# GNU/Linux UFW VPN kill switch tutorial
This is a quick guide for setting up a kill switch using UFW (Uncomplicated FireWall).
It is assumed you are using OpenVPN and optionally Network-Manager with network-manager-openvpn.

## 1. (Optional) IP Addresses
Before we can start we're going to need the IP address (or the IP addresses) of your VPN so that we can whitelist those later on, write them down. They are obviously going to be different for every VPN and VPNs with multiple servers, so I'll leave this up to you.

## 2. Install & Enable UFW
On some systems UFW is installed and enabled by default (Ubuntu, for example). Installation procedure is going to be different for every distribution of GNU/Linux, but once you've got it installed enabling it is easy (assuming you have sudo):

```
sudo ufw enable
```

## 3. Block All Traffic
Block all outgoing traffic:

```
sudo ufw default deny outgoing
```

And also block all incoming traffic:

```
sudo ufw default deny incoming
```

## 4. Make an exception for OpenVPN
It is assumed you are using TUN as a network adapter (if you're unsure you most definitely are).
Allow outgoing traffic on tun0:

```
sudo ufw allow out on tun0 from any to any
```

And optionally allow incoming traffic on tun0 (if you're a seeder, for example):

```
sudo ufw allow in on tun0 from any to any
```

Choose 5.A. or 5.B. depending on your VPN situation

## 5.A. (Optional) Make an exception for your VPN with a static address or range
At this point you're technically done, but with this setup you would need to disable UFW every time OpenVPN needed to connect to your VPN and then re-enable UFW when it has connected. Instead of doing that you could add the IP addresses mentioned earlier as exceptions to UFW.

To add a single IP:

```
sudo ufw allow out from any to 123.123.123.123
```   
   
To add a range, use a mask:

```
sudo ufw allow out from any to 123.123.123.0/24
```

Go to step 6.

## 5.B.1 (Optional) Make another exception for OpenVPN

If you didn't follow 5.A, and your VPN service changes/rotates IP addresses you will at least need to allow OpenVPN to somehow communicate to the outside:

```
sudo ufw allow out 1198/udp
sudo ufw allow in 1198/udp
```

Here, `1198` is the port number that OpenVPN uses, but be careful as default is actually `1194`, you might have to check your VPN configuration files (the line that begins with `remote {server} {port} ...` or a line with `rport {port}`) for the actual port number used. Might also need to add the same rules for tcp.

This will allow you to at least disable ufw, connect to your VPN, and then enable ufw again to turn the kill switch back on. You will, however, have to do this every time you want to connect or if you're disconnected, which isn't entirely desirable.

## 5.B.2 (Optional) Force OpenVPN to use a specific port when authenticating to allow reconnecting

By default, OpenVPN will use a random port when connecting to the VPN.
Replace the `ǹobind` option from your VPN configuration files with `bind` to force OpenVPN to use the desired port (`1194` by default), and add the desired port (for example `port 1198`). But beware; this won't work on a system with multiple VPN clients on the same host, e.g. it will only work if you connect to one VPN at a time (unless you specifically bind different ports for different VPNs, of course, but you need to be aware of this).

Example; replace `nobind` in a `/etc/openvpn/client/{name}.conf` with:

```
local 0.0.0.0
lport 1198
bind
```

The local option is required (trivia: because "the C API" - bind() always takes an address *and*
a port number, so you can't just bind to an address alone.)

There's a high possibility openvpn will try to resolve a host address, in that case add a rule for DNS:

```
sudo ufw allow out 53
sudo ufw allow in 53
```

This works fine with `sudo openvpn /etc/openvpn/client/{name}.conf`, but I have yet to make it work with NetworkManager+OpenVPN, I'll try looking into it in the future, but at first glance it seems only `--nobind` is supported by the plugin, meaning someone will probably have to implement support for bind. 

## 6. Check that it's working

```
sudo ufw status
```

And test your internet connection!

## 7. You're done!
Congratulations, you've configured a VPN Kill switch on your GNU/Linux system!


---

Thanks to:

- Me!
- [formeroosid](https://github.com/formeroosid)