What is a User-Agent?
A User-Agent refers to the software used on the client side to access servers based on specific protocols. Typically, when discussing "User-Agent," it refers to HTTP User-Agent, which is transmitted during access to web servers using HTTP.
In HTTP communication, the User-Agent header sent with client requests identifies the User-Agent. For example, when accessing a web server using the latest version of Google Chrome on Windows 10 (as of February 2016), the User-Agent header contains the following information:
Mozilla/5.0 (Windows NT 10.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.109 Safari/537.36
Web servers utilize this information for various purposes, such as conducting access analysis to determine the most frequently used browsers or tailoring webpages based on the operating system (e.g., displaying different pages for PCs vs. smartphones).
In dynamic websites that use JavaScript or other scripts, the behavior of script execution can vary depending on the browser. To address this, cross-browser compatibility requires identifying the browser type (often using User-Agent information) and running appropriate code for each browser. However, User-Agent information can be spoofed, so modern techniques involve executing certain browser-dependent code and identifying the browser type based on the results.
Previously, cross-browser compatibility was a major challenge due to the widespread use of proprietary browser features. However, advancements in browser standardization have eased these difficulties. Some modern websites explicitly exclude support for older browsers, reducing compatibility requirements for outdated technology.
With F5’s BIG-IP iRules, analyzing User-Agent strings becomes simple, allowing actions such as blocking access from specific browsers and redirecting users to a Sorry page.