Failed to send ping to

Elastic Stack Elasticsearch
6

And now I changed it the configuration again using fully qualified domain and discovery.zen.minimum_master_nodes: 1 even if I have 5 server nodes + 2 client nodes'

Still getting the same error after a couple of hours:

[2015-05-26 13:33:37,024][WARN ][discovery.zen.ping.unicast] [hostname038.domain] failed to send ping to [[#zen_un
icast_3#][hostname038.domain][inet[hostname507.domain/xx.xx.xx.35:9300]]]
org.elasticsearch.transport.ReceiveTimeoutTransportException: [hostname507.domain][inet[/xx.xx.xx.35:9300]][inter
nal:discovery/zen/unicast] request_id [5521] timed out after [37501ms]
        at org.elasticsearch.transport.TransportService$TimeoutHandler.run(TransportService.java:531)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

and many more ..

What kind of check could I do over the network? (using tcpdump or other tools..)

Any advice is appreciated.

Thanks