I had never heard of ipmasq
until I read your post. But i just looked it up
to see what it was. The description sounds reasonable, but, of course, I can't vouch for something I haven't looked at. Most (but not all) of the information about what it is doing would be in indicated by the firewall rules it sets up. On the running system, you can list the rules with the following commands run as root
iptables -t nat -nvL
I omitted the command for listing rules for the "mangle table" since 1) I doubt they are used,and 2) I have no experience with them.
If you could post the output of the above two commands, I (and others) would have a chance to asses the rules.
But since you are running this (if I understand correctly) on a PC whose sole purpose is to act as a firewall, I suggest you look into distrubtions that are made specifically for that purpose. This really isn't something that fits my current setup, so I am not well versed in what is available. But a little over a year I did take some time to look at a distro called IPCop. After playing with it for several hours I was fairly impressed with it. You can Google for it if you are interested. There may be others that are as good or better. I can't say ...