How to block all websites except for a few using IP Tables

#!/bin/sh
### BEGIN INIT INFO
# Provides:          custom_iptables
# Required-Start:    sudo /etc/init.d/custom_iptables start
# Required-Stop:     sudo /etc/init.d/custom_iptables stop
# Default-Start:     37 S
# Default-Stop:      37 0
# Short-Description: Run script at boot time
# Description:       Enables custom iptable rules
### END INIT INFO
 
IPTABLES_SAVE="/etc/custom_iptables/iptables.up.rules"
SAVE_RESTORE_OPTIONS="-c"
SAVE_ON_STOP="yes"
 
checkrules() {
	if [ ! -f ${IPTABLES_SAVE} ]
	then
		echo "Not starting iptables. First create some rules then run"
		echo "\"/etc/init.d/custom_iptables save\""
		return 1
	fi
}
 
save() {
  echo "Saving iptables state"
  /sbin/iptables-save ${SAVE_RESTORE_OPTIONS} > ${IPTABLES_SAVE}
}
 
start(){
	checkrules || return 1
	  echo  "Loading iptables state and starting firewall"
	  echo -n "Restoring iptables ruleset"
	  start-stop-daemon --start --quiet --exec /sbin/iptables-restore -- ${SAVE_RESTORE_OPTIONS} < ${IPTABLES_SAVE}
}
 
case "$1" in
  save)
        save
        echo "."
        ;;
 
  start)
  	start
	echo "."
	;;
  stop)
	  if [ "${SAVE_ON_STOP}" = "yes" ]; then
		  save || exit 1
	  fi
	  echo -n "Stopping firewall"
		for a in `cat /proc/net/ip_tables_names`; do
			/sbin/iptables -F -t $a
			/sbin/iptables -X -t $a
	
			if [ $a == nat ]; then
				/sbin/iptables -t nat -P PREROUTING ACCEPT
				/sbin/iptables -t nat -P POSTROUTING ACCEPT
				/sbin/iptables -t nat -P OUTPUT ACCEPT
			elif [ $a == mangle ]; then
				/sbin/iptables -t mangle -P PREROUTING ACCEPT
				/sbin/iptables -t mangle -P INPUT ACCEPT
				/sbin/iptables -t mangle -P FORWARD ACCEPT
				/sbin/iptables -t mangle -P OUTPUT ACCEPT
				/sbin/iptables -t mangle -P POSTROUTING ACCEPT
			elif [ $a == filter ]; then
				/sbin/iptables -t filter -P INPUT ACCEPT
				/sbin/iptables -t filter -P FORWARD ACCEPT
				/sbin/iptables -t filter -P OUTPUT ACCEPT
			fi
		done
    start-stop-daemon --stop --quiet --pidfile /var/run/iptables.pid --exec /sbin/iptables
    echo "."
    ;;
 
  restart)
	  echo -n "Flushing firewall"
		  for a in `cat /proc/net/ip_tables_names`; do
			  /sbin/iptables -F -t $a
  			  /sbin/iptables -X -t $a
	  	done;
	  start
    echo "."
    ;;
  *)
    echo "Usage: /etc/init.d/custom_iptables {start|stop|restart|save}" >&2
    exit 1
    ;;
esac
 
exit 0

                                              

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