Description:

The rwall command sends a message to all of the users logged into
a specified host. Actually, your machine's rwall client sends the
message to the rwall daemon running on the specified host, and the
rwall daemon relays the message to all of the users logged in to
that host. The rwall daemon is run from /etc/inetd.conf and is
disabled by default on Mandrake Linux systems (it can be very annoying
to keep getting all those messages when you're trying to play
Quake--I mean trying to get some work done).

Install rwall if you'd like the ability to send messages to users
logged in to a specified host machine.