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๐ŸŒ Chapter 12: Networking Fundamentals

Intermediate Chapter 12

๐Ÿ“‘ Table of Contents

Networking Concepts

The OSI Model (Simplified)

Layer 7 โ”‚ Application  โ”‚ HTTP, SSH, DNS, FTP       โ”‚ curl, wget
Layer 4 โ”‚ Transport    โ”‚ TCP, UDP                   โ”‚ ss, netstat
Layer 3 โ”‚ Network      โ”‚ IP, ICMP, routing          โ”‚ ip, ping, traceroute
Layer 2 โ”‚ Data Link    โ”‚ Ethernet, ARP, MAC address โ”‚ arp, bridge
Layer 1 โ”‚ Physical     โ”‚ Cables, WiFi signals       โ”‚ ethtool

Key Terms

Term Description
IP Address Unique network identifier (e.g., 192.168.1.100)
Subnet A network segment (e.g., 192.168.1.0/24)
Gateway Router that connects to other networks
DNS Translates domain names โ†’ IP addresses
DHCP Auto-assigns IP addresses
Port Endpoint for communication (0โ€“65535)
MAC Address Hardware address (aa:bb:cc:dd:ee:ff)

Common Ports

Port Service Protocol
22 SSH TCP
80 HTTP TCP
443 HTTPS TCP
53 DNS TCP/UDP
25 SMTP TCP
3306 MySQL TCP
5432 PostgreSQL TCP
6379 Redis TCP

Network Interfaces

# List interfaces with ip command (modern)
ip link show
ip -brief link show          # Concise output
ip -c addr show              # With colors

# List interfaces (legacy)
ifconfig                     # May need: sudo apt install net-tools

# Bring interface up/down
sudo ip link set eth0 up
sudo ip link set eth0 down

IP Addressing

# Show IP addresses
ip addr show                 # All interfaces
ip -4 addr show              # IPv4 only
ip -6 addr show              # IPv6 only
ip addr show eth0            # Specific interface
hostname -I                  # Quick โ€” just the IPs

# Add/Remove IP address (temporary)
sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip addr del 192.168.1.100/24 dev eth0

# Get your public IP
curl -s ifconfig.me
curl -s icanhazip.com
curl -s ipinfo.io/ip

Routing

# Show routing table
ip route show                # Or: ip r
route -n                     # Legacy format

# Default gateway
ip route | grep default

# Add a route
sudo ip route add 10.0.0.0/8 via 192.168.1.1 dev eth0

# Delete a route
sudo ip route del 10.0.0.0/8

# Trace the route to a host
traceroute google.com        # Shows every hop
tracepath google.com         # Similar, no root needed
mtr google.com               # Real-time traceroute (interactive)

DNS (Domain Name System)

# Check DNS settings
cat /etc/resolv.conf
resolvectl status            # systemd-resolved status

# Resolve hostnames
nslookup google.com          # Simple lookup
dig google.com               # Detailed lookup
dig +short google.com        # Just the IP
dig -x 8.8.8.8               # Reverse DNS lookup
host google.com              # Simple resolution

# /etc/hosts โ€” local DNS override
cat /etc/hosts
# 127.0.0.1    localhost
# 192.168.1.50 myserver.local

# Test resolution order
getent hosts google.com

Network Diagnostic Tools

ping โ€” Test Connectivity

ping google.com              # Continuous ping (Ctrl+C to stop)
ping -c 5 google.com         # Send 5 packets
ping -i 0.5 google.com       # 0.5 second interval
ping -s 1024 google.com      # Custom packet size

ss โ€” Socket Statistics (Modern netstat)

ss                           # All connections
ss -t                        # TCP connections
ss -u                        # UDP connections
ss -l                        # Listening sockets
ss -tuln                     # TCP/UDP listening, numeric ports
ss -tulnp                    # Include process name (need sudo)
ss -s                        # Summary statistics
ss state established         # Only established connections
ss -t dst 192.168.1.1        # Connections to specific host

netstat โ€” Legacy (Still Useful)

sudo apt install net-tools
netstat -tuln                # TCP/UDP listening, numeric
netstat -an                  # All connections, numeric
netstat -rn                  # Routing table

curl & wget โ€” HTTP Tools

# curl โ€” transfer data
curl https://example.com                  # GET request
curl -I https://example.com               # Headers only
curl -o file.html https://example.com     # Save to file
curl -O https://example.com/file.zip      # Save with original name
curl -L https://short.url                 # Follow redirects
curl -s https://api.github.com/users/sovon | jq .  # JSON API

