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Troubleshooting: Server is not reachable or unable to connect

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This runbook is designed to methodically diagnose and resolve server connectivity issues. It caters to various scenarios, from simple DNS problems to more complex network or hardware failures

LinuxTroubleshooting

  1. 1

    Determine Connectivity for server connectivity issues

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    LinuxTroubleshooting
    1

    1. 1.1

      Ping the Server for server connectivity issues

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      • Objective: Verify if the server is reachable.
      • Outcomes and Actions:
      1. If Both Hostname and IP Address are Pingable: The issue may lie on the client side, as the server is reachable. Investigate client firewall or application configurations.
      2. If Only IP Address is Pingable: Indicates a possible DNS resolution issue. Proceed to Step 2 for DNS troubleshooting.
      3. If Neither Hostname Nor IP Address is Pingable: Suggests a network or server-side issue. Proceed to Step 3 for further diagnostics.
      ping -c 4 <hostname> ping -c 4 <IP_Address>
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      LinuxTroubleshooting
      1.1

  2. 2

    DNS Troubleshooting for server connectivity issues

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    LinuxTroubleshooting
    2

    1. 2.1

      Verify Local DNS Configuration for server connectivity issues

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      • Objective: Identify misconfigurations in DNS settings.
      • Checks:
      1. /etc/hosts: cat /etc/hosts
      2. /etc/resolv.conf: cat /etc/resolv.conf
      3. /etc/nsswitch.conf: cat /etc/nsswitch.conf
      4. (Optional) Check interface-specific DNS settings in /etc/sysconfig/network-scripts/ifcfg-<interface>.
      • Outcome:
      1. Correct any misconfigurations found in DNS settings and retest connectivity by pinging the hostname.


      cat /etc/hosts cat /etc/resolv.conf cat /etc/nsswitch.conf cat /etc/sysconfig/network-scripts/ifcfg-<interface>
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      LinuxTroubleshooting
      2.1

  3. 3

    Network Diagnostics for server connectivity issues

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    LinuxTroubleshooting
    3

    1. 3.1

      Broad Network Check for server connectivity issues

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      • Objective: Determine if the issue is isolated to the server or affects the wider network.
      • Action: Ping another server within the same network.
      • Outcome:
      1. False: Issue is not overall network side but its with that host/server
      2. True: Might be overall network side issue


      ping <ip_within_the_same_network>
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      LinuxTroubleshooting
      3.1

    2. 3.2

      Server-Side Checks for server connectivity issues

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      • Objective: Assess server's network configuration and operational status.
      • Actions:
      1. Virtual Console Check
      2. Network Interface and IP Configuration


      LinuxTroubleshooting
      3.2

      1. 3.2.1

        Virtual Console Check

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        Log into the server via virtual console. Check if the server is powered on and operational by verifying its uptime.

        uptime
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        LinuxTroubleshooting
        3.2.1

      2. 3.2.2

        Network Interface and IP Configuration for server connectivity issues

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        LinuxTroubleshooting
        3.2.2

        1. 3.2.2.1

          Verify network interface status for server connectivity issues

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          ip addr show
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          LinuxTroubleshooting
          3.2.2.1

        2. 3.2.2.2

          (Optional) Review IP configuration in /etc/sysconfig/network-scripts/ifcfg-<interface>

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          cat /etc/sysconfig/network-scripts/ifcfg-<interface>
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          LinuxTroubleshooting
          3.2.2.2

    3. 3.3

      Connectivity Checks for server connectivity issues

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      LinuxTroubleshooting
      3.3

      1. 3.3.1

        Ping the Gateway for server connectivity issues

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        ping -c 4 <gateway_IP>
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        LinuxTroubleshooting
        3.3.1

      2. 3.3.2

        Check Routes for server connectivity issues

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        ip route show
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        LinuxTroubleshooting
        3.3.2

    4. 3.4

      Security Configuration for server connectivity issues

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      LinuxTroubleshooting
      3.4

      1. 3.4.1

        Check SELinux Status for server connectivity issues

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        Action: Ensure SELinux is not blocking access to the service/server.

        getenforce
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        LinuxTroubleshooting
        3.4.1

      2. 3.4.2

        Review Firewall Rules for server connectivity issues

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        Command:

        • sudo iptables -L (for iptables)
        • sudo firewall-cmd --list-all (for firewalld)
        sudo iptables -L sudo firewall-cmd --list-all
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        LinuxTroubleshooting
        3.4.2

    5. 3.5

      Physical Connection

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      • Objective: Ensure there's no physical disruption to the network connection.
      • Action: Perform a physical inspection of network cables and hardware. Verify the link lights on the network interface card (NIC).


      LinuxTroubleshooting
      3.5

  4. 4

    Outcomes and Resolutions for server connectivity issues

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    • DNS Issue: Correct DNS configurations as needed and verify resolution by pinging the hostname.
    • Client-Side Issue: Adjust client firewall or network settings as necessary.
    • Network or Server-Side Issue: If broader network issues are identified, work with network administrators to resolve. If isolated to the server, ensure correct network configuration and operational status, checking for any security policy blockages.
    • Hardware Failure: Replace faulty cables or network components as identified during physical inspection.
    LinuxTroubleshooting
    4

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