This Cosa library is an implementation of the Telnet Protocol. It allow Arduino to act as a Telnet Server.
To use this library you must download and install the [Arduino IDE] (http://www.arduino.cc/en/Main/Software) (or GCC AVR toolchain) and [Cosa] (https://github.com/mikaelpatel/Cosa).
Download and unzip the Cosa-Telnet library into your sketchbook libraries directory. Rename from Cosa-Telnet-master to Telnet.
The AVR library and examples should be found in the Arduino IDE File>Sketchbook menu. Open the CosaTelnetServer example sketch. Select the Cosa core by selecting one of the Cosa boards in the Tools>Board menu.
See also Cosa-Shell for additional example sketch.