# wget โ€” download files
wget https://example.com/file.zip         # Download file
wget -c https://example.com/large.zip     # Resume download
wget -r https://example.com               # Recursive download
wget -q -O - https://example.com          # Output to stdout

Other Tools

# ARP table
ip neighbor show             # Modern
arp -a                       # Legacy

# Network statistics
ss -s                        # Socket statistics
cat /proc/net/dev            # Interface statistics
nstat                        # Kernel network statistics

# Bandwidth test
sudo apt install speedtest-cli
speedtest                    # Test internet speed

# Port scanning (with nmap)
sudo apt install nmap
nmap localhost               # Scan localhost
nmap 192.168.1.0/24          # Scan entire subnet
nmap -sV 192.168.1.1         # Detect service versions

Network Configuration

NetworkManager (Most Desktops)

# Using nmcli (NetworkManager CLI)
nmcli device status          # List network devices
nmcli connection show        # List connections
nmcli device wifi list       # List available WiFi networks

# Connect to WiFi
nmcli device wifi connect "MyNetwork" password "mypassword"

# Set static IP
nmcli connection modify "Wired connection 1" \
  ipv4.method manual \
  ipv4.addresses "192.168.1.100/24" \
  ipv4.gateway "192.168.1.1" \
  ipv4.dns "8.8.8.8,8.8.4.4"

# Apply changes
nmcli connection up "Wired connection 1"

Netplan (Ubuntu Servers)

# /etc/netplan/01-config.yaml
network:
  version: 2
  renderer: networkd
  ethernets:
    eth0:
      dhcp4: false
      addresses:
        - 192.168.1.100/24
      routes:
        - to: default
          via: 192.168.1.1
      nameservers:
        addresses: [8.8.8.8, 8.8.4.4]
sudo netplan apply            # Apply netplan config
sudo netplan try              # Test (auto-reverts in 120s)

Firewall Basics (ufw)

ufw (Uncomplicated Firewall) is the easy frontend for iptables.

# Install & enable
sudo apt install ufw
sudo ufw enable               # Enable firewall
sudo ufw disable              # Disable firewall
sudo ufw status verbose       # Show current rules

# Default policies
sudo ufw default deny incoming
sudo ufw default allow outgoing

# Allow/Deny rules
sudo ufw allow 22/tcp         # Allow SSH
sudo ufw allow 80/tcp         # Allow HTTP
sudo ufw allow 443/tcp        # Allow HTTPS
sudo ufw allow from 192.168.1.0/24  # Allow entire subnet
sudo ufw deny 3306/tcp        # Deny MySQL from outside

# App profiles
sudo ufw app list             # List available profiles
sudo ufw allow 'Nginx Full'   # Allow HTTP + HTTPS

# Delete rules
sudo ufw status numbered      # Show rules with numbers
sudo ufw delete 3             # Delete rule #3

# Reset everything
sudo ufw reset

๐Ÿ‹๏ธ Practice Exercises

  1. IP: Find your IP address with ip addr show and hostname -I
  2. DNS: Use dig to find the IP of github.com
  3. Ping: Ping 8.8.8.8 with 5 packets and note the latency
  4. Ports: List all listening ports with ss -tuln
  5. Route: Find your default gateway with ip route
  6. curl: Download a web page and save it to a file
  7. Firewall: Enable ufw and allow only SSH (port 22)
  8. Public IP: Find your public IP using curl

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