Table of Contents

Network Troubleshooting Guide

This guide provides a structured approach to diagnosing and resolving common network connectivity issues, from basic to advanced problems.

Troubleshooting Methodology

The OSI Model Approach

A systematic way to troubleshoot network issues is to work through the OSI model layers from bottom to top:

  1. Physical Layer (Layer 1)

    • Check physical connections (cables, ports)
    • Verify power and link lights
    • Test alternative cables/ports

  2. Data Link Layer (Layer 2)

    • Check interface status
    • Verify MAC addresses
    • Examine for switching loops

  3. Network Layer (Layer 3)

    • Verify IP configuration
    • Check routing tables
    • Test basic connectivity

  4. Transport Layer (Layer 4)

    • Verify port availability
    • Check for port filtering
    • Test service responsiveness

  5. Session/Presentation/Application Layers (Layers 5-7)

    • Check application configurations
    • Verify DNS resolution
    • Test application-specific requirements

Common Network Issues and Solutions

Connectivity Problems

No Network Access

Symptoms:

  • Unable to access any network resources
  • "No internet" error messages

Diagnostic Steps:

  1. Check physical connections (cables, Wi-Fi signal)

  2. Verify network interface status:

    # Linux/macOS
ifconfig
# or
ip addr show

# Windows
ipconfig /all

  3. Test loopback interface:

    ping 127.0.0.1

Common Solutions:

  • Reconnect or replace network cables

  • Restart network interface

  • Reset network adapter:

    # Linux
sudo ifdown eth0 && sudo ifup eth0
# or
sudo ip link set eth0 down && sudo ip link set eth0 up

# Windows
ipconfig /release
ipconfig /renew

Local Network Access Only

Symptoms:

  • Can access local resources but not internet
  • Ping to local IPs works but not to external domains

Diagnostic Steps:

  1. Check default gateway configuration:

    # Linux/macOS
ip route | grep default

# Windows
ipconfig | findstr Gateway

  2. Try pinging the gateway:

    ping 192.168.1.1  # Replace with your gateway

  3. Check DNS settings:

    # Linux
cat /etc/resolv.conf

# Windows
ipconfig /all | findstr DNS

Common Solutions:

  • Reset router/modem

  • Correct gateway settings

  • Set alternative DNS servers:

    # Linux
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf

# Windows
netsh interface ip set dns "Wi-Fi" static 8.8.8.8

DNS Issues

Cannot Resolve Domain Names

Symptoms:

  • IP addresses work but domain names don't
  • "DNS server not responding" errors

Diagnostic Steps:

  1. Test DNS resolution:

    nslookup google.com
# or
dig google.com

  2. Check DNS server configuration

  3. Try alternative DNS servers

Common Solutions:

  • Switch to public DNS servers (8.8.8.8, 1.1.1.1)

  • Flush DNS cache:

    # Linux
sudo systemd-resolve --flush-caches

# macOS
sudo killall -HUP mDNSResponder

# Windows
ipconfig /flushdns

  • Check firewall rules for DNS traffic (port 53)

Performance Issues

Slow Network Performance

Symptoms:

  • High latency
  • Slow file transfers
  • Intermittent connectivity

Diagnostic Steps:

  1. Test network speed:

    speedtest-cli

  2. Check for network congestion:

    ping -c 100 8.8.8.8 | grep time

  3. Monitor bandwidth usage:

    # Linux
iftop
# or
nethogs

# Windows
netstat -e -t 5

Common Solutions:

  • Identify and limit bandwidth-heavy applications
  • Update network drivers
  • Check for interference (Wi-Fi channels)
  • Implement QoS settings on router

Advanced Troubleshooting Tools

Network Diagnostics

Traceroute

Traces the path packets take to a destination:

# Linux/macOS
traceroute google.com

# Windows
tracert google.com

MTR (My Traceroute)

Combines ping and traceroute functionality:

mtr google.com

Netstat

Displays network connections and routing tables:

# Show all connections
netstat -a

# Show listening ports
netstat -l

# Show statistics by protocol
netstat -s

Packet Analysis

Tcpdump

Capture and analyze network traffic:

# Capture packets on interface eth0
sudo tcpdump -i eth0

# Capture packets to/from specific host
sudo tcpdump host 192.168.1.10

# Capture specific protocol
sudo tcpdump tcp port 80

Wireshark

For GUI-based packet analysis:

  1. Capture traffic on the relevant interface
  2. Apply display filters (e.g., http, dns, ip.addr==192.168.1.10)
  3. Inspect packet details and conversations

Network Configuration Validation

Interface Configuration

Verify interface settings:

# Linux
ethtool eth0

# Windows
netsh interface show interface

Routing Table

Check routing configuration:

# Linux/macOS
ip route
# or
route -n

# Windows
route print

Firewall Rules

Verify firewall settings:

# Linux (iptables)
sudo iptables -L

# Linux (firewalld)
sudo firewall-cmd --list-all

# Windows
netsh advfirewall show allprofiles
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