UDP & IPChains

hi all ! i've got a little problem, at the moment i'm setting up RRDTool with NRG as grafik frontend. Problem is now, the host on wich i've run RRDtool is behind a firewall, so i think i've not to manny options, i'll only allow answer packages that was send from my RRDTool host, is there a reason to bring this working with IPChains ? at the moment i allow all udp packages from extern(temporarily) think thats not so good, other reason is an UDP Proxy i think, but i've only found some udp forwarding Proxys with few security. in advance THX