I think FakeTruth's method might not work for highly hacked clients - the IsOnGround bit is sent by the client and the server doesn't check it in any way, only stores that in the cPlayer object
If you want to be bullet-proof, you need to fully simulate the physics for each player, and compare the simulated and client positions; if they differ to much, make them reset (that's what causes the occasional jerks in the official server). Don't be too tough, though, because lag can do bad things to this kind of simulation.
If you want to be bullet-proof, you need to fully simulate the physics for each player, and compare the simulated and client positions; if they differ to much, make them reset (that's what causes the occasional jerks in the official server). Don't be too tough, though, because lag can do bad things to this kind of simulation.