What ways are there of setting up a firewall that can be managed with a GUI that allows you to firewall (filter*) incoming traffic routed to other machines? Can GUFW do it? FireStarter? (or should that be avoided because it is supposedly no longer updated?)
*By filtering, I'm mean the traffic I am setting rules up for is not destined for this computer. It is either from or to other computers on my LAN.
Say, for (a simplified, hypothetical) example:
I have an ethernet connection from my ISP that I have plugged into eth0. It gets an address of 18.104.22.168 and I also have 22.214.171.124/24 routed to me, so I don't have to do any NAT. I have a switch attached to my second ethernet port (eth1). Attached to it are various PCs (that do not necessarily have a host-based firewall on them) that have addresses out of my 126.96.36.199/24 block statically set. How can I use my Ubuntu box (which is doing routing and no NAT) to block incoming connections from eth0 that are being routed to my various PCs and let through just a few specific ports on a per host basis?