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.