Adding a VPN “KillSwitch” to NetworkManager

5 September 2014

I use a VPN connection most days, and leave it unattended. Sometimes, however, it is not as stable as I would like it to be, and it disconnects, for instance because of wireless network problems. The wireless connection is brought up again, but the VPN connection stays down, causing potentially sensible data to be exchanged through an insecure channel. Bad.

How can we solve this?

In jargon, a mechanism that disconnects you from the Internet when your VPN connection disconnects, is called a “VPN Kill Switch”. There is a simple way to add one to NetworkManager, thankfully, via a dispatcher script. These scripts will be automatically called when a connection goes up or down.

You can save the following in /etc/NetworkManager/dispatcher.d/99vpnkillswitch, and give it 0755 permissions (it must be owned by root):

#!/bin/sh

IFFILE=/var/run/vpnkillswitch.iface

interface=$1 status=$2
case $status in
    vpn-up)
        # Get the physical device associated with the VPN connection
        nmcli -f type,device c | awk '$1~/^vpn$/ && $2~/[^\-][^\-]/ { print $2; }' > "${IFFILE}"
    ;;
    vpn-down)
        xargs -n 1 -a "${IFFILE}" nmcli device disconnect
    ;;
esac

You’re good to go! Try stopping your VPN interface, and your associated physical interface should be brought down too.

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