Fig.03: Checking firewall configuration for error
Read answer to: "Linux Configure Firewall Using Shorewall Under RHEL / CentOS"
Troubleshoot Linux / UNIX bind dns server zone problems with named-checkzone tool
HowTo: Check If a Directory Exists In a Shell Script
Ubuntu Linux Save / Restore Iptables Rules
Iptables setup masquerading for Linux firewall
Linux Firewall: Display Status and Rules of Iptables Firewall
If sshd is restart will any connections be disconnected 
OpenBSD / FreeBSD / NetBSD: PF Firewall List Rules
Linux Deleting Firewall RulesIptables: Unblock / Delete an IP Address Listed in IPtables Tables
Test Iptables Script Remotely 
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