Network troubleshoot

What to check if 2 servers can not communicate on ports.

1. How to check network connectivity between 2 servers:

   1. open a port on one server:

      1. nc -l PORT
         

   2. from the second server test the connectivity:

      1. check if there is direct connection on tcp:

         nc -zv IP PORT
         

      2. check if there is any proxy setup (tcp might work but http or https will not work).

         wget -T 5 -t 1 --spider http://${to}:${port}
         wget -T 5 -t 1 --spider https://${to}:${port}
         

      3. please also try to run, without installing it, try to install a package; if there are errors, then it needs to be fixed, based on the case…

         yum install zip
         

2. What can go wrong:

   1. there is a network issues - needs to be fixed by client

   2. there is routing issues - needs to be fixed by client

   3. there is firewall on host, so please stop/disable:

      1. systemctl stop firewalld; systemctl disable firewalld
         

      2. there are proxies on the servers, so local http and https traffic is routed:

         1. http_proxy setup for the system: /etc/environment → please comment it.

         2. http_proxy setup for repos: /etc/yum.conf → please comment it

         3. http_proxy at user level: /etc/profile.d/proxy.conf → please comment it

         4. you can do a general search: grep -ri http_proxy /etc/ → there are many places where on one env, this parameter can be set (also check in /home for http_proxy)