Quick script to enable connection sharing (i.e. nat) on an interface in Linux. Based on http://xmodulo.com/2014/06/internet-connection-sharing-iptables-linux.html

iptables-connection-sharing.sh

#!/bin/bash

while getopts "i:t:" opt; do
    case $opt in
        i)
            $(ip link show $OPTARG 1> /dev/null 2>&1)
            if [[ $? != 0 ]]; then
                echo "Argument to -${opt} should be an network device."
                exit -1
            else
                interface=$OPTARG
            fi
            ;;
        t)
            if [[ $OPTARG != 'on' ]] && [[ $OPTARG != 'off' ]]; then
                echo "Need to select either 'on' or 'off'."
                exit -1
            else
                toggle=$OPTARG
            fi
            ;;
        \?)
            echo "Invalid option: -$OPTARG" >&2
            ;;
  esac
done

# Get/update cached sudo credentials
sudo -v

## Internet connection shating script

case $toggle in
    on)
        sudo -n sysctl -q -w net.ipv4.ip_forward=1
        sudo -n iptables -X
        sudo -n iptables -F
        sudo -n iptables -t nat -X
        sudo -n iptables -t nat -F
        sudo -n iptables -I INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
        sudo -n iptables -I FORWARD  -m state --state RELATED,ESTABLISHED -j ACCEPT
        sudo -n iptables -t nat -I POSTROUTING -o $interface -j MASQUERADE
        ;;
    off)
        sudo -n sysctl -q -w net.ipv4.ip_forward=0
        sudo -n iptables -X
        sudo -n iptables -F
        sudo -n iptables -t nat -X
        sudo -n iptables -t nat -F
        ;;
esac

exit 0