sshd keep alive

Hello, I have Suse 9.0 and I use ssh to create tunnels to access several services of the server. My problem is that ssh close the connections if there is no activity in the tunnel. I have the property KeepAlive yes in the sshd_config but I doesnt work. I have ipchains as firewall, perhaps this is my problem but I don´t know, ¿any idea? Thaks -- Emiliano Sutil García