LWM-Linux/04 - Networking in Linux/Network Troubleshooting.md

Linux Network Troubleshooting Guide

1. Basic Network Configuration Check:

• Check IP address and network interface status:

  ip addr show
  

  This command displays all network interfaces, their IP addresses, and status.

• Verify default gateway:

  ip route show
  

  Ensures your system knows how to route traffic outside the local network.

• Check DNS configuration:

  cat /etc/resolv.conf
  

  Displays the DNS servers your system is using.

2. Connectivity Tests:

• Ping test:

  ping -c 4 8.8.8.8
  

  Tests basic connectivity to Google's DNS server (or any other IP).

• Traceroute:

  traceroute google.com
  

  Shows the path packets take to reach a destination.

• DNS resolution test:

  nslookup google.com
  

  or

  dig google.com
  

  These test DNS resolution capabilities.

3. Advanced Diagnostic Tools:

• netstat or ss:

  netstat -tuln
  

  or

  ss -tuln
  

  Display active network connections and listening ports.

• tcpdump:

  sudo tcpdump -i eth0
  

  Captures and displays packet data on a specified interface.

• nmap:

  nmap -p- localhost
  

  Scans for open ports on the local machine (or any specified target).

4. Firewall Configuration:

• Check iptables rules:

  sudo iptables -L -v -n
  

  Displays current firewall rules.

• Temporarily disable firewall (for testing):

  sudo systemctl stop firewalld  # for systems using firewalld
  

  or

  sudo ufw disable  # for systems using ufw
  

5. Network Service Diagnostics:

• Check service status:

  systemctl status networking
  

  or

  systemctl status NetworkManager
  

• Restart network service:

  sudo systemctl restart networking
  

  or

  sudo systemctl restart NetworkManager
  

6. Network Interface Configuration:

• Edit network interface configuration:

  sudo nano /etc/network/interfaces  # for Debian-based systems
  

  or

  sudo nano /etc/sysconfig/network-scripts/ifcfg-eth0  # for Red Hat-based systems
  

• Restart specific network interface:

  sudo ifdown eth0 && sudo ifup eth0
  

  or

  sudo ip link set eth0 down && sudo ip link set eth0 up
  

7. Wireless Network Troubleshooting:

• List available wireless networks:

  sudo iwlist wlan0 scan
  

• Check wireless interface details:

  iwconfig
  

• Monitor wireless connection in real-time:

  watch -n 1 iwconfig
  

8. Advanced Network Analysis:

• Wireshark: GUI-based packet analyzer Install with:

  sudo apt-get install wireshark  # on Debian-based systems
  

  or

  sudo yum install wireshark  # on Red Hat-based systems
  

• iftop: Displays bandwidth usage on an interface

  sudo iftop -i eth0
  

• nethogs: Groups bandwidth by process

  sudo nethogs eth0
  

9. Performance Testing:

• iperf: Network performance measurement tool

  iperf -s  # on server
  iperf -c server_ip  # on client
  

• speedtest-cli: Command-line interface for testing internet speed

  speedtest-cli
  

10. Log Analysis:

- System logs:
   ```
   sudo tail -f /var/log/syslog  # on Debian-based systems
   ```
   or
   ```
   sudo tail -f /var/log/messages  # on Red Hat-based systems
   ```
   or
   ```
   sudo journalctl -b0
   ```
   or
   ```
   sudo dmesg -k
   ```

- Network-specific logs:
   ```
   sudo tail -f /var/log/daemon.log
   ```

11. Network Configuration Backup and Restore:

- Backup network configuration:
   ```
   sudo tar -czvf network_config_backup.tar.gz /etc/network
   ```

- Restore network configuration:
   ```
   sudo tar -xzvf network_config_backup.tar.gz -C /
   ```

12. Troubleshooting Specific Issues:

- High latency: Use ping and traceroute to identify where delays occur.
- Packet loss: Use mtr (My TraceRoute) for a combination of ping and traceroute.
- DNS issues: Check /etc/hosts file and DNS server configurations.
- IP conflicts: Use arping to detect duplicate IP addresses on the network.