So google It, I am very bad at explaning things, but basically since like 2000 or so, there was a open doom mod called Skulltag. It evolved to a game itself, so to say, because the modifications on Doom engine gives you the ability to create completly different games that doom, apart from just deathmatch which Skulltag kinda had also in mind, creating IDese, which contains live info on any servers that are online there.