OpenVPN 2.4 treats "udp" and "tcp" as dual stack now, move old preference to udp4/tcp4
OpenVPN 2.4 changed the meaning of "udp" and "tcp" to be dual stack, listening on IPv6 on all interfaces at once. "local" can still nudge the behavior but to preserve the user's choice, it's safest to move the user preference to udp4/tcp4 which reflect the previous behavior.
Also it's a good time to introduce "multihome" for those who want to run dual stack which will make OpenVPN reply from the address it received the packet on.
To me, I have a patch
Update OpenVPN protocol preferences to match the new behavior of OpenVPN 2.4. Fixes #7062
Fix #7074: Fix automatic port number guessing
Rework openvpn_port_used() to take care of following conflicts after
ticket #7062 was committed:
- "UDP" is dual stack and conflicts with "UDP4" and "UDP6"
- "TCP" is dual stack and conflicts with "TCP4" and "TCP6"
- "UDP4" and "UDP6" do not conflict unless interface is set to "any"
- "TCP4" and "TCP6" do not conflict unless interface is set to "any"