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".
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. An example of this is faking being a major search engine web crawler bot.
By faking being a search engine crawler bot via the appropriate user agent string, one may be able to access restricted web content in this manner. While this technique MAY still work, most web hosts are aware of this trick so you may have to spoof your IP address to be that of the corresponding search engine web crawler IP address as well.
For example, if the user agent reveals to the web service that it is a mobile phone communicating with them (i.e. Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_4 like Mac OS X) Apple WebKit/536.26 (HTML, like Gecko) Version/6.0 Mobile/10B350 Safari/8536.25) it will send a mobile friendly web page (smaller, loads faster ...) which would be totally different from an user agent string indicating the web browser is desktop based. Not that we condone this method of online dishonesty, for your curiosity and use, here are the major search engine web crawler user agent string.
You can tell which search engine each of the user agents represent because their name is contained within the HTTP user agent string. 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 is a software program that acts on behalf of a user, such as a web browser or e-mail application.
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. 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. 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. Following the lifting of export restrictions, most vendors supported 256-bit encryption.
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.
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/
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.