/etc/init.d/enable-nat is in debian-edu-config 1.818+deb8u2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 | #! /bin/sh
### BEGIN INIT INFO
# Provides:          enable-nat
# Required-Start:    $remote_fs
# Should-Start:      $network $syslog
# Required-Stop:     $remote_fs
# Should-Stop:       $network $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Enabling NAT for clients behind eth1
# Description:       Enabling Network Address Translation for clients
#                    sitting in the thin client network behind eth1
### END INIT INFO
IPTABLES=/sbin/iptables
NETWORK_TO_NAT=
OUTSIDE_IF=eth0
# Only enable by default if LTSP is installed
if [ -e /opt/ltsp ] ; then
    NETWORK_TO_NAT="192.168.0.0/24"
fi
. /lib/lsb/init-functions
if [ -f /etc/default/enable-nat ] ; then
    . /etc/default/enable-nat
fi
# Bail out if no iptables binary or no configuration
[ -x ${IPTABLES} -a "$NETWORK_TO_NAT" ] || exit 0
do_status() {
    $IPTABLES -L -t nat |grep -A3 POSTROUTING
}
is_enabled() {
    if do_status | grep -q "$NETWORK_TO_NAT" ; then
	true
    else
	false
    fi
}
do_start() {
    if is_enabled ; then
	log_action_msg "NAT for clients on network $NETWORK_TO_NAT already enabled"
    else
	log_action_begin_msg "Enabling NAT for clients on network $NETWORK_TO_NAT."
	$IPTABLES -t nat -A POSTROUTING -s $NETWORK_TO_NAT -o $OUTSIDE_IF -j MASQUERADE
	log_action_end_msg $?
    fi
    # Enable IP-forwarding if it isn't enabled already.
    if [ 0 = "`cat /proc/sys/net/ipv4/ip_forward`" ]; then
	log_action_begin_msg "Enabling IPv4 forwarding."
	echo 1 > /proc/sys/net/ipv4/ip_forward
	log_action_end_msg $?
    fi
}
do_stop() {
    if is_enabled ; then
	log_action_begin_msg "Disabling NAT for clients on network $NETWORK_TO_NAT."
	$IPTABLES -F -t nat
	log_action_end_msg $?
    else
	log_action_msg "NAT for clients on network $NETWORK_TO_NAT already disabled"
    fi
}
case "$1" in
    start)
        do_start
        ;;
    stop)
        do_stop
        ;;
    restart|force-reload)
        do_stop
        do_start
        ;;
    status)
        do_status
        ;;
    *)
        echo "Usage: $0 {start|stop|restart|force-reload|status}"
        exit 2
        ;;
esac
exit 0
 |