xhost
The X server's original access control, which admits whole machines — and whose best-known invocation switches control off entirely.
X11 sends drawing commands and keystrokes across the network, so the server has to decide who may connect. xhost manages a list of permitted hosts: xhost +lab3 lets in anything coming from the machine lab3, whoever happens to be logged in there.
The famous form is a bare xhost +, which disables access control altogether. It appeared in countless install guides as the quick way to get a remote program on screen, and with it anyone on the network could grab the display, log keystrokes or inject their own. The later si:localuser:name admits a local user rather than a machine. In practice the approach gave way to the cookies of Xauth and to X11 forwarding over SSH.