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[The ttylinux firewall structure is being re-organized.]
The default settings of the ttylinux firewall script assume that the network interface to the rest of the world is called ippp0 or ppp0, depending on whether you use ISDN or modem dialup. By default, the firewall will allow the world to access the SSH server, but will deny access to the telnet server. Other incoming connection attempts will also be denied and logged, rate limited to an average of 3 messages per hour, except for connections to the ftp-data port, which is used for active FTP sessions and allowed in. This applies to TCP connections. For the UDP protocol, incoming packets to ports above 1024 are allowed. Also, the outside world is not allowed to ping the ttylinux machine.
Outgoing traffic is not firewalled at all.
You can change the settings of the firewall in /etc/firewall.conf. The only configuration options are whether to allow or deny access to the SSH and telnet servers.
Note that this firewall script is very simple and assumes that local users are always trusted. There is no protection against users running server daemons on UDP ports above 1024. If you don't trust your users, this firewall script is not good enough. Also note that only the ppp0 or ippp0 device is protected, all other interfaces are not firewalled at all.