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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. CODE
        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. CODE
        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)

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