The Linux Firewall Administration ProgramsBuild versus Buy: The Linux KernelSource and Destination Addressing OptionsInitializing the FirewallSymbolic Constants Used in the Firewall ExamplesEnabling Kernel-Monitoring SupportRemoving Any Preexisting RulesResetting Default Policies and Stopping the FirewallEnabling the Loopback InterfaceDefining the Default PolicyUsing Connection State to Bypass Rule CheckingSource Address Spoofing and Other Bad AddressesProtecting Services on Assigned Unprivileged PortsCommon Local TCP Services Assigned to Unprivileged PortsCommon Local UDP Services Assigned to Unprivileged PortsEnabling Basic, Required Internet ServicesAllowing DNS (UDP/TCP Port 53)Enabling Common TCP ServicesEmail (TCP SMTP Port 25, POP Port 110, IMAP Port 143)SSH (TCP Port 22)FTP (TCP Ports 21, 20)Generic TCP ServiceEnabling Common UDP ServicesAccessing Your ISP’s DHCP Server (UDP Ports 67, 68)Accessing Remote Network Time Servers (UDP Port 123)Logging Dropped Incoming PacketsLogging Dropped Outgoing PacketsInstalling the FirewallTips for Debugging the Firewall ScriptStarting the Firewall on Boot with Red Hat and SUSEStarting the Firewall on Boot with DebianInstalling a Firewall with a Dynamic IP AddressSummary