🌐 Complete Networking Guide

Comprehensive reference covering network protocols, server administration, security, and troubleshooting.

📡 Network Fundamentals

🌐 OSI Model

7-layer network architecture

Physical → Application

🔗 TCP/IP Model

4-layer practical model

Network Access → Application

🔄 Protocols

Communication rules

TCP, UDP, HTTP, DNS, DHCP

🛡️ Security

Network protection

Firewall, VPN, Encryption

📊 OSI Model Layers

Layer Name Function Protocols/Devices
7 Application User interface, network services HTTP, FTP, SMTP, DNS
6 Presentation Data formatting, encryption SSL, TLS, JPEG, MPEG
5 Session Connection establishment, management NetBIOS, RPC, SIP
4 Transport End-to-end communication, reliability TCP, UDP, SCTP
3 Network Logical addressing, routing IP, ICMP, OSPF, Router
2 Data Link Physical addressing, error detection Ethernet, MAC, Switch, Bridge
1 Physical Electrical signals, physical connections RJ45, Fiber, Hub, Repeater

💻 Network Programming Languages

🐍 Python

Strengths

Common Uses

Example: Basic Python Socket Server

import socket
import threading

def handle_client(client_socket, address):
    print(f"[+] Connection from {address}")
    client_socket.send(b"Welcome to the server!\n")
    while True:
        data = client_socket.recv(1024)
        if not data:
            break
        print(f"[{address}] {data.decode()}")
        client_socket.send(b"Message received\n")
    client_socket.close()

def start_server(host='0.0.0.0', port=9999):
    server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server.bind((host, port))
    server.listen(5)
    print(f"[*] Listening on {host}:{port}")
    
    while True:
        client, addr = server.accept()
        client_handler = threading.Thread(target=handle_client, args=(client, addr))
        client_handler.start()

if __name__ == "__main__":
    start_server()

☕ Java

Strengths

Common Uses

Example: Java HTTP Server

import com.sun.net.httpserver.HttpServer;
import com.sun.net.httpserver.HttpHandler;
import com.sun.net.httpserver.HttpExchange;
import java.io.*;
import java.net.InetSocketAddress;

public class SimpleHttpServer {
    public static void main(String[] args) throws IOException {
        HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
        
        server.createContext("/", new HttpHandler() {
            @Override
            public void handle(HttpExchange exchange) throws IOException {
                String response = "<html><body><h1>Java HTTP Server</h1>" +
                               "<p>Request Method: " + exchange.getRequestMethod() + "</p>" +
                               "<p>Remote Address: " + exchange.getRemoteAddress() + "</p>" +
                               "</body></html>";
                
                exchange.sendResponseHeaders(200, response.length());
                OutputStream os = exchange.getResponseBody();
                os.write(response.getBytes());
                os.close();
            }
        });
        
        server.createContext("/api", new HttpHandler() {
            @Override
            public void handle(HttpExchange exchange) throws IOException {
                String response = "{\"status\": \"ok\", \"message\": \"API endpoint\"}";
                exchange.getResponseHeaders().set("Content-Type", "application/json");
                exchange.sendResponseHeaders(200, response.length());
                OutputStream os = exchange.getResponseBody();
                os.write(response.getBytes());
                os.close();
            }
        });
        
        server.setExecutor(null);
        server.start();
        System.out.println("Server started on port 8080");
    }
}

🚀 Go (Golang)

Strengths

Common Uses

Example: Go Concurrent TCP Server

package main

import (
    "bufio"
    "fmt"
    "net"
    "strings"
    "time"
)

func handleConnection(conn net.Conn) {
    defer conn.Close()
    
    remoteAddr := conn.RemoteAddr().String()
    fmt.Printf("New connection from %s\n", remoteAddr)
    
    conn.Write([]byte("Welcome to Go TCP Server!\n"))
    conn.Write([]byte("Type 'quit' to exit\n"))
    
    reader := bufio.NewReader(conn)
    for {
        conn.SetDeadline(time.Now().Add(60 * time.Second))
        
