The netfilter firewall is responsible for routing incoming traffic through the ppp0 interface.
First, I loaded all the required modules using a bash script, including ip_tables, ip_conntrack, iptable_nat, and ipt_MASQUERADE. Then, I flushed all the rules to start with a clean slate.
Next, I appended a rule to the NAT table to MASQUERADE the connection for all packets going out ppp0 (-o ppp0) after routing (POSTROUTING). I also set a rule to route any traffic to 192.168.1.x.
To allow certain traffic to pass through, I added additional FORWARD rules to accept packets from 192.168.1.0/24 and drop any traffic not originating from this subnet. I also implemented a DROP rule to disallow NEW and INVALID incoming or forwarded packets from ppp0.
To further secure my system, I blocked any traffic using port 113.
Finally, I turned on IP forwarding and set rules to route incoming traffic at port 80 and port 21 to specific destinations.
Overall, this netfilter firewall was effective and easy to use. I highly recommend it to anyone looking for a reliable way to manage incoming traffic.