Linux is now one of the most widespread operating system for embedded devices due
to its openess as well as the wide variety of platforms it can run on. Many
manufacturer actually use it in firmware you can find on many devices: DVB-T 
decoders, routers, print servers, DVD players ... Most of the time the stock 
firmware is not really open to the consumer, even if it uses open source software.

You might be interested in running a Linux based firmware for your router for
various reasons: extending the use of a network protocol (such as IPv6), having
new features, new piece of software inside, or for security reasons. A fully 
open-source firmware is de-facto needed for such applications, since you want to
be free to use this or that version of a particular reason, be able to correct a
particular bug. Few manufacturers do ship their routers with a Sample Development Kit,
that would allow you to create your own and custom firmware and most of the time,
when they do, you will most likely not be able to complete the firmware creation process.

This is one of the reasons why OpenWrt and other firmware exists: providing a 
version independent, and tools independent firmware, that can be run on various 
platforms, known to be running Linux originally.

\subsection{Which Operating System does this device run?}

There is a lot of methods to ensure your device is running Linux. Some of them do
need your router to be unscrewed and open, some can be done by probing the device
using its external network interface  upstream-18.06.7.zip