        message, err := reader.ReadString('\n')
        if err != nil {
            fmt.Printf("Connection closed: %s\n", remoteAddr)
            return
        }
        
        message = strings.TrimSpace(message)
        fmt.Printf("[%s] %s\n", remoteAddr, message)
        
        if strings.ToLower(message) == "quit" {
            conn.Write([]byte("Goodbye!\n"))
            return
        }
        
        response := fmt.Sprintf("Echo: %s\n", message)
        conn.Write([]byte(response))
    }
}

func main() {
    listener, err := net.Listen("tcp", ":9999")
    if err != nil {
        fmt.Println("Error starting server:", err)
        return
    }
    defer listener.Close()
    
    fmt.Println("TCP Server listening on :9999")
    
    for {
        conn, err := listener.Accept()
        if err != nil {
            fmt.Println("Error accepting connection:", err)
            continue
        }
        
        go handleConnection(conn)
    }
}

🐚 Bash/Shell Scripting

Strengths

Common Uses

Example: Network Monitor Script

#!/bin/bash

# Network Monitoring Script
LOG_FILE="/var/log/network_monitor.log"
CHECK_INTERVAL=60
TARGETS=("8.8.8.8" "1.1.1.1" "google.com" "localhost")

log_message() {
    echo "$(date '+%Y-%m-%d %H:%M:%S') - $1" >> "$LOG_FILE"
}

check_ping() {
    local target=$1
    if ping -c 1 -W 2 "$target" &> /dev/null; then
        echo "✓ $target: Reachable"
        return 0
    else
        echo "✗ $target: Unreachable"
        return 1
    fi
}

check_port() {
    local host=$1
    local port=$2
    timeout 2 bash -c "</dev/tcp/$host/$port" &>/dev/null
    if [ $? -eq 0 ]; then
        echo "✓ $host:$port: Open"
        return 0
    else
        echo "✗ $host:$port: Closed"
        return 1
    fi
}

check_dns() {
    if nslookup google.com &> /dev/null; then
        echo "✓ DNS: Working"
        return 0
    else
        echo "✗ DNS: Failed"
        return 1
    fi
}

monitor_network() {
    echo "=== Network Status Check ==="
    
    # Check connectivity
    for target in "${TARGETS[@]}"; do
        check_ping "$target"
    done
    
    # Check common ports
    check_port "localhost" 22
    check_port "localhost" 80
    check_port "localhost" 443
    
    # Check DNS
    check_dns
    
    # Check bandwidth (requires iftop or similar)
    echo "=== Current Connections ==="
    netstat -tun | grep ESTABLISHED | head -5
    
    echo "=== Interface Statistics ==="
    ip -s link show | grep -A 3 "state UP"
}

# Main monitoring loop
while true; do
    echo "Starting network check..."
    monitor_network | tee -a "$LOG_FILE"
    echo "Waiting $CHECK_INTERVAL seconds..."
    echo ""
    sleep $CHECK_INTERVAL
done

🔧 Server Architecture & Operations

Server Types and Functions

Web Server

  • Examples: Nginx, Apache, Caddy
  • Function: Serves HTTP/HTTPS content
  • Ports: 80 (HTTP), 443 (HTTPS)
  • Config: Virtual hosts, SSL certificates

Database Server

  • Examples: MySQL, PostgreSQL, MongoDB
  • Function: Data storage and retrieval
  • Ports: 3306 (MySQL), 5432 (PostgreSQL)
  • Config: User permissions, replication

Application Server

  • Examples: Node.js, Tomcat, Django
  • Function: Runs business logic
  • Ports: 3000 (Node), 8080 (Tomcat)
  • Config: Environment variables, scaling

File Server

  • Examples: Samba, NFS, SFTP
  • Function: File sharing and storage
  • Ports: 445 (SMB), 2049 (NFS)
  • Config: Share permissions, quotas

