I can't see the interfaces in DHCPv6 relay, but i dont have this problem in Dhcp
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Picture dhcp relay (ipv4) show the interfaces.
Picture dhcpV6 relay, no show the interfaces.


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See where it says interfaces without ip address will not be shown..
Are you trying to set up a dhcpv6 relay on interface without ipv6 address? BTW I would find it unlikely you would want to relay dhcp requests on your "wan" interface!!
So for example I have ipv6 on some of my interfaces.. You can see they are listed