internet-sharing.sh

#!/bin/bash

## This is a setup script for temporary sharing of an internet connection
## Configure the variables at the top of this file, then run:
##   sudo ./internet-sharing.sh setup
## And optionally start the DHCP server:
##   sudo ./internet-sharing.sh dhcp
## Connect your other computer or LAN to the second network device.
## Dnsmasq will output the IP address(es) of connected clients as they request them.
## Press Ctrl-C to quit dnsmasq when you no longer need DHCP.
## When all done, run teardown:
##   sudo ./internet-sharing.sh teardown

### Configure these variables for yourself:
## The name of your network device connected to internet:
INTERNET_DEV=wlp3s0
## The name of the secondary network device to share with:
SHARE_DEV=enp0s25
## The new IP address of this computer, on the sharing network:
SHARE_GATEWAY=192.168.123.1
## Subnet:
SHARE_SUBNET_CIDR=24
SHARE_SUBNET=192.168.123.0/24
## DHCP:
DNSMASQ=$(which dnsmasq)
DHCP_DNS=1.0.0.1
DHCP_RANGE=192.168.123.50,192.168.123.100,12h



exe() { ( echo "## $*"; $*; ) }

error() {
  printf '\E[31m'; echo "$@"; printf '\E[0m'
}

setup() {
    ## Assign IP address
    exe ip link set up dev ${SHARE_DEV}
    exe ip addr add ${SHARE_GATEWAY}/${SHARE_SUBNET_CIDR} dev ${SHARE_DEV}

    ## Enable IP forwarding:
    exe sysctl net.ipv4.ip_forward=1
    exe iptables -t nat -A POSTROUTING -o ${INTERNET_DEV} -j MASQUERADE
    exe iptables -A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
    exe iptables -A FORWARD -i ${SHARE_DEV} -o ${INTERNET_DEV} -j ACCEPT
    ## Enable access for DHCP server:
    exe iptables -I INPUT -p udp --dport 67 -i ${SHARE_DEV} -j ACCEPT
    exe iptables -I INPUT -p udp --dport 53 -s ${SHARE_SUBNET} -j ACCEPT
    exe iptables -I INPUT -p tcp --dport 53 -s ${SHARE_SUBNET} -j ACCEPT
}

teardown() {
    (
        set +e
        exe ip link set down dev ${SHARE_DEV}
        exe ip addr del ${SHARE_GATEWAY}/${SHARE_SUBNET_CIDR} dev ${SHARE_DEV}
        exe iptables -t nat -D POSTROUTING -o ${INTERNET_DEV} -j MASQUERADE
        exe iptables -D FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
        exe iptables -D FORWARD -i ${SHARE_DEV} -o ${INTERNET_DEV} -j ACCEPT
        exe iptables -D INPUT -p udp --dport 67 -i ${SHARE_DEV} -j ACCEPT
        exe iptables -D INPUT -p udp --dport 53 -s ${SHARE_SUBNET} -j ACCEPT
        exe iptables -D INPUT -p tcp --dport 53 -s ${SHARE_SUBNET} -j ACCEPT
    )
}

dhcp() {
    if [[ $DNSMASQ == "" ]]; then
        error "Could not find dnsmasq installed. Please install dnsmasq and try again."
        exit 1
    fi

    ## Start dnsmasq:
    echo "Starting dnsmasq DHCP server, press Ctrl-C to quit when done."
    exe ${DNSMASQ} -i ${SHARE_DEV} --port 0 --bind-interfaces --dhcp-option=3,${SHARE_GATEWAY} \
        --dhcp-option=6,${DHCP_DNS} --dhcp-range=${DHCP_RANGE} --no-daemon
}

main() {
    set -e
    if [[ $EUID -ne 0 ]]; then
        error "This script must be run as root."
        exit 1
    fi
    if [[ $# -gt 0 ]]; then
        $*
    else
        echo "## Must specify a command: setup, dhcp, teardown"
    fi
}

main $*