How a Server Works: Complete Flow

HTTP Request Processing

  1. Client Request: User types URL → Browser creates HTTP request
  2. DNS Resolution: Browser queries DNS for IP address
  3. TCP Handshake: SYN → SYN-ACK → ACK (3-way handshake)
  4. Request Transmission: HTTP request sent over established connection
  5. Server Processing:
    • Web server (Nginx/Apache) receives request
    • Checks virtual host configuration
    • Routes to appropriate application handler
    • Application processes request (database queries, business logic)
    • Generates HTTP response
  6. Response: Server sends HTTP response with status code, headers, body
  7. Connection Closure: FIN → FIN-ACK → ACK (graceful termination)

Example: Nginx Configuration

# /etc/nginx/nginx.conf
user www-data;
worker_processes auto;
pid /run/nginx.pid;

events {
    worker_connections 1024;
    multi_accept on;
    use epoll;
}

http {
    # Basic settings
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    server_tokens off;
    
    # MIME types
    include /etc/nginx/mime.types;
    default_type application/octet-stream;
    
    # SSL Settings
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers ECDHE-RSA-AES256-GCM-SHA512:DHE-RSA-AES256-GCM-SHA512;
    ssl_prefer_server_ciphers off;
    ssl_session_cache shared:SSL:10m;
    ssl_session_timeout 10m;
    
    # Logging
    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;
    
    # Gzip compression
    gzip on;
    gzip_vary on;
    gzip_min_length 1024;
    gzip_types text/plain text/css application/json application/javascript;
    
    # Virtual Host Configuration
    server {
        listen 80;
        server_name example.com www.example.com;
        root /var/www/html;
        index index.html index.htm;
        
        location / {
            try_files $uri $uri/ =404;
        }
        
        location /api {
            proxy_pass http://localhost:3000;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_cache_bypass $http_upgrade;
        }
        
        location ~ \.php$ {
            include snippets/fastcgi-php.conf;
            fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
        }
        
        location ~ /\.ht {
            deny all;
        }
    }
    
    # HTTPS Configuration
    server {
        listen 443 ssl http2;
        server_name example.com www.example.com;
        
        ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
        
        # Rest of configuration same as HTTP server
        root /var/www/html;
        index index.html index.htm;
        
        location / {
            try_files $uri $uri/ =404;
        }
    }
}

🔀 Port Forwarding & NAT

Understanding NAT (Network Address Translation)

NAT Types

Port Forwarding Configuration Examples

Router Configuration (Generic)
# Typical home router port forwarding setup
# Access router at 192.168.1.1 → Advanced → Port Forwarding

Service Name: Web Server
External Port: 80
Internal Port: 80
Protocol: TCP
Internal IP: 192.168.1.100
Enabled: ✓

Service Name: SSH Server
External Port: 2222
Internal Port: 22
Protocol: TCP
Internal IP: 192.168.1.101
Enabled: ✓

Service Name: Minecraft Server
External Port: 25565
Internal Port: 25565
Protocol: TCP/UDP
Internal IP: 192.168.1.102
Enabled: ✓

Service Name: Security Cameras
External Port Range: 8000-8010
Internal Port Range: 8000-8010
Protocol: TCP
Internal IP: 192.168.1.150
Enabled: ✓
Linux iptables Port Forwarding
#!/bin/bash
# Linux server acting as router with iptables

# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Flush existing rules
iptables -F
iptables -t nat -F
iptables -X

# Default policies
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow loopback
iptables -A INPUT -i lo -j ACCEPT

# Port forwarding rules
# Forward external port 80 to internal web server
iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 192.168.1.100:80
iptables -A FORWARD -p tcp -d 192.168.1.100 --dport 80 -j ACCEPT

# Forward external port 2222 to internal SSH
iptables -t nat -A PREROUTING -p tcp --dport 2222 -j DNAT --to-destination 192.168.1.101:22
iptables -A FORWARD -p tcp -d 192.168.1.101 --dport 22 -j ACCEPT

# Masquerade outgoing traffic
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# Save rules
iptables-save > /etc/iptables/rules.v4

# For IPv6 (if needed)
ip6tables-save > /etc/iptables/rules.v6
Windows Firewall Port Forwarding
# PowerShell as Administrator

