Which HTTP request header indicates the browser used by the client sending the request?

The HTTP request header that indicates the browser used by the client sending the request is the "User-Agent" header.

The HTTP request header that indicates the browser used by the client sending the request is called "User-Agent". This header field contains a characteristic string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent.

The HTTP request header that indicates the browser used by the client sending the request is the "User-Agent" header. Here are the steps to understand this:

1. HTTP headers let the client and the server pass additional information with an HTTP request or response. They define the operating parameters of an HTTP transaction.

2. The "User-Agent" header field is part of the HTTP request sent by your web browser or other client to the server hosting the website you're visiting.

3. This header contains a characteristic string that allows the network protocol peers to identify the application type, operating system, software vendor, or software version of the requesting software user agent.

4. So, when a client (like a web browser) sends a request to a server, it includes this "User-Agent" header to let the server know what browser and operating system it's using.

5. For example, if you're using Chrome on a Windows machine, your User-Agent header might look something like this: "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3".

6. The server can use this information to serve different web pages to different types of browsers, or to track which browsers are being used to access the server, among other things.