OpenRA is an open-source reimplementation of Westwood’s original Command & Conquer: Red Alert game engine where RA stands for Red Alert. The open denotes the game being an open-source game. The game is updated to use the hardware acceleration of advanced video-cards using OpenGL.
Going beyond the little implementation, however, OpenRA became a more significant project using mods that allow players to go back to the defining days of the RTS genre and bring back the Tiberian Dawn (the first Command and Conquer) as well as Dune 2000.
It is a modern remake of the game engine created by the fans, available for Linux, macOS, Windows and is written C#. The primary focus is on multi-player gaming with partial support of the single-player gaming. You may not find few features of the original game like replay recording and an observer mode have been added and given rise to a small hobbyist e-sports scene.