Logstash node name show '0.0.0.0' on kibana

OS version: CentOS 6.9
kibana version: 6.3.0
logstash version: 6.3.0

Add two logstash nodes, one node is normal, and the other show '0.0.0.0'

How does kibana get the hostname of the operation system?

Thanks!!!

I found that we can set a node name manually by set node.name in the config file logstash.yml.

I followed this page:

Now it shows the specified node name, but I still wonder how kibana get the OS hostname.

If I do not set node name manually, what happened?