[eduVPN-deploy] Dropping support for OpenVPN 2.3 clients

[François Kooman]

Hi,

Real soon now, I want to push a release that will break support for
OpenVPN 2.3 clients by tightening up some crypto related settings and
dropping IPv6 default routing hacks.

The good news is, OpenVPN 2.3 wasn't supported since forever in the
default server installation, so for most people it won't make any
difference. But I happen to know that there are still some deployments
out there that operate in "2.3 compat" mode, i.e. use "tls-auth" and not
(yet) "tls-crypt".

The official eduVPN/Let's Connect! applications all use OpenVPN 2.4, or
3 and are thus not affected.

For an up to date list of compatible clients, see [1]. If you have
experience with an OpenVPN client not mentioned here, feel free to
contribute any notes regarding it!

Please let me know if dropping OpenVPN 2.3 support will be problem for
you! I'll be testing the changes in the next few days across a variety
of deployed servers with different clients.

Regards,
François

[1] https://github.com/eduvpn/documentation/blob/master/CLIENT_COMPAT.md