# Enable port forwarding
netsh interface portproxy add v4tov4 listenport=80 listenaddress=0.0.0.0 connectport=80 connectaddress=192.168.1.100
netsh interface portproxy add v4tov4 listenport=443 listenaddress=0.0.0.0 connectport=443 connectaddress=192.168.1.100
netsh interface portproxy add v4tov4 listenport=3389 listenaddress=0.0.0.0 connectport=3389 connectaddress=192.168.1.150

# Show current port forwarding rules
netsh interface portproxy show all

# Windows Firewall rules to allow forwarded ports
New-NetFirewallRule -DisplayName "Allow Port 80" -Direction Inbound -Protocol TCP -LocalPort 80 -Action Allow
New-NetFirewallRule -DisplayName "Allow Port 443" -Direction Inbound -Protocol TCP -LocalPort 443 -Action Allow
New-NetFirewallRule -DisplayName "Allow Port 3389" -Direction Inbound -Protocol TCP -LocalPort 3389 -Action Allow

# For RDP forwarding specifically
Set-ItemProperty -Path 'HKLM:\System\CurrentControlSet\Control\Terminal Server' -name "fDenyTSConnections" -Value 0
Enable-NetFirewallRule -DisplayGroup "Remote Desktop"

Port Forwarding Security Considerations

⚠️ Security Risks

🔒 Security Best Practices

  1. Change Default Ports: Use non-standard ports for common services
  2. Implement Rate Limiting: Use fail2ban or similar tools
  3. Use VPN Instead: Access services through VPN rather than direct exposure
  4. Regular Updates: Keep all exposed software patched
  5. Strong Authentication: Use key-based auth for SSH, strong passwords elsewhere
  6. Firewall Rules: Restrict source IPs when possible
  7. Monitoring: Log and monitor all forwarded port access
  8. Reverse Proxy: Use Nginx as reverse proxy with WAF capabilities

📶 Wireless Networking

Wi-Fi Standards and Specifications

Standard Year Frequency Max Speed Range Features
802.11b 1999 2.4 GHz 11 Mbps 35m indoor First widespread adoption
802.11a 1999 5 GHz 54 Mbps 25m indoor Less interference, shorter range
802.11g 2003 2.4 GHz 54 Mbps 38m indoor Backward compatible with 802.11b
802.11n (Wi-Fi 4) 2009 2.4/5 GHz 600 Mbps 70m indoor MIMO, channel bonding
802.11ac (Wi-Fi 5) 2013 5 GHz 3.5 Gbps 35m indoor MU-MIMO, wider channels
802.11ax (Wi-Fi 6) 2019 2.4/5/6 GHz 9.6 Gbps 30m indoor OFDMA, TWT, better efficiency

Wi-Fi Channel Planning

2.4 GHz Band Channels

5 GHz Band Channels

Wi-Fi Site Survey Tools

# Linux wireless tools
sudo apt install wireless-tools iw iwlist wavemon

# Scan for networks
sudo iwlist wlan0 scan

# Check current connection
iwconfig wlan0

# Monitor mode for packet analysis
sudo airmon-ng start wlan0
sudo airodump-ng wlan0mon

# Windows tools
# netsh wlan show networks mode=bssid
# NirSoft WirelessNetView

# macOS tools
# /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s

# Professional tools
# Ekahau Site Survey
# NetSpot
# Acrylic Wi-Fi

Wireless Security Protocols

WEP

Status: Broken, never use

Crack Time: Minutes

Flaws: Weak IV, RC4 stream cipher

WPA

Status: Deprecated

Crack Time: Hours with good wordlist

Flaws: TKIP vulnerabilities

WPA2

Status: Current standard

Crack Time: Days with KRACK attack

Security: AES-CCMP, strong when using WPA2-Enterprise

WPA3

Status: Latest standard

Security: SAE, 192-bit encryption

Features: Forward secrecy, brute force protection

🔐 Network Security

Firewall Configuration Examples

UFW (Uncomplicated Firewall) - Ubuntu

# Basic UFW setup
sudo ufw default deny incoming
sudo ufw default allow outgoing

# Allow SSH
sudo ufw allow 22/tcp
sudo ufw limit 22/tcp  # Rate limiting

# Allow web traffic
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp

# Allow specific IP ranges
sudo ufw allow from 192.168.1.0/24
sudo ufw allow from 10.0.0.0/8 to any port 22

# Allow Docker
sudo ufw allow 2376/tcp  # Docker daemon
sudo ufw allow 7946/tcp  # Docker swarm
sudo ufw allow 7946/udp
sudo ufw allow 4789/udp  # VXLAN

