masq

sharewall is very good

#
# Shorewall version 4 - Masq file
#
# For information about entries in this file, type "man shorewall-masq"
#
# The manpage is also online at
# http://www.shorewall.net/manpages/shorewall-masq.html
#
###############################################################################
#INTERFACE		SOURCE		ADDRESS		PROTO	PORT(S)	IPSEC	MARK
#LAST LINE -- ADD YOUR ENTRIES ABOVE THIS LINE -- DO NOT REMOVE