netstat

To display open ports, enter:

netstat --listen

To display open ports and established TCP connections on a Linux host, enter:

netstat -vatn

Run these on the hosts that stop responding.

Additionally, you can use strace to trace system calls and signals. Specify the "-e trace=network" trace the network-related system calls. As an example, you can run the following command to trace the network signals for the ping -c 3 10.0.0.1command and output to a file called "ping.trace".

strace -e trace=network -o ping.trace ping -c 3 10.0.0.1

$ nc -z -v mystite.com 80

Connection to mystite.com 80 port [tcp/http] succeeded!

$ curl -I mystite.com

HTTP/1.1 404 Not Found Content-length: 169 Content-Type: text/html Date: Thu, 12 Jun 2014 23:42:23 GMT Server: nginx/1.1.19 Connection: keep-alive