Same here, I used to play it a few years back. Still seems fairly active server wise. It's multiplayer only, but I do love the simple class system and the endless maps made for it.
I tried Red orchestra, couldn't tell the difference between teams and kept getting killed as I didn't want to TK.
There's also a petition for a new DOD like they did with CS:GO
https://www.change.org/p/day-of-defeat-2 Get signing!
DOD Source is one of my favourite online FPS games. I tried to sign but just get 3 dots loading doing nothing and wont let me sign. I tried
Red Orchestra is a bit more hardcore in that it doesnt highlight friends / enemies, much like it wouldnt in real life. You have to look on the map for friendlies or just see what they are wearing / look like