Shorewall
Shorewall is an open source firewall tool for Linux that builds upon the Netfilter ( iptables/ipchains ) system built into the Linux kernel , making it easier to manage more complex configuration schemes by providing a higher level of abstraction for describing rules using text files . 
<triplet> Shorewall <subj> firewall <obj> instance of <triplet> Linux <subj> Linux kernel <obj> based on <triplet> Netfilter <subj> Linux <obj> operating system <triplet> iptables <subj> Linux <obj> operating system <triplet> ipchains <subj> firewall <obj> instance of <subj> Linux <obj> operating system <triplet> Linux kernel <subj> Linux <obj> part of