Iptables-Client http-access only for few domains
 

Hello!
I have a small network in which I would like to share one internet-access through a linux-machine. I use iptables and an extern DNS to configure a firewall.
Currently the clients can ping my firewall-server and any server outside the lan without problems.
The trouble ist that the http connection for the cliens seems only to work for a few domains (google.com, cnn.com, etc... Proxy?!??). SSH and ftp connections also only work partially (works fine for a while and then there the connection drops dead).

Is there anyone who could help me with this?

Greetings from France,
Michael