# Application profiles
sudo ufw app list
sudo ufw allow 'Nginx Full'
sudo ufw allow 'Apache Secure'
sudo ufw allow 'OpenSSH'

# Enable logging
sudo ufw logging on

# View status
sudo ufw status verbose
sudo ufw status numbered

# Delete rule by number
sudo ufw delete 2

# Disable/Enable
sudo ufw disable
sudo ufw enable

firewalld - CentOS/RHEL/Fedora

# Basic firewalld commands
sudo systemctl start firewalld
sudo systemctl enable firewalld

# Zones
sudo firewall-cmd --get-default-zone
sudo firewall-cmd --get-active-zones
sudo firewall-cmd --set-default-zone=public

# List services
sudo firewall-cmd --list-services
sudo firewall-cmd --list-all

# Add services
sudo firewall-cmd --permanent --add-service=http
sudo firewall-cmd --permanent --add-service=https
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --permanent --add-service=cockpit

# Add ports
sudo firewall-cmd --permanent --add-port=3000/tcp
sudo firewall-cmd --permanent --add-port=8080/tcp
sudo firewall-cmd --permanent --add-port=9000-9010/tcp

# Rich rules (advanced)
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.1.0/24" port port="22" protocol="tcp" accept'
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="10.0.0.5" reject'

# Port forwarding
sudo firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toport=8080
sudo firewall-cmd --permanent --add-forward-port=port=80:proto=tcp:toaddr=192.168.1.100

# Masquerading
sudo firewall-cmd --permanent --add-masquerade

# Reload
sudo firewall-cmd --reload

# Check
sudo firewall-cmd --list-all --zone=public

VPN Technologies

OpenVPN

  • Type: SSL/TLS based
  • Ports: 1194 UDP (default), 443 TCP
  • Strengths: Mature, cross-platform, very configurable
  • Weaknesses: Slower than WireGuard, complex setup

WireGuard

  • Type: Modern UDP-based
  • Ports: 51820 UDP (default)
  • Strengths: Fast, simple, modern cryptography
  • Weaknesses: Newer, fewer GUI tools

IPsec

  • Type: Network layer
  • Ports: 500 UDP, 4500 UDP
  • Strengths: Enterprise standard, hardware acceleration
  • Weaknesses: Complex, NAT traversal issues

Tailscale

  • Type: WireGuard-based mesh
  • Ports: Various
  • Strengths: Zero-config, easy setup, built-in coordination server
  • Weaknesses: Proprietary control plane

🔍 Network Troubleshooting

Complete Troubleshooting Methodology

Step-by-Step Troubleshooting Process

Step 1: Define the Problem
Step 2: Gather Information
Step 3: Isolate the Problem
Step 4: Systematic Testing
# OSI Layer Troubleshooting
1. Physical Layer (Layer 1)
   - Check cable connections
   - Verify link lights on NIC/router
   - Test with different cable
   - Check power to devices

2. Data Link Layer (Layer 2)
   - Check MAC address table
   - Verify VLAN configuration
   - Check for spanning tree issues
   - Test ARP resolution

3. Network Layer (Layer 3)
   - Verify IP configuration (ipconfig/ifconfig)
   - Check routing tables
   - Test with ping to gateway
   - Verify subnet masks

4. Transport Layer (Layer 4)
   - Test with telnet/nc to specific ports
   - Check firewall rules
   - Verify TCP handshake
   - Check for MTU issues

5. Application Layer (Layers 5-7)
   - Verify DNS resolution
   - Check application logs
   - Test with different client
   - Verify authentication

Common Network Issues and Solutions

Issue: "No Internet Connection"

