Start the master

ssh hal
roscore

Start the listener

ssh hal
export ROS_MASTER_URI=http://hal:11311
rosrun rospy_tutorials listener.py

Start the talker

ssh marvin
export ROS_MASTER_URI=http://hal:11311
rosrun rospy_tutorials talker.py

rostopic

rostopic list # You get a list of all available topics
rostopic 
 
echo /topic_name # check if there is a connection and messages still come.

When something goes wrong

echo $ROS_IP 

Leaning ROS: Running ROS across multiple machines