OpenVPN Allowing Cross-NAT traffic over two NICs

Tylor Pater

1/10/23, 9:21 PM

I'm running OpenVPN on Ubuntu 20.04, and I'm trying to allow users to access servers that are on a different subnet. Here is a diagram of what the network looks like:

Diagram of network

So, eth1 is connected to DHCP and the wider internet, and eth0 is connected to a private network with the servers on them. So the VPN is running on eth1 (192.168.1.0/24), but I need to be able to access the machinery on eth0 (192.168.177.0/24) when connected to the VPN.

I've tried adding temporary iptables rules, but I'm not sure if they were working because they are on two NICs. Everything I've been able to find on Google is all about when they're on the same NIC.

Edit: I've also added push "route 192.168.177.0 255.255.255.0" to my server.conf for OpenVPN but that does not seem to have worked.

Edit 2: I've also tried the following iptables rules:

sudo iptables -A INPUT -i tun0 -j ACCEPT
sudo iptables -A INPUT -i eth0 -j ACCEPT
sudo iptables -A INPUT -i eth1 -j ACCEPT
sudo iptables -A FORWARD -i eth0 -j ACCEPT
sudo iptables -A FORWARD -i eth1 -j ACCEPT
sudo iptables -A FORWARD -i tun0 -j ACCEPT

as well as:

sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
sudo iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
sudo iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

I can see my route listed if I run route print on the Windows client:

 192.168.177.0    255.255.255.0         10.8.0.1         10.8.0.2    259

0 + 0

ubuntu

iptables

openvpn

Michael Hampton

1/10/23, 9:27 PM

Just set up normal routes. NAT or iptables is not appropriate here.

Tylor Pater

1/10/23, 9:31 PM

I've tried just adding `push "route 192.168.177.0 255.255.255.0"` to server.conf but that doesn't seem to work (if that is what you meant)

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: OpenVPN Allowing Cross-NAT traffic over two NICs

TH: OpenVPN อนุญาตการรับส่งข้อมูลข้าม NAT ผ่านสอง NIC

RO: OpenVPN Permite trafic Cross-NAT peste două NIC-uri

RU: OpenVPN, разрешающий трафик Cross-NAT через два сетевых адаптера

VI: OpenVPN Cho phép lưu lượng Cross-NAT trên hai NIC

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