i wrote this little script to get the firewall to a "safe" configuration. I'm not accepting any incoming connections but i allow my pc to connect to anyone. This is like being behind a router with no forwarding: browsing should be ok, but filesharing and such might give some problems. here's the script:
#!/bin/bash
iptables -A INPUT -s 127.0.0.1/255.255.255.255 -j ACCEPT
iptables -A INPUT -s 192.168.0.0/255.255.255.0 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/255.255.255.0 -j ACCEPT
iptables -A INPUT -s 0.0.0.0/0.0.0.0 -j DROP

192.168.0.0/255.255.255.0 is a net between an eth dsl modem and my pc with eth0 and 192.168.1.0/255.255.255.0 is a net connecting my pc and another p with eth1. The second pc can access the internet using squid on the first pc.

The problem is that i can't lookup my hostname or browse the web using this configuration. Without the firewall everything runs fine. The dsl modem has a static ip 192.168.0.1 and it lends a dynamic ip to my pc (usually lends the ip obtained from the dsl provider).
What's wrong here? After i get the basic setup working, adding a few other entries for my filesharing programs shouldn't be hard...

a little more info:

this is my routing table:
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
82.48.147.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 82.48.147.117 0.0.0.0 UG 0 0 0 eth0


i really can't figure out what's wrong. I haven't blocked any outgoing connections!

and of course, everything runs fine if i add
iptables -A INPUT -d 82.48.147.116 -j ACCEPT (x.x.x.116 is my ip, x.x.x.117 is the gateway, misterious modem settings)
before the last line, but then i could just flush iptables down the toilet because every packet is accepted

What's your default OUTPUT policy? Also, you'll need to allow ESTABLISHED,RELATED connections coming in from the outside.

Maybe this one line will help you:

iptables -A INPUT -p all -m state --state ESTABLISHED,RELATED -j ACCEPT

but of course things get a lot more complex as you make your iptables better and better.