Another request header worth knowing about is the User-Agent header. Any client that communicates using HTTP can be referred to as a user agent. RFC 7231 suggests that user agents should use the User-Agent header to identify themselves in every request. What goes in there is up to the software that makes the request, though it usually comprises a string that identifies the program and version, and possibly the operating system and the hardware that it's running on. For example, the user agent for my current version of Firefox is shown here:

Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20140722 Firefox/24.0 Iceweasel/24.7.0

Although it has been broken over two lines here, it is a single long string. As you can probably decipher, I'm running Iceweasel (Debian's version of Firefox) version 24 on a 64-bit Linux system. User agent strings aren't intended for identifying individual users. They only identify the product that was used for making the request.

We can view the user agent...