User Agent in HTTP protocol
The term User Agent refers to a program that downloads a website page from a server. As a rule, this program is a browser. But not always - there are other programs that receive website pages. For example, search robots that analyze the text of a page and show the site in the search.
Programs accessing the server usually transmit a special header User-Agent, which contains the name of the program, the computer's operating system and the user's language.
The general header format for browsers looks like this:
User-Agent: Mozilla/5.0 (system-information) platform
As you can see, for any browser the header value starts with Mozilla/5.0. This is necessary for historical reasons.
Example of the header value in the Firefox browser on Windows:
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0
Example of the header value in the Firefox browser under the MAC axis:
Mozilla/5.0 (Macintosh; Intel Mac OS X x.y; rv:42.0) Gecko/20100101 Firefox/42.0
Example of header value in Chrome browser on Linux:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36
Example of a title value from Googlebot:
Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)
Open your browser debugger and examine the contents of the User-Agent header.
