What is a User Agent?

User Agent - a string that identifies the client application making a request to the server (e.g., a web browser, search robot, etc.). It is sent in the HTTP header User-Agent.

Example structure (may vary):

ProductName/Version [Comments, OS Type, Language...]

Uses of User Agent:

• Client type identification: The server can adapt content for a specific browser or device.
• Analytics: Web analytics use User Agent to collect statistics about site visits.
• Blocking: Site owners can block access based on User Agent (e.g., for bots).
• Testing: QA uses modified User Agents to simulate different browsers/devices.

Example HTTP header:

GET /index.html HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.124 Safari/537.36

In this example:

• Mozilla/5.0 - a common marker, not always matching the actual engine.
• Windows NT 10.0; Win64; x64 - information about the operating system.
• AppleWebKit/537.36 (KHTML, like Gecko) - rendering engine information.
• Chrome/91.0.4472.124 - browser name and version.
• Safari/537.36 - often included for compatibility with older sites that check for "Safari" in User Agent.