Troubleshooting Steps
  1. Check physical connection - Cable plugged in? Wi-Fi connected?
  2. Test local connectivity - Can you ping the router?
    ping 192.168.1.1 or ping 10.0.0.1
  3. Check IP configuration
    Windows: ipconfig /all
    Linux: ip addr show or ifconfig
  4. Test DNS resolution
    nslookup google.com
    dig google.com
  5. Test gateway connectivity
    traceroute 8.8.8.8
  6. Check firewall rules
    sudo ufw status or netsh advfirewall show allprofiles
  7. Test with different device - Is it just this computer?
  8. Restart network services
    Windows: ipconfig /release && ipconfig /renew
    Linux: sudo systemctl restart NetworkManager
  9. Check router/modem - Restart if necessary
  10. Contact ISP - Last resort, check for outages

Issue: "Slow Network Speeds"

Troubleshooting Steps
  1. Test baseline speed
    speedtest-cli or curl -s https://raw.githubusercontent.com/sivel/speedtest-cli/master/speedtest.py | python -
  2. Check for bandwidth hogs
    Linux: nethogs or iftop
    Windows: Resource Monitor → Network tab
  3. Test at different times - Peak vs off-peak hours
  4. Check for duplex mismatches
    ethtool eth0 (Linux)
    Get-NetAdapterAdvancedProperty -Name "*" (PowerShell)
  5. Test with wired connection - Isolate Wi-Fi issues
  6. Check for interference (Wi-Fi)
    Use Wi-Fi analyzer tools
  7. Test different servers
    ping -c 10 google.com
    mtr 8.8.8.8
  8. Check QoS settings - Is traffic being throttled?
  9. Update network drivers
  10. Test with different cables/ports

Issue: "Intermittent Connectivity"

Troubleshooting Steps
  1. Monitor connection over time
    ping -t 8.8.8.8 (Windows)
    ping 8.8.8.8 (Linux/macOS, use Ctrl+C to stop)
  2. Check for packet loss
    ping -c 100 8.8.8.8
  3. Test during different activities - Streaming vs browsing
  4. Check event logs
    Windows: Event Viewer → System logs
    Linux: journalctl -f
  5. Test with continuous trace
    mtr --report 8.8.8.8
  6. Check for IP conflicts
    arp -a
    Look for duplicate IP addresses
  7. Test DHCP lease times - Are they too short?
  8. Check for thermal issues - Overheating equipment
  9. Test power supply - Brownouts can cause resets
  10. Replace suspect hardware - Cables, NIC, switch ports

Advanced Troubleshooting Commands

Linux Network Diagnostics

# Comprehensive network check script
#!/bin/bash

echo "=== Network Diagnostics ==="
echo "Timestamp: $(date)"
echo ""

# 1. Interface Information
echo "1. Network Interfaces:"
ip -c addr show
echo ""

# 2. Routing Table
echo "2. Routing Table:"
ip -c route show
echo ""

# 3. DNS Configuration
echo "3. DNS Configuration:"
cat /etc/resolv.conf
echo "Nameservers:"
systemd-resolve --status | grep "DNS Servers" -A 5
echo ""

# 4. Connectivity Tests
echo "4. Connectivity Tests:"
echo "Testing gateway..."
ping -c 3 $(ip route | grep default | awk '{print $3}') 2>&1
echo ""
echo "Testing external connectivity..."
ping -c 3 8.8.8.8 2>&1
echo ""

# 5. DNS Resolution
echo "5. DNS Resolution:"
echo "Testing DNS..."
dig google.com +short 2>&1
nslookup google.com 2>&1
echo ""

# 6. Listening Ports
echo "6. Listening Ports:"
ss -tulpn | head -20
echo ""

# 7. Current Connections
echo "7. Current Connections:"
ss -tn | head -20
echo ""

# 8. Firewall Status
echo "8. Firewall Status:"
if command -v ufw &> /dev/null; then
    sudo ufw status verbose
elif command -v firewall-cmd &> /dev/null; then
    sudo firewall-cmd --list-all
fi
echo ""

