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Troubleshooting Scenarios

This page outlines a systematic approach to troubleshooting common network issues.

No Internet Access​

  1. Check Physical Connection: Ensure network cables are properly connected and that the network interface is enabled (ip link show).
  2. Check IP Address: Verify that the device has a valid IP address (ip addr show). If not, check DHCP.
  3. Check Default Gateway: Use ip route show to ensure a default gateway is configured.
  4. Test Local Connectivity: Ping the default gateway to check if the local network is reachable.
  5. Test DNS Resolution: Use dig or nslookup to see if domain names are resolving correctly.
  6. Test External Connectivity: Ping a public IP address (e.g., 8.8.8.8) to check for internet connectivity beyond your local network.

DNS Not Resolving​

  1. Check DNS Server Configuration: Look at /etc/resolv.conf to see which DNS servers are being used.
  2. Test DNS Server Reachability: Ping the DNS servers listed in /etc/resolv.conf.
  3. Query a Specific DNS Server: Use dig @<dns-server> <domain-name> to test a specific server.
  4. Check for Firewall Rules: Ensure that firewall rules are not blocking DNS traffic (port 53).
  1. Check Interface Status: Use ip link show or ethtool <interface-name> to check the status of the network interface.
  2. Check Physical Layer: Inspect cables and switch ports for issues.
  3. Check System Logs: Look for relevant error messages in dmesg or journalctl.
  4. Check for Duplex Mismatch: A duplex mismatch between the device and the switch can cause link flapping.

MTU / Packet Loss Issues​

  1. Check for Packet Loss: Use ping with a large number of packets (ping -c 100 <host>) to check for packet loss.
  2. Check MTU Size: The Maximum Transmission Unit (MTU) is the size of the largest packet that can be transmitted over a network. You can use ping with the "don't fragment" bit set to find the optimal MTU size. 
    ping -s <packet-size> -M do <host>
    Start with a large packet size and gradually decrease it until the packets are no longer fragmented.
  3. Check for Network Congestion: Use tools like mtr or traceroute to identify potential points of congestion in the network path.