Speed up OpenVPN by Disabling IPv6 (Update June 2021)

Just a quicky,

P.P.P.S As of 9th June, the results are inconsistent. Sometimes disabling IPv6 is substantially faster (24 mbps vs 5mbps), sometimes it is about the same. It seems to be generally faster with it disabled.

P.P.S According to my testing as of 29th May 2021, on kernel 5.12.7-arch1-1, Network speeds are substantially slower when IPv6 is disabled. Please ignore this article if you are not having performance issues. According to https://ipv6leak.com/, IPv6 does not seem to leak either.

P.S Discussion is here

https://www.reddit.com/r/linux/comments/lw1ko9/disabling_ipv6_for_openvpn_tunnels_can_speed_up/

All Linux users manually connecting through OpenVPN should know that many VPN providers don’t yet support IPv6 or have it enabled in their configurations. I encountered this after a couple of days of near native speeds of 90Mbps to just around 1.5-15Mbps fluctuation.

Many articles such as this one show you how to disable IPv6

https://protonvpn.com/support/disable-ipv6-protocol-linux/

The problem is they tend to disable all IPv6 Functionality. While IPv6 is not yet widely used or supported I do like having the technology enabled for learning and testing.

And from what I discovered, the only interface that needs IPv6 disabled is the VPN tunnel itself.

If you run ip a in a terminal

You will see the VPN tunnel tun0 which has an IPv6 address fe80::7297:2fd6:baaf:6e1d. This is no good for OpenVPN, so we can simply disable IPv6 for virtual interface tun0.

P.S as user u/sarcism points out, the above statement is very flawed. I do agree. It’s not that it is no good for OpenVPN, just that it is no good for the server side configuration used by VPN providers. IPv6 is supported in OpenVPN, just that many providers are not onto it yet.

The sysctl is used to interact with some kernel module parameters at runtime, in our case, we want to disable IPv6 for tun0.

$ sudo sysctl net.ipv6.conf.tun0.disable_ipv6=1

Now if you test to see if your IPv6 address was removed. Running ip a again gives me,

And as you can see, the inet6 address has been removed. Then test your internet speeds, which should improve or at least become more stable.

To make the change permanent on reboot, you need to include the option in the /etc/sysctl.conf file by appending

net.ipv6.conf.tun0.disable_ipv6=1

to the end of the file.

Knowing how to use sysctl is a great boon to runtime testing of kernel parameters, and saves time from changing the GRUB config or rebooting.

To see all kernel parameters that can be changed, run

$ sysctl -a

To narrow down the selection, run

$ sysctl -a -r {{ pattern }}

Example here

$ sysctl -a -r ipv6 | grep disabled

Be sure to read the man pages and the Arch Wiki for more information!

Happy web surfing!