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Use iptables to Configure a VPN Kill Switch on OpenVPN Clients

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A virtual private network is often used to evade censorship, surveillance, or geolocation by routing internet traffic from your local device to the remote VPN server through an encrypted tunnel. In this scenario, the VPN server is the internet gateway for all connected client devices, and it forwards traffic from clients out to the internet, then receives and routes the traffic back to the client devices. However, there is always a risk that the VPN connection will unexpectedly drop, which can result in your traffic being communicated over the public internet instead of through the encrypted VPN connection.

For this reason, VPN clients often use firewall rules to ensure that internet traffic is allowed only to the VPN gateway. This protects the client’s traffic from being compromised in the event of a sudden disconnection from the VPN server. This functionality is sometimes referred to as a kill switch, because it has the effect of instantly blocking all connections to the internet if the VPN connection should fail.

Before You Begin

This guide assumes that you already have an OpenVPN server running on your Linode, and have at least one client configured to connect to it. If you need help doing this, see our three-part series on setting up an OpenVPN environment:

  1. Set Up a Hardened OpenVPN Server with Debian

  2. Tunnel Your Internet Traffic Through an OpenVPN Server

  3. Configuring OpenVPN Client Devices

Gather Client Device Information

Before configuring a client device, you will need to know:

  • The device’s network interface name.
  • The client’s local network’s subnet.

You can find this information by running the route command on your Linux client.

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         gw-li938.linode 0.0.0.0         UG    0      0        0 eth0
45.56.120.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0

The output shows the network interface name under the Iface column (eth0), and the LAN’s subnet under the Genmask (255.255.255.0). These values will be used throughout the remainder of this guide, so replace wlp6s0 and 198.168.0.1/24 with the interface and IP address/subnet found by running route on your client.

For macOS, the commands networksetup -listallhardwareports and ifconfig will show all your possible network interfaces and associated network information. From that list, you can find your ethernet and WiFi device names and their local subnet.

Configure client.ovpn

On your client, change the client.ovpn configuration file as follows:

  1. You should already have the setting dev tun to specify the virtual network adapter. Change it to tun0 so it can be referred to in firewall rules:

    client.ovpn 
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    dev tun0

  2. Make sure your VPN server is listed by its IP address instead of a hostname. For example:

    client.ovpn 
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    remote 198.51.100.0 1194

GNU/Linux Clients

The majority of GNU/Linux users use either iptables or ufw to manage their firewall. This guide will cover configuration for both of these options.

VPN firewall using iptables

Caution
You may want to back up your current iptables ruleset with iptables-save.

  1. Create a shell script with the following iptables ruleset:

    iptables-vpn.sh 
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    #!/bin/bash
iptables --flush
iptables --delete-chain
iptables -t nat --flush
iptables -t nat --delete-chain
iptables -P OUTPUT DROP
iptables -A INPUT -j ACCEPT -i lo
iptables -A OUTPUT -j ACCEPT -o lo
iptables -A INPUT --src 192.168.0.0/24 -j ACCEPT -i wlp6s0
iptables -A OUTPUT -d 192.168.0.0/24 -j ACCEPT -o wlp6s0
iptables -A OUTPUT -j ACCEPT -d 198.51.100.0 -o wlp6s0 -p udp -m udp --dport 1194
iptables -A INPUT -j ACCEPT -s 198.51.100.0 -i wlp6s0 -p udp -m udp --sport 1194
iptables -A INPUT -j ACCEPT -i tun0
iptables -A OUTPUT -j ACCEPT -o tun0

  2. Save the script as iptables-vpn.sh, then set the permissions using chmod and execute the script:

    chmod +x iptables-vpn.sh
./iptables-vpn.sh

This ruleset replaces the pre-exiting iptables rules and instructs the firewall to drop every outgoing connection other than loopback traffic, the local network’s subnet and UDP traffic to and from your OpenVPN server’s IP on port 1194. In addition, all incoming and outgoing connections are allowed over the virtual network interface tun0.

Your VPN firewall is now active, but this ruleset is only temporary and will be cleared when you reboot your Linode. To make the firewall permanent, you can install the iptables-persistent package for Debian or Ubuntu-based distributions, or you can see our iptables or Firewalld guides to create permanent rulesets and/or profiles.

VPN Firewall using ufw

Caution
You may want to back up your current firewall ruleset.

  1. Create a new shell script containing the following commands:

    ufw-vpn.sh 
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    ufw --force reset
ufw default deny incoming
ufw default deny outgoing
ufw allow in on tun0
ufw allow out on tun0
ufw allow in on wlp6s0 from 192.168.0.0/24
ufw allow out on wlp6s0 to 192.168.0.0/24
ufw allow out on wlp6s0 to 198.51.100.0 port 1194  proto udp
ufw allow in on wlp6s0 from 198.51.100.0 port 1194 proto udp
ufw enable

  2. Save the script as ufw-vpn.sh, then set the permissions using chmod and execute the script:

    chmod +x ufw-ks.sh
./ufw-ks.sh

Your VPN firewall is now active. Use ufw disable if you want to disable the firewall.

Apple OS X / macOS

Pf is the firewall application used by newer versions of OS X and macOS.

  1. Edit the pf configuration file:

    /etc/pf.conf 
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    block drop all
pass on lo0
pass on utun0
pass out proto udp from any to 198.51.100.0 port 1194

  2. Import the newly added rules as follows:

    pfctl -f /etc/pf.conf

  3. Turn on the firewall:

    pfctl -e

Once pf is enabled, your VPN firewall is active. Use pfctl -d if you need to deactivate the firewall.

In recent versions of OS X or macOS with the Tunnelblick OpenVPN client, you might have an unused utun interface, in which case you will not be able to connect to the VPN server. You can check for unused interfaces with ifconfig.

If you have an unused utun0, for example, then change pass on utun0 in pf.conf:

pass on utun1

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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This guide is published under a CC BY-ND 4.0 license.