I implemented something like this recently on my home network, on Tomato (shibby) v138. Here's a diagram:
Before getting into the VPN setup, I initially had both the 2.4GHz and 5GHz networks on the same SSID, let's call it "public". The internal network assigned devices to addresses in the range 192.168.1.2-254. This is what you see in the top half of the diagram.
These are the changes I made to add a new subnet that was routed through the VPN:
- Under Basic/Network/LAN, I added a new bridge named "br1". I gave it the IP address 192.168.2.1, netmask 255.255.255.0, DHCP enabled, and IP Range 192.168.2.2-254.
- Under Advanced/Virtual Wireless, I added two new virtual wireless interfaces, wl0.1 and wl1.1, for the 2.4GHz and 5GHz interfaces respectively. Both are assigned to the new bridge "LAN1 (br1)". I gave both the same new SSID, e.g. "private". You can also give them a different password from the public network if you like.
- Under VPN Tunneling/OpenVPN Client/Basic, I configured the VPN client (my VPN provider is Private Internet Access, so I followed this guide). I also enabled "Start with WAN" so it will start up automatically.
- Under VPN Tunneling/OpenVPN Client/Advanced, I set the "Ignore Redirect Gateway" option so the client won't route everything to the VPN.
- Under VPN Tunneling/OpenVPN Client/Routing Policy, I checked "Redirect through VPN" and added a line with type "From Source IP" and value "192.168.2.0/24" so all hosts on the new subnet get routed through the VPN.
At that point, I can start the VPN client, then pick up a wireless device, connect to the "private" network and confirm that my internet-facing IP is behind the VPN, and connect to "public" and stream Netflix/Amazon Prime video without getting geographic restriction errors.
Now you can set up each device to connect to either SSID according to their needs. In our house, the media streamer that serves Netflix streams to the TV set stays on the public network. My phone and laptop connect to the private network. In most cases you should pick one or the other--you don't want the device auto-connecting to either one arbitrarily.
Getting wired: If you want a physical Ethernet port to connect through the VPN, you can add a new VLAN under Advanced/VLAN and assign it to the new bridge (br1). At this point you can move one or more physical Ethernet ports on the router to your secure VLAN if you want. I didn't, so only wireless clients will be able to join my private subnet.
Internal Routing: After following the steps above, you may find that clients on the public and private networks can't talk to each other. Setting up the VPN client's routing policy as I did above adds this rule:
iptables -t mangle -A PREROUTING -s 192.168.2.0/24 -j MARK --set-mark 311
to tomato's firewall script. That marks every packet originating on the 192.168.2.0/24 network, and everything with the mark 311 gets routed through the VPN. This meant that any devices on the "public" subnet (192.168.1.0/24) couldn't talk to devices on the "private" subnet through the internal network, because though the request would get through, the response would get diverted to the VPN and lost. In my case I wanted to be able to access file shares from a server on the private network, so I decided to clear the mark for anything that should be sent to the public network. I did that by adding the line:
iptables -t mangle -A PREROUTING -s 192.168.2.0/24 -d 192.168.1.0/24 -j MARK --set-mark 0
to Administration/Scripts/Firewall. You can add a similar rule for any ports you intend to forward to the router from the private network.
Fail-safe: Also known as a "kill switch," I added a couple additional rules to Administration/Scripts/Firewall that are meant to prevent anything from the private network going to the unprotected WAN (vlan2). This means that if the VPN goes down for some reason, clients connecting to the private network can't accidentally communicate over the unprotected WAN interface.
iptables -I FORWARD -s 192.168.2.0/24 -o vlan2 -m state --state NEW -j REJECT --reject-with icmp-host-prohibited
iptables -I FORWARD -p tcp -s 192.168.2.0/24 -o vlan2 -m state --state NEW -j REJECT --reject-with tcp-reset