Red Hat Enterprise Linux 5 Administration Unleashed

504CHAPTER 24Configuring a FirewallFIGURE 24.2Adding Additional PortsIf you are logged in remotely via SSH, be sure to select SSH as a trusted device so you remainconnected to the system. Finally, click OK in the main window to enable the firewall.The security level tool uses IPTables rules to configure the firewall for commonly usedservices. After setting which services to allow, the rules are written to /etc/sysconfig/iptables. The iptables service must be running as discussed earlier in the chapter for thefirewall to work. When the firewall is enabled in the system-config-securitylevel tool,the system is automatically configured to start the iptables service at boot-time, and it isimmediately started if it is not already on.CAUTIONDo not use the system-config-securitylevel program after writing andsaving custom rules because the custom rules will be overwritten when a new/etc/sysconfig/iptables file is written by the program.SummaryAs you have read, IPTables offers very simple to extremely complex packet filtering. It can beused to block all connection requests, only allow requests for a specific port through, limitthe number of simultaneous connections per client while logging the state of the connections,forward requests to a different server, modify the destination of a packet, and more.If you just need a simple firewall to block all connections except ones on specific ports,you can enable the basic Red Hat Enterprise Linux firewall using the Security Level graphicalapplication.