you don't want to write your own software for this. instead, pick up PortSentry.

it is a package designed to do exactly this. it is not entirely Free Software, but it is free for use and redistribution (unmodified) by corporations (and everyone else).

for the exclusion by port part, you may want to look at the ipchains package which provides firewalling and masquerading capabilities (ipfwadm under 2.0).