Linux - routing examples

set ip_forward to 1 to enable routing between nics. type ‘route’ to insure both networks are routed as expected (connected route added with nic)

Setup a script with the following (CHMOD 0755 the script so it executes)

iptables -F (clears previous iptables stuff) iptables -P INPUT DROP (will set default policy to DROP all INPUT packets - Incoming to local NICs) iptables -P OUTPUT DROP (will set default policy to DROP all OUTPUT packets - Outgoing to local NICs) iptables -P FORWARD DROP (will set default policy to DROP all FORWARDed packets - Routed via local NICs)

allow rules; iptables -A FORWARD -s 172.23.23.1 -d 172.23.23.2 -p tcp –dport 80 -j ACCEPT (append forward rules - self explanatory)

Add more as required.

Want to use NAT outbound?

wlan0 => external network - route to Internet eth0 => internal network

#setup masquerading iptables -P INPUT ACCEPT iptables -P OUTPUT ACCEPT iptables -P FORWARD ACCEPT iptables -A FORWARD -m state –state ESTABLISHED,RELATED -j ACCEPT iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

A very handy tool and the current best way i’ve found to test firewall rules is to fire up tcpdump. Check man for details but here is an example of monitoring a specific destination port and host on a specific NIC;

tcpdump -i eth1 dst port 22 and dst host 192.168.4.34

If you can see the traffic leaving the interface outbound it has made it past the firewall, if not then you need to have a look at your firewall rules or check your routing table. Type route to see current routing table

Done