pfSense

Found out when enabling RADVD on ovpns1 (a tun virtual interface), that the configuration page does not add UnicastOnly on; to the radvd.conf, resulting in nonworking IPv6 and log spam like this:
radvd: interface ovpns1 does not support broadcast
radvd: do you need to add the UnicastOnly flag?
radvd: interface ovpns1 does not support broadcast
radvd: do you need to add the UnicastOnly flag?
radvd: interface ovpns1 does not support broadcast
radvd: do you need to add the UnicastOnly flag?

Fix:
Add:
if (strstr($realif, "ovpns")) {
$radvdconf .= "\tUnicastOnly on;\n";
}

right after:
$ifcfgipv6 = get_interface_ipv6($dhcpv6if);
if (!is_ipaddrv6($ifcfgipv6))
continue;

$ifcfgsnv6 = get_interface_subnetv6($dhcpv6if);
        $subnetv6 = gen_subnetv6($ifcfgipv6, $ifcfgsnv6);
        $radvdifs[$realif] = $realif;

$radvdconf .= "# Generated for DHCPv6 Server $dhcpv6if\n";
        $radvdconf .= "interface {$realif} {\n";

in /etc/inc/services.inc