#!/bin/bash

set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?" >&2' ERR

do-forward() {
    cmd=$1; shift
    for port; do
        /sbin/iptables -t nat $cmd PREROUTING -i eth0 -p tcp -m tcp --dport $port -j DNAT --to-destination 10.8.0.4:$port
    done
}

ports=(25 143 587)
case $1 in
    start)
        do-forward -A ${ports[@]}
        ;;
    stop)
        do-forward -D ${ports[@]}
        ;;
    *)
        echo "$0: error: expected 1 argument of start or stop"
        exit 1
        ;;
esac