards.

(VendorProductToken | VendorComment) Product tokens for applications based on Mozilla. Format and content shall be vendor-specific, but should adhere to the HTTP standards.

A release Mozilla/5.001 (windows; U; NT4.0; en-us) Gecko/25250101 A branded release based on the same codebase as the browser above Mozilla/5.001 (Macintosh; N; PPC; JA) Gecko/25250101 MegaCorpBrowser/1.0 (Mega Corp, Inc.) A re-branded release Mozilla/9.876 (X11; U; Linux 2.2.12-20 i686, en) Gecko/25250101 Netscape/5.432b1 (C-MindSpring) A Gecko-based browser TinyBrowser/2.0 (TinyBrowser Comment) Gecko/20201231 Starting with Mozilla 1.8 beta2, the best way for applications, vendors, and extensions (if needed) to add to default preferences to add VendorProductToken s or VendorComment s is to add a default preference of the form general.user agent.extra. Just one character out of place can wreak havoc on your SEO and prevent search engines from accessing important content on your site.

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Primarily, it lists all the content you want to lock away from search engines like Google. You can also tell some search engines (not Google) how they can crawl allowed content.

Unless you’re careful, disallow and allow directives can easily conflict with one another. If you’re unfamiliar with sitemaps, they generally include the pages that you want search engines to crawl and index.

So you’re best to include sitemap directives at the beginning or end of your robots.txt file. Google supports the sitemap directive, as do Ask, Bing, and Yahoo.

For example, if you wanted Google bot to wait 5 seconds after each crawl action, you’d set the crawl-delay to 5 like so: Google no longer supports this directive, but Bing and Yandex do.

If you set a crawl-delay of 5 seconds, then you’re limiting bots to crawl a maximum of 17,280 URLs a day. That’s not very helpful if you have millions of pages, but it could save bandwidth if you have a small website.

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However, until recently, it’s thought that Google had some “code that handles unsupported and unpublished rules (such as no index).” So if you wanted to prevent Google from indexing all posts on your blog, you could use the following directive: If you want to exclude a page or file from search engines, use the meta robots tag or robots HTTP header instead.

No follow This is another directive that Never Google officially supported, and was used to instruct search engines not to follow links on pages and files under a specific path. If you want to no follow all links on a page now, you should use the robots meta tag or robots header.

Having a robots.txt file isn’t crucial for a lot of websites, especially small ones. Note that while Google doesn’t typically index web pages that are blocked in robots.txt, there’s no way to guarantee exclusion from search results using the robots.txt file.

This example blocks search engines from crawling all URLs under the /product/ subfolder that contain a question mark. In this example, search engines can’t access any URLs ending with .pdf.

In other words, you’re less likely to make critical mistakes by keeping things neat and simple. Failure to provide specific instructions when setting directives can result in easily-missed mistakes that can have a catastrophic impact on your SEO.

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For example, let’s assume that you have a multilingual site, and you’re working on a German version that will be available under the /DE/ subdirectory. Because it isn’t quite ready to go, you want to prevent search engines from accessing it.

The robots.txt file below will prevent search engines from accessing that subfolder and everything in it: But it will also prevent search engines from crawling of any pages or files beginning with /DE.

These are mainly for inspiration but if one happens to match your requirements, copy-paste it into a text document, save it as “robots.txt” and upload it to the appropriate directory. Robots.txt mistakes can slip through the net fairly easily, so it pays to keep an eye out for issues.

To do this, regularly check for issues related to robots.txt in the “Coverage” report in Search Console. It’s easy to make mistakes that affect other pages and files.

This means you have content blocked by robots.txt that isn’t currently indexed in Google. If this content is important and should be indexed, remove the crawl block in robots.txt.

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Once again, if you’re trying to exclude this content from Google’s search results, robots.txt isn’t the correct solution. Remove the crawl block and instead use a meta robots tag or xrobots-tag HTTP header to prevent indexing.

This may help to improve the visibility of the content in Google search. Here are a few frequently asked questions that didn’t fit naturally elsewhere in our guide.

As well as this, we've also got resources (such as our Frontend Library) dedicated to detecting frontend web browser features and settings, so that you can understand what technology your website visitors are using and help solve their technical problems. The sad reality is that most webmasters have no idea what a robots.txt file is.

A robot in this sense is a “spider.” It’s what search engines use to crawl and index websites on the internet. Once that’s complete, the robot will then move on to external links and continue its indexing.

This is how search engines find other websites and build such an extensive index of sites. When a search engine (or robot, or spider) hits a site, the first thing it will look for is a robots.txt file.


Remember to keep this file in your root directory. Keeping it in the root directory will ensure that the robot will be able to find the file and use it correctly.

White spaces and comment lines can be used but are not supported by most robots. Notice the “Disallow:” command is blank; this tells robots that nothing is off limits.

It also doesn’t allow the admin.php file to be indexed, which is located in the root directory. This list tells the Google Bot not to index the admin folder.

Just punch in a URL and add robots.txt to the end to find out if a site uses it or not. It will display their robots.txt file in plain text so anyone can read it.

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