I've setup a server for fixed DHCPv6 addresses for a number of clients using the Client DUID that appears in /var/lib/dhcp/dhclient-*.lease when the client first connects. I'm using the ISC DHCP server.

However, at this point the client has already received an undesired IPv6 address.

How do I determine the Client DUID before the client performs its first DHCPv6 request?