Can't recognize masters in VM

I followed http://digital.csic.es/bitstream/10261/133333/1/ROS-systems.pdf and I can echo the topic created in the VM but not the other way around. And now when I try to check the master inside the VM I get this (rosservice call /master_discovery/list_masters)

masters:

name: "ubuntu"
uri: "http://ubuntu:11311/"
timestamp: 1556551349.94
timestamp_local: 1556551349.94
online: True
discoverer_name: "/master_discovery"
monitoruri: "http://localhost:11611"

Exception in thread Thread-3:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rospy/impl/tcpros_base.py", line 154, in run
    (client_sock, client_addr) = self.server_sock.accept()
  File "/usr/lib/python2.7/socket.py", line 206, in accept
    sock, addr = self._sock.accept()
  File "/usr/lib/python2.7/socket.py", line 174, in _dummy
    raise error(EBADF, 'Bad file descriptor')
error: [Errno 9] Bad file descriptor`

[atiderko]

I have no Idea where the error comes from. I can’t reproduce it in VirtualBox 6.0 with Ubuntu 18.04. But I mean it is not the problem why you can’t discover/see the topics in VM.

Try to check your Multicast traffic: https://taosecurity.blogspot.com/2006/09/generating-multicast-traffic.html

or add robot_hosts parameter to master_discovery: rosrun master_discovery_fkie master_discovery _robot_hosts:=[VM_HOST_NAME]

This adds explicit an unicast communication to given hosts.

[Afladimir1]

Thank you! Adding robot_host in main worked, and now the VM recognizes it. The error from the first post still appears but it detects the other master. But I still have no idea why multicast isn’t working.