Link: http://wiki.ros.org/multimaster_fkie

apt install ros-melodic-master-discovery-fkie  ros-melodic-master-sync-fkie

On each machine, run:

rosrun master_discovery_fkie master_discovery& rosrun master_sync_fkie master_sync

Each one of the ROS hosts involved should know each other ROS host by name. This can be accomplished either by a dedicated DNS server in the network, e.g., dnsmasq or by adding the appropriate DNS entries in each machine's /etc/hosts file. If we're working with two computers, named AAA and BBB, then:

• Add an entry with the IP of AAA in /etc/hosts of BBB
• Add an entry with the IP of BBB in /etc/hosts of AAA

To verify that the above works, you should be able to ping each machine from each other machine by their hostname.

• In the computer that will serve as the master, launch a roscore
• In the other computer(s) you have to export the following variable, pointing to the DNS name of the computer serving as master. In our case, the master is AAA, so we're adding a line like the following in the .bashrc of BBB.
  • export ROS_MASTER_URI=http://AAA:1131

After that's in place, you can verify that you can see all the topics and nodes active in AAA by running rostopic list or rosnode list from a shell in BBB.