deny unknown-clients in DHCPv6

[dqq]

Hello:
 Now I want to use dhcp-4.2.3-PI as my dhcpv6 server.
 In my dhcpv6 server,I want to use MAC address to control the access of users,but when I use the "deny unknown-clients" in my config file,The unknown-clients also get the address.I known it works well in dhcpv4 , I don't known why it can't work in HDCPv6. Can you give me any ideas?
  
 P.s
 In the mailing list ,I see the same question, but unfortunately there is no answer corresponding to this question.
  
 And the related config in  my conffile look like this:
 subnet6 2001:db8:1111::/64 {
                option dhcp6.domain-search "nic";
        range6 2001:db8:1111::10 2001:db8:1111::fff;
         # Use the whole /64 prefix for temporary addresses
        #  (i.e., direct application of RFC 4941)
        range6 2001:db8::/64 temporary;
         # Some /64 prefixes available for Prefix Delegation (RFC 3633)
        prefix6 2000:: 2001:db8:: /80;
     # deny unknown-clients;
        deny unknown-clients;
}
 host{
 hardware xx:xx:xx:xx:xx:xx;
 }

 Look forward to your reply...
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