# 9. Network Statistics
echo "9. Network Statistics:"
ip -s link show
echo ""

# 10. MTU Check
echo "10. MTU Check:"
ip link | grep mtu
echo ""

echo "=== Diagnostics Complete ==="

Windows Network Diagnostics

# Windows PowerShell Network Diagnostics
# Save as network-diag.ps1

Write-Host "=== Windows Network Diagnostics ===" -ForegroundColor Green
Write-Host "Timestamp: $(Get-Date)`n" -ForegroundColor Yellow

# 1. Network Adapters
Write-Host "1. Network Adapters:" -ForegroundColor Cyan
Get-NetAdapter | Format-Table Name, InterfaceDescription, Status, LinkSpeed -AutoSize
Write-Host ""

# 2. IP Configuration
Write-Host "2. IP Configuration:" -ForegroundColor Cyan
Get-NetIPConfiguration | Format-List
Write-Host ""

# 3. DNS Client Cache
Write-Host "3. DNS Cache:" -ForegroundColor Cyan
Get-DnsClientCache | Select-Object -First 10 Entry,RecordName,RecordType,Data | Format-Table -AutoSize
Write-Host ""

# 4. Routing Table
Write-Host "4. Routing Table:" -ForegroundColor Cyan
Get-NetRoute | Select-Object -First 20 DestinationPrefix,NextHop,RouteMetric,InterfaceAlias | Format-Table -AutoSize
Write-Host ""

# 5. ARP Cache
Write-Host "5. ARP Cache:" -ForegroundColor Cyan
Get-NetNeighbor | Select-Object -First 15 IPAddress,LinkLayerAddress,State,InterfaceAlias | Format-Table -AutoSize
Write-Host ""

# 6. Firewall Rules
Write-Host "6. Active Firewall Rules:" -ForegroundColor Cyan
Get-NetFirewallRule -Enabled True | Select-Object -First 10 DisplayName,Profile,Direction,Action | Format-Table -AutoSize
Write-Host ""

# 7. Network Connections
Write-Host "7. Active TCP Connections:" -ForegroundColor Cyan
Get-NetTCPConnection -State Established | Select-Object -First 10 LocalAddress,LocalPort,RemoteAddress,RemotePort,State | Format-Table -AutoSize
Write-Host ""

# 8. Connectivity Tests
Write-Host "8. Connectivity Tests:" -ForegroundColor Cyan
$gateway = (Get-NetRoute -DestinationPrefix "0.0.0.0/0").NextHop
Write-Host "Testing gateway ($gateway)..." -ForegroundColor Yellow
Test-Connection -ComputerName $gateway -Count 3 -Quiet
Write-Host "Testing external (8.8.8.8)..." -ForegroundColor Yellow
Test-Connection -ComputerName "8.8.8.8" -Count 3 -Quiet
Write-Host "Testing DNS (google.com)..." -ForegroundColor Yellow
Test-Connection -ComputerName "google.com" -Count 3 -Quiet
Write-Host ""

# 9. DNS Resolution
Write-Host "9. DNS Resolution Tests:" -ForegroundColor Cyan
Resolve-DnsName -Name "google.com" -Type A | Select-Object -First 3
Write-Host ""

# 10. Network Adapter Statistics
Write-Host "10. Adapter Statistics:" -ForegroundColor Cyan
Get-NetAdapterStatistics | Select-Object Name,ReceivedBytes,SentBytes,ReceivedUnicastPackets,SentUnicastPackets | Format-Table -AutoSize
Write-Host ""

Write-Host "=== Diagnostics Complete ===" -ForegroundColor Green

# Additional troubleshooting commands
Write-Host "`nQuick Fix Commands:" -ForegroundColor Magenta
Write-Host "Release/Renew IP: ipconfig /release && ipconfig /renew"
Write-Host "Flush DNS: ipconfig /flushdns"
Write-Host "Reset Winsock: netsh winsock reset"
Write-Host "Reset TCP/IP: netsh int ip reset"
Write-Host "Restart Network Adapter: Restart-NetAdapter -Name '*'"

