FirewallBuilder

My firewall has virtual interface eth0:1. In fwbuilder I added the interface, however, when I want to apply the new rules I get the error "Interface eth0:1 does not exist"

eth0:1 is not a real interface on Linux, it is just a label for a second IP address of the interface eth0. One way to solve this is not to create it in the OS (remove file /etc/sysconfig/network-scripts/ifcfg-eth0:1 and restart networking), but rather add its IP address in the Firewall Builder GUI as a second address to interface eth0.

In any case you should not add interface "eth0:1" in fwbuilder because it really does not exist. Iptables will not recognize this interface either, so even if Firewall Builder did allow you to use it, you would get an error from iptables. You see "eth0:1" in the output of ifconfig only because ifconfig has been modified on Linux to show virtual IP addresses as pseudo-interfaces. The command "ip addr show" will reveal that the eth0:1 is really a label on the second IP address of eth0.