Networking :: Ubuntu Iptables - Restricting Access Assistance

I have a small home network with a router to the outside world and an ubuntu server through which traffic passes first.My ISP limits my download usage during the day, which traditionally has not been an issue, but now the children come in from school, boot up the internet and up goes my usage!Ideally I would like to be able to restrict them to IM and maybe certain specified URLs (I think the latter probably needs to use Squid though?). Once the download limits are lifted, I would like my iptables to allow HTTP, etc, but pretty much block most other things.

I have two sets of iptables currently to approach this issue, with a cron job that runs to swap between one and the other.Chains run in order, so if rule A says allow x, and rule B says drop all, then X should still be allowed. However, try as I may, this is not what happens in practice. I have even tried changing the overall order from ALLOW to DROP in FORWARD and then approach from the other angle. That didn't work either. *IS* it actually possible to block all but http / https and IM? These are myrules:

Code:

# Generated by iptables-save v1.4.4 on Sat Jan 9 19:15:49 2010
*nat
:PREROUTING ACCEPT [583:45175]
:POSTROUTING ACCEPT [694:60887]
:OUTPUT ACCEPT [143:18642]

[code]....