it's seems that opnsense default install block multicast traffic on lan ... why ?
(ffe80:: xxxxxx f02::16 ip Default deny rule ...)
if i do a factory reset, then let default config ....
LAN computer with IPV6 ONLy can't connect to internet (for sure local link is working ...) ... (ipv4 can !)
finally get it wprking buit DHCPv only give ip not dns or gw parameter !