I'm trying to secure my Debian etch 4.0r1 machine. I've built some rules for iptables, but I think that they are blocking responses from the servers I am connecting to. For example, if I do a lookup on google.ca the request is sent by my PC, but the reply comes in on a port that IPTABLES isn't allowing. How do I let these responses through?

I can connect to SSH on this PC from another PC on my LAN, so I know that the interfaces are working.

My IPTABLES rules, according to WebMin:

# INPUT

Accept If protocol is ICMP and ICMP type is echo-request
Accept If protocol is TCP and input interface is eth2 and destination port is 22
Accept If protocol is UDP and input interface is eth2 and destination port is 22
Accept If protocol is TCP and input interface is eth2 and destination port is 20:21
Accept If protocol is TCP and input interface is eth2 and source port is 14400:14499
Drop If input interface is eth2

#eth1 is behind a router, so SMB shares are NOT public.
Accept If protocol is TCP and input interface is eth1 and destination port is 22
Accept If protocol is UDP and input interface is eth1 and destination port is 22
Accept If protocol is TCP and source is 10.0.11.0/24 and input interface is eth1 and destination port is 139
Accept If protocol is UDP and source is 10.0.11.0/24 and input interface is eth1 and destination port is 139
Accept If protocol is TCP and source is 10.0.11.0/24 and input interface is eth1 and destination port is 445
Accept If protocol is UDP and source is 10.0.11.0/24 and input interface is eth1 and destination port is 445
Accept If protocol is TCP and input interface is eth1 and destination port is 55555
Drop If input interface is eth1

Accept If protocol is UDP and input interface is eth0 and destination port is 18800:18899
Accept If protocol is TCP and input interface is eth0 and destination port is 18800:18899
Accept If protocol is TCP and input interface is eth0 and destination port is 6881:6999
Accept If protocol is TCP and input interface is eth0 and destination port is 22
Accept If protocol is UDP and input interface is eth0 and destination port is 22
Drop If input interface is eth0

# OUTPUT
Default = Accept

# FORWARDED
Default = Accept

Hi.
I don't know much about security or iptables, so this may not be the best solution, but it works fine for me.

iptables -I INPUT -m conntrack --cstate ESTABLISHED -j ACCEPT

This will let all incoming connections if we have stablished that connection. Which means (I hope :P) that only those connections that we have started are allowed.

I'm not familiar with the cstate match (in fact, the command you posted won't work at all on my Ubuntu box), so I'll post the more traditional way to do this:

Ups. It is always better to copy and paste than using memmory :P.
It is ctstate. I'm sorry for the mistake. The command would be (with copy and paste)

iptables -I INPUT -m conntrack --ctstate ESTABLISHED -j ACCEPT

It should work now, I'm using kubuntu feisty.
regards.

'established' is not enough for ftp, since ftp uses 2 connections, which need to be recognized as 'related', so if you want ftp, use the command from win32sucks. It doesn't matter if you use 'ctstate' or 'state' match, the difference is, that ctstate provides some more matches.

This is working... What is the difference between this and iptables -I INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT ?

there is absolutely no difference between them.