ERROR: Unable to communicate with master!

Example:

• rosnode ping rosout
  rosnode: node is [/rosout]
  ERROR: Unable to communicate with master!

Solution:

• Is your Master running? If no, please run roscore

• Is your ROS_MASTER_URI set correctly? If no, please set the ROS_MASTER_URI tothe location of your Master.

• Can you contact the address specified in the ROS_MASTER_URI?

  $ echo $ROS_MASTER_URI
  http://foo:11311
  $ ping foo
  ping: unknown host foo

  If you get the "unknown host" error, please check your network settings or change the ROS_MASTER_URI to a resolvable address.

rosnode machine doesn't find all nodes on a machine

The machine name argument to rosnode machine must resolve to the same address as the machine reports. Thus "localhost" probably will not match local nodes.

You can determine the address a node is using by running rosnode info <node-name>:

$ rosnode info rosout
--------------------------------------------------------------------------------
Node [/rosout]

... stuff you can ignore ...

contacting node http://foo:46621/ ...

In the above example, the machine name is "foo".