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Browser sniffing (also known as browser detection) is a set of techniques used in websites and web applications in order to determine the web browser a visitor is using, and to serve browser-appropriate content to the visitor. It is also used to detect mobile browsers and send them mobile-optimized websites.
Feature detection (also feature testing) is a technique used in web development for handling differences between runtime environments (typically web browsers or user agents), by programmatically testing for clues that the environment may or may not offer certain functionality.
On the Web, a user agent is a software agent responsible for retrieving and facilitating end-user interaction with Web content. [1] This includes all web browsers , such as Google Chrome and Safari , some email clients , standalone download managers like youtube-dl , and other command-line utilities like cURL .
This function returns a string with one or more access method specifications. These specifications cause the user agent to use a particular proxy server or to connect directly. [1] Multiple specifications provide a fallback when a proxy fails to respond. The browser fetches this PAC file before requesting other URLs.
Often web shells detect the user-agent and the content presented to the search engine spider is different from that presented to the user's browser. To find a web shell a user-agent change of the crawler bot is usually required. Once the web shell is identified, it can be deleted easily. [2]
The user agent string format is currently specified by section 10.1.5 of HTTP Semantics. The format of the user agent string in HTTP is a list of product tokens (keywords) with optional comments. For example, if a user's product were called WikiBrowser, their user agent string might be WikiBrowser/1.0 Gecko/1.0. The "most important" product ...
This instructs the user agent that the content is stale and should be validated before use. The header field Cache-Control: no-store is intended to instruct a browser application to make a best effort not to write it to disk (i.e not to cache it). The request that a resource should not be cached is no guarantee that it will not be written to disk.
Spoofing some of the information exposed to the fingerprinter (e.g. the user agent) may create a reduction in diversity, [51]: 13 but the contrary could be also achieved if the spoofed information differentiates the user from all the others who do not use such a strategy more than the real browser information. [10]: 552