Simple ruleset for a home router

This example shows the configuration of an IPv4-only home router using a ppp interface to go out to the Internet.

flush ruleset

define DEV_PRIVATE = eth1
define DEV_WORLD = ppp0
define NET_PRIVATE = 192.168.0.0/16

table ip filter {

    chain inbound_world {
        # accepting ping (icmp-echo-request) for diagnostic purposes.
        # However, it also lets probes discover this host is alive.
        # This sample accepts them within a certain rate limit:
        #
        # icmp type echo-request limit rate 5/second accept

        # allow SSH connections from some well-known internet host
        ip saddr 81.209.165.42 tcp dport ssh accept
    }

    chain inbound_private {
        # accepting ping (icmp-echo-request) for diagnostic purposes.
        icmp type echo-request limit rate 5/second accept

        # allow DHCP, DNS and SSH from the private network
        ip protocol . th dport vmap { tcp . 22 : accept, udp . 53 : accept, tcp . 53 : accept, udp . 67 : accept}
    }

    chain inbound {
        type filter hook input priority 0; policy drop;

        # Allow traffic from established and related packets, drop invalid
        ct state vmap { established : accept, related : accept, invalid : drop }

        # allow loopback traffic, anything else jump to chain for further evaluation
        iifname vmap { lo : accept, $DEV_WORLD : jump inbound_world, $DEV_PRIVATE : jump inbound_private }

        # the rest is dropped by the above policy
    }

    chain forward {
        type filter hook forward priority 0; policy drop;

        # Allow traffic from established and related packets, drop invalid
        ct state vmap { established : accept, related : accept, invalid : drop }

        # connections from the internal net to the internet or to other
        # internal nets are allowed
        iifname $DEV_PRIVATE accept

        # the rest is dropped by the above policy
    }

    chain postrouting {
        type nat hook postrouting priority 100; policy accept;

        # masquerade private IP addresses
        ip saddr $NET_PRIVATE oifname $DEV_WORLD masquerade
    }
}

Do not forget to:

echo 1 > /proc/sys/net/ipv4/ip_forward

to enable IPv4 forwarding in your router or enable it through /etc/sysctl.conf for persistency