Routing incoming ppp0

Routing incoming ppp0 is a netfilter firewall.

Sample:

#!/bin/bash

# Load required modules
insmod ip_tables
insmod ip_conntrack
insmod iptable_nat
insmod ipt_MASQUERADE

# Then flush all rules
iptables -F
iptables -t nat -F

# In the NAT table (-t nat), Append a rule (-A) after routing
# (POSTROUTING) for all packets going out ppp0 (-o ppp0) which says to
# MASQUERADE the connection (-j MASQUERADE).

#iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# Below means route 192.168.1.x
iptables -t nat -A POSTROUTING -d ! 192.168.1.0/24 -j MASQUERADE

iptables -A FORWARD -s 192.168.1.0/24 -j ACCEPT
iptables -A FORWARD -d 192.168.1.0/24 -j ACCEPT
iptables -A FORWARD -s ! 192.168.1.0/24 -j DROP

# Disallow NEW and INVALID incoming or forwarded packets from ppp0.
#iptables -A INPUT -i ppp0 -m state --state NEW,INVALID -j DROP
#iptables -A FORWARD -i ppp0 -m state --state NEW,INVALID -j DROP

# port 113 is evil ;)
iptables -A INPUT --protocol udp --source-port 113 -j DROP
iptables -A INPUT --protocol udp --destination-port 113 -j DROP

# Turn on IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

#iptables -A INPUT --protocol udp --source-port 113 -j DROP

# Route incoming ppp0 at port 80, to 192.168.1.18:80
iptables -A PREROUTING -t nat -p tcp -i ppp0 --dport 80 -j DNAT --to 192.168.1.18:80

# Route incoming ppp0 at port 21, to 192.168.1.18:21
iptables -A PREROUTING -t nat -p tcp -i ppp0 --dport 21 -j DNAT --to 192.168.1.18:21