This bash script is used to set up and tear down a simple internet gateway. It uses the ip, iptables, ufw, and dnsmasq utilities to configure network interfaces, set up NAT (Network Address Translation), configure a firewall, and set up a DHCP server.

computer_gateway.sh

#!/bin/bash

# Check if script is run as root
if [ "$EUID" -ne 0 ]
  then echo "Please run as root"
  exit
fi

# Check if required utilities are installed
for util in ip iptables ufw dnsmasq; do
  if ! command -v $util &> /dev/null
  then
    echo "$util could not be found, please install it"
    exit
  fi
done

# Set network interfaces
ETH_INTERFACE="enp0s25"
WIFI_INTERFACE="wlp3s0"

ETH_IP_ADDRESS="192.168.0.1"
ETH_SUBNET_MASK="255.255.255.0"
DHCP_RANGE="192.168.0.100,192.168.0.200"

start_gateway() {
    echo "Setting up internet sharing and DHCP server..."
    
    # Configure Ethernet interface with a static IP address
    ip addr add "$ETH_IP_ADDRESS/$ETH_SUBNET_MASK" dev "$ETH_INTERFACE"
    ip link set dev "$ETH_INTERFACE" up
    
    # Enable IP Forwarding
    sysctl net.ipv4.ip_forward=1
    
    # Set up NAT using iptables
    iptables -t nat -A POSTROUTING -o "$WIFI_INTERFACE" -j MASQUERADE
    
    # Allow traffic through UFW
    ufw allow in on "$ETH_INTERFACE"
    ufw allow out on "$WIFI_INTERFACE"
    ufw enable
    
    # Configure DHCP server (dnsmasq)
    echo "interface=$ETH_INTERFACE" | sudo tee /etc/dnsmasq.conf
    echo "dhcp-range=$DHCP_RANGE,12h" | sudo tee -a /etc/dnsmasq.conf
    sudo systemctl restart dnsmasq
    
    echo "Internet sharing and DHCP server are now active."
}

stop_gateway() {
    echo "Stopping internet sharing and DHCP server..."
    
    # Disable IP Forwarding
    sysctl net.ipv4.ip_forward=0
    
    # Remove NAT rule using iptables
    iptables -t nat -D POSTROUTING -o "$WIFI_INTERFACE" -j MASQUERADE
    
    # Remove UFW rules
    ufw delete allow in on "$ETH_INTERFACE"
    ufw delete allow out on "$WIFI_INTERFACE"
    ufw disable
    
    # Disable DHCP server (dnsmasq)
    sudo systemctl stop dnsmasq
    
    # Remove IP configuration from Ethernet interface
    ip addr del "$ETH_IP_ADDRESS/$ETH_SUBNET_MASK" dev "$ETH_INTERFACE"
    
    echo "Internet sharing and DHCP server have been stopped."
}

case "$1" in
    start)
        start_gateway
        ;;
    stop)
        stop_gateway
        ;;
    *)
        echo "Usage: $0 {start|stop}"
        exit 1
esac

exit 0