I came across the following example of how to use iptables to block internet access when the VPN connection is terminated abruptly:
iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT #allow loopback access iptables -A OUTPUT -d 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server iptables -A INPUT -s 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server iptables -A INPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT #make sure that you can communicate within your own network iptables -A OUTPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT iptables -A FORWARD -i eth+ -o tun+ -j ACCEPT iptables -A FORWARD -i tun+ -o eth+ -j ACCEPT # make sure that eth+ and tun+ can communicate iptables -t nat -A POSTROUTING -o tun+ -j MASQUERADE # in the POSTROUTING chain of the NAT table, map the tun+ interface outgoing packet IP address, cease examining rules and let the header be modified, so that we don't have to worry about ports or any other issue - please check this rule with care if you have already a NAT table in your chain iptables -A OUTPUT -o eth+ ! -d a.b.c.d -j DROP # if destination for outgoing packet on eth+ is NOT a.b.c.d, drop the packet, so that nothing leaks if VPN disconnects
I have installed iptables-persistence and would like to know how to use the above to work with iptables.
Any help would be much appreciated.
P.S.: I do not have much IT knowledge and even less of Ubuntu. Could someone explain to me how does one obtain the value 192.168.0.0/16 ?