Trying to run rviz: Getting error "Could not contact ROS master..."

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You need some instance of roscore running, either on your machine or externally. One option is what someone else already suggested, which is to roscore in a terminal on your machine. What we frequently have to do in our lab is to remotely access robot data. In this case, we need to access the instance of roscore running on the robot (PR2, just for reference). In this case set the environment variable ROS_MASTER_URI to the appropriate value. This value is usually http://[name]:11311.