I'm using a dedicated host. Unfortunately, when I use x windows with xming, it tends to freeze up the x windows session. This is keeping me from calling system-config-securitylevel and making changes to the firewall. Is there a way to do it on the command line?
Yes, it is called
The CentOS wiki has written a nice piece together about it: http://wiki.centos.org/HowTos/Network/IPTables