📈 Network Monitoring Tools

Open Source Monitoring Solutions

Zabbix

  • Type: Enterprise monitoring
  • Best For: Large networks, custom monitoring
  • Features: Auto-discovery, alerting, graphing
  • Complexity: High

Prometheus + Grafana

  • Type: Time-series monitoring
  • Best For: Cloud-native, containers
  • Features: Pull-based, powerful query language
  • Complexity: Medium-High

Nagios

  • Type: Classic monitoring
  • Best For: Service monitoring
  • Features: Plugin architecture, mature
  • Complexity: High

LibreNMS

  • Type: Network monitoring
  • Best For: Network device monitoring
  • Features: Auto-discovery, Oxidized integration
  • Complexity: Medium

Command Line Monitoring Tools

# Real-time monitoring commands

# Network traffic
sudo iftop -i eth0          # Bandwidth usage per connection
sudo nethogs                # Bandwidth per process
sudo bmon                   # Multiple interface monitor
sudo iptraf-ng              # Advanced traffic analysis

# Connection monitoring
sudo netstat -tunap         # All connections with processes
sudo ss -tunap              # Faster netstat replacement
sudo lsof -i                # Open files (sockets)
sudo tcpdump -i eth0 -n     # Packet capture

# Bandwidth testing
iperf3 -s                   # Server mode
iperf3 -c server_ip         # Client mode
speedtest-cli               # Internet speed test
mtr --report google.com     # Continuous traceroute

# SNMP monitoring
snmpwalk -v 2c -c public router_ip system
snmpget -v 2c -c public router_ip ifInOctets.1

# Web server monitoring
ab -n 1000 -c 10 http://example.com/  # Apache benchmark
siege -c 10 -t 1M http://example.com  # Load testing

# DNS monitoring
dig +trace example.com      # Full DNS trace
dnstop -l 5 eth0            # DNS traffic monitor

# SSL monitoring
openssl s_client -connect example.com:443 -servername example.com
sslscan example.com:443

# Port scanning
nmap -sS -sV -O target_ip   # Stealth scan with version detection
masscan -p1-65535 target_ip --rate=1000

📚 Protocols Deep Dive

TCP vs UDP Comparison

Feature TCP UDP
Connection Connection-oriented Connectionless
Reliability Guaranteed delivery Best-effort delivery
Ordering In-order delivery No ordering guarantee
Error Checking Checksum, retransmission Checksum only
Flow Control Sliding window None
Congestion Control Multiple algorithms None
Overhead High (20-60 bytes) Low (8 bytes)
Speed Slower Faster
Use Cases Web, email, file transfer DNS, VoIP, gaming, streaming
Header Size 20-60 bytes 8 bytes

Common Ports Reference

Web & Email

  • 20/21: FTP (Data/Control)
  • 22: SSH/SFTP
  • 25: SMTP
  • 53: DNS
  • 80: HTTP
  • 110: POP3
  • 143: IMAP
  • 443: HTTPS
  • 465: SMTPS
  • 587: SMTP Submission
  • 993: IMAPS
  • 995: POP3S

Remote Access

  • 22: SSH
  • 23: Telnet
  • 3389: RDP
  • 5900: VNC
  • 5985: WinRM HTTP
  • 5986: WinRM HTTPS

Database

  • 1433: MS SQL Server
  • 1521: Oracle
  • 3306: MySQL/MariaDB
  • 5432: PostgreSQL
  • 27017: MongoDB
  • 6379: Redis

Gaming & Media

  • 25565: Minecraft
  • 27015: Steam/CS:GO
  • 3074: Xbox Live
  • 3478: STUN
  • 5060: SIP
  • 1935: RTMP
  • 554: RTSP

⚠️ Security Best Practices

• Always use HTTPS instead of HTTP
• Disable unused services and ports
• Regularly update all network equipment firmware
• Implement network segmentation (VLANs)
• Use strong, unique passwords for all devices
• Enable logging and monitor logs regularly
• Implement intrusion detection/prevention systems
• Regularly backup network device configurations
• Use VPNs for remote access instead of port forwarding
• Conduct regular security audits and penetration tests

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