OpenVPN: Add support for IPv6 dynamic prefix selection
When WAN is obtaining an IPv6 prefix that allows multiple prefix IDs (i.e. smaller than /64), allow selection of an IPv6 prefix ID in OpenVPN setup so that if the prefix changes in the future, OpenVPN would be automatically updated to support the new prefix. Currently, manually entering a valid /64 prefix is required.
Basically, this is extending the "Track Interface" functionality found in the standard network interfaces to OpenVPN.
My ISP provides me with 2001:aaaa:bbbb:cc00::/56.
I want to select prefix ID f0 for OpenVPN use (2001:aaaa:bbbb:ccf0::/64)
ISP prefix in the future changes to 2001:abcd:1234:aa00::/56
OpenVPN should automatically be updated to new prefix (2001:abcd:1234:aaf0::/64)
Obviously there should be validation to make sure that the prefix ID selected isn't already in use on another interface.
The only downside to this is that automatically restarting the OpenVPN service when such a change occurs would unexpectedly disconnect any users that are connected. Maybe show this as a warning if specifying a prefix ID is chosen over manually entering a prefix?