This proposed solution is for Debian-based distributions. We recommend disabling IPv6 if you are using a manual VPN connection, such as OpenVPN.
1) Open Terminal and enter the command sudo nano /etc/sysctl.conf
2) Add all the lines to the bottom of the file and save it:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.tun0.disable_ipv6 = 1
3) Now make the system read the file and confirm the changes using this line:
sudo sysctl -p
That’s it! In order to check whether IPv6 was disabled on your computer, you can do an IPv6 leak test here.