Firewall

This is a basic firewall which works for most Linux distro's. It's based on the Setting up a simple Debian gateway article from debian-administration.org and assumes eth0 to be the external WAN interface and eth1 the internal LAN interface. On Debian based machines, save this script as /etc/network/if-up.d/00-firewall and it will execute whenever the networking starts up.

<bash> !/bin/sh delete all existing rules. iptables -F iptables -t nat -F iptables -t mangle -F iptables -X Always accept loopback traffic iptables -A INPUT -i lo -j ACCEPT Allow established connections, and those not coming from the outside iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -m state --state NEW ! -i eth0 -j ACCEPT iptables -A FORWARD -i eth0 -o eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT Allow outgoing connections from the LAN side. iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT Masquerade. iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE Dont forward from the outside to the inside. iptables -A FORWARD -i eth0 -o eth0 -j REJECT Example pinhole: forward packets on port 5900 to 192.168.1.100 iptables -A FORWARD -i eth0 -o eth1 -p tcp --dport 5900 -j ACCEPT iptables -A PREROUTING -t nat -p tcp --dport 5900 -j DNAT --to-destination 192.168.1.100 Enable routing. echo 1 > /proc/sys/net/ipv4/ip_forward </bash>

See also

• dnsmasq