Combination of NAT, DHCP, and OpenVPN on lubuntu 14.04

Question

Okay, so I've got a set up, where eth0 my internal IP address (192.168.1.3) it has been configured as my default gateway using the iptables this

sudo sysctl -w net.ipv4.ip_forward=1

sudo iptables -P FORWARD ACCEPT

sudo iptables –-table nat -A POSTROUTING -o eth2 -j MASQUERADE

Where eth2 is my static ip for internet

That works fine, I can access internet from any computer on my network via DHCP (192.168.1.10-200)

Everything stops working when I run openvpn

Here's my config file http://pastebin.ubuntu.com/7556511/

That config works without the NAT, as in on the computer I can browse the web, and if I do

curl ifconfig.me

it gives me a remote ip address from my VPN provider.

When I say fails on other computers, I mean I can't load pages, I can't ping, nothing works outside of the network (But I can still SSH in on the internal interface

Answer 1

The error is in your iptables, the command:

  iptables –-table nat -A POSTROUTING -o eth2 -j MASQUERADE

should be modified to

 iptables –-table nat -A POSTROUTING -o tun0 -j MASQUERADE

If your interface is not called tun0, please change accordingly (BTW, the very same applies if you have a tap interface, just use tap0 in tun0 stead).

In OpenVPN, all traffic exits your pc through the bridge, not the ethernet interface. You cannot however apply iptables to a bridge (that requires ebtables), but you can circumvent the problem by applying iptables to the virtual interface.