Whilst I agree with most of what you say - including the need to update the port forwarding article, the above statement is incomplete. Roon ARC now prefers IPv6 for external connectivity when both IPv6 and IPv4 are available at both ends of the connection.
In the UK many cellular service providers and publuc WiFi access points only provide an IPv4 connection. In these circumstances Roon ARC will continue to use the IPv4 connection even when ipv6 is avaliable on the Roon Server.
With regard to your recommendations, I would just like to add one thing more:
With IPv6, it is possible for devices to be allocated multiple global ipv6 addresses (e.g. One via DHCPv6 and one via SLAAC). In these circumstances, I have always err’d on the safe side and opened ipv6 firewall exceptions for all public ip addresses on the basis that I dont know which one will be used.
Is this necessary? And if it is not, how do I know which ipv6 address is going to be used? Does the Roon ARC settings page just show the one ip address that will be used by ARC (I can’t tell at present because my Roon Server only has one IPv6 address) or does it show all IPv6 addresses.
Finally, if ipv6 is now the preferred transport, is it still appropriate to bury the ipv6 address in the ‘advanced’ section of the Roon ARC settings page.
Edit: if ipv6 is now going to be preferred, I also think that making the diagnostics in the Roon ARC settings page needs to be reworked as I suggested a while back in my feature suggestion post at:
At present, if I disable my ipv4 port forwarding rule, then I cannot use ARC from my ipv4 only cellular phone service but the Roon ARC settings page still reports ‘Ready’ because the ipv6 connectivity is working even though I can’t use it when using ARC on cellular. I presume the converse is also true but since it is rare to be without an IPv4 connection, this does not matter.