dnsmasq-full does not work in IPv6 relay mode

We definitely need DHCPv6 relay as some ISP only provides a /64 prefix, where relay is the only way to grant IPv6 accesses to LAN devices.

By default, OpenWrt uses dnsmasq for IPv4 DHCP and odhcpd-ipv6only for IPv6 DHCP. The DHCPv6 relay (powered by odhcpd-ipv6only) works as expected and we only need to change /etc/config/dhcp to be like this to make it work:

config dhcp 'lan'
        option interface 'lan'
...IPv4 stuff omitted...
        option ra 'relay'
        option dhcpv6 'relay'
        option ndp 'relay'

config dhcp 'wan'
        option interface 'wan'
        option ignore '1'

config dhcp 'wan6'
        option interface 'wan6'
        option dhcpv6 'relay'
        option ra 'relay'
        option ndp 'relay'
        option master '1

If one uses dnsmasq-full to handle DHCP for both IPv4 and IPv6(opkg remove dnsmasq odhcpd-ipv6only and opkg install dnsmasq-full), the DHCPv6 relay stops working. The /etc/init.d/dnsmasq script does not seem to generate dnsmasq configurations for these IPv6 options:

Confirmed on OpenWrt 23.05.3 x86-64.

Start by inspecting /etc/init.d/dnsmasq and the /etc/config/dhcp relay settings shown in the report, then compare the generated configuration with the working odhcpd-ipv6only setup. Done means dnsmasq-full supports the documented IPv6 relay options and DHCPv6 relay works on OpenWrt 23.05.3 x86-64.