I've got a server on my LAN providing DHCP for IPv4. I want to use OpenWrt to relay IPv6 DHCP assignments from my Xfinity cable modem to my LAN, but I don't want it to issue IPv4 leases.
On the LAN interface I have DHCP set to Ignore interface and have the IPv6 tab set to relay for RA, DHCPv6, and NDP proxy. I'm also trying without ignore interface but with dynamic DHCP disabled. Is there some other combination I can use to get IPv6 addresses but not IPv4 DHCP?
Noted. I changed the subject line to reflect that it's not the specific program I'm concerned with, but the options in the LUCI page. I'm not seeing v6 addresses on my LAN, but the router has an address on its WAN interface.
This situation is effectively broken, your ISP should provide you an prefix for your own disposal - and the ISP's CPE should allow you to sub-let prefixes from this range via DHCPv6-PD (proprietary firmware often get this wrong though).
BRouter setups using NDP relaying are possible (as documented in the wiki), but only a measure of last resort. At least try to configure your ISP CPE to provide DHCPv6-PD correctly.